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Ms. Pratyasha Sahani
Prof. R. Vijaya
16109275
Anisotropy of spectrally-resolved polarized light in two- and three-dimensional photonic crystals
13/09/2021
10:00 AM
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Photonic crystals have fascinating properties that enable the control and modification of light to such extents unknown in bulk optics. Recent advances in photonic crystals have revolutionized the studies further, pushing the frontier towards miniature lasers and chip-level integrated optoelectronic devices. They also provide a vital platform for tailoring the polarization properties of electromagnetic waves in a manner suitable to widen their application prospects. This thesis traverses from the basic understanding of the polarization properties of light in three-dimensional (3D) colloidal photonic crystals (or opals) to the design of a wide-angle polarizer based on a two-dimensional (2D) photonic crystal slab at the conventional telecommunication wavelength.
The first part of the seminar will be on the experimental studies that analyze the polarization state of light emitted from an active opal, when it is pumped in the absorption band of its emitter molecule. This is studied when the emitted light lies within the stopband and outside the stopband of the opal, by collecting the angle-dependent light emission. We observe the emitted light to be predominantly unpolarized with a small polarized component, even if the incident light is linearly polarized. In addition, a comprehensive quantitative analysis of the polarization properties of the transmitted light from a passive opal is carried out by performing Mueller matrix polarimetric measurements. The effects of the coherent scattering regime (inside the stopband) and the incoherent scattering regime (outside the stopband) are demonstrated on the polarization state of the transmitted light with a suitable choice of the incident/transmission wavelengths. The polarization properties of the light are quantified by calculating the degree of polarization (DOP) and therefore, Stokes vector of the polarized component and the polarization ellipse parameters for linearly and circularly polarized incident states. For linearly polarized incident light, the DOP of the transmitted light is observed to gradually decrease from unity when the transmission wavelength moves from inside the stopband to outside the stopband of the opal, and the polarized component evolves from linear inside the stopband to elliptical outside the stopband. On the other hand, DOP remains nearly the same and close to unity, and the polarized component has a dominant circular-polarization character, independent of the spectral position of the transmitted light, for circularly polarized incident light. Suitable reference samples are also studied in the same experimental set-up for comparison.
The second part of the seminar will be on the numerical analysis and design of a 2D photonic crystal slab-based polarizer. The polarizer is designed on a silicon wafer and silicon-on-insulator wafer, enabling its realization using silicon-based nanofabrication techniques. Different polarization states are possible from the designed structure, merely dependent on the wavelength of operation, direction of propagation, and the angle of incidence of light. The advantage of this work is that it does not require the difficult in-plane coupling geometry for light into the slab, and the much-easier out-of-plane incidence is sufficient. It is further demonstrated that the photonic crystal slab can be structurally engineered for its polarizer action over wider angles of incidence at the desired telecommunication wavelength.
All interested are welcome to attend.
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Divya Rawat
Prof. Pankaj Jain
16109266
Timing and spectral study of highly spinning black hole x-ray binary systems
15/03/2021
10:00 AM
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Black-hole X-ray Binaries (BHXBs) are systems consisting of a black hole and a companion star orbiting around the centre of mass of the combined system. The material from the companion accretes to the compact object via either Roche lobe flow or stellar wind accretion. The gravitational potential energy of the accreting material is released in the form of electromagnetic radiations.
As a part of my PhD thesis, my work has been centred on the temporal and spectral study of these systems in the X-ray band. These systems emit dominantly in X-ray and show high X-ray variability. The X-ray variability study of these systems involves PDS (Power-Density Spectra), which is the Fourier transform of the light curve, time-lag spectra (the study of the time lag between the photons of different energy bands) and RMS spectra (study of the distribution of photons in different energy bands). The LFQPOs (Low-frequency Quasi-periodic oscillations) are the ubiquitous feature of black hole X-ray binaries. There are several models that have been proposed till now in literature that claims to explain the origin and these LFQPOs. These involved the Relativistic Precession Model (RPM), The precessing inner flow model, Corrugation modes, Accretion ejection instability (AEI), Propagating oscillatory shock, Pressure or accretion rate modes.
The energy spectra (spectral study) of these systems consist of the thermal and non-thermal component. The thermal component originates from the inner part of the accretion disk and is modelled with a multi-temperature blackbody. While the non-thermal component is usually modelled as a power law (PL). This component originates as a result of inverse comptonization that is the interaction of thermal component with hot corona (Temperature~MeV).
I have analysed the data from AstroSat and NICER (Neutron Star Interior Composition Explorer) in 0.3-80.0 keV and 0.2-12.0 keV energy bands respectively. I have studied the highly spinning microquasar source GRS 1915+105 temporally when it was making a transition from non-variable to heartbeat state via the intermediate state. We reported a LFQPO with varying frequency (3-5 Hz) and studied the temporal properties. We found that the temporal properties remain unaffected as the source made a transition. Then, we did a spectro-temporal study and identified the origin of the C-type LFQPO as the dynamical frequency which is inverse of the sound crossing time. Our result supports the Pressure or accretion rate modes model. Next, I did time-resolved spectroscopy to study the variation of spectral components in details in the heartbeat state. We found that the flux of the non-thermal component varies with accretion rate as Flux ∝ Ṁ^(1/2) and nearly independent of the inner disk radii. We argue that this may imply that the corona is powered by the spin of the black hole rather than accretion.
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Vimalesh Kumar Vimal
Prof. V Subrahmanyam and Prof. H. Wanare
14109886
Dynamics of Quantum Correlation in Kitaev Spin Chain
02/03/2021
10:00 AM
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Quantum many body systems show the correlation that does not have a classical counterpart. It is known as entanglement. It is a physical phenomenon in which a quantum state of many subsystems can not be described independently of each other even when they are spatially separated. Entanglement is a key resource in quantum information processing, quantum cryptography, and quantum teleportation. In the last two decades, a lot of work has been done to conceptualize entanglement in the quantum spin chains. Quantifying and understanding the structure of the entanglement is a major interest in this field. Many such systems have been explored extensively from a quantum information perspective. 1D spin chains have been in the center of research for its simplicity and resemblance properties with the real systems. We present the study of one such 1D spin model which has a Kitaev type spin interaction. The Hamiltonian of the system is defined by the nearest neighbour interactions of spins with a global magnetic field term in the transverse direction of the interactions. The Hamiltonian can be block diagonalized in the momentum space for each mode by the usual steps of diagonalization used for the 1D spin chains. The Hamiltonian shows macroscopic degeneracy in the ground state which can be split by the transverse magnetic field. We quantify different local and global correlation measures like pairwise entanglement, quantum discord, and the global entanglement in the GS. These correlation measures have been used to detect the quantum phase transitions for many other spin-1/2 models. The results of this model show a bit of contrast to the others spin chains. It doesn't show a very pronounced signal for detecting QPT. Although, the analysis of macroscopic entanglement b/w spins up and spins down shows the trace of the QTP at the critical point.
We also discuss the dynamics of these correlations along with magnetization. The entanglement is generated quickly after the evolution starts. At the arbitrary parameters, the entanglement and the other correlation measures show the revivals peaks after a sufficient time of revivals. In the critical zone, it shows the envelope decay of these correlations. The dynamics of the system have some interesting features when we use kicked magnetic field in the system. For some specific kick values, it can generate entanglement at the alternate kicks or it may not generate entanglement at all throughout the evolution. We also discuss some characteristics of the Loschmidt echo for this Hamiltonian with different initial states.
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Krishnendu Dandapat
Prof. Saurabh Mani Tripathi
16209861
Studies on optical waveguide long-period gratings and their application as sensors
30/12/2020
10:00 AM - 11:00 AM
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Long-period gratings, in their simplest form, are periodic refractive index variations created within the core region of optical waveguides along the direction of propagation of optical modes. They are used for resonant power coupling between co-propagating modes, mostly from fundamental core mode to higher-order cladding modes, and have attracted considerable research attention in many areas, including a variety of sensors, bandpass and bandstop filter, EDFA gain flattening etc.. The large evanescent field associated with the cladding modes makes them an ideal choice for bio-sensing applications. In this work, excitation of various cladding modes using novel grating designs to obtain extremely high sensitivity, complete temperature insensitivity, and simultaneous temperature and strain insensitivity are investigated in detail. Fundamental physics to achieve inherent temperature insensitivity by controlling and manipulating temperature-induced phase changes in the grating regions is explored. We have shown that by suitably selecting and exciting two cladding modes and tailoring their dispersion properties, the temperature-induced phase difference accumulated between the fundamental core mode and one cladding mode can be compensated by that of the other cladding mode.
A new design of concatenated long period gratings is proposed to reduce the temperature and strain cross-sensitivity simultaneously. The insensitivity has been achieved by inherently nullifying the (i) temperature-induced phase inside the fiber by properly adjusting the core dopants and their concentrations, and (ii) strain-induced phase changes inside one LPFG with that inside the other LPFG by optimising their grating parameters. Our study should find application in designing inherently temperature and strain insensitive high precision bio/chemical sensors.
Further, tailoring the dispersion properties of cladding modes by optimizing the core and cladding regions' Opto-geometric properties and related grating parameters, we show that two distinct turn-around wavelengths, where the resonance wavelength suddenly flips its spectral nature, can be achieved for the same pair of core-cladding modes. The Discovery of this unique feature enhances the sensitivity by many folds and can also be used in designing specific perturbation insensitive sensors.
Some specific applications of optical fiber gratings for quantitative detection of E. Coli bacteria, trace concentrations of microcystin-LR, and trace concentrations of methanol and water present in bio-fuel are also discussed in detail.
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Swayamshree Patra
Prof. Debashish Chowdhury
15109863
Rulers, timers and transport for length control in long cell protrusions
21/12/2020
12:00 noon
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A living cell uses its long cylindrical appendages for locomotion and sensory purposes. Hence, assembling and maintaining a protrusion of correct length is crucial for its survival and overall performance. Usually the protrusions lack the machinery for the synthesis of building blocks and imports them from the cell body.
What are the unique features of the transport logistics which facilitate the exchange of these building blocks between the cell and the protrusion? What kind of ‘rulers’ and ‘timers’ does the cell use for constructing its appendages of correct length on time? How do the multiple appendages coordinate and communicate among themselves during different stages of their existence? How frequently do the fluctuations drive the length of these dynamic protrusions out of the acceptable bounds?
I will address some of these questions in the context of a specific cell appendage called eukaryotic flagellum (also called cilium). Motivated by real experiments, we frame minimal models for capturing these wide range of interesting phenomena in the context of flagellar length control. We analyse our models by borrowing tools from non equilibrium statistical mechanics and stochastic processes.
All interested are welcome to attend this (virtual) open seminar.
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Krishanu Sadhukhan
Prof. Amit Agarwal
14109878
A study of anisotropic plasmon in tilted Dirac materials
03/01/2020
11:00 AM
FB 382
Plasmons are collective density oscillation of the electronic charges. Collective behavior of electrons are characterized by elementary excitations which are long lived eigenstates evolving independently with a definite energy-momentum relation. Plasmon is one of such excitations. On the other hand after the remarkable discovery of graphene in the last decade people become interested in the field of topological Dirac materials and many 2D and 3D anisotropic Dirac materials have been discovered. In this work, the plasmonic properties of 2D and 3D anisotropic tilted Dirac materials are investigated within random phase approximation using a generalized classical hydrodynamic theory and rigorous quantum mechanical framework. 2D anisotropic Dirac fermions are realized in 8-Pmmn borophene which exhibit anisotropic plasmon, dynamical screening and Friedel oscillation. 3D Dirac plasmon is experimentally observed in type-II Dirac semimetal (DSM) PtTe2 and our theoretical findings perfectly matches with the experimental data. We show the existence of a novel undamped gapless plasmon mode in type-II DSM when the momentum transfer is along the direction of the tilt. We also generalize the classical hydrodynamic theory of plasmon to hydrodynamic theory for multi-component electron liquid and apply this to known systems like spin polarized electron gasses, spatially separated 2DEG, tilted DSM etc. These classical predictions are in good agreement with the quantum mechanical results.
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Sayid Mondal
Prof. Gautam Sengupta
14209863
Aspects of Holographic Entanglement Negativity in AdS/CFT
12/12/2019
11:00 AM
FB 382
Entanglement is a fundamental property that distinguishes quantum system from classical ones. For bipartite pure states it is characterized by the von Neumann entropy of the reduced density matrix. However it fails to be a valid entanglement measure for bipartite mixed states. In this context Entanglement Negativity is a computable measure for characterizing mixed state entanglement in quantum information theory which provides an upper bound on the distillable entanglement.
We propose a covariant holographic entanglement negativity construction for time dependent mixed states of adjacent intervals in (1+1)-dimensional conformal field theories (CFT1+1) dual to non-static bulk anti-de Sitter (AdS) geometries through the AdS3/CFT2 correspondence. Subsequently, we explore the time evolution of the holographic entanglement negativity following a global quench for mixed states in (CFT1+1)s dual to bulk eternal BTZ black holes sliced in half by an end of the world (ETW) brane. Finally we describe a possible higher dimensional generalization of our construction and its application to an example of mixed states of adjacent subsystems in CFTds dual to bulk pure AdSd+1 space times and AdSd+1-Schwarzschild black holes.
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Shubhajyoti Mohapatra
Prof. Avinash Singh
13109075
Three-orbital-model investigation of electronic structure, magnetic excitations, and anisotropy effects in strongly spin-orbit coupled iridates and osmates
06/12/2019
03:00 PM
FB 382
Since the discovery of the spin-orbit assisted Jeff=1/2 Mott insulator Sr2IrO4 in 2008, the 5d family of transition-metal oxides, such as iridates and osmates with a variety of two and three dimensional lattices, octahedral sharing geometry and varying degrees of frustration, have been a subject of intensive study for novel physical phenomena that arise from the combined influence of spin-orbit coupling (SOC), electron correlations, and crystal field. As the SOC entangles real space geometry with spin space magnetism, the resulting J = 1/2 and 3/2 spin-orbital isospins are very sensitive to structural distortion and play key role in allowing various anisotropic magnetic interactions such as pseudo-dipolar, Dzyaloshinski-Moriya, Kitaev, and spin-off-diagonal (SOD) interactions. As a result, subtle interplay of these on-site interactions and the electron kinetic energy related to the hopping integrals is expected to drive various exotic quantum phenomena such as superconductivity, topological insulators, spin-liquid state etc. Intensive studies have been undertaken recently in an effort to better understand the combined effects of SOC, electronic correlations and structural distortions.
In this thesis, material-specific itinerant-electron models are investigated with respect to electronic structures, magnetic ground states, excitations, and magnetic anisotropy effects in square-lattice perovskites (Sr2IrO4, Sr3Ir2O7), honeycomb-lattice systems (Na2IrO3,RuCl3), and cubic perovskite (NaOsO3). Compared to the existing phenomenological spin models, the microscopic three-orbital-model approach with realistic parameters provides a unified description of the electronic and novel magnetic properties exhibited by the strongly spin-orbit coupled systems by simultaneously incorporating: (i) finite U + finite SOC effects, (ii) structural distortion effects, (iii) Hund's coupling, and (iv) intra- and inter-orbital pseudo-spin dynamics involving J=1/2 and 3/2 sectors. More importantly, our non-perturbative approach provides insight into the magnetic anisotropy effects induced by mixing between the J=1/2 and 3/2 sectors, not considered in earlier works.
The three-orbital-model approach provides a good account of the experimentally measured magnon energies, anisotropic effects, and high-energy spin-orbit exciton modes in both mono- and bi-layer iridates. Octahedral tilting-induced spin-dependent hoppings and associated anisotropic interactions in iridate heterostructures strongly suppress the magnon gap, driving the systems to the verge of a isospin reorientation transition. Bond directional anisotropic Kitaev and SOD interactions generated by the J=1/2 and 3/2 mixing stabilize novel magnetic orders in the honeycomb lattice iridates and ruthenium based compounds without Hund's coupling. Despite the nominally orbitally-quenched osmium ions in the 5d3 compound NaOsO3, the SOC and octahedral rotations play crucial role in explaining electronic band structure, insulating behavior and magnetic anisotropy effects.
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Meenaxi Sharma
Prof. Krishnacharya
14109267
Aqueous Drops on Thin Lubricating Fluid Coated Slippery Surfaces: Statics and Dynamics
28/11/2019
11:00 AM
FB 382
In recent years, interfacial science has emerged as an important research topic due to the increased concernment of micro- and nanotechnology, where surface effects are dominant compared to bulk or inertial. Surface or interfacial phenomena are observed in nature as well as in daily life in the form of water drops on various plant leaves due to moisture, gecko’s adhesion on a wall, formation of soap solutions, to name a few. All these phenomena can be understood on the basis of the interaction of the two objects and it depends on the physical and chemical nature of both the surfaces. The process involved in bringing a drop, placed on a solid substrate, into the equilibrium is mainly governed by the surface tension force. Over past few decades, a considerable amount of research has been carried out to create liquid shedding surfaces based on solid-liquid interaction and requirement of such surface is fulfilled with the development of superhydrophobic and superoleophobic surfaces using micro/nano-textures with appropriate surface chemistry. However, when a thin liquid film is deposited on a solid substrate, overall wetting behavior becomes quite different due to the appearance of a new liquid-liquid interface. This technique provides an alternative approach to create liquid shedding surfaces, where the thin liquid film behaves as a lubricating layer thus creating lubricating fluid infused slippery surfaces. Due to the involvement of four different phases, slippery surfaces offer enormous scope for fundamental research. The word done in the present thesis deals with the static and dynamic behavior of aqueous drops on lubricant coated slippery surfaces (LCS) to understand the role of various interfacial energies and also various physical parameters such as viscosity, roughness, etc.
