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| Speaker: |
Arpan Hait |
| Roll no.: |
20109264 |
| Title: |
The hunt for an elusive form of matter |
| Date: |
Apr 27, 2022 |
| Time: |
10:30 AM (Wed) |
| Venue: |
FB 382 |
| Abstract: |
Astrophysical and cosmological observations indicate the existence of an elusive form of matter, permeating the universe in a vast amount, yet invisible to all of us. This form of matter has been aptly dubbed "Dark Matter". There is quite a lot of conclusive evidence for the existence of dark matter. A tremendous amount of progress has been made on theoretical and experimental fonts in hunting for this missing matter. Cosmological observations tell us that dark matter constitutes nearly 85% of the universe's matter density and contributes approximately 25% of the total energy budget of the present universe. Apart from this, not much is known about dark matter.
In this talk, I will give a basic introduction to dark matter physics. Then various production mechanisms of dark matter in the early universe will be discussed in a model-independent way. Dark matter production in non-standard scenarios will also be sketched. |
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| Speaker: |
Abhijeet Kishore |
| Roll no.: |
19109261 |
| Title: |
Lattice QCD with Minimally Doubled Fermion and Random Matrix Theory |
| Date: |
03.12.2021 (Friday) |
| Time: |
11:00 AM |
| Venue: |
Online, Zoom Meeting
https://iitk-ac-in.zoom.us/j/99096259897?pwd=NkQvelhiLzhDYmxsVjdSUnB2Smt3QT09
Meeting ID: 990 9625 9897
Passcode: 534798 |
| Abstract: |
The dynamics of quarks and gluons inside a hadron is described by the SU(3) non-Abelian gauge theory, known as Quantum Chromodynamics(QCD). At low energies, QCD is a strongly interacting theory and required non-perturbative technique to solve. Lattice QCD where the theory is formulated on a discrete space-time lattice is the most successful nonperturbative technique. But fermions on a discrete space-time lattice suffer from the notorious doubling problem i.e, appearance of extra fermion species. To overcome that problem several remedies are proposed such as Wilson fermion, staggered fermion, minimally doubled fermion etc. In this talk, I will discuss the minimally double fermion proposed by Borici and Creutz (Borici-Creutz fermion). Borici-Creutz fermion preserves the chiral symmetry at the expense of hyper cubic symmetry. I’ll discuss the restoration of the hyper cubic symmetries for Borici-Creutz fermion.
The low lying eigenvalues of the Dirac operators are important to understand the spontaneous chiral symmetry breaking. Banks and Casher showed that the chiral condensate which is the order parameter of the phase transition is directly related to the eigenvalue density of the QCD Dirac operator. In the chiral limit, the spectral properties of lattice QCD Dirac operator are universal and can be predicted by Random Matrix Theory which does not know about the microscopic dynamics of the theory but deals with the universal properties. After discussing some basics of RMT, I’ll discuss the possible application of RMT to study the spectral density of Dirac operator with Borici-Creutz fermion. |
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| Speaker: |
Bheemsehan Gurjar |
| Roll no.: |
19109270 |
| Title: |
Internal structure of the nucleons and relation between various parton distributions functions |
| Date: |
26.11.2021 (Friday) |
| Time: |
11:00 AM |
| Venue: |
Online, Zoom Meeting
https://iitk-ac-in.zoom.us/j/91652145357?pwd=WUVzUVNRVW9VNWQwNlZKR2JzeXdSZz09
Meeting ID: 916 5214 5357
Passcode: 115348 |
| Abstract: |
The three dimensional structure of the nucleons is one of the most interesting and challenging task in particle physics. Even after more than fifty years of QCD, the internal structure of the nucleons are not yet well understood. All types of deep inelastic scattering (DIS) experiments (inclusive, exclusive and semi-inclusive DIS experiments) provide the crucial information about the nucleon structure.
To explain the experimental data we need to understand the spatial structure as well as spin and orbital angular momentum distribution of the quarks and gluons inside a nucleon. The internal structure of the hadrons (nucleons) are encoded into different distribution functions e.g., Transverse Momentum dependent Distribution functions (TMDs), Generalized Transverse Momentum dependent Distribution functions (GTMDs), Generalized Parton Distribution functions (GPDs) and Wigner distributions etc. Since these distributions are non-perturbative in nature and difficult to be calculated in full QCD, So peoples study those distributions in QCD inspired models. Model investigation of these distribution functions provide information about the internal structure of the nucleons.
In this talk we will discuss about the investigation of the relations between transverse momentum dependent parton distributions (TMDs) and generalized parton distributions (GPDs) and the orbital angular momentum distribution of the quarks inside the nucleon, by using a simple Light-Front quark-diquark model in the light-front formalism. The seminar is based on: Phys.Rev.D 104 (2021) 7,076028, e-Print:2107.02216 [hep-ph] |
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| Speaker: |
Yogesh Kumar Verma |
| Roll no.: |
17109872 |
| Title: |
|
| Date: |
12.09.2019 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Microring resonators (MRR) are widely used photonic component having applications in optical switches, wavelength filters, bio & chemical sensors, and optical communication networks. It consists of a closed-loop waveguide and input straight waveguide placed in the vicinity. By interaction of the evanescent field of the input waveguide, modes into the close-loop waveguide are excited. If these modal fields are phase-matched in a round trip in the coupling region, a resonant field coupling takes place between the straight and ring waveguides. The dependency of this wavelength selection on refractive index (RI) of the cladding material is the basis of its usefulness as a RI sensor and wavelength filter. Spectral response of such devices is well simulated by Finite-Difference Time-Domain (FDTD) numerical technique. In this seminar I will present the basics of MRR, some recent exciting results, a brief account of FDTD method and some regenerated spectral response of MRR already reported in literature. I will also present some of my recent results of ultra low FWHM laser line filters based on MRR. |
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| Speaker: |
Anan Bari Sarkar |
| Roll no.: |
17209871 |
| Title: |
Exploration of low-dimensional and topological magnetic materials |
| Date: |
28.08.2019 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Following the recent discovery of monolayers of magnetic materials, the interest in magnetism in 2D materials has seen a revival. Furthermore, the ‘marriage’ between magnetism and topology is giving rise to the new paradigm of magnetic topological materials. In this SOTA seminar, I will review the physics of 1) 2D magnetic materials, 2) magnetic topological systems (for example, magnetic Weyl semimetals) and 3) their exploration using density functional theory.
Generally, the realization of long range magnetic order in two-dimensional systems has been difficult, as the magnetic ordering is suppressed by thermal fluctuations in presence of spin-rotational symmetry at finite T. However, the presence of magnetic anisotropy, and dipolar interaction allow the emergence of long range magnetic ordering in 2D materials at finite T. Another related emerging area is topological magnetic materials, where the combination of topologically protected surface states and magnetism promises potential application in spintronics, and data storage devices. |
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| Speaker: |
Sudipta Dutta |
| Roll no.: |
17109273 |
| Title: |
Vacuum structures and spectrum of null strings |
| Date: |
21.08.2019 (Wednesday) |
| Time: |
03:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Tensionless strings or null strings refer to strings in the ultra high energy limit of string theory where the tension of the fundamental string becomes zero. In this limit the worldsheet metric becomes degenerate and Polyakov action fails to describe the dynamics. However there is way of rewriting the action, where the problems that arises due to degenerate nature of worldsheet, can be avoided. This new action , called ''LST action", also allows us to understand the worldsheet symmetries that arises in this limit. This set of symmetries surprisingly, are the same set of symmetries of asymptotically flat spacetimes in 3 dimensions and known as BMS3. Now with the aid of this symmetry analysis, physical state conditions can be derived which restrict vacuum configuration along with the spectrum built around the vacua. Unlike the case of tensile strings, the physical state conditions can be implemented in different ways which correspond to different inequivalent vacua. We discuss these different vacua and some of their properties. |
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| Speaker: |
Kamal Das |
| Roll no.: |
17209872 |
| Title: |
Magneto-transport in Weyl semimetals and the realization of quantum anomalies |
| Date: |
21.08.2019 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Topological materials in condensed matter systems have opened up a new paradigm, for the exploration of relativistic quasi-particles in table-top experiments. Amongst these, Weyl semimetals host chiral quasi-particle excitations, and Berry-curvature monopoles that lead to very interesting physics. One experimental probe of the diverse phenomenology of Weyl physics is charge and heat transport experiments, in presence of a magnetic field. This is where this SOTA seminar is focussed.
We review the current status of magneto-transport in Weyl semimetals. The Berry curvature monopoles act as an effective magnetic field in the momentum space, and lead to several new effects in Weyl semimetals. These include anomalous Hall effect, Chiral magnetic effect, Chiral electric effect, negative magneto-resistance, and planar Hall effect, among others. More interestingly, several quantum anomalies such as axial (or chiral) anomaly, and axial-gravitational anomaly, predicted in relativistic chiral fluids in presence of magnetic field, also manifest in magneto-transport experiments in Weyl semimetals. We review these experiments, and explain them within the semiclassical Boltzman transport formalism in presence of Berry curvature. |
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| Speaker: |
Rekha Kumari |
| Roll no.: |
17109867 |
| Title: |
Quantum Transport in Nano-structures of Topological Systems |
| Date: |
May 29, 2019 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Topological systems in 1D (such as topological superconductors), 2D (such as quantum Hall systems) as well as 3D (such as Topological insulators and semimetals) has been a center of research in resent time. Such topological systems can be in equilibrium as well as in far from equilibrium. One of the most useful way to identify their topological nature is by understanding their behavior in quantum transport. Various quantum transport formalism, broadly based on Landauer-Büttiker formalism, are used to tackle problems in different dimensions as well as when explicit time-dependent systems are concerned. In this state-of-the-art seminar I'll describe some of the recent progress in quantum transport, specifically in Weyl semimetalic heterostructure. In particular I'll emphasize on quantum transport with superconducting interface and how signatures of non-trivial band-structures can be understood through such study. |
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| Speaker: |
Souvik Bandyopadhyay |
| Roll no.: |
17109271 |
| Title: |
Out of equilibrium dynamics of topological quantum many body systems |
| Date: |
May 17, 2019 (Friday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Topological phases of quantum matter are of great interest from both theoretical and experimental perspectives The topological properties of these systems are protected by a gap in the bulk spectrum and are manifested in the occurrence of the robust edge states localised at the boundaries. The so called topological invariant, characterising the topological phases, can undergo a change only when the system is tuned across a gapless (topological) quantum critical point. Whether these exotic phases of matter and the associated bulk boundary correspondence survive in the presence of global time dependent perturbations is an interesting area of recent interest. At the same time, these studies also open up the possibility of dynamical preparation of topologically protected non-equilibrium states.
In this talk, I shall review some recent developments in this field and further elaborate on the non-equilibrium response of topological systems to time dependent driving and the fate of equilibrium topological properties. Finally, I shall illustrate the possibility of dynamical preparation of 1D topological insulators under unitary driving. |
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| Speaker: |
Sayantan Ghosh |
| Roll no.: |
17109268 |
| Title: |
Nematic electronic state and it's connection to electronic and magnetic properties of materials |
| Date: |
May 14, 2019 (Tuesday) |
| Time: |
09:30 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Occurrence of Nematic phase is well known in the context of liquid crystal, where rod shaped molecules which have no positional order can transform into a state where they begin to point in a given direction collectively. The transformation is described as a phase transition, and the phase is a Nematically ordered phase. Recent, evidences show that there exists an exotic Nematic phase even in hard condensed matter systems, where in the electronic phase in the materials develops Nematic ordering with the system spontaneously breaking rotational symmetry. The emergence of a Nematic phase seems to have connections with onset of a special type of magnetic order as well as possible connections to development of superconducting order. The Nematic phase also clearly affects electrical transport properties in the materials. I will use Iron Pnictide system as a test bed material to discuss the phenomena of electronic Nematic state and how it affects magnetism, Superconductivity and transport property in a material. The underlying correlations among all of these three phenomena’s is an open question, and I will try and explore the issues. The phenomena of Nematic ordering in the electronic state has also been seen in High Tc superconductors. In the presentation I will also attempt to show some of my recent studies in high Tc Cuprates. |
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| Speaker: |
Sourav Bhattacharjee |
| Roll no.: |
17109269 |
| Title: |
Quenched quantum systems: non-analyticities, quantum work and engines |
| Date: |
May 14, 2019 (Tuesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
The study of non-equilibrium dynamics of quantum systems is an exciting field of recent research from both experimental and theoretical viewpoints. A quantity of interest central to the understanding of the evolution of a system following a quench (i.e., a change in parameter of the Hamiltonian) is the Loschmidt overlap amplitude (LOA) which measures the overlap between the initial state and the evolved state. The short time dynamics of the LOA may be associated with novel non-analyticities occurring at those instants of time where it vanishes; these are known as dynamical quantum phase transitions. On the contrary, the long time dynamics of the same LOA encodes emergent universalities in the so-called work distribution function. Another recent and interconnected area of research in the domain of non-equilibrium dynamics is the cyclic operation of quantum systems as quantum heat engines or quantum refrigerators. In this seminar, I shall present a brief review of the recent developments in the aforementioned topics and discuss some of my related recent works. |
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| Speaker: |
Arnab Sarkar |
| Roll no.: |
17109262 |
| Title: |
Dynamical strong coupling in micro and nano-mechanical nonlinear resonators |
| Date: |
May 6, 2019 (Monday) |
| Time: |
10:15 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Micro and nano mechanical resonators have been at the focus of research in multidisciplinary fields, due to the wide range applications they offer, from accelerometers, sensors and frequency references in microwave domains to experiments in testing limits of superposition in macroscopic quantum states and using quantum bits for faster and secure processing of information. To generate controlled and efficient information gates with mechanical resonator modes, studies have been undertaken in understanding nonlinearity in these systems, along with transition of their driven dynamics to controlled chaos. Possibilities of processing information with such mechanical modes as bits in classical domain in both strong and weak driving, have been studied extensively. Primary signature of such engineered interaction between mechanical modes are usually observed in frequency spectrum of the resonator, where strongly coupled modes with high quality factor hybridize into normal modes with typical signature of avoided level crossing as a function of frequency or energy mismatch. It is therefore necessary to understand such strong coupling regime in mechanical resonators, towards engineering and using efficient information gates.
In this talk, I will review a range of qualitatively different physical couplings between resonator modes and their corresponding experimental signatures. In particular I will review resonant and off resonant parametric coupling of two resonator modes and analyse corresponding signatures of avoided crossing,avoided level crossing and level repulsion in such systems.I will end with a discussion and analysis of some of our recent experimental observations in atomically thin, suspended graphene resonator and a large area, heavy SiNx membrane, strongly cupled via electrostatic foces and driven parametrically via gate voltage modulation. |
|
| Speaker: |
Debasmita Giri |
| Roll no.: |
17109264 |
| Title: |
Collective modes and correlations in Dirac and Weyl systems |
| Date: |
May 7, 2019 (Tuesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Response of a system to a weak external perturbation is given by the susceptibility of the system. For a system of unpolarized electrons, the indirect interaction (named as RKKY interaction) among impurity spins present in the system is given by the spin-spin susceptibility of that electronic system and may lead the system of spins to be ferro or anti-ferro magnetic in nature. For a weakly interacting system, the system reacts to an external perturbation through the particle-hole excitation as well as through excitations which are the collective modes of the system. Such collective modes are typically of charge-density waves or of spin-density waves in nature differentiated by their appearance in charge-charge susceptibility or spin-spin susceptibility of the system. Dirac and Weyl systems have low-energy electronic excitations that follow relativistic equations with spin-momentum coupling as well as with topological surface states. These may lead to exotic properties of the system, where we are interested, in particular, to the signatures in collective modes of such systems. |
|
| Speaker: |
Anisha |
| Roll no.: |
17109261 |
| Title: |
Capturing New physics effects through Effective Field theory |
| Date: |
May 3, 2019 (Friday) |
| Time: |
04:30 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
The Standard Model (SM) is known to be a very successful theory till date, but due to deviations of a few Electroweak Precision data and Higgs data from SM predictions, the presence of New Physics (NP) is hinted. Since the existence of any New Physics particle is not yet confirmed, one possible way to search for NP is through Effective Field Theory. It is a tool to describe NP effects by parameterizing it in terms of operators with a mass dimension greater than 4 each supplemented with its own Wilson coefficient (WC). Based upon the SM gauge symmetries and particle content, different bases of dimension six operators for Standard Model Effective field theory (SMEFT) are available in the literature. In my talk, I will talk about these bases and how to translate between different bases by using the SM equation of motions. Then I will discuss the effects of NP on the low energy Electroweak observables (Fermi Constant, Mass of Z and W boson, weak mixing angle etc) through non-zero values of WC by using different classes of operators. In the end, I will discuss briefly that using the same approach in some Beyond Standard Model (BSM) scenarios at a high energy scale, BSM-EFT can be constructed and NP effects can be analyzed in a similar manner |
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| Speaker: |
Sourav Biswas |
| Roll no.: |
17109270 |
| Title: |
Interacting Fermions in one dimension |
| Date: |
April 22, 2019 (Monday) |
| Time: |
10:30 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
In one dimension Fermi liquid theory (FLT) fails to explain interaction among electrons due to breaking down of adiabatic continuity. I will show how quasi particles in 1D can be thought, in terms of wave like excitations instead of particle-hole pairs of FLT, where, Bosonization formalisms can be used successfully to describe the low-energy behavior of such systems. In particular, interaction among Fermions in 1D lead to Luttinger liquid (LL) phase, predicting exotic phenomena, such as charge-spin separation. We will see how signatures of Luttinger physics can be found in scaling of the correlations. I will also discuss some experimental signatures of LL phase.
Such Bosonization techniques are being applied in recent times, especially, in Dirac or Weyl systems, where it is usefull in understanding the low-energy properties of the system, either in quasi one dimensional systems or at the edge of such systems. |
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| Speaker: |
Deepika |
| Roll no.: |
17109265 |
| Title: |
Low-temperature atmospheric pressure micro-plasmas: diagnostics, fluctuations, and recombination physics |
| Date: |
March 25, 2019 (Monday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Atmospheric pressure microplasmas are becoming increasingly important because of their applications in several fields such as flexible electronics, medicine, biology, and environment. However, the physics of these plasmas have not been investigated in details. The non-equilibrium plasmas are termed as “cold” plasmas, with widely different electron (Te ~ 0.5 eV) and ion temperatures (Ti ~ 0.025 eV). The coupling parameter (gc) which dictates the ratio between the Coulomb interaction energy to kinetic energy is relatively large ~ 0.18 as compared to conventional low pressure plasma. The plasma frequency (ωp) is in the THz regime, because of the very high plasma densities.
To create the micro-plasmas, we have built two different experimental configurations based upon dielectric barrier capillary discharges, with helium as a working gas. The first configuration, is a ring to ring electrode system which is operated at 10 kHz frequency and 15 kV (p-p) voltage. The second system is a needle to ring electrode configuration operated at a comparatively higher frequency (80 - 100 kHz) and lower voltage (~ 4 kV). The plasma emerges out from the capillary as a micro-jet, and consists of reactive species of atmospheric nitrogen, oxygen, including those of helium and OH radical.
The present work proposes basic investigation of plasma parameters, such as electron temperature and plasma density by two methods: (i) optical emission spectroscopy and (ii) high pressure Langmuir probe diagnostics. By comparing the plasma parameters obtained by these two methods, we can find out the validity of high-pressure probe theory. Further, the plasma plume velocity emerging out of the capillary, will be determined by a single electrostatic probe by time of flight measurements. The characteristics of the electric field fluctuations along the axial, radial and azimuthal directions, will be determined by employing a double electrostatic probe operated in the floating mode. Finally, the effect of recombination on plasma decay will be investigated. At atmospheric pressures, the recombination rate coefficients are large due to the high plasma density. The plasma can recombine by dissociative and three body recombination processes at atmospheric pressure. Our aim is to study these recombination processes, by diagnosing the afterglow region of a pulsed power micro-plasma jet. The presentation will comprise of literature survey, work accomplished so far and future plan of research. |
|
| Speaker: |
Bashab Dey |
| Roll no.: |
17109861 |
| Title: |
Exotic properties of radiation-driven 2D Dirac semimetals |
| Date: |
27-02-19 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Two-dimensional electron systems driven by intense terahertz radia- tion exhibit several noble phenomena like zero-resistance states in semiconduc- tor heterojunctions, semi-metal to insulator transition, photovoltaic Hall effect, etc. Floquet theory is used to address the dynamics of such periodically-driven systems. We have investigated the effect of circularly polarized radiation on the band structure of a 2D Dirac semimetal called α-T3 lattice. This lattice is an extension of honeycomb lattice with an additional atom at the centre of each hexagon. The hopping parameter α governs a continuous evolution of the low energy Dirac Hamiltonian from pseudospin 1/2 (graphene) to pseudospin 1 (dice lattice).
We have studied the effect of radiation in this system under different fre- quency regimes - (a) On-resonant (Ω ≈ vf k) and (b) Off-resonant ( ̄hΩ > 6τ ), where 6τ is bandwidth of the Bloch band structure. Under on-resonant driving, we obtain the quasienergy band structure, time-averaged energy spectrum and time-averaged density of states by solving the Floquet Hamiltonian numerically. We deduce exact analytical expressions of the quasienergies at the Dirac points for all values of α and field strength. Approximate forms of quasienergy and band gaps at single and multi-photon resonant points are derived using rotating wave approximation. The expressions reveal a stark dependence of quasienergy on the Berry phase acquired by the charge carrier. The quasienergy flat band remains unaltered in presence of radiation for dice lattice (α = 1). The valley degeneracy and electron-hole symmetry in the quasienergy spectrum are broken for 0 < α < 1.
The topological characteristics of the photon-dressed band structure are studied under the off resonant condition. We obtain exact analytical expressions of the quasienergy bands over the first Brillouin zone. Band gaps created at the Dirac points vary with α similar to the on-resonant case. At α = 1/√2, the gap between flat and valence bands closes at K, while that between conduction and flat bands closes at K', thereby restoring a semimetalic phase. At the gap closing point (α = 1/√2), there is a reappearance of low-energy Dirac cones around K and KK' points. Under the influence of the circularly polarized radiation, the α-T3 lattice is transformed from semimetal to a Haldane-like Chern insulator characterized by non-zero Chern number. The system undergoes a topological phase transition from C = 1(−1) to C = 2(−2) at α = 1/√2, where C is the Chern number of the valence (conduction) band. This sets an example of a multiband system having larger Chern number. These results are supported by the appearance of chiral edge states in irradiated α-T3 nanoribbon. |
|
| Speaker: |
Bidisha Bhatt |
| Roll no.: |
17109263 |
| Title: |
Investigation of Interfacial Phenomenon in Soft Matter Using Optical Probes |
| Date: |
27-02-19 (Wednesday) |
| Time: |
09:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Most of the systems which we see in our daily life are neither completely solid nor completely liquid. They are classified under special class of material, commonly known as “Soft Matter”. Mechanical properties of such materials and/or systems are intermediate to ideal solids and liquids. Also typical length scale associated with soft matter system is few microns which correspond to tens or hundreds of molecules (e.g. length of a polymer). Among the soft matter system, my research interest involves investigation of surface and interfacial phenomenon using specialized optical techniques. In this seminar, I would mainly discuss about three type of system: liquid/solid interaction, liquid-liquid interaction and manipulation of drops/bubbles in air and other medium.
Equilibrium behavior of a liquid drop on a solid surface is determined in terms of their wetting properties. Contact angle goniometer is a very powerful technique to measure surface/interfacial energies of various liquid or solids and their wetting behavior in terms of the contact angle. But this technique has limitation in special resolution due to the wavelength of the light used. In many non-equilibrium situations, wetting drops are associated with thin and long pre-wetting films with thickness about 50 nm. Origin of these pre-wetting thin films is generally due to non-equilibrium heterogeneous solid surfaces and adsorption. Thin film interference based technique has been shown to be very useful to investigate statics as well as dynamics of such pre-wetting films. I will present, how such technique has been used to measure thickness of pre-wetting things as low as 25 nm. I will also present how confocal microscopy can be used to image liquid drops to measure the drop profile as well as its contact angle with much higher accuracy.
I would also discuss liquid-liquid interaction in the form of slippery surfaces. A thin layer of a suitable fluid can act as lubricating film where another liquid can slip effortlessly with almost no friction. Such slippery behavior also depends strongly on various system parameters including wetting behavior of the solid surface and surface tension and viscosity of test and lubricating fluids. Again, thin film interference based technique is very useful in analyzing the thin liquid film which is sandwiched between solid surface and the top liquid which can vary from 10 micron down to 25 nm.
Towards the end, I will discuss how the interaction between drops or particles can be measured or manipulated using a powerful technique called as “Optical Tweezers”. I will present some latest results on the measurement of interaction of two oil drops having the force resolution about pico-Newton. |
|
| Speaker: |
Fahim Varsi |
| Roll no.: |
17109266 |
| Title: |
Measurement of the energy spectrum, composition and (an)isotropy of cosmic rays by the GRAPES-3 experiment |
| Date: |
13-02-19 (Wednesday) |
| Time: |
04:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Cosmic rays are highly energetic relativistic charge particles (mainly protons) which fill the space and originate outside our solar system. The all particles energy spectrum ranges from 10 GeV to 100 EeV and follows a power law. The power index of energy spectrum changes from ~ -2.7 to ~ -3.1 at around few 1 PeV. The transition region is called the knee. It further changes from ~ -3.1 to ~ -2.6 at around few 1 EeV, which is called the ankle. Even a century after the discovery of cosmic rays, the basic information about their origin, propagation in interstaller medium, change in power index of energy spectrum at knee and ankle is not very well understood. Many balloon borne and satellite observations have been made for lower energy range (upto few 1 TeV) and ground based observations have been made for higher energy range (10 TeV onwards) to understand the origin, acceleration, propagation and other basic properties of cosmic rays.