I will first discuss the stability analysis of aqueous drops on lubricated surfaces with varying surface energies. Our investigations reveal that surface forces play a dominant role in determining the final state of the drop on the surface: whether stable (float) or unstable (sink). I will discuss in detail the dynamic of the sinking of aqueous drops with effect of lubricating fluid viscosity and substrate wettability along with the underlying mechanism. Further, I will talk about the other case, for which drops are stable on LCS and do not sink. Here I will discuss drop mobility on LCSs and investigate the effect of lubricating film thickness, surface tension and viscosity on the drop mobility together with the thickness optimization for the best slippery behavior. I have also developed a theoretical model to predict the drop velocity by comparing viscous dissipations in various regions of the two liquids.
The motion of the three-phase contact line along with sinking or slipping is further achieved by evaporation of liquid drops on dry and lubricated surfaces. Hence, I will discuss in detail a comparative study of evaporation of aqueous and binary mixture drops on dry and lubricated surfaces. Here I will mainly talk about different evaporation modes on dry and lubricated surfaces along with additional phenomena arising on lubricated surfaces such as wetting ridge: its formation and dynamics for an evaporating drop. I will also present a diffusion-based theoretical model to understand the whole evaporation process.
Towards the end, I will discuss the stability of aqueous drops on another kind of lubricated surfaces in which a homogenous solid surface is replaced by a heterogeneous surface (made of patterns of hydrophobic and hydrophilic regions). I will focus here on the static and dynamic behavior of aqueous drops on such heterogeneous surfaces by varying area fraction of hydrophobic and hydrophilic regions and draw a conclusion based on the comparison with homogenous one. I will present anisotropic slippage of water drops on such surfaces showing stick-slip type motion. I will also discuss a phase diagram based on the drop motion on chemically patterned lubricated surfaces.
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Ankit Kumar
Prof. Satyajit Banerjee
12209062
Exploring low magnetic field instabilities in the static and driven vortex state in K-doped BaFe2As2 superconductor
27/11/2019
12:00 PM
FB 382
In the vortex state of superconductor, there are various competing energies, viz., elastic energy, pinning energy, and thermal energy. Depending on their competition different phases of static vortex matter evolve in the field – temperature vortex matter phase diagram. Melting of a crystal of vortices is a unique and interesting phenomenon in superconductors. In the first part of my talk, I will present our work on thermal fluctuation induced low field vortex lattice melting phenomenon in an optimally doped Ba0.6K0.4Fe2As2 single crystal. While high field melting has been well studied, we report the first observation of a low-field vortex solid to liquid phase transition boundary. We show a close match with the theoretically predicted behaviour of the low field phase boundary. All of this study is via high-sensitivity differential magneto-optical imaging technique. Using scaling analysis and other angular dependent magnetization studies we show there is a lowering of the vortex dimensionality in this Pnictide. The dimensionality change we propose is due to presence of planar pins in the system. It is the lowering of the vortex dimensionality through planar pins in a weak interaction regime, which triggers the low field melting phenomenon. We have also imaged and explored the effect of a drive on the melting phase transition in Ba0.6K0.4Fe2As2 sample. Here using high sensitivity Differential self-field magneto-optical imaging technique, we explore the behaviour of melting under the influence of an external current induced drive. We find a highly nonhomogeneous flow of current associated with low field vortex solid to liquid transformation. We show the presence of weak transport currents causes a lowering of the melting phase boundary to nominally zero fields. In Ba0.6K0.4Fe2As2, we show the low-field glassy phase present below the liquid in the equilibrium phase diagram disappears and the liquid phase extend all the way down to the Meissner state when a current is kept on. We model the data to show that for transport current I > 50 mA Joule heating plays a pivotal role in lowering the melting phase boundary while for I < 50 mA there is a dynamic vortex melting scenario which leads to a novel drive induced melting. If time permits, there will be a very brief overview of results related to a small part of my thesis, viz., work relating to transport studies of high resistance natural fibers.
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Mohammad Zaffar
Prof. Asima Pradhan
12109873
Investigation of Mueller matrix in connection with detection and classification of cervical precancer
13/11/2019
11:30 AM
FB 382
Cervical cancer is one of the major causes of women’s mortality according to WHO. Cervical cancer originates in the epithelial layer and also affect the stromal just beneath it separated through basal layer. Various diagnostic and therapeutic techniques are directing towards the early detection and cure of cervical precancer. Optical techniques based on Mueller matrix imaging (MMI) polarimetry has shown its tremendous potential in the early detection of cervical cancer.
Diattenuation, Polarizance and Retardance vectors have been extracted and studied in connection with the diagnosis and staging of cervical precancer. The comparative results of mapping of these vectors on Poincare sphere reveals that the changes in the polarization states shown by these normalized Stokes vectors corresponds to the degradation of linearly arranged collagen fibers and the breakage of the collagen cross links in the stromal region as well as to the change in the density of scattering sites when cervical cancer evolves. It has been found that versatility of these vectors for normal and precancerous cervical tissue of various grades may be utilized as a key distinction for qualitative staging of cervical precancer tissue. Quantitative classification of precancerous stages of cervical precancer has been determined with 95%-100% sensitivity and 93%-100% specificity through the evaluation of linear and circular diattenuation, linear polarizance and linear birefringence from the components of the respective vectors.
The various anisotropic structures of the stromal region of normal cervical tissues deteriorate during the evolution of cervical precancer. The same information has been accessed through the application of Fast Fourier Transform (FFT) which has displayed its capability to detect various grades of cervical precancer through Fourier spectra of MM images and linear birefringence and circular diattenuation along with their spatial frequency profiles. Fourier spectra of differential polarization gated images are limited to only one orientation of collagen. Fourier spectra of first row elements M11, M12, M13 & M14 and first column elements M11,M21, M31& M41 discriminates CIN-I from normal cervical tissue samples with 95% to 100% sensitivity and specificity. FFT Spectra of first and fourth row elements classify CIN-I and CIN-II grades of cervical cancerous tissues with 90% -100% sensitivity and 87%-100% specificity. Normal and CIN-II grade samples are successfully discriminated through Fourier spectra of every MM elements while that of M31 element arises as the key classifier among normal, CIN-I and CIN-II grades of cervical cancer with 100% sensitivity and specificity. These results demonstrate the promise of spatial frequency analysis of Mueller matrix images as a potentially novel approach for cancer / pre-cancer detection.
The optically anisotropic stromal region of cervical is also highly birefringent and chiral. The variations of different refractive indices of various biological components in the stroma due to the spatial autocorrelation that has been found to exhibit significant differences correlated with pathological changes during the progression of cervical precancer. It is seen that the spatially varying polarizance from different regions of anisotropic stromal region gets correlated within a given spatial lags of 500 during the pathological changes. The diattenuation governing elements M12, M13 and M14 clearly discriminate normal and various grade of precancerous cervical tissue through their autocorrelation profile and correlation map. Evaluation of autocorrelation of spatially varying linear birefringence and linear-45 birefringence characterized by MM elements M34 & M43 and M24 & M42 are not found to differ between the precancer grades, indicating that these changes are arising from highly directional collagen network.
The connective tissue region of cervical tissue section has found to be multifractal in nature. A wavelet based Multifractal Detrended Fluctuation analysis has been applied to study the self-similarities of MM images of stromal region of cervical tissue section. The MM images corresponding to diattenuation have been found to exhibit increasing multifractality which is inconsistent with the previously reported results while other MM images are consistent with earlier studies displaying decreasing trends of tissue multifractality with increasing grades of cervical precancer.
Mueller matrix polarimetric measurements have been generally studied in the steady state. A picosecond time-resolved Mueller matrix imaging polarimetry has also been performed as a pilot study to observe time evolution of the scattered light which is influenced by the variation in refractive indices in the cervical tissue. The time-gated measurements from both the epithelium and stromal region of cervical tissue have shown promising potential towards the detection of cervical precancer.
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Pramod Ghising
Prof. Zakir Hossain
13109071
Study of Interface Induced Phenomena in Perovskite Oxide Thin Films
26/08/2019
11:00 AM
FB 382
Oxide materials have garnered tremendous interest due to its exotic properties which include high temperature superconductivity, colossal magnetoresistance and ferromagnetism among others. With the advent of thin film deposition techniques like pulsed laser deposition, sputtering, molecular beam epitaxy etc., interest in oxide materials have been further rekindled. Of particular interest in oxide thin films is the interface between various overlayers that play host to exciting physics. In this talk, we will explore the interface phenomena in perovskite oxide thin films, fabricated by pulsed laser deposition.
Perovskite oxides have the formula ABO3. Our work focuses on tailoring interfaces of various perovskite oxide films and investigating the emergent phenomena therein. LaTiO3 (LTO)/SrTiO3 (STO) have been known to exhibit metallic behaviour due to the formation of high density 2D electron gas (2DEG) at the interface. We find that on doping the LTO/STO interface with thin layers of CeTiO3 (CTO), the interface shows interesting properties like Kondo effect and spin orbit interaction (SOI); which can be tuned by the CTO layer thickness. The interfacial phenomena gets more fascinating in La0.7Sr0.3MnO3 (LSMO)/LTO/STO heterostructures, where we demonstrate direct control of its electrical properties using light and gate voltage. With the use of light and gate voltage, we have shown that the electrical properties can be tuned between resistive and conductive states at room temperature. In LSMO, the charge and spin degrees of freedom are strongly correlated. Thus, our work shows strong prospect for control of magnetic properties of LSMO with light and external electric field. Our investigation unleashes new possibilities for spintronic applications.
Furthermore, a unique variant of the hysteresis loop (known as the inverted hysteresis loop) is observed in LSMO/STO thin film, which exhibit negative remanence and coercivity values. The origin of the inverted hysteresis loop (IHL) can be traced to the presence of the LSMO/STO interface. Ferromagnetic resonance studies show competing anisotropies in the LSMO film, while magnetic measurements confirm the presence of positive exchange bias. Based on our experimental results, we show that EB leads to the observed IHL.
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Prabodh Kumar Pandey
Prof. Asima Pradhan
10209063
Reconstruction and characterization algorithms for optoacoustic and fluorescence-optoacoustic tomography
16/08/2019
11:00 AM
FB 382
The use of fluorescence in optical based tomographic imaging helps us to obtain physiological markers of metabolism and pathology before their structural manifestation. Also the combining of acoustic and optical modalities yields reconstructed images with good contrast(optically based) and resolution(acoustic based).
We set up fluorescence photoacoustic tomographic (FPAT) algorithms for reconstruction of the absorption coefficient of fluorescent markers at excitation wavelength. We report first results of FPAT reconstructions from pressure data with diffusion approximation(DA) modelled light propagation in tissue in Jacobian and gradient type algorithm settings. For non-scattering-dominant media, as found in several tissues of interest, where the DA is not valid, we have developed full radiative transport equation modelled Jacobian and gradient based FPAT algorithms.
With objectives of system design, we have then developed characterization algorithms in the context of universal backprojection based photoacoustic reconstructions, which distinguish between data of different noise levels, as well as provide an appropriate cut-off frequency for photoacoustic reconstructions.
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Ashish Kumar
Prof. Manoj Kumar Harbola
13109062
Obtaining exact exchange-correlation potential in density functional theory; New methods and insights
03/08/2019(Tuesday)
11:30 AM
FB 382
Density functional theory (DFT) is the most widely used theory of electronic structure of many-electron systems. The basic variable of the theory is the ground-state density of a system and the DFT is formulated to get the density directly bypassing the need to solve the many-electron Schrodinger equation. In its Kohn-Sham version, the interacting electron system is mapped to a system of non-interacting electrons in which the particles move in a local potential known as the Kohn-Sham potential. It is the sum of the external, the Hartree and the exchange-correlation potential. The latter two potentials are calculated as the functional derivatives of the corresponding energy functionals. Of these, the exchange-correlation functional and therefore the potential are not known exactly and hence are treated approximately. Over the years the many accurate exchange-correlation energy functionals have been developed. The way to test their accuracy is to compare them with the exact results wherever possible. It is in this context that obtaining the exact exchange-correlation potential, wherever possible, is significant.
Over the past three decades many different methods have been proposed to construct the Kohn-Sham potential for systems where its density or wavefunction can be calculated accurately. In our work we prove that all density-based methods can be derived from one general algorithm based on fundamental equations of DFT and obtain a condition for their convergence. The proposed general algorithm also paves the way of generating a method of one’s choice. As an extreme example of our work, we show how density-to-potential inversion can be carried out using random numbers.
When constructing the exchange-correlation potential from the density, it is seen that even slight deviation of the exact density causes the potential to deviate significantly from its exact behaviour. This is problematic when densities generated from Basis-set based calculations are used to obtain Kohn-Sham potential. This problem, however, does not arise when wavefunction based methods are employed even with an approximate wavefunction. Thus the latter methods have become popular over the past five years. However, there hasn’t been a clear understanding of why it happens. In our work we provide an insight into why this is so by analysing the wavefunction methods on the basis of Levy-Perdew-Sahni equation for the density. In the process we also propose how one can obtain accurate ground-state densities starting from approximate wavefunctions. These densities are more accurate than those calculated directly from these wavefunctions.
Having gained insights into the construction of exchange-correlation potential from wavefunction, we next show how this can be employed to find exchange-only potential from Hartree-Fock densities that have been obtained from basis set calculations. Examples of its application to atoms are given.
Finally, we use Levy’s constrained search approach to get the exchange-correlation potential using penalty functional based approach. For this we identify many penalty functionals on the basis of our previous work on density-to-potential inversion algorithm. The proposed method can be seen as the generalization of Zhao-Morrison-Parr method of obtaining exchange-correlation potential.
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Purna Chandra Patra
Prof. Y.N.Mohapatra
12209066
Thin Film Graphitic Carbon Nitride: Suitability for Device Applications
02/08/2019
12:00 PM
FB 382
Polymeric graphitic carbon nitride (g-C3N4) is an emerging 2D n-type widegap semiconductor consisting of fem atomic layers linked through sp2 hybridized C and N atoms which has attracted much attention as a catalyst over the years. Among the different allotropes of g-C3N4, heptazine based structural unit is considered to be most stable in ambient conditions which exhibits high chemical and thermal stability. However, the synthesis and its applications are limited to the bulk. The electrical properties of this material especially in thin film form is yet to be studied in detail.
In this thesis, we seek to study g-C3N4 in different forms including bulk, exfoliated layers and with specific focus on evaporated thin films keeping in view of its possible applications in optoelectronic devices. Photo luminescence (PL) spectroscopy is used to study differences in all three forms. The exfoliated samples and evaporated thin films shown common excitonic origin and quantum confinement.
In order to study the electrical properties of the film, we fabricate sandwich devices consisting of ITO/g-C3N4/Al. The J-V shows symmetric nature with low leakage current density. Impedance spectroscopy is deployed to characterize the dielctric nature of the film. The dielectric constant is measured from the linear behavior of the capacitance-voltage (C-V) characteristics. To further enhance its dielectric nature, ITO/Al2O3/g-C3N4/Al device is fabricated and characterized through temperature dependent J-V and C-V. However, Al2O3/g-C3N4 heterostructure shows relatively high permittivity as compared to their individual permittivity.
In the last section, we studied tempearture dependent C-V characteristics of a metal-insulator-semiconductor (MIS) device using a-IGZO where Al2O3 and g-C3N4/Al2O3 heterostructure are used as dielectrics. Both frequency and temperature dependent C-V characteristics are used to determine carrier concentration of a-IGZO using conventional Mott-Schottky equation in the depletion region. The respective permittivity of their dielectrics are measured as a function of temperature. The high permittivity of the heterostructure is openly makes a path for dielectrics in possible thin film transistor (TFT) devices.
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Ravindra Kumar Verma
Prof. Pankaj Jain
13109883
Search for a charged Higgs Boson at 13 TeV in the CMS experiment at the LHC, CERN
22/07/2019
04:00 PM
FB 382
A search is conducted for a low-mass charged Higgs boson produced in a top quark decay and subsequently decaying into a charm and an antistrange quark. The data sample was recorded in proton-proton collisions at center of mass energy 13 TeV by the CMS experiment at the LHC and correspond to an integrated luminosity of 35.9 /fb. The signal search is conducted in the process of top-quark pair production, where one top quark decays into a bottom quark and a charged Higgs boson, and the other to a bottom quark and a W-boson. With the W boson decaying to a charged lepton (electron or muon) and a neutrino, the final state comprises an isolated lepton, missing transverse momentum, and at least four jets, of which two are tagged as b-jets. To enhance the search sensitivity, one of the jets originating from the charged Higgs boson is required to satisfy a charm tagging requirement. No significant excess beyond standard model predictions is found in the dijet invariant mass distribution. An upper limit is set on the branching fraction of the top quark decay to the charged Higgs boson and bottom quark for a Higgs mass between 80 and 160 GeV.
In this talk, first I will briefly discuss the theoretical details of the standard model of particle physics and its extension in which the charged Higgs boson is produced. Then, a short description of the LHC and CMS experiment will be presented. Finally, I will discuss the detailed analysis for the search of light charged Higgs boson in the c and sbar channel at center of mass energy of 13 TeV from the collision data recorded in 2016 by the CMS experiment.
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Annwesha Dutta
Prof. Debashish Chowdhury
13109061
Stochastic Thermodynamics and Kinetics of Molecular Machines: principles and applications to ribosome
18/07/2019
09:30 AM
FB 382
A molecular machine is either a single protein or a macromolecular complex. Natural nano-machines operate in the aqueous environment in living cells under conditions away from thermodynamic equilibrium. Their operational mechanisms are governed by the laws of stochastic thermodynamics and kinetics. The ribosome is one of the most complex, efficient and robust molecular machines inside the living cell. These molecular machines are responsible for protein synthesis in every organism. Therefore, understanding its intricate structure and dynamics has been the focus of many researchers all over the world.
The past few decades have witnessed a tremendous evolution of experimental investigation techniques, such as cryo-electron microscopy (cryo-EM), X-ray crystallography, and single molecule Fluorescence Resonance Energy Transfer (smFRET). While cryo-EM, along with the X ray crystallography, furnished us with a wealth of structural information at high, often near atomic resolution, the smFRET studies were able to characterize the kinetic and thermodynamic parameters by monitoring the large scale dynamics of the ribosome in real time. Motivated by some of the recent experimental results reported in the literature, this thesis primarily focuses on understanding the dynamics of ribosome during protein synthesis using an approach based on non-equilibrium statistical mechanics.
A major portion of the thesis is based on theoretical chemo-mechanical modelling of the ribosome kinetics during the elongation cycle. We have developed a stochastic kinetic model that captures the possibilities "mis-sense" error i.e., mis-reading of mRNA codon and prior mis-charging of a tRNA. The corresponding exact analytical expression for the average rate of elongation of a nascent protein turned out to be a 'biologically motivated' generalization of the Michaelis-Menten (MM) formula for the average rate of enzymatic reactions. This formula displayed the interplay of four different branched pathways corresponding to selection of four different types of tRNA. Furthermore, this result stimulated us to understand when does the MM equation hold in general? It has already been found that MM like formula appears not just in enzyme biochemistry but also in gene regulation, mRNA transcription, protein translation and molecular motors; they involve not only the macroscopic, but also the single-molecule, level. We have analysed the question of validity of MM equation for ribosome in the light of a graph-theory based linear framework for timescale separation. Using graph theoretic approach, we have also identified the various possible modes of operation of a ribosome in terms of its average velocity and mean rate of GTP hydrolysis. We have also discussed the stochastic thermodynamics of a ribosome by computing entropy production as functions of the rates of the interstate transitions.
We present a method of extracting kinetic information from the complex free energy landscape of the ribosome's elongation cycle which was obtained by Dashti et al, PNAS, 2014, using a technique of manifold embedding of single-particle data collected by cryo-EM. The minimum free energy path followed by the ribosome on this complex landscape was identified by employing two efficient search algorithms, POLARIS and MEPSA. As expected, this reaction trajectory passes through all the conformational states which were identified in the Dashti et al's paper. The kinetic rates for transitions between the intermediate, relatively long-lived, conformational states of the ribosome's translation elongation cycle were obtained via two different numerical approaches. The first approach is based on Kramer's rate theory. The second approach computes the rates of transitions between the local minima by carrying out Monte Carlo simulation based on Wang-Peskin-Elston prescription. The rates of transitions extracted by our analysis of the landscape has been compared with published data.
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Anupam Ghosh
Prof. Sagar Chakraborty
14109876
Comprehending Occasional Uncoupling Induced Chaotic Synchronization
15/07/2019
02:30 PM
FB 382
The word `synchronization’ implies the evolution of two or more than two interacting systems in unison. In our real-life, lightning of fireflies, metabolic processes in cells, etc., are some examples where we observe this phenomenon. Synchronization in coupled chaotic systems got popular after 1990; although, in dynamical systems, this phenomenon was first discovered in the seventeenth century by Huygens. Further, one of the most counter-intuitive ideas came in 1993—-the occasional uncoupling-—where the participating systems are coupled occasionally, instead of continuously, and synchronization is observed. Interestingly, it is also observed that the occasional uncoupling leads to robust synchronization, unlike the continuous coupling. There are various occasional uncoupling schemes reported in the literature, viz., the stochastic on-off coupling, the on-off coupling, the transient uncoupling, etc. In many of the cases, the corresponding scheme’s parameter(s) for a given coupled chaotic systems is (are) chosen purely on the basis of trial and error. In this talk, we investigate and understand why and how the aforesaid occasional uncoupling schemes work.
First, we understand how the transient uncoupling works for coupled chaotic oscillators from an imposed local analysis. The real part of the eigenvalues of the local Jacobian of the coupled oscillators are the indicators of the local expanding directions, and if we suppress the local expansions (due to the inherent chaotic nature of the oscillator) using a accurately calculated coupling term, the synchronized state could be obtained. We use the aforementioned local analysis to explain the reasons behind the particular predefined clipping region used in the transient uncoupling to obtain synchrony at higher coupling strength unlike the continuous coupling.
Going further, we study synchronization in coupled Hamiltonian systems. The absence of attractor in Hamiltonian systems leads to a different kind of synchronization: measure synchronization, when two Hamiltonian systems are coupled properly. Further, we extend the concept of occasional uncoupling to the coupled Hamiltonian systems and employ the on-off coupling to overcome measure desynchronized state. To date, in literature, the applicability of the occasional uncoupling is restricted in the domain of dissipative chaotic systems, we introduce it in the territory of conservative systems. In addition, the fast switching of the coupling term, we observe that the on-off coupling is equivalent to the continuous coupling with a proper scaling of the coupling strength.
In addition, there is stochastic occasional uncoupling scheme, where synchronization is observed even after coupling interacting systems randomly. In the process of our attempt to understand the mechanisms behind the success of the occasional uncoupling schemes, we devise a hybrid between the transient uncoupling and the stochastic on-off coupling, and aptly name it the transient stochastic uncoupling—yet another stochastic occasional uncoupling method. Through the transient stochastic uncoupling, we establish that the autocorrelation function—-a non-local indicator of the dynamics-—of the corresponding response system’s chaotic time-series dictates when the deterministic uncoupling could be successful.
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Manohar Kumar Sharma
Prof. Sagar Chakraborty
13109881
Statistical Properties of Rotating Turbulence
15/07/2019
11:45 AM
FB 382
Turbulence is a ubiquitous phenomenon in nature. We can observe its manifestation at every scale, from a cup of tea being stirred to the galaxy formation. Turbulent flows are characterized by randomness in velocity fields, by enhanced diffusivity, and strong nonlinearity in the flow. It has been studied analytically, numerically, and phenomenologically. One of the most popular models to study the statistical properties, like kinetic energy spectrum of the flow was proposed by Kolmogorov in 1941. Kolmogorov, through his model, predicted the scaling of kinetic energy spectrum as E(k) ~ k^-5/3 (where k is wavenumber) in the inertial range at high Reynolds number. This scaling is validated in numerical and experimental studies. The nonlinear interactions of velocity fields is responsible for the scale-to-scale transfer of kinetic energy in the inertial range. In three dimensional homogeneous and isotropic turbulence, a local and forward cascade of kinetic energy is reported in literature.
Though homogeneous and isotropic turbulence itself is an intricate problem, the effect of rotation makes even more intricate due to the anisotropy in the flow. The rotation does not contribute to the total energy of the system. However, it modifies the energy distribution in different scales of the system such that the flow behaves like quasi-two-dimensional at very high rotation rate. As a result, the scaling of kinetic energy spectrum changes and does not show Kolmogorov power scaling. We study the statistical properties of the rotating turbulent flow numerically. We solve the governing equation of the system in a three-dimensional cubic periodic box of box-size (2π)3. We used pseudo-spectral code Tarang developed by Prof. Mahendra K. Verma's group for our numerical simulations.
Initially, we discuss the statistical behavior of the decaying rotating turbulence and our proposed model. Here, we show that our proposed model is in good agreement with numerical results. We also show the most dominant energetic modes in the system are (1,0,0) and (0,1,0) in Fourier space .The most of the energy is trapped in the large scales.
Further, we discuss the behavior of kinetic energy spectrum for forced rotating turbulence. In this part of the talk, we show that the energy spectrum in the wavenumber region smaller than the forcing wavenumber shows Kunznestov—Zakharov—Kolmogorov scaling. Among the wavenumbers larger than the Kolmogorov dissipation wavenumber, the energy is distributed such that the suitably non-dimensionalized energy spectrum is Ē(k̄) ≈ exp(−0.05k̄), where the overbar denotes appropriate non-dimensionalization.
We end with the discussion on a comparative study of the anisotropic energy transfer for decaying and forced rotating turbulence. Here, we show that the energy transfer takes place from the pole to the equator in both the decaying and forced rotating turbulence.
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Khun Sang Phukon
Prof. Pankaj Jain
12109065
Study of inspiral dynamics of precessing compact binaries and low
latency searches of Gravitational Waves
10/07/2019
04:00 PM
FB 382
Astronomy with gravitational-wave observations is now a reality with the detections of gravitational waves (GWs) from 11 compact binary coalescences (CBCs) by the advanced gravitational-wave detector network during the first and second observing runs. In the coming years, the detection rate of compact binaries will increase multifold with the addition of the currently under-construction ground-based detectors (KARGRA, LIGO-India) and the planned space-based detector (LISA) to the network of gravitational-wave detectors. Compact binaries comprised of black hole pairs, neutron star pairs, and neutron star-black hole pairs are the most abundant sources of GWs for the ground-based detectors. In this talk, I will discuss some aspects of gravitational-wave astronomy with CBCs.
I will first discuss the dynamics of spinning compact binaries, particularly binary black holes (BBHs) during inspiral. Detailed understanding of the complex spin dynamics of the generic spinning BBHs is expected to play a crucial role in maximizing the scientific outputs of the observations of populations of BBHs. I will discuss how inspiral spin dynamics affects distributions of parameters of BBHs and their daughter BHs. Further, I will talk about spin morphologies or phases of spinning BBHs in elliptical orbits. I will present results on the transition probabilities of binaries among different morphologies and their implications.
With the addition of the spin variable the complexity of GW data analysis increases many folds. Several algorithmic and hardware optimization ideas are required to attain real-time implementation of CBC searches in the multi-detectors data to enable quick follow-up of electromagnetic signals associated with GW events. In this talk, I will introduce Random Projections based method that holds promise to perform computationally-intensive GW data analysis in a scalable and efficient way. Alongside algorithmic ideas, I will discuss the uses of advanced hardware architecture in accelerating CBC searches. I present speed-up achieved by implementing parts of GW data analysis pipeline on GPUs, in comparison to not just single CPUs but also multiple MPI processes running on a CPU cluster.
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Sougata Mardanya
Prof. Amit Agarwal
13109074
Griffiths Phase and Quantum Criticality in Ce(Cu1-xCox)2Ge2
27/06/2019
11:00 AM
FB 382
After the discovery of superconductivity in CeCu2Si2 by Steglich et al. strong progress have been made towards understanding the low-temperature properties of heavy fermion (HF) metals. The system CeCu2Ge2 is an example of HF metal, shows interesting physical properties close to its anti-ferromagnetic (AFM) quantum critical point (QCP), such as non-Fermi-liquid (NFL) behavior and unconventional superconductivity. Slight changes of external parameters, such as doping or application of pressure or magnetic field, can easily tune this system from magnetically ordered to paramagnetic states. At the point where magnetism is suppressed to T = 0 is known as QCP. In this context, the substitution of Co at Cu site makes the compound Ce(Cu1-xNix)2Ge2 interesting to look for QCP and some novel phases around it. This work is the results of numerous experiments and observations on Ce(Cu1-xCox)2Ge2. The macroscopic measurements like thermal transport, magnetic susceptibility, and specific heat are supplemented by microscopic measurements such as μSR and neutron investigations. The long-range anti-ferromagnetic order established in CeCu2Ge2 at TN = 4.1 K, can be suppressed by Co-doping, and at critical composition xc = 0.6 (TN → 0 K) a QCP has been observed. The magnetic susceptibility and specific heat data reveal the signatures of quantum Griffiths phase near the AFM QCP, accompanied by non-Fermi-liquid behavior inferred from the power-law dependence of heat capacity and susceptibility i.e., C(T)/T and χ(T) ∝ T(-1+α) down to 0.4 K. The QCP is also manifested in the power law divergence of exponential depolarization, i.e. λ ∝ T(-1+α) down to 0.1 K. The relaxation rate of x = 0.6, obeys the time-field scaling relation Gz(t,H) = Gz(t,Hγ), which is considered to be a characteristic feature of quantum critical magnetic fluctuations. Furthermore, for x = 0.6, the exponent of M ∼ Hη and magnetization-field-temperature scaling is consistent with the ZF-µSR data. These results show that around the quantum phase transition (at x = 0.6), the Griffiths phase crucially controls the low-temperature spin dynamics and is responsible for NFL behavior.
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Sougata Mardanya
Prof. Amit Agarwal
14109882
First principle investigation of topological phase transition in crystalline
materials
27/05/2019
11:00 AM
FB 382
Following the discovery of topological insulator, the materials with topologically protected surface state have drawn significant attention for their tremendous application potential in the field of spintronics, electronics and quantum computation. The topological protection of these surface states makes them robust against the local perturbation and ensure a backscattering free dissipation less flow of current at the boundary. In recent years, the focus has gradually shifted from the gapped insulating state to the gapless semi-metallic state, following the experimental realization of Weyl and Dirac semimetals. In condensed matter systems, the Weyl and Dirac states arise either due to accidental band crossings, or due to crystal symmetry enforced band crossings in the bulk, along with linearly dispersing quasiparticles in vicinity of the band-crossing points. Additionally, due to the non-trivial bulk and surface electronic states, the topological semimetals exhibits intriguing transport properties, such as large magneto-resistance, Chiral anomaly, and non-local transport signature.
In this work, our main aim is to explore the different topological materials in crystalline materials, and their origin and protection by specific crystalline and other symmetries. We also focus on the topological phase transitions arising from selective symmetry breaking in different materials. In particular, we will explore different topological and ’normal’ phases 2D allotropes of Arsenic, and in hexagonal 3D crystalline materials such as CaAuAs, BaAgAs and their alloys.
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Utso Bhattacharya
Prof. Amit Dutta
14109885
Aspects of non-equilibrium dynamics of closed quantum systems: information, phase transitions and topology
16/05/2019
11:00 AM
FB 382
In a quantum phase transition (QPT) one changes a non-thermal parameter of the Hamiltonian so as to drive a transition from one phase to the other at absolute zero temperature. Conventionally, phase transitions are studied based on the celebrated Landau-Ginzburg theory of spontaneous symmetry breaking. However, recently there has been a burst of activities in the field of topological phase transitions, a novel field of QPTs that lies beyond the Landau-Ginzburg paradigm. Although there are many theoretical proposals which unravel the possibility of many kinds of exotic topological phases, it is appreciably challenging to realize them experimentally. Therefore, people are now looking into alternative out of equilibrium ways of realizing such phases and for probing their topological behavior. But, it is difficult to study non-equilibrium criticality in the same footing as equilibrium due to the lack of a well established machinery. Quantum information theoretic measures ) such as the Loschmidt echo (LE) comes to rescue as it efficiently incorporates non-equilibrium behavior of quantum systems and serves as a natural candidate in the search for phase transitions out of equilibrium. The goal of this talk is therefore to look at different aspects of non-equilibrium dynamics brought about by quenches and time-periodic drives, focussing on the preparation and study of ``hard to attain" interacting quantum light-matter system, dynamical quantum phase transitions and non-interacting topological systems. We use quantum information theoretic measures to not only look for out of equilibrium criticality but also to look for signs of classical chaoticity and the emergence of novel topological behavior. This interconnection is, thus, the central theme of this open seminar.
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Arif Warsi Laskar
Prof. Saikat Ghosh
12209063
Quantifying and using quantum coherence in hot and cold atoms
30/04/2019
11:00 AM
FB 382
Coherent superposed quantum states are basic building blocks in quantum-enhanced technologies. Accordingly, there has been a recent major thrust in research towards generating and controlling stable superposed quantum states in a range of physical systems. A widely used technique to generate such coherent superposition in atomic system is based on the phenomenon of electromagnetically induced transparency (EIT), where a strong (control) optical field drives atoms in a ground state superposition in presence of a weak probe field. However, due to presence of a range of classical channels, the generated superposition in EIT becomes short-lived: decoherence due to environment sets in. This thesis primarily focuses on understanding the interplay between such classical and quantum dynamics and in the process, quantifying the generated coherence in the system and applying it in SU(1,1) interferometry, towards generating correlated photons.