GRAPES-3 is a ground based experiment having a high density Extensive Air Shower (EAS) array and a large area muon detector located at Ooty, TamilNadu. By using the GRAPES-3 EAS data, we plan to measure the energy spectrum of cosmic rays well below and above the knee region which will overlap with direct measurements at lower energies as well as with other cosmic rays measurements by the ground based experiments at higher energies. One of the objectives is to identify the hadronic interaction models that best describes the experimental data. The high quality data from GRAPES-3 EAS array coupled with information from the muon detectors should permit the study of the composition of the primary cosmic rays. We have already studied the trigger efficiency for each GRAPES-3 scintillator detectors, the distribution of energies of secondary particles recorded by the array with the aid of Monte Carlo simulations generated by CORSIKA in the primary energy range 1 TeV to 10 PeV. |
|
| Speaker: |
Hukam Singh |
| Roll no.: |
16109268 |
| Title: |
Long Period Gratings in Photonic Crystal Fibers and their application as sensors |
| Date: |
19-12-18 (Wednesday) |
| Time: |
10:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
One the most exciting development in the field of guided wave optics in the recent years has been the emergence of Photonic Crystal Fibers (PCFs) which are all-in-silica optical fibers. The dielectric cross-section of the PCF incorporates a periodic array of air-holes in the cladding region along the entire length of the fiber. The desired modal properties of PCFs can be engineered simply by tailoring the air-hole diameter and the spacing between them. Owing to their unique characteristics, various optical devices have been investigated including those based on mode coupling. Incorporation of Long-Period Gratings (LPGs) into the core of PCF makes it an intrinsic passive device that has a significant impact on optical sensing. PCFs have been exploited as a base in which LPGs have been inscribed as they provide enhanced sensitivity and offer a far more versatile platform for LPGs than traditional fibers. In this seminar, I will discuss about the field model approach to model the modal characteristics of the PCFs, with the main focus on optimizing the PCF based LPGs opto-geometrical parameters. I will then discuss about the recent experimental studies on the PCF based LPGs exploring their applications in: sensing refractive index of gaseous media, as well as their application in tailoring the transmission spectrum of the LPG for their possible application as band rejection filter. |
|
| Speaker: |
Supratim Das Bakshi |
| Roll no.: |
17109870 |
| Title: |
Effective Field Theory -- Tool to capture the new physics effects |
| Date: |
05-12-18 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Effective Field Theory (EFT) is the key to new physics when there is lack of experimental data to determine the pattern of new physics beyond SM. This is also a fantastic tool to capture the new physics effects through the effective operators and respective Wilson Coefficients. I will discuss the EFT formalism based on functional methods, and importantly avoiding the Feynman diagram computation. Then I will talk about how this functional method in the context of EFT can be used to understand SM-EFT starting from a BSM Lagrangian. In this context I will introduce briefly the Mathematica based package "CoDEx" that we have developed along with some example BSM models. |
|
| Speaker: |
Sana Afrin |
| Roll no.: |
16209863 |
| Title: |
Acousto-optic and photo-acoustic imaging of soft tissues |
| Date: |
26-10-18 (Friday) |
| Time: |
09:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
In vivo high resolution imaging of soft tissues is important in diagnosis of many conditions including cancer, autistic spectral disorder and many more. Currently available techniques either have good resolution but poor penetration depth, e.g. OCT or relatively better penetration depth but poor resolution, e.g. CT, MRI and fMRI etc.. Merging the advantages of high spatial resolution offered by optical waves and low scattering losses of acoustic waves, high resolution images deep inside the tissue can be recorded. In this seminar, at first I will be giving a brief overview of the current imaging techniques which are being used clinically such as MRI, fMRI, CT Scan, OCT etc.. The basic principle behind their working and their resolution will be discussed. In the second section, I will describe the acousto-optical and photo-acoustical Imaging techniques. Both these techniques combine the use of optical and ultrasound waves. But in principle they are very different from each other. I will discuss the theoretical modeling, the different methods of detection of the signal, their resolution and the ways to improve them. I will also present our simulated results of light interaction with a turbid medium and will discuss Monte-Carlo simulations of light diffusion inside biological tissues. |
|
| Speaker: |
Nitesh Kumar Jaiswal |
| Roll no.: |
16209263 |
| Title: |
Information Theory, AdS/CFT and Quantum Phase Transitions |
| Date: |
11-10-18 (Thursday) |
| Time: |
04:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
A quantum mechanical system can undergo a phase transition at the absolute zero of temperature as some parameters of its Hamiltonian are varied. Even at absolute zero temperature, quantum fluctuations are present in the system and they can trigger Quantum Phase Transition. As an example we review how to diagonalise the XY spin chain Hamiltonian to calculate its eigenvalues and eigenstates. We also discuss the Quantum Fidelity - which is just the overlap between two quantum states. Series expansion of the overlap gives Quantum Metric Tensor and Berry Phase. Next we discuss the Quantum Information Metric (Fidelity Susceptibility) in Conformal Field Theory with respect to a small perturbation by a primary operator. When the perturbation is exactly marginal, the gravity dual of metric is given by a volume of maximal time slice in AdS space time. As an application of gravity dual we calculate Holographic complexity- which roughly measures the difficulty of turning a quantum state into another. |
|
| Speaker: |
Shubhadeep Sadhukhan |
| Roll no.: |
16109864 |
| Title: |
Cooperation in evolutionary dynamics and its different aspects |
| Date: |
08-10-18 (Monday) |
| Time: |
03:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Cooperation is a fundamental aspect of any human or animal society; and its emergence, within the Darwinian theory of evolution, is still an unsolved problem. Animals cooperate with each other to construct social structures, groups, and societies. Thus, the evolution of the co-operators is an important and interesting topic of study. After a brief introduction, three topics---1) population genetics, 2) evolutionary game theory, and 3) the synchronization in evolutionary game dynamics---will be discussed. Specifically, we will see how the role of cooperation can be studied in the overlapping area of the three aforementioned topics. Finally, I will address the future problems that I want to address in this area of research during my PhD research. |
|
| Speaker: |
Anurag |
| Roll no.: |
16109264 |
| Title: |
Understanding the dynamics of non-integrable bounded Hamiltonian system |
| Date: |
08-10-18 (Monday) |
| Time: |
04:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Hamiltonian formulation provides a natural framework to investigate the idea of integrability and non-integrability in a wide class of mechanical systems. Integrability of any N-degree of freedom Hamiltonian system requires the existence of N appropriate constants of motion. Completely integrable Hamiltonian systems are rather rare but they play an important role in understanding the dynamics of the non-integrable systems: at times it may be convenient to represent a non-integrable Hamiltonian system in the form of an integrable Hamiltonian plus a small non-integrable part. In this talk, I will be talking about the dynamics of the non-integrable bounded Hamiltonian system in the light of the KAM theorem and Poincaré-Birkhoff fixed point theorem. I will also present the Chirikov criterion of overlapping resonance which explains the widespread chaos in a non-integrable Hamiltonian system. Subsequently, I will talk about my research work, in which I have tried to find the reason behind the order-chaos-order phenomenon in the planar elastic pendulum. Finally, I shall talk about the future directions what I want to take up during my PhD period. |
|
| Speaker: |
Sandeep Bari |
| Roll no.: |
16109282 |
| Title: |
Probing resonances in atomic and molecular systems |
| Date: |
05-10-18 (Friday) |
| Time: |
03:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
The evolution of atomic (molecular) systems under various external perturbations is fundamental to all processes. In nature collisional interactions or scattering of neutral particles with charged particles or photons is the main source of this perturbation. The interaction results in various channels of electronic excitation in the target-projectile system, and although the basic underlying interaction is the well-known Coulomb interaction, a detailed investigation of various modes of electronic excitations provides insight into the dynamics of the scattering process as well as modifications of the Coulomb interaction due to many body character of the interaction.
One such mode of electronic excitations is classified as resonances. These resonances cover a wide range of phenomena ranging from single particle interactions (eg. Auger decay) to many electron correlations (eg. shape resonance). In larger systems like fullerenes and other atomic clusters collective effects give rise to another class of resonance phenomena termed as giant dipole resonance. The dominant mode of relaxation upon resonant excitation is via electron emission.
In this seminar, I shall present a review of experimental and theoretical studies focusing on the origin and importance of resonance processes in atomic and molecular systems. I will also discuss the general experimental scheme we propose to employ to investigate these processes. |
|
| Speaker: |
Divya Rawat |
| Roll no.: |
16109266 |
| Title: |
Study of X-ray variability in Black-Hole Binaries using AstroSat |
| Date: |
16-08-18 (Thursday) |
| Time: |
04:00 PM (Tea @ 03:45) |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Black hole X-ray binaries (BHXBs) are binary systems in which X-rays are produced by falling of matter from a donor star (companion) to a black hole. The falling matter before losing its angular momentum forms a disk-like structure around the black hole known as accretion disk. These binaries system shows high X-ray variability which is closely related to fluctuation is accretion rate. To probe the rapid variation in X-ray variability near the vicinity of black hole the analysis tool commonly used is Power Density Spectra (PDS). PDS in BHXBs also exhibit transient, discrete features known as Quasi Periodic Orbits (QPOs) which may range in 0.01 Hz to 450 Hz. Energy Spectra of BHXBs often exhibit a composite structure consisting of thermal component and power-law component (non-thermal).