The first half of this work studies generation of quantum coherence in a thermal ensemble of atoms at room temperature, where thermal velocity of atoms tend to wash out the generated coherence. Traditionally, ground state coherence in such systems have been probed and characterized via measurement of width of a typical transparency window in probe absorption spectra. However, it is well appreciated that such characterization can lead to ambiguity in differentiating classical and quantum dynamics. Here we develop a methodology which directly probes the dynamics of coherence in time domain. In particular, we find that in a thermal ensemble, quantum coherence builds up in a timescale that is significantly faster than the corresponding classical dynamics. The time trace develops a typical experimental signature of the precise amount of generated coherence in the system. Using this experimental signature, we define a phenomenological measure of coherence and show that it satisfies the conditions to be qualified as a measure.We apply the quantifier to precisely distinguish the transition point from EIT to Autler-Townes regime where the stable ground state superposition is washed out. Furthermore, we develop a technique to compensate decoherence in the system by adding a pair of highly detuned Raman fields. The corresponding frozen coherence in the system is again well captured by the quantifier.
The second half of the thesis studies generated coherence in an ensemble of atoms, laser-cooled to a temperature of 50 micro-Kelvin. Low sample temperature reduces thermal decoherence while large optical depth (due to a million atoms trapped in a microscopic trap) results in strong atom-photon coupling. Accordingly, we have experimentally observed slowing, stopping and storing of light pulses in the atomic medium. In particular, the optical field gets stored in the form of a spin wave (magnon) in the system. We can further retrieve the stored optical pulse with an efficiency in excess of 90%.
We use this observed long coherence time to develop a new kind of interferometer. We store two optical fields in the form of two spin waves in the system. While stored, the magnons pick up relative phase with respect to each other, akin to two arms of a Mach-Zender interferometer. When we simultaneously retrieve, we observe phase coherent beating and interference fringes with large contrast. The developed interferometer can be used for precision measurement of spin phases, magnetic fields and interactions. Furthermore, when excited with few photon pulses, we have observed initial signatures that hint at controlled quantum correlations of photons at the output ports.
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Saikat Sur
Prof. V. Subrahmanyam and Prof. H. Wanare
14109272
Effects of local dynamical processes on spin chain dynamics
03/04/2019
10:00 AM
FB 382
Quantum Information and communication aspect of quantum spin chains has been investigated over the last few years. From quantum information theory point of view, a quantum spin chain is a many-qubit system that can undergo various multiparty operations, both global and local along with background unitary evolution. A local operation that interrupts the background evolution can occur from a local quantum decohering process or local coherent operation. Any local quantum dynamical process occurring on a many qubit state can change the distribution of correlations and entanglement structure. The various sub parts of a multiqubit system undergoing non unitary operations can be thought of as a simple model for decoherence in many body systems.
We will focus on the propagation of the signal of the intervening QDP in the chain. The possibility of detecting the occurrence of the QDP at farther sites from the dynamical evolution of the state after the epoch time of the QDP will be discussed for various models.The local QDP can interfere with the quantum state propagation in the system and change the quantum state transfer fidelity. The effect of QDP on state transfer fidelity will be highlighted for integrable and nonintegrable models.
The Loschmidt Echo is used as a measure of revival of a quantum state when imperfect time reversal procedures take place during the evolution. Multiple incoherent QDPs intervening the dynamics at regular intervals can be thought of as if the system is interacting with an external decohering environment, while the action of multiple number of coherent QDPs though does not cause decoherence can generate nonintegrability in the system.
We will discuss various measures of two party correlations and some multiparty correlations in spin systems. Depending on the location and time of QDP how does these correlation measures between two parties change will be addressed.
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Ritu Gupta
Prof. Zakir Hossain and Prof. K.P. Rajeev
12109068
Physical Properties of Charge Density Wave Superconductor LaPt2Si2
11/02/2019
10:00 AM
FB 382
Charge density wave (CDW) is a periodic modulation in electronic charge density which is found in some low dimensional materials due to strong Fermi surface nesting. However, three dimensional materials with layered structures also sometimes show CDWs due to their quasi-low dimensional Fermi surfaces. CDW state opens up a gap (or a partial gap) at the Fermi surface which leads to a metal to insulator (or semimetal) transition in contrast to a superconducting transition which manifests itself with an infinite electrical conductivity. So, systems which exhibit both CDW and superconductivity (SC) are of great interest. LaPt2Si2 shows a first order structural transition from tetragonal to orthorhombic accompanied by a CDW transition at 112 K along with a superconducting transition at around 1.5 K.
After a brief introduction of the CDW, I shall present the structural details of LaPt2Si2 which crystallizes in CaBe2Ge2 type structure. Absence of mirror symmetry in this material makes it suitable candidates for non-centrosymmetric (NC) superconductivity. We present temperature dependent electrical resistivity, magnetization, thermoelectric power, thermal conductivity, thermal expansion measurements etc. Competing nature of CDW and SC has been presented through application of negative chemical pressure in LaPt2(Si1-xGex)2. We have also measured anisotropy in various physical properties on the single crystals grown using Czochralski pulling method. Unconventional nature of the superconductivity has been confirmed in temperature dependent upper critical field and specific heat measurements. BCS fitting with various models including nodal gap (d-wave), isotropic gap (s-wave) and multigap (s+s or s+d) to the specific heat below TC will be discussed.
Further analysis of the superconducting gap has been done through muon spin relaxation/rotation (MUSR) technique. Through zero field MUSR spectra, it has been shown that time reversal symmetry is preserved in the system. Magnetic penetration depth, which has been calculated through transverse field MUSR spectra, further provides the symmetry of the superconducting gap.
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Rajan Singh
Prof. Saikat Ghosh
12209067
Hybrid nano-mechanical resonators
31/01/2019
11:00 AM
FB 382
Mechanical resonators have played important role in the field of sensing and metrology, dating back to the classic 18th century experiment by Cavendish measuring gravitational constant, to present day NEMS(nano-electro-mechanical-systems) resonator based accelerometers, gyroscopes and sensors, sensitive to masses at the level of single molecules or spins. Furthermore, with advancement in fabrication techniques and instrumentation, engineered high quality factor (Q) resonators at microwave frequencies have shown immense potential towards reaching quantum limited force sensitivities at room-temperature. Additionally they offer on-chip integrability with optical, microwave and mechanical systems towards classical and quantum information processors. In this seminar, I will introduce a hybrid platform consisting of high-Q Silicon Nitride (SiNx) resonator coupled to atomically thin freely suspended graphene resonator with high mechanical non-linearity and show the efficacy of such a platform in a range of scenarios.
We will start with a description of an amplifier that we have developed using this hybrid platform, to overcome traditional limitations in motion transduction. We amplify motion of a driven SiNx mode. When measured on graphene, we observe large amplitude oscillation of the transduced motion and measure a gain of 38 dB in displacement power spectrum. With additional parametric driving, we demonstrate thermo-mechanical squeezing of noise, with a measurement sensitivity of 3.8 fm/Hz1/2, a record at room-temperature.
We will discuss our experiments with resolved side-band cooling of the amplifier mechanical mode by 100 K and amplification due to intra-modal coupling, towards improved performance of the amplifier.
We have also demonstrated graphene induced tunable, giant mechanical nonlinearity in large area SiNx resonators. Along with a range of rich physics due to interplay between non-linear damping and Duffing response, we show that such tunable “on-off” nonlinearities can be used to process information in classical and quantum domains. I will discuss how the coupled, hybrid graphene-SiNx hybrid mode, when parametrically driven at sum of their resonance frequencies, show parametric amplification along with a rich spectrum of cascaded four wave mixing in the unstable regime. Measured dispersion profile match excellently with numerical simulations. Furthermore, we find that the back-action of graphene on SiNx resonator results in giant, induced non-linearity. We estimate an effective induced non-linear coefficient of SiNx at 7×10^20 kg m2/Hz2. We will end with a discussion on how the demonstrated hybrid platform can be integrated as an essential element to process, amplify and store information, in the growing field of quantum opto and electro-mechanics.
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Pulastya Parekh
Prof. Arjun Bagchi
16109277
Tensionless strings : A perspective from the worldsheet
14/12/2018
11:30 AM
FB 382
I will be speaking about the construction of the tensionless limit of closed bosonic string theory in the covariant formulation in the light of Galilean conformal symmetry that rises as the residual gauge symmetry on the tensionless worldsheet. I will show how the analysis of the fundamental tensionless theory is related to the tensionless limit that is viewed as a contraction of worldsheet coordinates. The connection to massless higher spin states can be seen naively. This analysis can be extended to the closed superstring to obtain the Super Gallilean Conformal Algebra (SGCA), that can be realised in two distinct ways : the Homogenous and the Inhomogenous SGCA. I will also comment on the hermiticity properties of fermions in case of the Inhomogeneous tensionless superstring. We will see that the analysis of the quantum regime uncovers interesting physics. The degrees of freedom that appear in the tensionless string are fundamentally different from the usual string states. Through a Bogoliubov transformation on the worldsheet, one can link the tensionless vacuum to the usual tensile vacuum. As applications, I will discuss how tensionless strings can be connected to Hagedorn Physics and Ambitwistors.
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Pankaj Singh
Prof. Asima Pradhan
10109067
Design, fabrication and validation of a probe and development of a frequency
domain tomography system for characterizing epithelial pre-cancers
07/12/2018
11:30 AM(Tea @ 11:15 AM)
FB 382
Cervical cancer is a type of epithelial cancer which is the fourth most common cancer in women. It has no clear symptoms at the precancerous stage and one has to go for regular screening tests. There is need for a substitute for conventional invasive and time-consuming screening techniques. Optical techniques can be used to extract the signature of abnormality present in the biological tissues. The spatially resolved reflectance technique is one of the most powerful techniques to extract optical properties of the tissue which directly correlate with the morphological and biochemical changes that occur with cancer progression. An epithelial tissue is considered to be a two-layered medium with ~300 um thick epithelium layer on top and stroma, the connective tissue underneath. Early signatures of precancer are contained in the epithelium layer. It is difficult to reach the internal organs such as the cervix, and take measurements. Hence, it is a challenge to recover the optical properties in vivo to characterize the epithelial tissue.
For practical purposes fiber-optic probes are the optimal choice. Monte-Carlo method based computational study has been carried out to study the efficacy of three common probe geometries for spatially resolved reflectance measurement based precancer detection of internal as well as external organs. Based on the results thus obtained, a fiber probe, suitable for spatially resolved reflectance measurement from the cervix, has been designed and fabricated. The configuration of the fibers in the probe has been done to optimize the signals from the epithelium layer. The fabricated prototype has been employed to carry out spatially resolved reflectance measurements on a cervical tissue mimicking two-layered phantoms and the optical parameters of the phantoms have been recovered using Monte-Carlo look up table.
Optical tomography is another technique to characterize two-layered media by reconstructing the cross-sectional maps of the optical parameters. Such a technique demands a large set of data. A frequency domain measurement system for optical tomography has been built and can be used in combination with the developed fiber probe to record both spatially resolved intensity as well as phase. A preliminary study using our system has been carried out to show the significance of the technique.
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Sourav Biswas
Prof. Anjan Gupta
12209070
Phase dynamic regime in micro-SQUIDs and its optimization for
probing nano-magnetism
28/11/2018
11:00 AM(Tea @ 10:45 AM)
FB 382
Superconducting QUantum interference devices (SQUID) are the most sensitive magnetic flux detectors to date. The miniaturized micro- or nano-SQUIDs, with weak links as Josephson junctions, can lead to magnetic moment resolution down to below an electron's magnetic moment. Thus, micro- or nano-SQUIDs are of recent research interest for probing nano-magnetism at low temperatures. The phase dynamic regime of a SQUID is the most suitable regime for its operation as a flux-to-voltage transducer. This regime offers good flux resolution, fast response and with conventional electronics. However, the Joule heating effects substantially affect this regime leading to even its non-existence at low temperatures unless the devices are carefully optimized.
In this thesis work, we first model the phase-dynamic regime by considering both the heating effects and phase dynamics. A new dynamic regime defined by two different retrapping currents is found. More importantly, we introduce a single dimensionless parameter beta that determines the amplitude of super-current in the dissipative phase-dynamic state and hence the voltage modulation magnitude. To understand and verify our dynamic thermal model (DTM), we fabricate Nb based micro-SQUIDs by electron beam lithography technique. Voltage oscillations with magnetic flux are observed even in the hysteretic regime of optimally designed micro-SQUIDs. We quantitatively explain all the experimental observations using the DTM.
In the next part, we analyze the DTM with a parallel resistive shunt with inductance. The competition between various time scales, namely Josephson, thermal and inductive times, leads to changes in retrapping current. As anticipated by the model, we observe large voltage modulation and improved flux resolution down to 1.3 K in micro-SQUIDs that are adequately shunted. Above a threshold inductance value, relaxation oscillations in SQUID voltage appear. Towards the objective of finding the noise characteristics of our devices, we also carry out a study on the distribution of critical and retrapping currents. The devices exhibit narrower distributions in both these currents in phase dynamic regime as compared to the static hysteretic regime. Thus, this thesis provides an understanding of the phase dynamic regime in micro-SQUIDs and weak-links and paves the way for further improved low temperature nano-magnetism studies.
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Girish Kulkarni
Prof. Anand Jha
14109264
Measurement and transfer of correlations in parametric down-conversion
11/09/2018
04:00 PM
FB 382
Coherence is the ability of a quantum system to superpose with itself and exhibit interference effects. On the other hand, entanglement is the inseparability of a composite quantum system into independent constituent systems. The quantification of entanglement for a general composite quantum system remains a long-standing open problem in quantum theory with important implications for fundamental physics and quantum technologies. We seek to approach this problem by investigating some intimate connections between coherence and entanglement in the physical setting of parametric down-conversion – a non-linear optical process in which a single photon is annihilated to create an entangled two-photon system. In this talk, I will present experimental and theoretical characterizations of the entanglement of the two-photon system via its relationship to the coherence of the individual constituent photons and to the coherence of the original annihilated source photon.
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Saikat Chakraborty
Prof. Kaushik Bhattacharya
14109881
Anisotropy evolution in f(R) gravity
01/08/2018
04:00 PM
FB 382
Although the present universe is homogeneous and isotropic in large scales, possibility of inhomogeneity and anisotropy at the very early stages of the universe cannot be ruled out. Indeed, any 'good' theory of the early universe, be it inflation or bounce, is demanded to possess some kind of isotropization mechanism inbuilt in it. The talk will mainly be focus on anisotropic universes, which are described by Bianchi models. In general relativity, behavior of anisotropy is relatively simple; in absence of any anisotropic source, the total amount of anisotropy varies simply as inverse of the cube of the scale factor, i.e., decreases in an expanding universe and increases in a contracting universe. However, in modified f(R) theories of gravity, behavior of anisotropy is not so simple; this is because of a strange intertwining between the definition of Ricci scalar for Bianchi models and the solution of the anisotropy parameter in f(R) gravity. The evolution of anisotropy in f(R) gravity becomes nonlinear and complicated. The talk will address the behavior of anisotropy in f(R) gravity from three different points of view; the dynamical systems analysis of f(R) gravity in Bianchi-I spacetimes, reconstruction method of f(R) gravity in presence of anisotropy, and explicit calculation of anisotropy evolution in a simple exactly solvable case. The talk is based on the my works arXiv numbered 1805.03237, 1803.01594, 1710.07906 .
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Ms. Rameshwari Naorem
Prof. S. Anantha Ramakrishna (PHY) and Prof. Anandh Subramaniam (MSE)
12209873
X-ray Scattering and Entropic effects in Systems with Atomic Disorder
31/07/2018
03:00 PM
FB 382
In multicomponent metallic alloys, configurational entropy may offset the tendency for compound formation, and give rise to disordered solid solutions. These alloys have been christened as high entropy alloys (HEA). The effect of atomic disorder (and the concomitant occurrence of strain) existing in concentrated solid solutions on the X-ray diffraction (XRD) pattern, especially the Bragg peaks, hitherto has not been characterized in detail. This issue has been addressed in the current work using computations and experiments. Concentrated multi-component alloys (CMA) like NiCoFeCrMn and CuNiCoFeV systems have been used as model systems for the study. It is proved that the intensity decrease is not insignificant; but is not anomalous either. A recipe is evolved to compare the Bragg peak intensities across the alloys of a CMA. It is shown that FWHM of the X-ray peaks can be identified as a “good measure” of the bond length distortion in the lattice, and it is not sufficient to model the effects of atomic disorder in a CMA as merely similar to the effects of an enhanced temperature. It is demonstrated that the true strain due to bond length distortion is of significantly lower magnitude than that given by a priori measures of lattice strain. In the scheme of categorization of defects in crystals, it is argued that CMA is a separate class: it should be construed as a defected solid, rather than as a defect in a solid. Further, the transition from the dilute to the concentrated regime is studied using binary alloys.Analogous to HEA with positional disorder, one may envisage systems with orientational disorder, wherein, in spite of the strain in the lattice, the enhanced configurational entropy due to multiple orientational variants can stabilize the system in a high symmetry phase. In the case of the cluster compounds (of type AM_2 O_4 , where A is a divalent transition metal like Ni, Fe and M is a trivalent atom like Cr, Al, Mn), the strain arises due to Jahn-Teller distortion of tetrahedral clusters of transition metal ions. Thus, compounds with both positional and orientational disorder can be envisaged. The role of entropy in the stabilization of the disordered phase in these systems is brought out though analysis of the XRD patterns and differential scanning calorimetric studies.
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Jitendra Kumar Pradhan
Prof. S. Anantha Ramakrishna
13109066
Spectral control of infrared absorption and emission by metamaterials
10/07/2018
04:00 PM
FB 382(A.K. Raychaudhuri Seminar Room, Physics Department)
Metamaterial perfect absorbers (MPA) consist of impedance matched subwavelength structural units that resonantly absorb the incident radiation and have been projected for various applications. We design band selective MPA for infrared wavelengths (2µm-15µm) and typically consist of a tri-layer with a top patterned layer separated from a continuous bottom conducting ground plane by a dielectric spacer layer. With vanadium dioxide (VO2) as a spacer layer, a high modulation of switching (> 78 %) is obtained in the reflectivity between the low and high-temperature phases of VO2. Ceramic-metals known as 'cermets' have been incorporated as spacer layers in the MPA for flexible control of the metamaterial resonances. The feasibility of a top protective (capping) layer to shield the MPA surface against harsh environments is examined. High absorption is obtained using an ultra-thin lossy coating of VO2 on a plasmonic substrate of indium doped tin oxide (ITO), circumventing the “quarter-wavelength” lower limit on the thickness of the planar antireflection coatings. This bi-layered design, VO2 /ITO exhibits “perfect” blackbody-like thermal emissivity peaked at 5 µm wavelength near room temperature. The structure emits less radiation upon heating and appears colder on an infrared camera. We will discuss the optical limiting of VO2 /ITO thin films when beamed by a sub-nanosecond pulsed laser at 1064 nm. A dielectric thin film of alumina (Al2O3) embedded metallic nanospheres of tungsten produces a band of large absorption at infrared wavelengths in a manner similar to the VO2 thin films. We expect our designs of MPA, as well as multilayer thin film will find applications in thermal infrared camouflage, infrared tagging, and thermal regulation, etc.
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Mr. Vinay M M
Prof. Gautam Sengupta
13109077
Holographic Entanglement Negativity in AdS3/CFT2
09/07/2018
11:00 AM
FB 382
Characterization of mixed state entanglement is a subtle and complex issue in quantum information theory as the usual measure of entanglement entropy ceases to be valid for these states. In this context entanglement negativity is one of the significant computable measures introduced in quantum information theory that characterizes the upper bound on the distillable entanglement for bipartite mixed states. We propose a holographic entanglement negativity conjecture for bipartite pure and mixed states of a (1+1)-dimensional conformal field theories (CFT) through the AdS3/CFT2 correspondence. Application of our conjecture to specific examples of the pure vacuum state and finite temperature mixed states exactly reproduces the corresponding CFT replica technique results, in the large central charge limit. Subsequently, we establish the consistency of our holographic conjecture through the large central charge analysis of the entanglement negativity for a mixed state configuration relevant to our conjecture. We further extend our holographic entanglement negativity conjecture for time-dependent bipartite states of (1+1) dimensional CFTs dual to the non-static bulk AdS3 configurations. Once again our results exactly reproduce the corresponding replica technique results for suitable examples in the large central charge limit. We briefly allude to a possible higher dimensional extension of our conjecture in a generic AdS(d+1)/CFTd scenario.
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Ashutosh Kumar Singh
Prof. Amit Agarwal
13109879
Nonlinear and anisotropic optical conductivity in Dirac materials
25/06/2018
11:00 AM
FB 382
Optical response, in general, can be quantified in terms of optical susceptibility or optical conductivity which reveal the system’s behavior to an external field in terms of Polarization or Current density respectively. Such description of response function is best provided in terms of density matrix, which is used to describe the carrier dynamics via Equation of Motion (EOM) approach. The EOM yields a set of coupled differential equations popularly known as Optical Bloch Equations (OBEs). Most of the studies in Nonlinear optics have been carried out either by direct integration of OBEs or they involve perturbative treatments in powers of the electric field. These kinds of study are very useful for ultrafast spectroscopy when the timescale over which a pulse lasts is approximate of the order of few femtoseconds. However, if a Continuous Wave (CW) is used instead of ultrafast pulses and different kind of decay channels are replaced by phenomenological damping in OBEs, the time evolution of the system attains quasi-equilibrium steady state owing to the balance between continuous excitation of carriers and relaxation due to damping. This approach leads to rather elegant and intuitive results which can easily be manifested in various kinds of optical transport measurements. The present thesis explores the nonlinear optical response in Dirac materials in the presence of a CW light. We develop an analytical framework to obtain the steady state density matrix of a two band system in general. Using the steady state solution of the photo-excited density matrix, we study in detail about the nonlinear optical conductivity, polarization rotation, anisotropic photo-conductivity and wave-mixing effects in graphene. Interestingly the well known Kubo formula for optical conductivity, appears as a limiting case (linear response and Infinite coherence time) of our more general non-linear formulation.
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Aditya Mehra
Prof. Arjun Bagchi
16109262
Galilean Conformal Field Theories
23/04/2018
4:00 PM
FB-382
I will be giving a detailed analysis on Galilean (non-relativistic) versions of conformal field theories. I will show that in all these cases, the field theories exhibit Galilean conformal structures in appropriate dimensions. Unlike relativistic conformal invariance, the surprising aspect of the analysis is that the non-relativistic conformal structure becomes infinite dimensional even when looking at spacetime dimensions greater than two. In every case, if the parent relativistic theory exhibited conformal invariance, we find an infinitely enhanced Galilean conformal invariance in the non-relativistic case. This leads to suggest that the enhancement of symmetries in the non-relativistic limit is a generic feature of conformal field theories in any dimension.
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Upkar Kumar Verma
Prof. Y. N. Mohapatra
Y8209064
Localized States in P3HT:PCBM based Bulk Heterojunction Device Structures
10th February 2016 (Wednesday)
12 noon
Samtel Center for Display Technologies (Seminar Room)
Blend of conjugated polymers and fullerene derivatives such as P3HT:PC60BM have emerged as model systems for organic photovoltaic applications. Device engineering and optimization has led to increase in the efficiency and stability of P3HT:PCBM solar cells over the past decades. However, there is a lack of understanding of physical mechanisms, specially the role of localized states in limiting performance and their electrical signature.
In this thesis, we seek to study possible manifestations of localized states in a variety of solar cell structures using P3HT:PCBM as the active material. We mainly used forward bias photo-capacitance response and open circuit voltage decay as tools among other conventional techniques. We demonstrate that useful information on deep localized states or recombination centers can be gained using this perspective.
We show that capacitance-voltage (C-V) characteristics in the first quadrant of current density-voltage (J-V) characteristics can be used as a probe to study the nature of defect states and their energetic distribution in conventional solar cell structures. Capacitance in space charge region is used to map the density of defect states distributed within the gap both under dark and white light illumination. The distribution parameters and the occupied density of defect states estimated as a function of temperature.
In order to isolate electron mobility and trapping mechanism in an otherwise bulk heterojunction material, electron only structures are fabricated and studied. Impedance spectroscopy is deployed to characterize the transport and storage of charge carriers in ITO|ZnO| P3HT:PCBM|Al (electron only) device. Complex part of impedance as a function of frequency is modeled to extract the mobility under dark and white light illumination. The defect states exposed under photo-excitation are interpreted as site service as dominant recombination centers. The recombination is mainly governed by Shockley Reed Hall recombination mechanism. The presence of recombination centre is further confirmed using the C-V characteristics. The mobility and capture cross-section are estimated as a function of temperature.
The photo-capacitance voltage characteristics of conventional as well as inverted solar cell incorporating P3HT:PCBM as active materials are modeled using the drift diffusion theory. The photo voltage and the carrier accumulation at the injecting interface are determined as a function of illumination intensity and compared for both structures.
Open circuit voltage decay transient have been used to identify internal loss mechanisms in P3HT:PCBM bulk heterojunction conventional and inverted solar cells. The transient describe using a model of decay based on a diode couple to a capacitor and a resistor. The diode ideality factor so obtain helps in the identification of dominant recombination mechanism. The diode current and dark shut current are isolated in the presence of excess carriers, and hence the intensity dependence of light induced recombination is determined independently for the two structures for comparison. The possible origins of such differences are discussed.
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Ms. Reeta Pant
Krishnacharya
Y9209066
Investigation of tunable wetting and slippery behavior on hydrophilic surfaces
Friday July 10, 2015
10:00 AM (tea @ 9:45 AM)
FB-382
Wetting of a liquid on a solid surfaces is one of the most common surface/interfacial phenomenon which can be primarily controlled by manipulating surface/interfacial energies and/or surface roughness. Here I demonstrate reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing. Static and dynamics of wetting transitions are investigated as a function of UV exposure, annealing temperature and time. Subsequently, the surface roughness was covered using an oil layer to prevent from Cassie to Wenzel transition which also acts as lubricant for enhanced slippery behavior. Slippage of water drops on smooth/rough hydrophilic surfaces will be discussed in details. Optimization of various experimental parameters will be presented to fabricate most efficient slippery surfaces in terms of performance and stability.
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Mr. Jitesh Barman
Krishnacharya
11109868
Electric Field Controlled Interfacial Phenomenon at Liquid-Solid/Liquid Interfaces
December 10, 2015 (Thursday)
11 am
FB-382
Wetting of liquids on solid surfaces is one of the most important interfacial phenomenon from fundamental as well as application point of view. Wettability of a surface can be controlled by manipulating the surface free energy of solids either passively by coating or actively using external stimulus e.g. temperature, radiation, electric filed etc. Here we report external electric field induced manipulation of liquids on solid and liquid surfaces. Electrowetting on dielectrics (EWOD) has been established as an effective tool to reversibly manipulate the surface wettability. Firstly I will talk about the application of EWOD in open-microfluidics. Surface grooves with triangular cross-section will be used as open-microfluidics. Static and dynamics of liquid transport in the grooves, actuated via electrowetting, will be discussed along with appropriate theoretical model. Subsequently, EWOD on dielectric lubricating fluid coated solid surfaces, to reduce hysteresis, will be presented. Nepenthes pitcher plants have motivated researchers to explore lubricating fluid coated slippery surfaces. I will demonstrate how using external electric field, electrically tunable slippery surfaces can be fabricated with electric field dependent slip velocity. Multiple aqueous droplets on such lubricating fluid coated surfaces show spontaneous coalescence or pseudo non-coalescence depending on the lubricating fluid film thickness. External electric field can also be used to control the coalescence or non-coalescence on such surfaces. Towards the end, I will talk about specially engineered superoleophobic surfaces which repel low surface tension liquid (oils and hydrocarbons). Ultraviolet radiation induced reversible wettability change, from superoleophobic to oleophilic, will also be discussed.
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Mr. Ambrish Pandey
Mahendra K. Verma
10109872
Scaling of large- and small-scale quantities in Rayleigh-Bénard convection
02-Dec-2015
11 am
FB-382
Thermal convection is responsible for heat transport in many natural systems, e.g., in stars, Earth's mantle, Earth's atmosphere, etc. We study Rayleigh-Bénard convection (RBC) in which a fluid placed between two horizontal plates is heated from below and cooled from top. Resulting motion of the confined fluid is primarily governed by the Prandtl number (Pr), a ratio of kinematic viscosity and thermal diffusivity, and the Rayleigh number (Ra), a ratio of buoyancy and viscous forces. The Nusselt number (Nu), a measure of the convective heat transport, and the Péclet number (Pe), a ratio of heat advection and heat diffusion, are important nondimensional parameters. We study the scalings of Nu(Ra,Pr) and Pe(Ra,Pr) by performing direct numerical simulations using pseudo-spectral solver Tarang. For Pr ≈ 1, we observe Nu ~ Ra0.27 and Pe ~ Ra0.50. However, Nu ~ Ra0.30 and Pe ~ Ra0.60 are observed for very large and infinite Pr. Moreover, for very large Pr the scalings of Nu and Pe are very similar in 3D and 2D RBC. We also study the scaling of kinetic energy and entropy spectra for very large Prandtl numbers. The kinetic energy spectrum scales with wavenumber (k) as k-13/3, and the entropy spectrum exhibits dual branches in the inertial range for both 2D and 3D simulations. The entropy flux remains constant in the inertial range for very large Pr.
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Mr. Gyanendra Kumar
R.Vijaya
Y9109068
Loss-modulated fibre ring laser and control of its dynamical features
05.11.2015 (Thursday)
10.30 am
FB-382
Highly-stable erbium-doped fibre amplifiers and lasers are useful in traditional fibre-optic communication due to their wavelength range of operation lying in the low-loss window of glass fibres. Fibre lasers operating in the chaotic domain are equally useful, but for widely disparate applications ranging from secure communication systems to chaos-based LiDARs and random number generation. An understanding of the conditions for switch-over from linear to nonlinear dynamical regime of fibre lasers, their tendency for chaotic operation, and possibilities of controlling their dynamics are essential for these applications. We study the fundamental as well as sub- and super-harmonic resonance characteristics of the erbium-doped fibre laser in the regime of its relaxation oscillation frequency, the evolution of its chaotic dynamics, and the conditions leading to its bi-stable and multi-stable operation using the cavity-loss modulation technique through experiments. For various modulation parameters, the laser exhibits different periodic states (period-1, period-2, period-4, etc) eventually leading to deterministic chaos through the period-doubling route. This work shows that we can repetitively drive the non-autonomous class-B laser into and out of linear, nonlinear (of different extents), and chaotic dynamics with a very simple approach as required for applications. The nonlinear oscillator model of the laser provides a good agreement between the theory and experiments. Convenient user-defined parameters such as the pumping ratio and the biasing voltage of modulation enable a fine control on the laser dynamics. The alternate technique of pump modulation is also analyzed for the sake of completeness, and the advantage of loss modulation is established.
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Mr. Rohit Kumar
Prof. Mahendra K. Verma
10109875
Energy transfers in dynamos with small and large magnetic Prandtl numbers
03.11.2015
11 am
FB-382
The presence of magnetic field in celestial bodies is explained by dynamo mechanism in which a conducting fluid in motion generates a self-sustained magnetic field. In dynamo, energy is transferred form velocity field to magnetic field and consequently the growth of magnetic energy takes place. The magnetic Prandtl number (Pm), the ratio of kinematic viscosity and magnetic diffusivity, is an important non-dimensional parameter for dynamos. We study energy transfers in dynamos with small- and large-Pm by performing direct numerical simulations (DNS) in a 3D periodic box on 10243 grid, using a pseudo-spectral solver Tarang. Energy fluxes and shell-to-shell energy transfers show that for dynamo with large-Pm, the growth of magnetic energy takes place due to nonlocal energy transfers from large-scale velocity field to small-scale magnetic field. For dynamo with small-Pm on the other hand, the magnetic energy grows due to local energy transfers from large-scale velocity field to large-scale magnetic field. We also use a shell model of dynamo to understand the energy transfers for extreme values of Pm, which are otherwise inaccessible to DNS due to the computational constraints. The energy fluxes in our shell model simulation are in qualitative agreement with the DNS results. We also construct a low-dimensional model to study the dynamo transition for very small to very large Pm and observe that the critical magnetic Reynolds number for dynamo transition, Rmc, saturates to constant values in the two limiting cases of Pm.
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Ms. Shraddha Sharma
Dr. Amit Dutta
Y9209067
Fidelity and Loschmidt echo: Quenches, non-analyticities and emergent thermodynamics
30/09/2015 (Wednesday)
11 am
FB-382
Our work focuses on relating quantum information theoretic measures such as the ground state fidelity and the Loschmidt echo (LE) to quantum phase transitions (QPTs) and dynamics. Fidelity is the modulus of the static overlap of the two ground states of a system at different parameter values; whereas, the LE is the modulus of the overlap of two states evolved from the same initial state with two different Hamiltonians. Therefore, one can map a connection between the two; both the tools effectively detects the quantum critical point (QCP) and follows universal scaling behavior close to it. Since, QPTs are low energy phenomena, (i.e., the gap is minimum for the low energy modes) the major contribution to the fidelity and the LE comes from these low energy modes. We shall explore the situation in which these tools fail to mark a QCP and address the question that in what situations high energy modes makes dominant contributions. In this so-called marginal situation, we shall explain how one ends up in a logarithmic scaling behavior of the fidelity. Furthermore, we shall introduce and analyze a quantity referred to as the dynamical fidelity (which is a version of the LE) obtained for a periodically driven quantum Hamiltonian. Exploring the entire regimes of the frequency and the number of periods, we shall show how it differs from any other thermodynamic observable obtained for a periodic Hamiltonian. We shall also explore dynamical phase transitions (DPT) using the LE as a tool for both sudden and slow quenching of an integrable quantum system; extending the work further to a non-integrable system, we shall explain the presence and the absence of DPTs by exploring the underlying integrability of the Hamiltonian. Also, we shall exhibit how the LE emerges as an useful measure not only to detect the QCP but also to investigate the work statistics at zero and finite temperature of a quenched system. Exploring the finite temperature LE, we shall further study the emerging thermodynamics and observe the behavior of the average irreversible work and the irreversible entropy in a many body quantum system following a sudden quench and propose their scaling behavior.