The NASA’s Rossi X-ray Timing Explorer (RXTE) observed the time-variation of many astronomical X-ray sources before its deactivation in 2012. After the demise of RXTE, AstroSat which is India’s first dedicated multi-wavelength observatory successfully launched by ISRO in 2015 is observing the celestial bodies in different wavelengths. We are using the data from Large Area X-ray Propotional Array(LAXPC) and Soft X-ray telescope (SXT) onboard the AstroSat for studying the accretion-disk physics of BHXBs. |
| Speaker: |
Krishn Pal Singh |
| Roll no.: |
16109271 |
| Title: |
Optical properties of atomically heterogeneous systems created by microwave plasma generated low energy ion beams |
| Date: |
21 May 2018 (Monday) |
| Time: |
11:00 AM (Tea @ 10:45) |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
The optical properties of metallic thin films (MTF) have long been a subject of scientific research. Metallic thin films behave in a different way as compared to the bulk, this is manifested in its various applications such as in perfect lenses, enhanced transmission through holey metal and in mirror coatings for space applications. Optical components are exposed to the flux of energetic particles when used in either space applications or nuclear energy plants. The study of their behaviour in such an environment is important to avoid failure of the components during their operation. It has been earlier observed that the surface properties of MTF can be modified by creating an atomically heterogeneous system using low energy (0 - 5 keV) ion beam irradiation. The ions are embedded into the lattice of the host metal and can substantially modify the surface properties of the MTF, such as electrical properties (sheet resistance) and wettability. There are reports on modification of optical properties such as transmittance, reflectance and absorption, in gold and iridium thin films implanted by helium ions. Change in the reflectance of optical aluminium film mirror by proton irradiation has also been reported. Similarly, the behaviour of coatings of metallic oxides thin films under proton and gamma irradiation have also been investigated. There is therefore a need to undertake a systematic investigation of the modification of the optical properties of atomically heterogeneous systems. Associated with the change in dielectric constant of the MTF, the frequency of the surface plasmon which are delocalised electron oscillations occurring between metal-dielectric and metal-air interfaces can be modified. It is therefore expected that the properties of surface plasmons (in particular their interaction with light) can be tuned and controlled, in such atomically heterogeneous systems. The first part of the present research proposes to carry out an investigation of the optical properties (such as reflectance, transmittance and absorption) of MTF of gold, silver, aluminium and copper in such atomically heterogeneous systems. The second part of the research will focus on the generated surface plasmons such as their frequency and its controllability in such systems. In the research carried out so far, measurement of ion beam flux have been made in the microwave plasma based ion source developed earlier in the laboratory. The wide range of available ionic species, their energies, fluxes and target MTF, can be utilised to create a variety of atomically heterogeneous systems and to further carry out investigations of the aforementioned optical properties. |
|
| Speaker: |
Punit Sharma |
| Roll no.: |
16109278 |
| Title: |
Tensionless Limit of Bosonic and Superstrings. |
| Date: |
March 21, 2018 |
| Time: |
4:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
We will motivate the tensionless limit of Bosonic closed string from the massless relativistic particle. This limit induces an infinite dimensional symmetry algebra on the worldsheet known as the (Super) Galilean conformal algebra as a residual symmetry after gauge fixing in (Superstring) Bosonic case. The theory is studied two ways, viz. as a fundamental tensionless theory or as an ultra-relativistic limit on the worldsheet. We see how the tensionless limit makes the mass of the states vanishing. The tensionless limit is then studied as a Bogoliubov transformation that connects tensile to tensionless states. Construction of vacuum state for both cases and their link is established. We encounter a problem with the tensionless Superstring fermionic modes which opens up a new direction for the future work. |
|
| Speaker: |
Krishnendu Dandapat |
| Roll no.: |
16209861 |
| Title: |
Long-period fiber gratings (LPFG) based bio-sensors. |
| Date: |
March 12, 2018 |
| Time: |
10:00 AM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Long-period fiber gratings (LPFGs) are periodic modulation of the refractive index along the axis of an optical fiber, and are most widely used for inline excitation of cladding modes in optical fibers. Owing to their modal field present beyond the cladding region (outside the optical fiber), they can interact with the ambient environment and are therefore very suitable for bio-sensing applications. It has been observed that some higher-order cladding modes can support dual-resonance. i.e. the same pair of core-cladding mode can have resonance at two distinct resonance wavelengths. Advantage of dual-resonance is two fold, (i) spectral shifts are of opposite nature for the two resonances, which effectively doubles the sensitivity, and (ii) the sensitivity increases as the two resonance wavelengths are brought closer to the turn-around wavelength of dual-resonance. I will discuss about the theoretical framework, based upon the coupled-mode theory, to analyze the transmission spectrum of LPFGs, their fabrication, recent developments of dual-resonance LPFG based sensor and our recent efforts to develop a temperature insensitive bio-sensor for the detection of E. Coli bacteria in water. |
|
| Speaker: |
Avijit Koley |
| Roll no.: |
16109265 |
| Title: |
Optical bistability in optomechanical systems |
| Date: |
March 16, 2018 |
| Time: |
02:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
The central mechanism in Optomechanics is transfer of momentum between light and mechanical degree of freedom. We need both unitary and nonunitary dynamics to describe the cavity optomechanical system, an open quantum system. The dynamical coupling between optical mode and mechanical mode are responsible for various phenomena, like optomechanically induced transparency, mode hybridization, optical bistability etc.. It can also be used in developing high precision sensors like mass sensor, force sensor (pN-fN) etc.. In this seminar I will discuss about different optomechanical system and their bistable/ multistable behavior. The generic optomechanical system itself provides nonlinearity, but bistability occurs at high photon number. The question that I am trying to address are: can we enhance the nonlinearity in order to get bistability at lower photon number? Up to what regime of parameters the system shows bistability. Could it be possible that by adding higher order nonlinearity the system shows multistable behaviour? I will also discuss my recent theoretical results of bistable behavior of phonon assisted hybrid optomechanical systems. |
|
| Speaker: |
Mamta Gautam |
| Roll no.: |
16109272 |
| Title: |
Magnetised Black holes |
| Date: |
Friday, 9th March |
| Time: |
2:30 PM |
| Venue: |
FB 382 (Raychaudhuri Seminar Hall) |
| Abstract: |
The study of black holes immersed in magnetic fields is of great importance in the understanding of accretion disc phenomena. In General Relativity, such situations can be considered in Ricci flat backgrounds, following the work of Wald. These serve as simplistic models for black holes in the presence of weak magnetic fields. In this talk, we will focus on two classes of black holes - static (Schwarzschild) and stationary (Kerr) that are surrounded by a magnetic field. The motion of charged particles in such situations will be considered, as geodesic trajectories. Implications for accretion disc physics will be discussed. |
|
| Speaker: |
Garima Bawa |
| Roll no.: |
16109863 |
| Title: |
Modal interference effect in optical waveguides and their application in sensors |
| Date: |
March 14, 2018 |
| Time: |
01:00 PM |
| Venue: |
Prof. Amal Kumar Raychaudhuri Seminar Room (FB382) |
| Abstract: |
Each mode in an optical waveguide propagates with its unique propagation constant, thereby traversing a different path length than other modes. Interference among these modes generates specific spatial and spectral intensity patterns, which are directly related to differential phase modulation between modes. Controlling and manipulating this differential phase modulation can give us control over the nature of spectral shifts produced by the modal-interference effect in optical waveguides. Analyzing fiber optic directional couplers and Single-Multi-Single (SMS) mode fiber structures, I will discuss about the possibility of observing two unique wavelengths (a critical wavelength and a cross-over wavelength) for the former case around which the spectral shifts are of opposite nature. I will discuss about the theoretical analysis explaining their existence, our recent experimental efforts to observe them and the sensors based on SMS structure. |
|
| Speaker: |
Pulastya Parekh |
| Roll no.: |
16109277 |
| Title: |
Tensionless Strings from Worldsheet Symmetries |
| Date: |
March 7, 2018 |
| Time: |
3:00 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
We revisit 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. We relate the analysis of the fundamentally tensionless theory to the tensionless limit that is viewed as a contraction of worldsheet coordinates. We see the connection to massless higher spin states. 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, we link the tensionless vacuum to the usual tensile vacuum. As applications, we discuss how our analysis can be connected to Hagedorn physics, left right entanglement. |
|
| Speaker: |
Aditya Mehra |
| Roll no.: |
16109262 |
| Title: |
Galilean Conformal Electrodynamics |
| Date: |
March 7, 2018 |
| Time: |
2:00 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
I will be talking about the detailed analysis of Galilean (non-relativistic) version of Electrodynamics. The Maxwell’s Electrodynamics in this limit admits two distinct sectors called the Electric and Magnetic limits. I will show that the equations of motion in both these limits are invariant under the Galilean Conformal Algebra in D = 4, thereby exhibiting non-relativistic conformal symmetries. 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. Thus Galilean Electrodynamics give us the first example of an infinitely extended Galilean Conformal Field Theory in D > 2. |
|
| Speaker: |
Himadri Roy |
| Roll no.: |
16109267 |
| Title: |
Phenomenology of Vector Boson Dark Matter with SU(2)N Extension |
| Date: |
28th February, 2018 |
| Time: |
4:00 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
About 26% of universe is made of an invisible massive particle called Dark Matter(DM). Inspite of several direct, indirect and collider searches, so far we are unable to pin point the exact nature of DM particles, i.e., whether it is scalar or fermion or vector boson. Models of fermion and scalar dark matter abound. Here we consider a vector boson dark matter, from an $SU(2)_N$ extension of the standard model. Apart from its interesting DM and collider phenomenology, this model also allows a possible implementation of the inverse seesaw mechanism for neutrino mass. |
|
| Speaker: |
Sandip Chowdhury |
| Roll no.: |
14209264 |
| Title: |
Non-linear perturbation in f(R) gravity |
| Date: |
22nd February 2018 |
| Time: |
3 PM |
| Venue: |
FB 382 (Raychaudhuri Seminar Room) |
| Abstract: |
Various cosmological observations have inferred that currently the universe is expanding, and it is expanding at an accelerated rate. Rather than using the standard prescription of GR, (i.e. the inclusion of a Dark Energy (DE) component in the cosmic energy budget) this late time acceleration phenomenon can well be explained with an alternate theory of gravity known as f(R) gravity. In the f(R) gravity framework the gravitational action is modified with an arbitrary scalar function of the Ricci scalar R. The background expansion history alone can not differentiate between the DE and different f(R) gravity models. To distinguish the gravitational dynamics between the above models one needs to consider the evolution of the cosmological perturbations obtained from the Large Scale Structures (LSS). In this presentation we propose to initiate the study of the non-linear evolution of the cosmological perturbation in f(R) gravity model. |
|
| Speaker: |
Nirmal Roy |
| Roll no.: |
16109274 |
| Title: |
Spin Configuration in Low Dimensional System and Topological excitation: Skyrmions |
| Date: |
22nd Feb, 2018 |
| Time: |
12:00 (Tea: 11:45) |
| Venue: |
FB382 (A. K. Raychaudhuri Seminar Room) |
| Abstract: |
Topological excitation in magnetic structures, like skyrmions have drawn a lot of attention in recent times, because of their thermal stability and their potential for application in low power consumption-high density magnetic memory devices. A novel aspect of the skyrmionic spin texture is their particle like nature and high mobility. This texture can be driven by current densities which are several orders of magnitude smaller than those required to drive conventional magnetic domain walls. Skyrmionic excitations are produced by Dzyaloshinskii–Moriya interaction (DMI). In this talk, I will discuss DMI, its effect on domain wall chirality, the relationship with skyrmions and experiments on real space observation as a skyrmion lattice. The electromagnetic properties associated with transport of skyrmions will also be discussed. |
|
| Speaker: |
Sagar Paul |
| Roll no.: |
16209862 |
| Title: |
Nano-particle magnetometry using micron size superconducting quantum interference devices (µ-SQUIDs) |
| Date: |
21st February, 2018 |
| Time: |
3:30 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Conventional VSM and SQUID magnetometers allow magnetic measurements of bulk sample or cluster of magnetic nano-particles. But for a growing demand in magnetic hard drive industries, bio-medicine as well as for the physics of magnetic interactions, anisotropies and quantum tunneling of magnetization in a single domain magnetic particle, it is essential to probe magnetism of single magnetic nano-particles avoiding inter particle interactions and averaging effects present in bulk measurements. For this purpose a micro-SQUID sensor with a flux resolution of 10-4Фo has been used by very limited number of groups since last two decades with one of the major issue being positioning of a single nano-particle at a nanometer distance from the micron size SQUID loop. A very successful group in this field, led by W. Wernsdorfer*, has used several techniques like low energy cluster beam deposition (LECBD) [1], dissolution of nano-porous polycarbonate membrane [2] to place single magnetic nano-particles and nano rods on a SQUID loop. Angle resolved three dimensional magnetization reversal studies [1,2] on Co nanoparticles, Ni rods, barium ferrite particles exhibit evidence of Stoner-Wohlfarth and Neel-Brown models of magnetization reversal which were milestones in theoretical development of magnetism. We are making a set-up for micro-SQUID magnetometry with (i) a direction controlled vector magnet (maximum field 1.2 T at 10 A), (ii) a cryostat capable of 1.3K base temperature (iii) a 2D positioner based manipulator used inside SEM to place nano-particles on µ-SQUID loop. We have fabricated Pb µ-SQUIDs in our lab [3] and we also have access to a good number of Nb µ-SQUIDs for magnetometry. Our initial goals are to probe ferromagnetic permalloy nano particles having shape anisotropy and superparamagnetic Fe3O4 particles for their vast proliferation on applications. References: [1] M. Jamet et al., Phys. Rev. Lett. 86, 4676 (2001). [2] W. Wernsdorfer et al., Phys. Rev. Lett. 77, 1873 (1996). [3] S. Paul et al., Supercond. Sci. Technol. 30, 025017 (2016). |
|
| Speaker: |
Venus Rai |
| Roll no.: |
16109864 |
| Title: |
Evolution of Cr Magnetism in Phosphorus Doped BaCr2As2 |
| Date: |
15th February, 2018 |
| Time: |
3:30 PM |
| Venue: |
FB382 (A. K. Raychaudhuri Seminar Room) |
| Abstract: |
High temperature superconductivity is an exciting research topic since decades, especially after the discovery of cuprate based superconductors with very high superconducting transition temperature (TC). In 2008, high temperature superconductivity of unconventional nature (TC ~50 K) was also found in Fe-pnictides. It was observed in the case of iron pnictides that the superconductivity (SC) starts to appear as antiferromagnetic order of the Fe moments are sufficiently suppressed. Magnetic ordering as well as ordering temperature (TN) can be suppressed by either chemical doping or applying very high external pressure. Phosphorus (P) doping in AFe2As2 (A = Ca, Sr, Ba and Eu) suppresses the magnetic moment of the Fe and SC appears as the antiferromagnetic ordering temperature starts to approach zero. P doping seems quite favorable to SC as superconducting transition starts to appear even without complete suppression of TN for the BaFe2As2 and EuFe2As2. Interestingly, P doped EuFe2As2 has shown coexistence of SC and ferromagnetism for some range of P doping concentration. Therefore, study of P doping in various compounds can be very exciting. Cr moments in BaCr2As2 orders antiferromagnetically in a G-type magnetic structure at TN = 580 K. Cr doped BaFe2As2 has not shown SC; rather increase in resistivity was observed, which is really surprising. Even as small as 2% Cr doping changes structural phase transition temperature significantly from 140 K to 110 K. Such unusual properties of Cr make it unique among all transition metal doping. Therefore, BaCr2As2 can also be a very interesting compound to explore as a function of chemical doping and pressure. Our future plan is to prepare unreported polycrystalline and single crystal samples of P doped BaCr2As2 and to characterize physical properties using magnetization, resistivity and other microscopic measurement techniques such as neutron and x-ray scattering. |
|
| Speaker: |
Sanchari Pal |
| Roll no.: |
16109281 |
| Title: |
Particle Production in gravitational background and thermodynamic aspects |
| Date: |
29th January 2018 |
| Time: |
4 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
QFT in curved space-time is a covariant generalization of QFT in flat space-time. Using a covariant form of Klein-Gordon equation it can be shown that particle production happens at the expense of gravitational energy in an expanding universe. I will mainly focus on the production of spin zero massive particles. Introducing Bogolyubov transformation the expression of particle production rate will be derived. A brief discussion on the definition of creation and annihilation operators during expansion will follow. Invoking Einstein's theory of gravity it will be shown that mass less particles of spin zero are not produced in a radiation dominated universe and massive particles of spin zero are not produced in dust filled universe. Gravity induced particle production can be studied as a thermodynamic open system in cosmological framework. Second law of thermodynamics dictates that this matter creation process is irreversible. It will be shown that this result holds true only when specific entropy remains constant. Modification to the energy momentum tensor due to emerging particles will be discussed and the form of temperature evolution law in presence of particle creation will be presented. |
|
| Speaker: |
Kalyani Barman |
| Roll no.: |
16109865 |
| Title: |
Low temperature atmospheric pressure micro-plasmas: physics and applications |
| Date: |
January 10, 2018 (Wednesday) |
| Time: |
10:30 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Atmospheric pressure micro-plasmas are becoming increasingly important these days because of their applications in various fields such as biology, medicine, material science and environment. These plasmas can be created with relative ease because they do not require expensive vacuum systems or large electromagnetic coils for plasma confinement. The non-equilibrium plasmas have electron temperatures (Te ~ 0.5 eV) and ions are at room temperature (Ti ~ 0.026 eV), with very small fractional ionization (~ 10^-8). The coupling parameter gc , which is the ratio of the Coulomb interaction energy to the kinetic energy is relatively small ~ 0.1, implying that two body Coulomb interactions still remain weak and non-local mean field is responsible for typical collective effects in the plasma. There are different experimental schemes to create micro-plasmas in ambient air. In this thesis work, micro-plasmas will be created inside a quartz capillary with ring electrode configuration in Helium gas. The plasma emerges out of the capillary as a jet, and consists of reactive oxygen and nitrogen species, including those of Helium. In the measured optical emission spectra (OES), prominent emission lines such as OH radicals (287 nm, 308 nm), He-I (501.6 nm, 587.6 nm, 667.8 nm, 506.5 nm, 728 nm), O-I (777.41 nm, 844.6 nm), N2+(391.4 nm), and N2 (337.1 nm) are observed. These reactive species and electrons can be transported by the jet to a specific area of interest, thereby favoring the aforementioned applications. The present research work proposes creation and optimization of low temperature atmospheric pressure micro-plasma, with respect to parameters such as gas flow rate, inter electrode spacing, wave frequency and discharge geometry. The characteristics of the jet will be investigated under the influence of strong electric and magnetic fields. As an important application, surface functionalization employing NH2 and COOH group on carbon nanotube (CNT) arrays will be investigated. These functional groups makes possible covalent bonding to biological systems such as DNA and protein, and can work as biosensors. Finally, an electrical model will be developed to understand the physics of the micro-plasma system. The presentation will comprise of a literature survey, work accomplished so far, and a future plan of research. |
|
| Speaker: |
Sudip Malick |
| Roll no.: |
16209865 |
| Title: |
Exotic Properties of Weyl Semimetal |
| Date: |
5th January, 2018 (Friday) |
| Time: |
11:00 AM (Tea at 10: 45 AM) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
The discovery of Dirac and Weyl semimetal is a massive breakthrough in condensed matter physics. In Weyl semimetal, either time reversal symmetry or inversion symmetry is broken, unlike Dirac semimetal. Massless fermion found in Weyl semimetal known as Weyl fermion shows some exotic properties such as chiral anomaly, magnetic monopoles, extremely large positive and unsaturated magnetoresistance and SdH oscillation at low temperature and high magnetic field. In this talk, we shall review experimental results on some of the recently discovered Weyl semimetals. |
|
| Speaker: |
Rohit Tyagi |
| Roll no.: |
16109280 |
| Title: |
Development of a combined electron-ion momentum spectrometer for investigation of ICD and other higher order relaxation processes in energetic ion – cluster collisions |
| Date: |
09/01/2018 (Wednesday) |
| Time: |
15:00 hrs |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
The field of ion –atom/molecule collision at non relativistic energies has received a fresh impetus in the 21st century due to its relevance to radiation damage associated processes. A multi-electronic target, upon excitation via ion collision, can relax in various ways. These are broadly clubbed as radiative and non-radiative decay. The most famous non radiative decay channel “Auger decay” has been studied extensively following inner shell ionization ion isolated atomic and molecular systems. In 1997 Cederbaum et al. predicted an analogous decay mechanism for van der Waal bonded dimers and termed it as Inter atomic (molecular) Coulombic decay or ICD. In ICD the de-excitation energy is transferred via virtual photon exchange to a neighboring atom, which releases it by electron emission. This talk I shall present a review of experimental studies focused on investigation of ICD process in various van der Waals and hydrogen bonded systems. I shall also discuss the general experimental scheme used for such studies and present a short outline of our experimental setup (under development) for performing the experiments at the 1.7MV tandetron accelerator. |
|
| Speaker: |
Sonu Verma |
| Roll no.: |
16109286 |
| Title: |
Magnetism in materials through indirect exchange interaction |
| Date: |
January 02, 2018 (Tuesday) |
| Time: |
4:00 PM (Tea at 3:45 PM) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
--- |
|
| Speaker: |
Pratik Adhikary |
| Roll no.: |
15209262 |
| Title: |
Controlled Sources of Single and Entangled Photons |
| Date: |
Tuesday, 19th September (2017) |
| Time: |
3:30 PM |
| Venue: |
Amal Kumar Raychaudhuri seminar room (FB382) |
| Abstract: |
Controlled sources of photons can transmit (quantum) bits of information over long distances while maintaining their relative phase and can also bring down measurement noise below the absolute shot noise level in any measurement that uses light to probe. While traditional lasers fail to provide any control on individual photons, there are several existing literature on generating such controlled, bright sources of on-demand and entangled photons. Here we will first review these existing methods, primarily in context of the growing field of quantum information. We will then review our recent ongoing experiments, generating on demand single photons as well as entangled photonic states with simultaneous correlations in various polarization basis, from a laser-cooled atomic ensemble at 100 micro-Kelvin in the laboratory. Finally we will present an outline of construction of a new experiment to generate a heralded entangled pair of photons, using a coupled, cavity-QED architecture. |
|
| Speaker: |
Avijit Duley |
| Roll no.: |
15109861 |
| Title: |
Investigating dynamics of Inter-atomic/molecular Coulombic Decay (ICD) using Momentum Spectroscopy |
| Date: |
21/07/2017 (Friday) |
| Time: |
14:30 hrs |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
ICD is a special kind of ionization channel which is observed only in weakly bonded systems e.g. inert gas dimers and hydrogen bonded clusters. This decay process is a highly non-local one, in the sense that it occurs from two atomic/molecular sites which are separated by a distance such that there is no overlap of electron orbital. This kind of process can be probed using Ion-Atom collision. This talk will give a brief introduction to ICD. The progress so far made in this field will be discussed. In later part of the talk the details of the Recoil Ion Momentum Spectrometer (RIMS), which we are constructing, will also be discussed. |
|
| Speaker: |
Pritam Banerjee |
| Roll no.: |
15109271 |
| Title: |
Curvature coupling in Analogue Gravity. |
| Date: |
3rd July, 2017 |
| Time: |
3 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
An analogue system can be convenient to study the Einstein's general relativity. It can hold similarities to the original curved space-time and produce results that can be compared with the reality. For example, an acoustic black hole or a 'dumb hole' in a non-relativistic, Newtonian fluid flow where acoustic perturbations plays the role of light, can be formed to describe black holes, event horizon or Hawking radiation processes in Einstein's gravity. Likewise Superfluid 3He-A system can be considered as the background space to study some properties of a stationary analogue black hole. One of such properties is the curvature coupling of a fermionic spin. When the superfluid vacuum is in motion, quasi-particles moving in a circular geodesic experience an effective magnetic field which may be measurable in experiments. |
|
| Speaker: |
Abhinandan Bhattacharjee |
| Roll no.: |
15209261 |
| Title: |
Investigating spatial-correlation properties of classical and quantum optical fields |
| Date: |
July 3rd, 2017 |
| Time: |
11:30 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Correlations properties of an optical field at two separate space points are studied through spatial correlations functions. As compared to temporal correlations, the spatial correlation properties of optical fields have not been investigated much. One of the main reasons is that it is relatively very hard to generate and detect fields with partial spatial correlations. However, more recently, several studies have shown some unique benefits of using spatially partially coherent fields for applications such as imaging, holography, optical trapping, free-space communication, etc. This talk will first present an overview of the existing ways of producing and detecting spatially partially coherent fields and of the unique benefits of using such fields for various applications. The talk will then present our recent theoretical and experimental efforts at finding more efficient ways of generating and detecting such fields. Finally, the talk will outline as to how these studies would be carried forward to investigating the entanglement in the spatial degree of freedom of the two-photon field produced by parametric down-conversion. |
| Speaker: |
Suvankar Paul |
| Roll no.: |
15109275 |
| Title: |
Formulation and Study of Rotating Bertrand Spacetime |
| Date: |
1st June 2017 |
| Time: |
3 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Bertrand Spacetime (BST) is one of the well-known static, spherically-symmetric spacetimes in General Relativity. Remarkably, it contains stable circular orbits through each point in space. Moreover, it also possesses rich gravitational structures like naked singularity, Dark Matter etc. But, in nature, rotating stationary spacetimes are always physically more realistic than the static ones. In this context, there is an algorithm in GR known as Newman-Janis Algorithm (NJA), which is used extensively to derive rotating counterparts of known static spacetimes. Here, we take BST as our seed metric and apply the NJA on it. This produces the rotating generalization of BST which we name as 'Rotating Bertrand Spacetime (RBST)'. We shall discuss about NJA, formulation of RBST and different important characteristics of RBST in the context of GR. |
|
| Speaker: |
Ravinder Kumar |
| Roll no.: |
14109880 |
| Title: |
Growth, structure and magnetic characterization of PLD grown Yttrium Iron Garnet thin films |
| Date: |
21 July, 2017 (Friday) |
| Time: |
11:0 AM (Tea at 10:50 AM) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
The development of Spintronics into a viable technology requires novel magnetic materials. Ferrimagnetic garnets having low spin wave damping are potential candidate materials. The garnet Y3Fe2(Fe.O4)3 (YIG) is known for its high Curie temperature (~ 560 K) and is of considerable scientific interest due to many remarkable properties that it possesses like low spin wave damping (SWD), high Faraday rotation (FR), high transmittance for infrared radiation etc., which make it suitable for microwave device applications, magneto-optical (M-O) recording and spintronics. The properties of YIG can be tuned by changing the elemental composition, doping and strain. In this talk we shall discuss literature results on YIG followed by our results on thin film growth, structural characterization and magnetization of YIG. |