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Gopal
Dr. Pankaj Jain
Y9109066
Cosmological and Colliders Implications of Conformal Symmetry
29/09/2015 (Tuesday)
4 pm
FB-382
Symmetry has always played an important role in understanding Nature. At present most of our theoretical knowledge comes from the symmetry principles. In this talk I will discuss the implications of scale or conformal symmetry. In particular, I will propose a possible solution to the fine tuning problem of cosmological constant within the framework of softly broken conformally symmetric model. Cosmological constant is potentially a source of Dark energy, which constitutes about 70% of the energy density of the Universe. A major problem with the cosmological constant is that it gets very large quantum contributions from the matter sector at each order in perturbation theory. So we need to cancel these large contributions in order to maintain the small value of observed dark energy density, leading to an acute fine tuning problem. In our proposed solution we have shown that the matter sector will not contribute to the cosmological constant and hence we have solved the fine tuning problem. Here we have not considered the quantum gravity effects since quantum theory of gravity is not well formulated.
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Shubhankar Das
Z. Hossain & R.C. Budhani
Y9109078
Magnetoresistance and Magnetothermopower studies of delta-doped
2-dimensional electron gas at the interface of LaTiO3/SrTiO3
30th April, 2015
11 am
FB-382
Abstract: Studies of the 2-dimensional electron gas (2DEG) at the interface of LaAlO3 (LAO) or LaTiO3 (LTO) and TiO2 terminated SrTiO3 (STO) has attracted much attention in recent years due to its interesting properties like metal-insulator transition, magnetism, superconductivity and strong spin-orbit interaction effects in the electronics transport. We have used a new approach to bring about modification in the electronic properties of this 2DEG. This involves delta (δ) doping at the interface of LTO/STO by an iso-structural antiferromagnetic perovskite (TN = 298 K) LaCrO3 (LCO). This δ-layer dramatically alters the properties of the 2DEG. The changes includes increase in room temperature sheet resistance (R□), drop in the sheet carrier density (n□) almost linearly with the layer thickness, and emergence of new features in the temperature dependence of R□ at T ≤ 50 K. Our spectroscopic measurement along with density functional theory (DFT) calculations show that the Cr-ions at the interface act like a trap for electrons which are transferred from the LTO to STO surface. Extensive measurements of out-of-plane and in-plane magnetoresistance (MR) have been carried out on all the samples to address issue such as weak antilocalization and Kondo scattering. We have also observed a gradual crossover from positive out-of-plane MR to negative in-plane MR when magnetic field is titled with respect to the film surface. The MR measurements are augmented by the measurement of thermopower (S) which increases dramatically with δ-layer thickness at ambient temperature. The linear temperature dependence of S in the temperature range 100 to 300 K is indicative of diffusion thermopower. We also observed a large enhancement in thermopower in the temperature range where a minimum in R□ is observed. This enhancement is attributed to Kondo scattering. The thermopower is suppressed in the presence of a magnetic field and the suppression is isotropic with respect to the field direction. We will also present a tunable Rashba S-O interaction in these interfaces by δ-doping with another iso-structural ferromagnetic perovskite LaCoO3 (LCoO). In LCoO-doped sample, the inelastic scattering time varies as 1/T and the S-O scattering time remains constant in temperature, which suggests that the spin relaxation follows the D’yakonov-Perel mechanism. The δ-doping also results in 3 order of magnitude decrease in τso whereas the inelastic scattering time increases very slowly with doping. A detailed analysis of anisotropic MR when the field is applied in the plane of the sample displays the effects of Zeeman interaction with conduction electron spin.
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Reeta Pant
Krishnacharya
Y9209066
Investigation of tunable wetting and slippery behavior on hydrophilic surfaces
10th July, 2015
10 am
FB-382
Wetting of a liquid on a solid surfaces is one of the most common surface/interfacial phenomenon which can be primarily controlled by manipulating surface/interfacial energies and/or surface roughness. Here I demonstrate reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing. Static and dynamics of wetting transitions are investigated as a function of UV exposure, annealing temperature and time. Subsequently, the surface roughness was covered using an oil layer to prevent from Cassie to Wenzel transition which also acts as lubricant for enhanced slippery behavior. Slippage of water drops on smooth/rough hydrophilic surfaces will be discussed in details. Optimization of various experimental parameters will be presented to fabricate most efficient slippery surfaces in terms of performance and stability.
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Dushyant Kumar
Prof. R. C. Budhani and Prof. Z. Hossain
Y9109064
Spin and electronic charge diffusion in superconducting NbN and disordered SrTiO3 under the influence of magnetic field and light
29th June, 2015
11 am
FB-382
Spin-based electronics generally called “spintronics†encodes and processes information in the quantum-mechanical spin of the electrons leading to reduced power consumption with faster operation. However, the generation of pure spin currents, its transport and detection remain a challenge. A comprehensive understanding of these phenomena requires study of spin diffusion in metals, semiconductors and superconductors. It has been of fundamental interest to search for the sources of pure spin currents and the materials which can allow long spin diffusion lengths. In this thesis work we have addressed both these issues. We used the full Heusler alloy Co2MnSi (CMS) as a source of polarized spins. This Heusler compound is a half metallic ferromagnet. Such a band structure suggests 100% spin polarization, thus making the ordered cubic phase of Co2MnSi a good candidate for spintronic devices. As for the material for efficient transport of spin current, we have concentrated on SrTiO3, a cubic band insulator well known for its useful dielectric and opto-electronic properties. The high mobility (∼2 x 103 cm2/Vs at 15 K) electron gas was induced in the few nanometer thickness of STO by irradiating its surface using Ar+-ions. The process converts ~4 nm thick surface layers of STO into a metal while over the remaining thickness of a 0.5 mm wafer remains insulating. The unperturbed base material has been used as a gate dielectric in a back gated geometry.
The NbN was used to study the effect of spin polarized current in superconductors. It is a BCS type of superconductor with high bulk critical temperature (TC ~ 16 K) and large upper critical field (>20 Tesla). The NbN thin films with fairly high TC (~15 K) was grown at relatively low growth temperature (~2000C) making this material possible to integrate with hybrid structures. Devices have been fabricated to study the spin diffusion in superconducting NbN as well as in metallic Ar+-ion irradiated STO (reduced STO).
The spin-polarized current (IS) injection into the superconductor drives it out of equilibrium and results in an excess quasi particle density, which leads to the suppression of the superconducting gap. This effect is realized in the form of suppression of superconducting critical current (IC). The IS-injection from CMS into NbN shows a large (≈67%) suppression of IC at T/TC ≈ 0.4. This correspond to a large dynamic gain, Gd = -dIC/dIS, of 36 at 3 K. A control device, Au/MgO/NbN, is also made to rule out the Joule heating effect.
The spin transport through reduced STO was studied in a Co-based 3-terminal device. The spin injected from one Co-electrode was allowed to diffuse through the surface electron gas of reduced STO and detected on the other Co-electrode located 10µm far from the injector. The change in resistance while ramping the magnetic field from positive to negative suggest that the reduced STO could remember the spin for 10µm distance.
For device perspective of the surface electron gas of reduced STO, it is desirable to study its response to photon and electrostatic fields. In this connection, we have studied the photoconductivity (PC) and photoluminescence (PL) of ion irradiated STO and tuned them using electrostatic gate field. The reduced STO showed a large UV sensitive persistent photoconductivity over the temperature range of 300 to 15 K, with a relaxation time of several hours. Interestingly, at 300 K, a negative gate field accelerates the post-illumination recovery process pushing the system to relax to its ground state quickly. This property has the potential for designing a solid state photoelectric switch. The PL spectra of the reduced STO showed multi-color emission at room temperature, whose intensity increases gradually with decreasing temperature with a sharp increase below 80 K. At 20 K, the negative as well as positive gate field is observed to increase PL intensity. These properties have been addressed in the light of structural phase transition in SrTiO3.
[1] Dushyant Kumar, P. C. Joshi, Z. Hossain, and R. C. Budhani, APL 102, 112409 (2013)
[2] Dushyant Kumar, Z. Hossain, and R. C. Budhani, PRB 91, 205117 (2015)
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Shubhankar Das
Z. Hossain & R.C. Budhani
Y9109078
Magnetoresistance and Magnetothermopower studies of delta-doped
2-dimensional electron gas at the interface of LaTiO3/SrTiO3
30th April, 2015
11 am
FB-382
Abstract: Studies of the 2-dimensional electron gas (2DEG) at the interface of LaAlO3 (LAO) or LaTiO3 (LTO) and TiO2 terminated SrTiO3 (STO) has attracted much attention in recent years due to its interesting properties like metal-insulator transition, magnetism, superconductivity and strong spin-orbit interaction effects in the electronics transport. We have used a new approach to bring about modification in the electronic properties of this 2DEG. This involves delta (δ) doping at the interface of LTO/STO by an iso-structural antiferromagnetic perovskite (TN = 298 K) LaCrO3 (LCO). This δ-layer dramatically alters the properties of the 2DEG. The changes includes increase in room temperature sheet resistance (R□), drop in the sheet carrier density (n□) almost linearly with the layer thickness, and emergence of new features in the temperature dependence of R□ at T ≤ 50 K. Our spectroscopic measurement along with density functional theory (DFT) calculations show that the Cr-ions at the interface act like a trap for electrons which are transferred from the LTO to STO surface. Extensive measurements of out-of-plane and in-plane magnetoresistance (MR) have been carried out on all the samples to address issue such as weak antilocalization and Kondo scattering. We have also observed a gradual crossover from positive out-of-plane MR to negative in-plane MR when magnetic field is titled with respect to the film surface. The MR measurements are augmented by the measurement of thermopower (S) which increases dramatically with δ-layer thickness at ambient temperature. The linear temperature dependence of S in the temperature range 100 to 300 K is indicative of diffusion thermopower. We also observed a large enhancement in thermopower in the temperature range where a minimum in R□ is observed. This enhancement is attributed to Kondo scattering. The thermopower is suppressed in the presence of a magnetic field and the suppression is isotropic with respect to the field direction. We will also present a tunable Rashba S-O interaction in these interfaces by δ-doping with another iso-structural ferromagnetic perovskite LaCoO3 (LCoO). In LCoO-doped sample, the inelastic scattering time varies as 1/T and the S-O scattering time remains constant in temperature, which suggests that the spin relaxation follows the D’yakonov-Perel mechanism. The δ-doping also results in 3 order of magnitude decrease in τso whereas the inelastic scattering time increases very slowly with doping. A detailed analysis of anisotropic MR when the field is applied in the plane of the sample displays the effects of Zeeman interaction with conduction electron spin.
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Nikhil Kumar
Anjan Kumar Gupta
Y9109074
Control of the thermally hysteretic regime in superconducting weak-links and quantum interference devices
21st Apr 2015 (Tuesday)
11:30 am
FB-382
Micron size-Superconducting Quantum Interference Devices (micro-SQUIDs) are the most suitable probes for nano-scale magnetism. Hysteresis in the current-voltage characteristics of such devices has been a limiting factor and my thesis explores ways to restrict this hysteretic regime. We have fabricated and studied the current-voltage characteristics of a number of Nb film based weak-link devices and SQUIDs showing a critical current and at least two re-trapping currents. We have proposed a new understanding for the re-traapping currents in terms of thermal instabilities in different portions of the device. We also find that the superconducting proximity effect and the phase-slip processes play an important role in dictating the temperature dependence of the critical current. The proximity effect also helps in widening the temperature range of hysteresis-free characteristics. Finally we demonstrate a control on temperature-range on hysteresis-free characteristics in two ways: 1) By using a parallel shunt resistor in close vicinity of the device, and 2) by reducing the weak-link width. Thus we demonstrate non-hysteretic regime down to 1.3 K temperature, which is usually restricted to 1-2 K below the critical temperature (~ 9 K, for Nb). We have also studied the SQUID oscillations with magnetic flux in both the hysteretic and non-hysteretic regimes.
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Bappa Saha
Sutapa Mukherji
10109063
Work distribution, entropy production and fluctuation theorems in
nonequilibrium systems
20th April, 2015 (Monday)
11 am
FB-382
Out of equilibrium processes are the most observed phenomena in the natural world. The presence of a nonzero flux of the energy or matter across the system, a nonzero entropy production in the surrounding medium are some of the special features of nonequilibrium processes. In addition, nonequilibrium systems exhibit interesting phenomena such as
boundary induced phase transition, spontaneous symmetry breaking, unusual critical behavior etc. Recently, there have been many efforts to characterize the nonequilibrium processes through a set of powerful theorems, known as fluctuation theorems (FT). The emphasis of FTs have been on quantifying the asymmetry between the probability distributions in the forward and time-reversed processes for the thermodynamic
quantities, such as entropy production, work done by external force etc.
In this talk, we shall study an asymmetric simple exclusion process(ASEP), a nonequilibrium process that involves driven motion of many particles on a one dimensional lattice. We show how the boundary induced phase transitions exhibited by this process can be systematically analyzed using a fixed-point based boundary layer analysis. In the next part of my talk, using the Onsager-Machlup functional integral approach, we discuss the work and heat distribution functions for a Brownian particle subjected to an oscillatory driving force. We verify whether these distributions satisfy FTs. Following the same line of approach, we next obtain the work distribution function for a Brownian particle subjected to a nonconservative force. Although the work distribution has a Gaussian form, it is found that the distribution does not satisfy the conventional work fluctuation theorem. In the last part of the talk, we discuss the entropy production and the associated fluctuation theorem for an ASEP with two sites and elucidate how the computation of the average entropy production rate plays a central role in characterizing the steady states of such non-equilibrium systems.
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Gangadhar Behera
S. Anantha Ramakrishna
Y9109065
Nanostructured Plasmonic thin films for enhanced optical properties
16th April, 2015 (Thursday)
11 am
FB-382
The unique optical properties of nano-structured plasmonic films have attracted great attention due to their potential applications in solar cells, photo-detectors, sensors, nano-imaging devices, thermal emitters and many more. Surface plasmon polaritons (SPPs) are surface electromagnetic waves that exist at the interface between a metal and a dielectric material. In this work, the significant role of the surface plasmons in understanding novel optical phenomena like enhanced transmission or enhanced absorption in nano-structured plasmonic thin films is investigated in detail. The extra-ordinary transmission of light through an array of holes in thin plasmonic films for use as conducting transparent electrodes is proposed. By experiments and simulations, these structured metallic electrodes are also shown to enhanced absorption for solar cell applications. Complementary layers of ladder-like plasmonic structures fabricated by laser interference lithography (LIL) are investigated for enhanced optical properties in the visible-IR bands. Possible applications as polarization dependent sensors at IR frequencies is also discussed.
Enhanced absorption from a trilayer plasmonic system consisting of structured hole arrays in gold film separted from the bottom gold layer by dielectric spacer is reported. The fabrication by LIL and optical characterization of these samples are presented. A new approach to design dual band perfect absorber in the visible to the NIR region with top metallic patches on a SiO2 coated Si substrate is reported. A physical model for these absorbers is presented. A combined structuring of metallic disc and grating arrays on glass substrates that give rise to triple-band perfect absorption at visible frequencies is also reported. The physics behind these enhanced phenomena are discussed. The results have good potential for realizing low-cost large area nanostructured plasmonic thin film devices for energy applications.
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Dheeraj Pratap
S. Anantha Ramakrishna
Y9109063
Anisotropic and Hyperbolic Metamaterials in the cylindrical geometry
30th March, 2015 (Monday)
4 pm
FB-382
Metamaterials are artificial structured composites, which have properties not usually found in natural materials. Electromagnetic metamaterials can be designed to show, for example, extreme anisotropy where the dielectric permittivity tensor components can even have opposite signs. In natural anisotropic crystals, the anisotropy is usually a very small fraction of the permittivity. The large anisotropy in metamaterials is attractive for novel applications ranging from super-resolution lenses to super-radiative sources due to the hyperbolic dispersion of modes. Oriented plasmonic nanowire assemblies and multilayers of alternating metallo-dielectric films are the common examples that show anisotropic behaviour. Fabrication of nanowire metamaterials is usually accomplished by electro-deposition of the metals into the nanopores of a nanoporous alumina template.
Nanoporous alumina microtubes in the cylindrical geometry have been developed by anodizing microwires of aluminum. These microtubes have nanopores radially emanating from the centre towards the periphery. These microtubes represent a new class of anisotropic optical fibers that have different kinds of modes described by uncommon Bessel functions with imaginary orders. This is due to the hyperbolic mode dispersion in these systems. Electro-deposition of plasmonic nanowires into the nanopores make hyperbolic disperison easily accessible. A homogenization of the nanostructure in the cylindrical geometry has been carried out using techniques of transformation optics. A detailed analysis of the mode structure and mode dispersions has been carried out.
Light scattering and absorption studies from nanoporous alumina surfaces in the flat and cylindrical geometries are reported here. Flouresence studies of molecules deposited on these nanoporous alumina surfaces with and without metallic inclusions in them are presented. Some unique applications of these alumina surfaces to thermal engineering will be mentioned.
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Govind Dayal Singh
Y9109067
Metamaterials for Infra-red Multi-spectral Absorbers
18th March, 2015 (Wednesday)
3:30 pm
FB-382
Electromagnetic absorbers have wide applicability and need to be impedance matched with free space to reduce the reflectance from their surface. Here we discuss the design and fabrication of metamaterials that show multi-band perfect absorbances at infra-red frequencies. The key a highly absorbing medium is to choose resonant structures that simultaneously have matched electrical and magnetic resonances. An array of metallic (conducting) particles separated from a conducting ground plane by a dielectric spacer layer constitutes the resonant structure. Proper choice of the particle size and layer thickness can result in perfect absorption that is reasonably independent of the excitation angle and polarization. Use of multi-layered stacks of particles that can also yield multi-band absorption. While most metamaterial designs utilize the fundamental mode in a sub-wavelength sized resonator, highly localized higher order modes can also be utilized for multi-band perfect absorption.