|
| Speaker: |
Bikash Ghosh |
| Roll no.: |
15109862 |
| Title: |
Topological phases in correlated Iridates |
| Date: |
25 May, 2017 |
| Time: |
11:00 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Pyrochlore oxides have generated immense research interest due to a number of unusual phenomena, some already realized experimentally, such as observation of residual entropy and monopole like excitations in spin ice pyrochlores, while emergence of novel topological phases such as Weyl Semimetals, chiral spin liquids, etc. in Pyrochlore Iridates (R2Ir2O7, where R corresponds to rare-earth elements or Yittrium) are still widely anticipated and debated. Pyrochlore Iridates are fundamentally appealing in the sense that they embody an interesting combination of interplay between Coulomb correlation and strong spin-orbit coupling (SOC) on one hand and the geometrical constraints imposed by the crystal structure on the magnetic configuration on the other. Breaking the cubic symmetry of the bulk crystal in low dimensional systems where surfaces/interfaces play an important role, could as well lead to interesting phenomenology. In this talk we shall discuss the physical origin and the possibility of experimental realization of novel topological phases in certain Pyrochlore Iridates. |
|
| Speaker: |
Boudhayan Paul |
| Roll no.: |
14109263 |
| Title: |
Geometry of AdS Spacetime and Conformal Quantum Field Theories. |
| Date: |
3rd, May 2017 |
| Time: |
11:00 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
The geometry and causal structure of negatively curved Anti De Sitter ( AdS) spacetimes of arbitrary dimensions will be reviewed. This will involve the description of the various coordinate systems including the global coordinates and the Poincare patch, also the conformal Penrose diagram and its relation to the Einstein Static Universe. Subsequently the structure and symmetries of conformally invariant quantum field theories in diverse dimensions will be described specializing to the case of two dimensions. The relation of the symmetries of the AdS space time with conformal symmetry in one lower dimensions will be elucidated with a brief mention of the AdS-CFT correspondence. |
|
| Speaker: |
Swayamshree Patra |
| Roll no.: |
15109863 |
| Title: |
Trip to the Tip with MAPs: Intracellular Motor Transport in cell protrusions |
| Date: |
23 FEBRUARY, 2017 |
| Time: |
12:00 NOON |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
A successful trip brings the picture all sort of luggage, well maintained highways, maps marked with the routes, traffic signals and also some unavoidable roadblocks. In this talk, I will give an overview of molecular motor driven cargo transport that are essential for sustaining long protrusions in living cells. Such cell protrusions with specialised tips include (i) Neurons (ii) Flagella & Cilia (iii) Pollen tubes & Root hairs. In these cell protrusions the motor tracks formed by microtubule filaments are decorated by microtubule associated proteins (MAPs). I’ll introduce asymmetric exclusion processes, a class of mathematical models for motor traffic far from equilibrium. I’ll show how the transported cargoes, along with the transporters, shape and nourish the tip. I’ll also summarize various alternative models developed in recent years for the temporal variation of the morphology of the cell protrusions in different stages of assembly, maturation and disintegration. Finally, I’ll briefly mention my original theoretical work on intraflagellar transport trains. |
|
| Speaker: |
Md. Arif Ali |
| Roll no.: |
15109267 |
| Title: |
Behavior of collection of vortices in low dimensional superconductors |
| Date: |
15/02/17, Wednesday |
| Time: |
5 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Vortices in an ideal bulk type II superconductor in the mixed state organize themselves into the well-known Abrikosov vortex configuration. Superconductivity and the behavior of vortices in them is intimately connected with two fundamental length scales, viz., the coherence length and the penetration depth. For nanostructured superconductors approaching the size of coherence length and/or penetration depth, edge effects become dominant and begin to affect the collective behavior of vortices in such superconductors. Early theoretical proposals suggested that vortices have the possibility to organize themselves into an ordered vortex row like configuration in the middle of a mesoscopic wire. Recent experimental work has shown that there exist critical current fluctuations as a function of magnetic field in superconducting Al nanowires along with the emergence of a magnetic field-induced dissipation-free state in these superconducting nanostructures. These features are thought to result from strong vortex confinement conditions produced at nanoscales. I will discuss these results and the direction of my work to investigate the statics and dynamical properties of vortices in low dimensional superconducting films with nanostructuring. |
|
| Speaker: |
Pritha Bari |
| Roll no.: |
15109272 |
| Title: |
Vector perturbations in bouncing cosmologies |
| Date: |
3rd February (Friday) 2017 |
| Time: |
11:30 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
The talk will focus on vector perturbations in cosmological models guided by Einstein's theory (GR) and a modified theory of gravitation called f(R) theory. Particular attention will be paid on bouncing cosmological models in f(R) gravity. |
|
| Speaker: |
Sanjeev Kumar Maurya |
| Roll no.: |
14209862 |
| Title: |
Creation and studies of high aspect ratio microstructures using noble gas plasma based focused ion beams |
| Date: |
October 5, 2016 (Wednesday) |
| Time: |
10:00 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Liquid metal ion source (LMIS) based Ga focused ion beams (FIB) is used for nanofabrication, however, volume milling at micrometer length scales is a major issue due to small beam currents (1 pA – 10 nA). For example, to mill out 2.5 x 10^5 µm^3 of Si at 10 nA and 30 kV requires more than 24 hours at normal incidence. Moreover, embedded Ga ions are metallic and introduces surface contamination. Many emerging applications require rapid processing of biomaterials and semiconductors, where non-toxic inert gas focused ion beams are necessary. A versatile microwave plasma based multielement FIB (MEFIB) system has been developed in our laboratory which may address such requirements. The MEFIB can deliver ions of a variety of gaseous elements such as Ar, Kr, Ne, and He of beam size ~ 1.5 µm, and significantly large currents in the range 1.5 nA – 1 µA. The possibility of employing ions of different masses allows a huge variation of momentum transferred to the substrate, so that size controlled microstructures with a wide variety of aspect ratio can be fabricated. Furthermore, if desirable, after the initial micron level structuring any finer features of nanometer length scales can be easily fabricated with a conventional FIB. This thesis work proposes to investigate micro-structuring with a variety of noble gas ions and establish scaling laws for milling rate, sputtering yield, and aspect ratio (width/depth) including some studies using these structures. In work accomplished so far, micro-structuring has been done on 50 nm Cu thin films using Ne, Ar, and Kr ion beams with different beam writing speed. Atomic force microscopy (AFM) measurement are used to determine line width and depth, from which the aspect ratio of the created microstructures are found to lie in the range 100 – 1000, by varying the ion beam species. The current normalized pressure on the substrate due to the ions vary in the range 60 – 120 Nm^-2nA^-1 with smallest value being for Ne and largest value for Kr. Similarly, the current normalized milling rate (µm^3nA^-1s^-1) on copper substrate increases with ion mass and has been found to be in the range 0.2 – 0.7. Further, micro glass capillary will be used for self focusing and almost loss less guiding of ion beams. The concept was verified earlier and will now be implemented in MEFIB. Additionally, quadrupole permanent magnets will be employed which together with capillary focusing is expected to further reduce the beam size. Finally, some optical and structural studies of different microstructured materials will be investigated. |
|
| Speaker: |
Archan Mukhopadhyay |
| Roll no.: |
14109877 |
| Title: |
Chaos in Evolutionary Game Theory. |
| Date: |
Friday, 2 September, 2016 |
| Time: |
02:30 P.M. |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Over the past few decades Evolutionary Game Theory (EGT) has been a very active research area. J. M. Smith and G. R. Price (1973) were first to introduce game theory into evolutionary biology. They analyzed evolutionary contests as competitions among different "strategies". They used mathematical aspects of game theory to predict the prevalence of such competing strategies. In short, EGT aims to to mathematically model Darwinian competition and evolution. Unlike classical game theory, it focuses more on the dynamics of evolutionary strategies that are not only dependent on the quality of the strategies but also on the frequencies of the strategies found in a population. In this context several types of Replicator Equations and their extensions have been introduced and analyzed. In this talk, first I will review some of the basic concepts of this field. Then I will discuss how chaos appears in EGT and how the nature of chaos is strongly dependent on the type of the replicator equation. |
|
| Speaker: |
Apurba Dutta |
| Roll no.: |
14209261 |
| Title: |
Biologically inspired Tape-copying Turing machines: interplay of accuracy and dissipation in Information Processing. |
| Date: |
12 August, 2016 |
| Time: |
12:00 NOON |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
As is well known in molecular biology, genetic information is encoded by a specific sequence of four letters of an alphabet on DNA molecule. This information is "transcribed" first into RNA and subsequently "translated" into a language based on a 20-letter alphabet. The molecular machines that carry our transcription and translation are called RNA polymerase and ribosome, respectively. Both of these machines have similarities with Turing machine, introduced by Alan Turing as a hypothetical computing device. However, in contrast to the Turing machine concept used in theory of computation where the output are numbers, the output of the RNA polymerase and Ribosome are "tapes" just like the input. Therefore, these machines have been referred to as "tape-copying Turing machines". However, because of the intrinsic stochasticity of the processes carried out by these nano-machines, the computation is error-prone. The accuracy of the computation by these machines, depends on the entropy production, i.e., dissipation of energy in the kinetic proofreading process. However, higher accuracy, achieved at the cost of dissipating more energy, reduces the speed of the computation. Thus interplay of dissipation and accuracy in the information processing by the tape-copying Turing machines is a fundamental problem of non-equilibrium statistical physics. In this talk I will review some of the earlier works on the stochastic operation of RNAP and Ribosome from the perspective of Turing machines and their limitations. |
|
| Speaker: |
Anupam Ghosh |
| Roll no.: |
14109876 |
| Title: |
Occasional coupling synchronization of chaotic systems. |
| Date: |
Wednesday, 3 August 2016 |
| Time: |
03:00 P.M. |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
One of the most important achievements in the field of nonlinear dynamics is the discovery of chaotic motion of dynamical systems. The term `chaotic' means that two trajectories in the phase space starting very close to each other exponentially diverge away from each other. Consequently, two chaotic systems evolving in synchrony might appear counter-intuitive in the light of this ultra-sensitivity to initial conditions.
In 1990, Pecora and Carroll reported that identical synchronization of two chaotic systems is also possible. Two chaotic systems are said to be identically synchronized when the systems eventually evolve identically owing to the coupling between them. Of course, in the absence of any coupling, these systems cease to be synchronized. Intriguingly, identical synchronization can be effected even if we couple two systems occasionally/intermittently/transiently. In this talk we shall discuss various occasional coupling schemes that are employed to synchronize two identical chaotic systems. |
|
| Speaker: |
Anuj Ram Baitha |
| Roll no.: |
14109875 |
| Title: |
Studies on plasma confined by a dipole magnet |
| Date: |
12 July 2016 (Tuesday) |
| Time: |
11:00 am (Tea at 10:45 am) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
There has been a long quest to understand charged particle generation, confinement and underlying complex processes in a plasma confined by a dipole magnetic field. Our earth’s magnetosphere is an example of such a naturally occurring system. The dipole confinement concept was motivated by spacecraft observations of planetary magnetospheres that sets the stage for charged particles to cause a variety of interesting phenomena. The energetic particles trapped in the earth's dipole magnetic field provides a remarkable and non-intuitive process in the random variation of the interaction with the solar wind. Another important outcome is the emission from the Van Allen radiation belts. Unlike most other approaches to magnetic confinement, in which stability requires good average curvature and magnetic shear, the stability in a dipole derives from plasma compressibility. The plasma is found to be stable to interchange and ballooning instabilities, when the pressure gradient is sufficiently gentle, even when the local plasma pressure exceeds the magnetic pressure.
There are a few laboratory experiments worldwide, that have been designed for such investigations. The Levitated Dipole Experiment (LDX) developed by MIT, the Terella experiment at Columbia university, and the Ring Trap-1 (RT-1) experiment at the University of Tokyo, are some examples, that have provided excellent efforts towards this direction. However, they are very large scale experiments, where the dipole magnetic field is created with superconducting coils, thereby, necessitating power supplies and stringent cryogenic requirements. In order to carry out the present research, we have built a table top experiment to investigate several important processes in a dipole plasma, such as plasma production and transport, particle energy distributions, fluctuations and confinement. A strong permanent magnet having a surface magnetic field of more than 0.6 T, is employed to create the magnetic dipole field inside the vacuum chamber. The magnet is suspended and cooled by circulating chilled water. The plasma is heated by electromagnetic waves of 2.45 GHz. In addition, dual frequency heating in the range 6 – 11 GHz have also been planned. The wave powers can be widely varied from a few hundred watts (~ 300 W) in the continuous wave mode to a few kilo watts (~ 7 kW) in the pulsed mode, thereby providing a large flexibility in the heating scheme.
In this seminar, a brief review of research carried out on LDX, Terella, and RT-1 will be presented and scope for further research will be elucidated, including work that has not been carried out and our proposed work. In addition, the dipole plasma research carried out in our laboratory until now will be presented, with measurements and numerical results for magnetic field, first plasma, and preliminary investigations of plasma density and the electron temperature profiles. |
| Speaker: |
Rohitashwa Chattopadhyay |
| Roll no.: |
14109271 |
| Title: |
Collective behaviour of electrons |
| Date: |
Friday, 24 June 2016 |
| Time: |
03:30 P.M. |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Hamiltonian formulation is one of the most fundamental formalisms in physics. It is widely used as a starting point in the analyses of systems in classical, quantum, relativistic, and statistical mechanics. Hamiltonian for a conservative system is usually given by the sum of the kinetic energy and the potential energy. For a dissipative system, there is no unique way (if there is one at all) of writing a Hamiltonian, and a corresponding satisfactory Hamiltonian formalism is yet to be formulated. Lienard system is a general class of nonlinear systems that encompass a plethora of nonlinear dissipative oscillators (for e.g. van der Pol oscillator, Duffing oscillator etc.). In this talk, we shall discuss the various methods that are employed to formulate Hamiltonians for such nonlinear dissipative systems. We shall then discuss an oxymoronic implementation of canonical perturbation theory to find frequency and amplitude perturbatively for the van der Pol oscillator. Thus, we shall overcome the limitation of canonical perturbation theory that it can only be applied to conservative systems. We shall conclude this talk by touching upon the methods and the implications of quantizing nonlinear dissipative systems. |
|
| Speaker: |
Krishanu SadhuKhan |
| Roll no.: |
14109878 |
| Title: |
Collective behaviour of electrons |
| Date: |
2nd June, 2016 (Wednesday) |
| Time: |
11 a.m. - 12 noon (Tea from 10.45 am) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Collective behavior of electrons are surprisingly different from the single particle nature and become dominant when the particles strongly interact with each other and also with the surroundings. The huge density of electrons in an electronic system discards the possibility of understanding it in a direct quantum mechanical approach. Mean field theory provides an intermediate way where we use different models and approximations for the interaction potential and try to find the response of the system to different time dependent perturbation. Here we review the derivations of the response functions for different systems e.g. electron gasses and Dirac materials of different dimensions. The nature of the response function determines the possibility of finding a collective excitation mode which is a direct consequence of the collective behavior of electrons. In most of the cases we study the density-density response function and plasmons. Friedel oscillation is an interesting phenomenon where we find an asymptotic oscillatory behavior of the local density of electrons under impurity effects. This is also reviewed for different electronic systems. |
|
| Speaker: |
Mr Amar Nath Sah |
| Roll no.: |
14109883 |
| Title: |
Development of Portable Synchronous Fluorescence Spectroscopy Device for Early Cancer Diagnosis |
| Date: |
20th May, 2016 |
| Time: |
4.00 pm (Tea @ 3:45 pm) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Biophotonics, the fusion of Photonics and Biology, involves the detailed study of the interactions of light with biological matter. Attempts have been made to detect cancerous and precancerous in-vitro and in-vivo lesions using Fluorescence spectroscopy over the past three decades. It has the ability to monitor the various metabolic and pathological changes related to the abnormal cells growth. It is a highly sensitive and accurate technique for detecting subtle changes in native fluorophore concentrations or the local environment in the tissues. These native fluorophores in turbid tissue include structural proteins, amino acids, coenzymes, lipids and porphyrin, whose contribution to the spectrum depends upon the choice of excitation and emission wavelengths. For human tissue, overlapping emission bands of multiple tissue fluorophores contribute to a single broad band fluorescence spectrum and mask signatures of certain fluorophores. So, vital information of individual fluorophore is difficult to extract and often missed. Synchronous fluorescence spectroscopy (SFS) offers a simple way to decouple the activity of individual fluorophore in a single acquisition with the selection of an appropriate offset wavelength between excitation and emission wavelength. In my seminar I will discuss about light tissue interaction, fluorescence, synchronous fluorescence spectroscopy, recent studies on synchronous fluorescence spectroscopy, diagnosis of cancer and future aim of developing portable SFS device. |
|
| Speaker: |
Ms. Meenaxi Sharma |
| Roll no.: |
14109267 |
| Title: |
Lubricating Fluid Infused Slippery Surfaces: Basics & Applications |
| Date: |
18/05/2016 (Wednesday) |
| Time: |
11 AM (Tea @ 10:45am) |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
--- |
|
| Speaker: |
Utso Bhattacharya |
| Roll no.: |
14109885 |
| Title: |
Topological Quantum Systems and Dynamics |
| Date: |
13th May, 2016 (Friday) |
| Time: |
11 a.m. - 12 p.m. |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
A topological insulator is a quantum phase of matter with insulating bulk but conductive surface states. Examples include the anomalous quantum Hall effect (quantum Hall effect in the absence of a net magnetic flux) and the spin quantum Hall (SQH) effect. Having reviewed these special states of matter, we will introduce the notion of the Berry phase and curvature bridging a connection with the topological invariants (e.g., the Chern number and the Z2 invariant) those distinguish the topological phases from the trivial ones. A discussion of the Floquet theory which reveals the physics of periodically driven systems will finally set the stage for analyzing the stroboscopic dynamics of such driven or kicked Floquet insulators. Interestingly, novel topological edge states can be induced by shining electromagnetic radiation on a topologically trivial insulator with no edge state in the static limit. It is also possible to open gaps at the Dirac point of graphene and even drive graphene into the topological phase by simple irradiation with suitably chosen parameters. We will conclude with a brief mention of some of our works those have already been done pointing to possible future endeavors. |
|
| Speaker: |
Poulami Nandi |
| Roll no.: |
14109879 |
| Title: |
AdS-CFT and Holographic Superconductors |
| Date: |
11th, May 2016 (Wednesday) |
| Time: |
11:00-12:00 Hours |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
The AdS-CFT or the Gauge Theory-Gravity correspondence holographically relates a bulk theory of weakly coupled gravity in Anti de Sitter (AdS) spacetime with a strongly coupled quantum field theory on the boundary in certain limits. The various constituents leading up to this correspondence would be reviewed including its application to the understanding of holographic superconductors arising from such strongly coupled boundary quantum field theories at a finite temperature. We will conclude with a brief description of our investigation to holographically characterize this superconducting phase transition in the boundary field theory in the framework of thermodynamic geometry. |
|
| Speaker: |
Tripurari Srivastava |
| Roll no.: |
14109883 |
| Title: |
Impact of Scalars on Beyond Standard Model Physics |
| Date: |
08/03/2016 (Tuesday) |
| Time: |
3:00 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
After discovery of Higgs boson at LHC, it has given hints for detection of scalars beyond standard model in near future. There are many BSM signatures at low scale such as muon g-2, charged lepton flavor violation (CLFV) decays,neutrino -oscillations etc. I will analyses muon g-2 and CLFV decays in context of doubly charged scalar. I will also present you the implication of di-photon excess observed recently at LHC run-2. I will briefly introduce the Left-Right symmetry model and analyses the scalar mass spectrum using vacuum stability and tree-unitarity bounds on couplings of scalar. |
|
| Speaker: |
Amit Kumar Jash |
| Roll no.: |
14109874 |
| Title: |
Exploring and imaging dynamic instabilities in the driven vortex state of a superconductor |
| Date: |
23rd February, 2016 (Saturday) |
| Time: |
12:00 PM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Vortex matter in type II superconductors provides a convenient system for the study of fundamental problems and as well as application oriented studies. In the vortex matter the density and interaction between vortices can be varied easily by changing the external magnetic field. The disorder in the vortex state can also be varied relatively easily. I will be discussing about two phenomena’s found in the driven vortex state. First, I will discuss about an old phenomena’ viz., the peak effect (PE) phenomena. While this phenomena has been well studied, origins of it are not well understood. Normally the current carrying capability, viz., the critical current density (Jc) of a type II superconductor should decrease monotonically with increase in temperature and magnetic field. However in a large variety of superconductors it has been found that in the PE regime, Jc abruptly increases close to the upper critical field of the superconductor. PE is considered to be associated with an order to disorder transformation in the vortex state, although as mentioned the origins of this phenomena are uncertain. Another phenomena is the newly discovered jamming transition found in the driven vortex state. In this phenomena a free flowing (FF) vortex state moving with a finite vortex velocity transforms suddenly into a state where the velocity goes to zero as a function of driving force. Such a phenomena is known as Jamming. The jamming phenomena has distinct signatures of a non – equilibrium transition where a transient time scale associated with the phenomena are found to diverge with a characteristic critical exponent. Under my thesis work I plan to search for superconductors where both these phenomena’s can be found simultaneously. I also plan to artificially control pinning center configuration carefully so that I can intentionally generate the jamming phenomena’s in the superconductor. Through such a study I hope to understand the above phenomena’s as well as search for connections between them. We would like to know whether these dynamical instabilities in the driven vortex state are generated by the injection of disordered states in the moving media. This work would also have an application angle as well, since we would like to search for new ways of controlling the current carrying capability locally in a superconductor and thereby search for device applications. Under my thesis I plan to employ magneto optic imaging studies, self field imaging and sensitive noise measurements techniques. |
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| Speaker: |
Vimlesh Kumar Vimal |
| Roll no.: |
14109886 |
| Title: |
Entanglement and its dynamics in quantum spin systems |
| Date: |
9th January, 2016 (Saturday) |
| Time: |
11:00 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
In recent years quantum entanglement of has been studied intensively because it has important application in quantum information processing. There are various approaches to detect entanglement in quantum state of a system, one such is concurrence. Using the concurrence measure, we will discuss entanglement and its evolution in Heisenberg XY model. In particular i will discuss a method to find concurrence in ground state of 1-dimensional Kitaev spin chain in transverse magnetic field. |
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| Speaker: |
Antu Laha |
| Roll no.: |
14109261 |
| Title: |
Transport and magnetic properties of Ce-based Compounds |
| Date: |
13th January, 2016 (Wednesday) |
| Time: |
11:00 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
Ce-based intermetallic compounds exhibits many interesting properties like heavy fermion, superconductivity, valance fluctuation, Kondo behavior, magnetic ordering. CeNi2Ge2 (Non- magnetic) and Ce2Ni3Ge5 (Antiferromagnetic) are two examples of Ce-based intermetallic compounds. CeNi2Ge2 shows a crossover from NFL to FL behavior at 250 mK and the NFL behavior is suppressed by high magnetic field or high pressure. It also shows large thermopower. On the other hand Ce2Ni3Ge5 exhibits two antiferromagnetic ordering at 5.1 K and 4.5 K. At high pressure (3.6 GPa) magnetic ordering is suppressed and superconductivity (Tsc= 0.26 K) is observed. In this seminar I shall present interesting properties of Ce-based intermetallic compounds including our own results on CeNi2Ge2 and Ce2Ni3Ge5. |
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| Speaker: |
Saikat Sur |
| Roll no.: |
14109272 |
| Title: |
Nonlocal detection of quantum dynamical processes in spin chains |
| Date: |
8 January 2016 |
| Time: |
11:00 AM |
| Venue: |
FB 382 (Physics Seminar Room) |
| Abstract: |
We study the dynamics of a one dimensional quantum spin chain evolving from unentangled or entangled initial state. At a given instant of time a quantum dynamical process (ex. measurement) is performed on a single spin at one end of the chain. The aim is detect whether the process occurred from a measurement on a far away spin. No communication theorem says it cannot be detected if there is no further evolution. From the dynamical evolution of the state from the epoch time we should be able to detect the occurrence of the dynamical process. We investigate the dynamics of the system from the epoch time, and from the evolution we determine how quickly the other sites 'receive' the information. Thus, we want to detect the speed of the communication that proceeds in the interacting quantum system. Intuitively, if the interaction is local, it is expected that the site, which is far from the ‘first’ one, feels the information at a later time compared to the near one. Obviously, the entire dynamics depends on the type of interaction of the system, strength of interaction among the spins and initial state of the system. It is also interesting to see how the speed depends on various parameters in the Hamiltonian of the system. |
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Manohar Kumar Sharma
13109881
Effect of rotation on statistically homogeneous helical turbulent fluid
27 August 2015 (Thursday)
11:30 am
FB-382
Turbulence is a ubiquitous phenomenon but even the simplest of turbulent systems, viz. statistically homogeneous and isotropic turbulence (SHIT), is far from being completely understood. While there is no universally accepted definition of turbulence, usually presence of randomness, vortical structures and strong nonlinearity characterizes turbulence. In the inviscid and the unforced limit, the three-dimensional (3D) SHIT is further characterized by the presence of two globally conserved quantities: total helicity and total energy. When helicity and rotation are externally imparted on 3D SHIT, the situation becomes even more interesting and new physical phenomena emerge as seen both experimentally and numerically. The rotation effectively seems to make turbulence "two-dimensionalized" in some sense. Researchers have tried to unravel the reasons behind two-dimensionalization by using experiments, numerics as well as analytical arguments. In this seminar, apart from literature survey, I shall present the works I have been doing in this context. First I shall discuss the numerical results obtained using relevant low-dimensional shell models and direct-numerical-simulations. I shall discuss the effect of rotation on the dynamics of 3D SHIT through the statistics of the energy and the helicity fluxes, energy spectrum, ring spectrum, etc. Then, I shall touch upon analytical work: specifically, I will discuss exact results concerning third-order-structure-functions in the presence of helicity. |
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Paramita Dasgupta Tewari
13109070
Extensive Air Showers and the Physics of Ultra High Energy Cosmic Rays
26th June, 2015
4 pm
FB-382
The origin of Cosmic Rays(CR) have been intriguing scientists since 1912 when V.Hess carried out his famous balloon flight to measure the ionisation rate in the upper atmosphere. The cosmic rays are energetic particles, which come to us from the outer space and are observed either through satellites or earth based detectors.The spectrum of Cosmic rays can be approximately described by a single power law from energy 10 GeV to the highest energies ever observed ~10^20 eV.During the last decade a significant progress has been made in experimental studies of high energy cosmic rays by means of extensive air shower (EAS) techniques. To a large extent this was due to a new strategy of data analysis, most consequently put forward by the KASCADE experiment.In this talk I will discuss about the particle interactions at ultra high energy using CORSIKA (Cosmic Ray Simulation for Kascade) simulation tools. With different particle interaction models employed, they significantly enhance the accuracy of data analysis, which allows us to obtain impressive results.