Multi-band metamaterial absorbers based on fundamental as well as higher order resonances for Infra-red frequencies have been designed, fabricated and characterized. The metamaterial absorbers have multiple absorption bands across the MWIR and LWIR bands with peak absorbances exceeding 90%. Metamaterial absorbers, with broadband absorption at the mid-infrared frequencies and high transmittance at visible frequencies, have been fabricated using a semiconductor Indium Tin Oxide (ITO) film as the ground plane.The metamaterials were fabricated by simple, cost-effective laser micromachining techniques, shadow mask deposition and oblique angle deposition technique. A strategy to tune or switch the metamaterial absorption by incorporated a phase change material such as VO2 in the metamaterial has been implemented and results in a thermochromic metamaterial.
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Pranati Rath
Y9109077
Large Scale Anisotropy in Cosmic Microwave Background Radiation and its Dependence on the Primordial Power Spectrum
20th March, 2015 (Friday)
11:30 am
FB-382
The Cosmological principle says that the universe at large distance scale is assumed to be statistically homogeneous and isotropic. The Cosmic Microwave Background Radiation (CMBR) is considered to be one of the best experimental evidence supporting this principle. However, there exists considerable evidence in cosmological data which suggests violation of this principle.
In this talk, I will revisit two of these anomalies observed in the CMBR temperature data provided by WMAP and recently by PLANCK team. These includes the alignment of the quadrupole (l=2) and octopole (l=3) and the hemispherical power asymmetry. To explain the low-l alignment, I will discuss some anisotropic inflationary models within the framework of the Big Bang cosmology. In these models, the anisotropy decays very quickly during the inflationary phase of expansion. The resulting direction dependent power spectrum in this anisotropic background leads to violation of isotropy and hence explain the alignment of the low l CMBR modes.
I will also discuss an inhomogeneous power spectrum model in order to explain the hemispherical power asymmetry, observed in the CMBR data. The hemispherical asymmetry can be parametrized in terms of the dipole modulation model. Alternatively, I will show that, an anisotropic dipolar imaginary primordial power spectrum, which is possible within the framework of noncommutative space-times, also provides a good description of the observed dipole modulation in CMBR data.
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Tutul Biswas
10109877
Wave packet dynamics and phonon scattering in spin-orbit coupled fermionic systems.
9th March, 2015 (Monday)
11 am
FB-382
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Subhash Mahapatra
10109069
Applications of the Gauge-Gravity duality : Superconductivity,
holographic Optics and Entanglement Thermodynamics.
10th February, 2015 (Tuesday)
11 am
FB-382
A remarkable connection, in the context of string theory, has emerged in last few years between gravity and the strongly coupled field theories. This connection, which is generally called as the AdS/CFT correspondence or the gauge/gravity duality, maps a quantum theory of gravity in (d +1) dimension Anti de sitter (AdS) spacetime to d dimensional conformal field theory (CFT) - living at the boundary of AdS spacetime. An important feature of this duality is that in certain approximation, the strongly coupled limit of field theory corresponds to the weakly coupled limit of the gravity theory. This strong-weak nature of the correspondence has provided a unique approach to address some questions in strongly coupled condensed matter systems which otherwise would be intractable.
In this thesis, we discuss applications of the AdS/CFT correspondence, on three main directions: superconductivity, optics and entanglement thermodynamics.
We generalize the minimal models of holographic superconductors in a gauge invariant way and explore the rich phase structure of the boundary theory. Then, we study the electromagnetic response functions and entanglement entropy of these generalized holographic superconductors for various space-time geometries.
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Uday Bhanu Paramanik
Y8109071
Magnetism and superconductivity of Eu-based "122" pnictides.
6th February, 2015 (Friday)
11 am
FB-382
The discovery of high-temperature superconductivity in Iron pinctides has created enormous interest leading to the finding of a wide range of layered pnictide compounds which exhibit a rich variety of magnetic and transport phenomena. Among the '122' pnictides, EuFe2As2 got particular attention as this system demonstrates interesting interplay between superconductivity and local moment magnetism of Eu moments. We have investigated the effect of Iridium doping on the magnetic and superconducting properties of EuFe2As2. The optimal Iridium doping suppresses the Fe antiferromagnetic order and induces superconductivity below 22 K. The Eu moments order magnetically at ~18K with strong ferromagnetic (FM) component. The competition between the superconducting and the magnetic state leads to the reentrance of superconductivity. While the superconducting properties of the Fe-As based materials have been studied extensively, very recently, unconventional superconductivity has been discovered in the vicinity of antiferromagnetic order in a class of CrAs based compounds. We present the synthesis and physical properties of a new compound EuCr2As2 in the '122' pnictide family which exhibits antiferromagntic ordering of Cr itinerant moments in addition to the ferromagnetic order of Eu local moment. This behavior is reminiscent of the parent compound of Fe based superconductor EuFe2As2 but with a slightly different magnetic structure. Furthermore, since the valence fluctuations in classical heavy-fermion systems are considered to be responsible for unconventional superconductivity, we discuss the important aspect of the evolution of magnetism upon Ge-doping in the valence fluctuating heavy fermion compound EuNi2P2. The rare-earth ion configuration changes from valence fluctuating (VF) state in EuNi2P2 to nearly integral valence Eu2+ (4f7) in EuNi2PGe which orders antiferromagnetically below TN = 12 K. No superconductivity was observed in the Ge-doped EuNi2P2 down to 2K.
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Bahadur Singh
Y9209061
Electronic structure and spin texture of non-trivial surface states
of topological insulators
28th Jan. 2015 (Wednesday)
4:0 pm
FB-382
A new paradigm for classifying condensed matter systems by topology of bulk band structure has spurred the discovery of several new states of matter having exotic properties. In particular, topological insulators are such new states of matter that are distinct from conventional insulators. These materials support spin-polarized conducting states at the boundaries/surfaces while remaining insulating in the bulk. The surface states often exhibit Dirac-cone energy dispersion with helical or chiral spin-texture and are protected by time reversal symmetry. The topological protection ensures the backscattering free transport at the boundaries of topological insulators. The recent developments in the field show that these nontrivial states not only offer potential applications in quantum computing and spintronics, but also provide platforms for realizing novel quantum phenomena such as Weyl semimetals, and Majorana fermions in a condensed matter system.
In this work, we present an analysis of topological surface state properties of several selected materials and predict new materials or thin films of materials that realize the quantum spin Hall state, Dirac semimetal, Weyl semimetal, or Rasbha effect, using the ab-initio density functional theory framework and k.p theory. Through systematic analysis of bulk and surface electronic structures, we show that thallium based ternary III-V-VI2 series of compounds, TlMQ2 (where M= Bi or Sb and Q= S, Se, or Te), contain both topological and normal insulators and therefore, it is possible to study the topological phase transition (TPT) by tuning the lattice parameters as well as spin-orbit coupling (SOC). The electronic structure and spin-texture analysis of topological surface states show that the surface states form an unusual “planer metal” that is essentially half of an ordinary two dimensional (2D) conductor and carry nontrivial π Berry phase. Furthermore, we discuss possible TPT in Ge(BixSb1-x)2Te4 thin films as a function of layer thickness and Bi concentration x. The systematic examination of band topologies suggests that thin films of Ge(BixSb1-x)2Te4 are viable candidates for 2D topological insulators, which would undergo a 2D-TPT as a function of x. Finally, our analysis shows that the inversion asymmetric compound, Sb2Se2Te, harbors both a novel nontrivial band topology and giant Rasbha-type spin splitting in its native form driven by strong SOC. The Rasbha splitting in the bulk bands of Sb2Se2Te is the largest that has been found to date and attributed to large polarization field (electric field), which arises from the broken inversion symmetry in the system.
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Jhuma Dutta
Y9109069
Photonic and Plasmonic properties of Periodically Patterned Columnar Thin Films
26 November, 2014 (Wednesday)
11 am
FB-382
Columnar thin films are known to be deposited by making incident a collimated vapor flux at arge oblique angles to a substrate. Using a substrate that is periodically patterned at icro/nanoscales, new kinds of periodically patterned columnar thin films (PP-CTFs) have been fabricated. The extremely anisotropic optical properties offered by the nanocolumnar structure with larger scale structures for photonic bandgap or diffractive effects provide for new physical phenomenon. These structures were also found to be extremely reconfigurable giving rise to enormous flexibility in applications.
The PP-CTF of various dielectrics like CaF2/ MgF2/SiO2 and metals like silver are shown to be conveniently fabricated by this method. The complete morphology of the PP-CTF is shown to be determined by controlling the angle and flux rate of depositions. PP-CTFs are shown to have enhanced photonic and plasmonic properties. Dielectric PP-CTFs are shown to function as blazed diffraction gratings with large asymmetric diffraction efficiencies. A CTF made of plasmonic metals like silver renders it an effectively biaxially anisotropic continuum. PP-CTFs of silver showed strong blazing action and asymmetrically coupled optical radiation to surface-plasmon-polariton(SPP) waves for both p- and s- polarizations propagating only along one direction supported by the CTF/dielectric interfaces.
The blazing effect of the gratings of dielectric CTFs as a result of the spatially linear phase shifts caused by prismatic air cavities was understood using Kirchhoff-Fresnel diffraction theory. Homogenization of the metallic CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves on both 1D and 2D gratings of such anisotropic hyperbolic media was found to be adequately described thereby. Detailed electromagnetic simulations of the grating structures reveal large changes in the photonic properties with the slant angle such as diffraction efficiencies and the electromagnetic near fields. Furthermore, these slanted nanocolumnar structure can be uniformly reconfigured by ion beam irradiation method and gives rise to reconfigurable blazed gratings, thereby maximizing the diffraction efficiencies for different wavelength bands by changing the blazing (angle) condition. A novel application using superstructures of CTFs to fingerprint visualization was developed. Visualization of latent fingerprints is enhanced by deposition of columnar thin films at large oblique angle of CaF2 and SiO2 on fingerprint marks on two nonporous surfaces such as smooth glass slides and highly reflecting rough aluminum sheets and further enhanced visualization is obtained by treating the deposited CTFs with a fluorescent dye and fluorescence imaging.
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Sourabh Barua
Y7109075
Electrical transport studies of topological insulator
materials.
24th November, 2014
11:30 am
FB-382
Topological insulators are a class of materials which have generated great interest due to their novel properties. The band structure of these materials has surface bands which span the bulk band gaps and the crossing of these bands at the time reversal invariant momentum is protected as long as time reversal symmetry is not broken. This ensures that the materials have a conducting surface despite possessing an insulating bulk. However experimental verification of these surface states in electrical transport studies has been difficult due to the fact that the bulk itself conducts due to defects. In the work done for this thesis we have performed electrical transport studies of the topological insulator materials Bi2Te3 , Sb2Te3 and Bi(2-x)Sb(x)Te(3-y)Se(y). In Bi2Te3, in spite of a metallic bulk, we observe signatures of a topological surface state from the non-zero Berry phase extracted from the Shubnikov-de Haas(SdH) oscillations in the magnetoresistance in addition to a large and linear magnetoresistance. Further angle dependent magnetoresistance studies have revealed interesting asymmetry for positive and negative fields for some angles although the SdH oscillations are more or less symmetric. Sb2Te3 shows initially a negative magnetoresistance which evolves with time to a positive one but the same SdH oscillations persist. There is also asymmetry in the positive and negative fields. In Bi(2-x)Sb(x)Te(3-y)Se(y)we obtained metallic single crystals which showed linear magnetoresistance. We have also made a comparison of the thickness dependence of resistivity reported for various topological insulators in literature with a simple model.
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Anirban Dutta
Y7109062
Temperature dependent scanning tunneling microscopy and spectroscopy (STM/S) studies of Iron-Arsenic based single crystals.
21st October, 2014 (Tuesday)
3 pm
FB-382
Iron-Arsenic (FeAs) based superconductors with high critical temperature have a spin ordered ground state with a spin density wave (SDW) order. Superconductivity emerges in close proximity to the SDW state by suppressing (or weakening) SDW order, thus the SDW order is found to anti-correlate with the superconducting order. My thesis work focuses on the variable temperature scanning tunneling microscopy and spectroscopy (STM/S) studies of AFe2As2 (A: rare earth or alkaline earth element), i.e. 122-series, compounds. We have studied two undoped parent compounds, EuFe2As2 and SrFe2As2, and two doped compounds, CaFe1.96Ni0.04As2 (underdoped) and SrFe1.6Co0.4As2 (optimally doped) with STM/S. The tunnel spectra show homogeneous energy gap in the undoped and underdoped crystals in the SDW state. However, we see spatially inhomogeneous spectra in the SC state in underdoped and optimally doped crystals. We observe an asymmetric suppression in densty of states only at some locations in the SC state for the underdoped crystal, while the optimally doped crystals show a superconducting gap everywhere with spatially varying magnitude. Moreover, for the undoped SrFe2As2 crystals, strength of the SDW gap weakens below 21K, where a small fraction superconducts as found from resistivity and suscptibility measurements. This compound could be more prone to superconducting order and small local perturbations, like strain, may nucleate superconductivity locally in this undoped crystal. Finally we discuss the posiible reasons behind the observed inhomogeneous superconducting state.
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Samit Paul
Y8209868
Micron Focusing, Diagnostics and Interaction of Multi-Element Ion Beams with Matter
22nd September, 2014 (Monday)
12:00 noon
FB-382
A focused ion beam (FIB) system is an inevitable tool for research and applications in nano-science and technology. However, the availability of only gallium (Ga) ions as projectile, in liquid metal ion source (LMIS) based FIB systems limits its applicability. Moreover, metallic Ga has contamination issues and provides less milling yield. In order to ameliorate the above problems, an emerging area in this field is development of focused ion beams of a variety of elements utilizing plasmas. A compact, high current density (~ 1 A/cm^2), microwave plasma based multi-element focused ion beam (MEFIB) system has been developed in the laboratory, i.e., capable of delivering beams of different ions (H2, Ar, Kr, Ne) of energies up to 18 keV and spot size of ~ 25 µm, using a 1 mm plasma electrode (PE) aperture. The beam has an emmittance of ~ 0.1 mm-mrad and brightness of ~ 10^5 Am-2sr-1V^-1, thereby making it an excellent candidate for FIB.
In this thesis our target has been to reduce the beam to submicron spot size and make it operate at higher energies (30 keV) comparable to that of conventional FIBs. In parallel, we developed a new diagnostics (spider probe) for measuring micron size beams and have investigated the time dependent physics of the interaction with matter, particularly, the milling capability to make micro-pores and microstructures on metallic foils and thin films.
In order to achieve the above, first the plasma has been optimized to maximize the extracted beam current density maintaining tolerable ion energy spread at the meniscus (~5 eV). The utilization of two Einzel lenses in conjunction with a PE and a beam limiting (BL) aperture, is found to have better control over the beam energy and current density. Controlled sculpting of micron scale pores in aluminum foil with Ar and Kr ion beams is demonstrated. The temporal evolution of the micro pores caused by beam interaction on metallic foils has been studied and smallest pore diameter of ~ 3 µm has been successfully created. This shows that with proper control of the irradiation time, submicron pores can be fabricated. Further experiments, using smaller PE apertures of 45 µm and 31 µm, created by MEFIB, have been used to obtain a focused beam size ~ 1 µm. A time-dependent exposure method to determine the exact location of the focal point of the beam has been invented that takes care of the over irradiation issue. Extensive use of Lab-View software is made to create the microstructures on 50 nm Cu thin films.
In order to reduce the beam spot size further, guiding and transmission of extracted Ar ion beams with the help of straight and tapered micro-glass-capillary have been investigated. The beam current through the capillary is found to have a threshold extraction voltage and observed to exhibit hysteresis, with a unique self-focusing capability. The temporal and dimensional dependence of the hysteresis have been studied. The guiding capability of the tapered capillary is found to be more effective where beam size reduction is desired without compromising total beam current unlike electrostatic beam limiters. For further understanding the phenomena, Particle-In-Cell simulations that solves Poisson's equation and equation of motion self consistently are carried out, and the experimental results are found to have a reasonable agreement with simulations.
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Ms. Vandana Yadav
Y9109082
Polymerization dynamics of microtubules: A boundary layer based approach
4th September, 2014
2:30 pm
FB-382
Microtubules are important components of the cytoskeleton of a cell. These are hollow, tube shaped biopolymers made of alpha,beta-tubulin heterodimers. Experimental studies reveal that microtubules alternate between persistent phases of polymerization and depolymerization. Since microtubules play an important role in various processes such as intracellular organisation, transport, exerting pushing and pulling forces etc., it is crucial to understand the polymerization dynamics of microtubules and its regulations by various other proteins.
In the first part of my seminar, I shall discuss a two-state model of microtubule polymerization with a length-dependent dynamics and obtain the length distributions of microtubules through a discrete formulation.
Next, I shall discuss the boundary layer method and show how it can be used to understand phases and phase transitions of driven many-particle exclusion processes. We develop a fixed point based boundary layer method and apply this to two distinct driven exclusion models to show how the shape of the particle distribution profile, the location of the boundary layer etc can be predicted from the stability properties of the fixed points of the boundary layer equation.
In the last part of my seminar, I shall use the boundary layer method to understand the polymerization dynamics of microtubules in the presence of diffusing tubulins. Since various rates associated with the polymerization dynamics are expected to be influenced by the number of available tubulins, the growth dynamics and the diffusive dynamics of tubulins are coupled in a nonlinear way. The boundary layer analysis emerges as a powerful method that allows us to obtain analytical solutions for the length distributions in a systematic way.
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Anirban Bagui
Y7109861
Effect of electric field annealing during solvent-drying of polymer layers.
30.07.2014 (Wednesday)
4:15 pm
SCDT seminar room
Controlling the nano-morphology of organic active layers for improved organic electronic devices has recently attracted immense attention. In this thesis the changes in the polymer film properties due to thermal annealed during the solvent drying in the presence of a constant electric field across the film are studied. Comparative studies of morphological and electro-optic properties of the P3HT: PCBM based blend film and bulk hetero-structure devices have been carried out to understand the role of this electric field annealing. The external quantum efficiency (EQE) and power conversion efficiency (PCE) of the solar cells made from the electrical field annealed films were found to be significantly enhanced due to better charge transport in the films.
In order to study the effect of electric-field on the mobility of P3HT, single carrier devices were made. The hole mobility in P3HT was found to increase monotonically with electric field strength up to 2E5 V/m. The J–V data corresponding to the space charge limited currents (SCLC) at low temperatures for P3HT based hole-only devices were fitted with the empirical model and Gaussian disorder model (GDM) for modeling the charge transport so as to interpret the findings of the XRD measurements. These confirmed an increase in crystallinity and crystallite size of the films for electric-field annealed samples.
Solar cells with the 'inverted structure' were fabricated to see if improvement in devices performance can be achieved by electric-field annealing of their active layers. Finally, electric–field assisted treatments were carried out on some other polymer - PTB7, PCDTBT, MEHPPV - layers and the effect of the treatments on the mobility in the films were studied.
In this thesis, the application of electric fields across the active layer of organic solar cells at the time of its formation has been shown to help achieve higher mobility polymer films for building devices that have improved performance.
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M. Hemanadhan
Y8209866
Study of excited-state energy density functionals constructed by splitting k-space for homogeneous electron gas
2.6.2014 (Monday)
4:00 pm
FB-382
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Jaidip M. Jagtap
Y8109066
Polarimetry, Optical and Fluorescence Confocal Microscopy: Study and Discrimination of Human Cervical Precancerous Tissue
2.5.2014 (Friday)
11.00 am
Biological tissue displays morphological and biochemical changes during transformation to diseased state. Optical techniques are ideal for minimally invasive detection of such changes. Light scattering has been seen to capture morphological changes, such as increase in size and density of nuclei in cells. Subtle precancerous (dysplasia) changes are probed via Mueller matrix imaging which strongly manifest in the depolarization power and retardance and are seen to differ significantly in normal and dysplastic tissues sections. Fluorescence Mueller matrix spectroscopy has been developed recently for measurements from the connective tissue regions of human cervical tissue and reveal intriguing fluorescence diattenuation and polarizance effects. The estimated fluorescence linear diattenuation and polarizance parameters were considerably reduced in the precancerous tissues as compared to the normal ones, which were also confirmed by control experiments.
Robust statistical parameters in the form of moments, specifically weighted mean, full width at half maxima, standard deviation, skewness, kurtosis and pixel counts characterizing these distributions are shown to unambiguously distinguish microscope captured images of various tissue types. Combining these parameters effectively improves the diagnosis and classifies quantitatively various classes of tissue. Optical spatial frequency spectra from different stages of human Cervical Intraepithelial Neoplasia (CIN) tissue are also evaluated as a potential quantitative pathological tool on confocal captured images. The degree of randomness of tissue structures from normal to different stages of CIN tissue can be recognized by spatial frequency analysis.
Fluctuations in the elastic light scattering spectra of normal and dysplastic human cervical tissues through wavelet transform based techniques have been analyzed and reveal clear signatures of self-similar behavior in the spectral fluctuations. The strong dependence of the elastic light scattering on the size distribution of the scatterers manifests in the angular variation of the scaling exponent. These findings using the multi-resolution analysis capability of the discrete wavelet transform can contribute to the recent surge in the exploration for non-invasive optical tools for pre-cancer detection.
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Rajyavardhan Ray
Y5209066
Disorder and Correlation Effects on a Honeycomb Lattice:
Application to Graphene
14th June 2013 (Friday)
11 am
FB-382
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SK Firoz Islam
Y8109070
Electronic and magnetotransport properties of spin-orbit coupled two-dimensional
electron system.
7th June, 2013 (Monday)
11:00 am
FB-382
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Amit Banerjee
Y7209864
Resonance behavior of FIB grown nanomechanical systems and the
role of microstructure
23rd April, 2013 (Tuesday)
9 AM
FB-382
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Pabitra Mandal
Y6109067
Anomalous magnetic response of CaFe1.94Co0.06As2 superconductor and nonlinear response of the driven vortex state in NbS2 superconductor.
25th April, 2013 (Thursday)
11 am
FB-382
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Suman Banerjee
Y5209865
Optoelectronic modelling of organic solar cells
12th March, 2013 (Tuesday)
5:15 PM
Samtel Centre Seminar Room
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Prince Gupta
10209064
Metamaterials and plasmonics with active gain medium
10/10/2012, 4:00-5:00 PM
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Mr. Gyanendra Singh
Y5109067
Spin reorientation transition and superconductivity in NbN based ferromagnet-superconductor thin film heterostructures.|
9th Fubruary, 2013 (Saturday)
12 pm - 1 pm
FB-382
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Sunil Kumar
Y9109079
Multiple Quantum wells in Organic Semiconductors
03/01/2012,Tuesday
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Mr. Gyanendra Singh
Y5109067
Spin reorientation transition and superconductivity in NbN based ferromagnet-superconductor thin film heterostructures.
23rd December (Friday), 2011
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Pavan Aluri
Y5109071
Large Scale Anomalies in the Cosmic Microwave Background Radiation
Thursday 1st December, 2011
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Ms. Anupam
Y6209061
Magnetic and Superconducting Properties of Novel Eu-based Arsenide and Gallide
29th November (Tuesday), 2011
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Naveen Kumar Singh
Y5109070
Scale Invariance and its Cosmological Implications
Thursday 20th October, 2011
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Bahadur Singh
Y9209061
Electronic band structure and Z2 topological Invariant of topological Insulators
September 16, 2011 (Friday)
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Ms. Reeta Pant
Y9209066
Responsive surfaces with tunable wetting properties and study of the wetting transition
8th September, 2011 (Thursday)
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Shraddha Sharma
Y9209067
FIDELITY APPROACH TO QUANTUM PHASE TRANSITION & QUENCHING
9th September 2011 Time: 1200 hrs
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Mr. Lalruatfela
Y9109071
Imaging in Random Media using Diffuse Light
12th August, 2011 (Friday)
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Dheerendra Yadav
Y4209861
SPECTROSCOPIC AND IMAGE DIAGNOSTICS OF LASER ABLATED PLASMA PLUME FOR ION AND ATOMIC SOURCE
November 29, 2010 (Monday)
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Indranuj Dey
Y5209863
Wave Interaction with Plasmas Confined in Multicusp Magnetic Fields
November 29, 2010 (Monday)
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Vijay Kumar Bisht
Y3809069
Studies on non-equilibrium magnetic behavior of antiferromagnetic oxide nanoparticles
November 29, Monday
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Dushyant Kumar
Y9109064
Heusler alloy based magnetic tunnel junctions
9th December (Thursday)
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Ms. Vandana Yadav
Y9109082
Driven Exclusion Processes in Physics and Biology: Models and Analysis
8th Dec, 2010
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Upkar Kumar Verma
Y8209064
Memories based on Quantum Dot Embedded Organic Semiconductors
Tuesday 16/11/2010
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Jhuma Dutta
Y9109069
DESIGN,FABRICATION AND CHARACTERIZATION OF PHOTONIC SCULPTURED THIN FILMS
10 November 2010 (Wednesday)
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Prabhakar Tiwari
Y9109075
Cosmic Ray anisotropy studies at the GRAPES observatory (Ooty).
November 3, 2010 (Wednesday)
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Govind Dayal Singh
Y9109067
Metamaterials for tunable multi-spectral absorbers
October 25, 2010 (Monday)
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Mr. Sunil Kumar Mishra
Y4109075
Size-dependent magnetization fluctuations and slow dynamics in NiO nanoparticles.
September 15, 2010 (Wednesday)
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Mr. Shyam Kumar Choudhary
Y4809063
Scanning Tunneling Microscopy and Spectroscopy Studies of Electronic Inhomogeneities in Graphite and Graphene
August 30, 2010 (Monday)
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Ashutosh Sharma
Y8209062
Electronic Structure of Topological Insulators
Sep 4, 2010
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Mr. Samit Paul
Y8209868
Submicron focusing, diagnostics and interaction of multi-element ion beams with matter
May 17, 2010 (Monday)
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Awnish Kumar Tripathi
Y3809065
Photo-electronic Properties & Localized States in Polymeric Semiconductors
7th May (Friday), 2010
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Devendra Kumar
Y280962
Non-equilibrium features in the solid state: A case study of the transition metal oxide NdNiO3
05-05-2010
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Jaivardhan Sinha
Y4209862
Properties of magnetic materials under extreme conditions
03-5-10(Monday)
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Anup Kumar Singh
Y8109062
Study of Electrical and optical properties of Indium Gallium Zinc Oxide: A novel amorphous oxide semiconductor
April 28, 2010 (Wednesday)
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Anurag Sahay
Y220961
Thermodynamic Geometry, Phase Transitions and Critical Phenomena in AdS black holes
23.04.2010
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Mr. Asrarul Haque
Y4109069
Causality in Nonlocal Quantum Field Theory, Noncommutative Quantum Field Theory and 1+1 Dimensional Yukawa Model
23rd April 2010 (Friday)
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Jose V Mathew
Y5109069
Multi-element focused ion beams from intense microwave plasmas
13th April, 2010 (Tuesday)
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Pankaj Kumar Mishra
Y3809066
Instabilities and Turbulence in Rayleigh-Bénard Convection: Numerical and Phenomenological studies
12th April, 2010 (Monday)
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Mr. Debjit Datta
Y4809062
Study of optical and morphological properties of thin films in CuPc/C60 solar cells
26th March 2010 (Friday)
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Jaidip Manikrao Jagtap
Y8109066
Mueller Imaging in Turbid Media and Human Tissues
15th March, 2010 (Monday); 11:00 AM (Tea at 10: 45 AM)
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Sk Firoz Islam
Y8109070
Electronic and transport properties of graphene
March 11, 2010 (Thursday)
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Ms. Sangeeta Chakrabarti
Y5209067
Controlling MetaMaterials with Radiation and Controlling Radiation with Metamaterials
8th March, 2010
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Speaker:
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Ajeet Kumar Sharma
Y8109061
Stochastic mechanics of molecular motors on nucleic acid strands
February 11, 2009 (Thursday)
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Mr. Mihir Sarkar
Y7209062
Tailoring the Optical Properties of Materials Using Ion Beams
27th Jan, 2010
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Amit Banerjee
Y7209864
Fabrication and Characterization of Nano Sensors using Focused ion beam
28th Jan, 2010.
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Ms. Arpana Prasad
Complex Magnetic Behaviour of Ce-, Pr-, Eu- and Yb-based Compounds
18th December (Friday), 2009
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Bhupendra Nath Tiwari
Y220961
Correlations, Stabilities and Black Holes in String Theory and M-Theory
04 November, 2009 (Wednessday)
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Sarvesh Kumar Tripathi
Y6109862
Investigation of focused ion beam induced processes and their utilization for nanofabrication
5th November, (Thursday)
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Ashok Garai
Y4109078
Molecular motor transport and traffic: effects of individual mechanochemistry and steric interactions
4th November, 2009 (Monday)
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Pramoda Kumar Samal
Y3209064
TESTING STATISTICAL ISOTROPY OF COSMIC MICROWAVE BACKGROUND RADIATION
October 19, 2009, (Monday)
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Bhaskar Kamble
Y4809061
A Quantum Expansion Parameter for Metallic Ferromagnets: Orbital Degeneracy, Hund's Coupling, and Quantum
Friday, 9 Oct 2009
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Himanshu Pandey
Y7109069
SPIN POLARIZATION AND SPIN DEPENDENT TUNNELING IN HEUSLER ALLOYS
September 29, 2009
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Nitul Singh Rajput
Y7209063
Fabrication of micro and nano structures by focused ion beam and their science and technology applications
September 30, 2009
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Surajit Sarkar
Y7109876
Oxide/CNT Heterostructures & Secondary Electron Emission
September 23, 2009
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Ankur Rastogi
Y7109063
Carrier Modulation by Field Effect and Electronic Phase Transitions in Perovskite Oxides
September 25, 2009
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Shail Kumari
Y7209063
Interaction of modulated electromagnetic waves with a pre-ionized medium
September 23, 2009
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Sourabh Barua
Y7109075
Dynamical studies of phase transitions in ferroelectrics and multiferroics
25 Sptember, 2009
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Speaker:
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Date:
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Surajit Sarkar
Y7109876
Oxide/CNT Heterostructures & Secondary Electron Emission
23 Sptember, 2009
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Speaker:
Roll No:
OS Title:
Date:
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Uma Divakaran
Y4209864
Slow quenching dynamics in quantum critical systems
22 Sptember, 2009
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Abhishek Chowdhury
Y7209061
Generation and studies of multi-element single and multiple focused ionbeamlets from an intense wave assisted plasma for creation of functional surfaces
17 Sptember, 2009
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Speaker:
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Prasanna Kumar Rout
Y7209866
Proximity Effect in Superconductor-Normal Metal and Superconductor-Ferromagnet Hybrids
14 Sptember, 2009
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Speaker:
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Anirban Dutta
Y7109062
STM/STS Studies of Strongly Correlated Systems
09 Sptember, 2009
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Speaker:
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Ajeet Kumar Sharma
Y8109061
Stochastic mechanics of molecular motors on nucleic acid strands
February 11, 2009 (Thursday)
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Speaker:
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Debaprasad Sahu
Y7109067
Negative ions: Volume generation, measurement and wave induced phenomena
4 May, 2009
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Prasanta Kumar Muduli
Y3109063
Spin polarized transport in planar structures and tunnel junctionsof perovskite oxides
23 April, 2009
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Speaker:
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Date:
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Anirban Bagui
Y7109861
Annealing Effects on Organic SolarCell Performance
31 March, 2009
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Speaker:
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Date:
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Shyam Mohan
Y4109074
Instabilities in the vortex state of a type II superconductor
31 March, 2009
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Speaker:
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Date:
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Vinod Chandra
Y3109068
Hot QCD Equations of state and Quark-Gluon Plasma in Heavy ion Collisions
23 March, 2009
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Speaker:
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Date:
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Udai Raj Singh
Y4109077
Low Temperature Scanning Tunneling Microscopy and Spectroscopy Study of ManganiteThin Fimls
4 March, 2009
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Speaker:
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Date:
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Shyam Lal Gupta
Y6209062
Experimental and Theoretical Studies of Strongly
Correlated Systems
3 March, 2009
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Speaker:
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Subhayan Mandal
Y280966
Pseudoscalar-Photon Mixing & It's Astrophysical Implications
23 February, 2009
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Speaker:
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Md. Shamim
Y6209861
Exchange and correlation energy functionals in excited-state densityfunctional theory
6 February, 2009
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Lipsa Nanda
Y3209063
Energy flow approach to propagation of electromagnetic pulses through highly dispersive media
27 January, 2009
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Speaker:
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Prashant Shukla
Y210964
Polarization Gated Imaging and Mueller Imaging in Turbid Media and Human Tissue
28, November 2008
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Speaker:
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Date:
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Tripti Tripathi
Y3809068
Template-Dictated Polymerization of Bio-Polymers: Machines and Mechanisms
26, November 2008
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Speaker:
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Date:
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Victor Mukherjee
Y6209864
Defect production and entropy generation in generalized quenching dynamics of spin systems
24, October 2008
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Sudhakar Pandey
Y210965
Correlated Electron Spin Dynamics in Metallic Ferromagnets
October 24, 2008
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Subhadip Mitra
Y280965
Two Photon Exchange Contributions to Elastic ep Scattering in the Non-local Field Formalism
September 24, 2008
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Gorky Shaw
Y6209863
Investigation of magnetic properties of superconducting nanostructures
September 22, 2008
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Pabitra Mandal
Y6109067
Effect of doping and disorder on superconducting properties
September 22, 2008
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Samar Layek
Y6209862
Investigations of Novel Rare Earth Compounds Using Magnetization, Electrical Resistivity and
Mössbauer Spectroscopy
August 20, 2008
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Somdutta Mukherjee
Y6109072
Electrical, Magnetic and Optical properties of multiferroic bulk and thin films
August 19, 2008
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