It is believed that supernovae (SN), supernovae remnants (SNR), pulsars, compact objects in close binary systems and stellar winds are sources to Cosmic rays(CR), but there is still some uncertainty about their origin. Cosmic-rays are energetic particles that have propagated for millions of years in the Interstellar medium (ISM). During this long period of time, the cosmic particles lose or gain energy and even change composition: CR nuclei can decay and become originators of secondary particles and electromagnetic radiation.
In the second part of my talk I will discuss The GALPROP code for cosmic-ray transport and diffuse emission production. Some results of our study on neutral and charged pion emissivity at different galactocentric radius and height will be presented at the end of my talk. |
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Purna Chandra Patra
12209066
Optoelectronic Properties of Graphitic Carbon Nitride as 2D Material
28th May, 2015
4 pm
SCDT Seminar Room
Polymeric graphitic carbon nitride (g-C3N4) is an emerging 2D material consisting of few atomic layers which has attracted much attention due to it's enhanced optical and electrical properties.
g-C3N4 consists of covalently linked sp2 hybridized C and N atoms in an alternating fashion. A variety of arrangements within planar structures are possible, though the actual structure realized in laboratory is still unclear. Heptazine based g-C3N4 is considered the most stable allotrope in ambient conditions which exhibits high chemical and thermal stability. We review the synthesis and optoelectronic properties of this material keeping in view of it's possible applications with focus on thin films. We will also report on some of our preliminary results on synthesis and characterization. Our aim is to explore its possible applications in flexible electronics. |
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M M Vinay
13109077
Holographic Quantum Entanglement Entropy
21st May, 2015 (Thursday)
11 am
FB-382
Entanglement entropy is used in quantum information as a measure of the quantum entanglement in bipartite quantum states. It was conjectured by Ryu and Takiyanagi few years ago, that the quantum entanglement entropy has a holographic geometrical interpretation through the AdS-CFT correspondence. This correspondence in string theory holographically relates a bulk theory of gravity in Anti de Sitter (AdS) space time with a conformal quantum gauge theory at the conformal boundary in some limit. The Ryu-Takiyanagi conjecture will be reviewed and specifically its implementation in the context of AdS3-CFT2 will be described. |
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Ashish Kumar
13109062
Magnetic Properties of Mn based Pnictogen Compounds
5th May, 2015 (Tuesday)
4.30 pm
FB-382
Recently Mn (d5) compounds such as LnMnXO (Ln=lanthanide, X=As,P) and BaMn2X2 (X=As, Bi,Sb) having same structure as transition metal pnictogen family have attracted considerable attention. These compounds are of interest as they have magnetic ordering like cuprates but their crystal structure is like iron pnictides. In this seminar, recent experimental investigations of magnetic properties of BaMn2As2, BaMn2Bi2, and LaMnPO compounds will be reviewed. Recent theoretical investigations of these systems starting with five band interacting electron model propose a rich phase diagram involving a variety of phases such as antiferromagnetic, stripe, flux, paramagnetic, and mixed phases |
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Shubhajyoti Mohapatra
13109075
Magnetic Excitations in Frustrated Quantum Spin Systems
5th May, 2015 (Tuesday)
3.30 pm
FB-382
Recent investigations of magnetic excitations and correlations near quantum phase transition (QPT) in antiferromagnetic frustrated spin systems with competing interactions and/or geometric frustration will be discussed, focussing on magnetic field- and pressure-induced QPT in the three dimensional coupled dimer system TlCuCl_3. Magnetic excitations in this system involving triplon excitations are theoretically modeled using the bond-operator technique, and the magnetic field-driven QPT is described through Bose-Einstein condensation in the lowest lying triplon mode. Triangular lattice organic salts will also be discussed, highlighting the QPT arising due to complex interplay between electron correlation, effect of low dimensionality, and magnetic frustration. |
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Sanghamitro Chatterjee
13109884
Plasma Based Low Energy Ion Beams: Extraction, Focusing and
Interaction with Matter
22th April, 2015 (Monday)
4 pm
FB-382
The application of low energy ion beams in science and technology has recently drawn attention of the scientific community. Low energy ion beams (0 – 5 keV) are interesting because of their unique interaction with matter. Unlike high energy beams, low energy ion beams do not penetrate deep into the substrate, but is confined within a few atomic layers below the surface, thereby bringing about interesting modification of material surface properties.
There are various methods of realizing low energy ion beams. A promising and useful technique is low energy extraction of ions from a plasma. In this research, a microwave plasma based multiple ion beamlet system (MIBS) capable of producing both broad beams (diameter ~ 1 cm) and focussed ion beamlets (diameter ~ 30 microns), developed in the laboratory will be utilized. The system provides control over each individual beamlet, to generate desired patterns on a substrate without employing a mask.
The characteristics of the plasma and the extracted ion beams in the MIBS have been investigated. At first, we have performed simulations on an inductively coupled plasma to understand the evolution of basic plasma properties like space potential, plasma density, electron energy distribution function, electron temperature, including phase space behavior of electrons and ions. From this study, we conclude that the discharge length (L) and pressure (p) are two important parameters that produce the same effect on discharge dynamics, a phenomenon named as “L-p similarity”. Thereafter, experiments are performed using the MIBS. While investigating the beam characteristics, it is found that the beam current profile is very similar to that of a vacuum diode, having two distinct regimes: (1) space charge limited flow regime and (2) extraction voltage limited flow regime. In the first regime, the profile deviates from the well-known Child-Langmuir law, whereas in the second regime, a Schottky-like behavior is observed, which is attributed to the fact that the collector potential actually perturbs the plasma potential through the plasma electrode apertures. The performance of the switching capable electrode for ion beam patterning has been investigated.
Finally, we expect to apply the low energy ion beams for modification of the electrical and field emission properties of Graphene and Carbon Nanotubes respectively. The development of a compact Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) system is proposed, for synthesis of these carbon based materials. A brief review on the structural, electronic, and morphological modification of these materials upon low energy ion implantation will be presented, followed by the plans for future research. |
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Annwesha Dutta
13109061
Degeneracy in genetic code: causes and consequences of codon usage
bias
13th April, 2015 (Monday)
4 pm
FB-382
The genetic information stored in DNA is a living archive of instructions that a cell uses in sustaining and propagating "life". Nature has designed wonderful machineries for polymerizing such macromolecules (e.g. DNA, RNA, protein) using another biopolymer as the corresponding template. Translation is such a process by which a protein is synthesized from the information contained in a the sequence of nucleotides on a mRNA molecule. Each triplet of nucleotides on the mRNA template constitutes a codon. The genetic code is a set of rules by which a ribosome translates the sequence of codons on the mRNA into the sequence of amino acids, the building blocks of a protein. There are 61 distinct codons corresponding to 20 species of amino acids, which implies that the genetic code is degenerate i.e there are more than one synonymous codon code for the same amino acid. For some time after the discovery of genetic code, it was often believed that synonymous codons were used randomly. With the availability of more and more sequence data, it came to light that synonymous codon usage was non random and that different genomes have different preferred synonyms for any given amino acid. This phenomenon is termed as codon usage bias (CUB). My aim is to explain the evolutionary causes and physiological consequences of CUB using stochastic kinetic models. In this talk I'll begin by reviewing some proposed models for explaining the consequences of CUB. Then I'll briefly mention a theoretical model that I have developed very recently, capturing codon usage bias. |
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Soumendu Ghosh
13109076
Polymerase traffic on nucleic acid tracks: effects of
stochasticity, queueing and interference
13th April, 2015 (Monday)
4 pm
FB-382
Nucleic acid strands, namely DNA and messenger RNA (mRNA) serve as templates for polymerization of "macromolecules of life". For example, a RNA polymerase (RNAP) synthesizes a mRNA strand which is complementary to a single-stranded DNA template. While polymerizing a RNA, the RNAP walks step-by-step on the ssDNA strand in a specific direction consuming input chemical energy. Thus, a RNAP can also be regarded as a molecular motor. A large number of such motors move simultaneously along the same track or on adjacent tracks; the collective movement of such motors on the template strand (track) is often referred to as traffic because of the superficial similarities with vehicular traffic. In this talk I'll first present an overview of the rules that govern these types of molecular motor traffic. Then I'll briefly present a theoretical model for RNAP traffic by incorporating the steric interactions between RNAPs and the mechano-chemical cycle of individual RNAPs during the elongation stage, including the possibility of RNAP backtracking. The results of this model explain the physical origin of the switch-like regulation of the two interfering genes in both co-directional and contra-directional traffic of the two groups of RNAPs |
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Himanshu Gupta
13209861
Rayleigh-Benard Convection Problem in Intracluster Medium
24th March, 2015 (Tuesday)
3 pm
FB-382
Galaxy clusters are the largest known gravitationally bound structures which consist of very hot (10-100 Mega-Kelvin) plasma, called intracluster medium (ICM). This plasma is mainly composed of Hydrogen & Helium. In ICM, both of the species are fully ionized. ICM is a weakly magnetized (~1 micro-Gauss), weakly-collisional and high-beta plasma in which the anisotropic transport of heat and momentum, and diffusion of ions is predominantly along the direction of the magnetic field. It is well known that ICM can become convectively unstable via either magneto-thermal instability or heat-flux-buoyancy-driven instability.
This talk concerns with aforementioned two instabilities in ICM. I plan to review them and discuss the outcomes of the linear stability analysis performed on the equations governing ICM modeled as a Rayleigh-Benard set-up. |
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Ashutosh Kumar Singh
13109879
Wave packet dynamics (collapse, revival etc.) and ultrafast carrier dynamics in
various two dimensional systems.
24th March, 2015 (Tuesday)
11 am
FB-382
This talk is divided into two parts. In the first part we will discuss about spontaneous collapse and consequent quantum revival of injected wave-packet in two dimensional systems with non-equidistant energy levels. Initially when a well localized wave packet is injected into a 2D system with non-equidistant Landau energy levels, it undergoes cyclotron motion and evolves quasi-classically for a number of cycles, with its probability density spreading around the quasi-classical trajectory. Non-equidistant nature of the discrete energy spectrum then leads to destructive quantum interference and consequently the collapse of the wave-packet. The collapsed wave-packet regain their initial waveform and oscillate again with the quasi-classical periodicity on a much longer time scale known as revival time.
In the second part we will talk about the carrier dynamics in Graphene under exposure to light beam which theoretically investigated either in the field theoretic language or is based on solving a set of extended optical Bloch equations obtained from time dependent Dirac/Schrodinger equation. |
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Pramod Ghising
13109071
MFM Studies on Ferromagnets and superconductors
27 February 2015 (Monday)
12:00 Noon
FB-382 |
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Rajesh Tripathi
13109074
Low temperature properties of rare earth based ‘122’ phosphide
27 February 2015 (Monday)
11:00 am
FB-382 |
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Jitendra Kumar Pradhan
13109066
Infrared sensors based on Metamaterial Perfect Absorbers
23 Feb. 2015 (Monday)
4.00 pm
FB-382 |
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Rabeet Singh
12109066
Study of adiabatic connection in density functional theory
29 December 2014 (Monday)
10.30 am
FB-382 |
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Arif Warsi Laskar
1220963
Manipulating states of photons with atoms
20 November 2014 (Thursday)
10.30 am
FB-382 |
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Rajan Singh
12209067
Optical detection of a weak scatterer
19 November 2014 (Wednesday)
11 am
FB-382 |
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Ankit Kumar
12209062
High Jc states in Iron Pnictides
7 November 2014 (Friday)
11 am
FB-382 |
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Sourav Biswas
12209070
Thermal Instability and Phase Slips in Superconducting Weak Links.
22 October, 2015
3 pm
FB-382
A superconducting weak-link, such as a constriction, between two bulk superconductors can act like a Josephson junction and can sustain a super-current, determining the phase difference across the weak-link, below a critical current. The phase-dynamics in weak-links , particularly phase slip processes due to quantum and classical fluctuations, is yet to be fully understood. Such weak-links are used in micron-size superconducting quantum interference device (SQUID), i.e. a sensitive detector of magnetic field. The characteristics of such devices are dictated by the phase dynamics in the weak-links. Current voltage characteristics of a weak-link shows hysteresis at low temperatures with a critical current and a re-trapping current, due to thermal instability. Since phase-slip processes generate heat and thus the details of heat evacuation from weak-link plays an important role in temperature and phase dynamics. Our recently reported time dependent thermal model found a new dynamic regime where the temperature and phase oscillate even in the thermally stable region and this gives rise to a new hysteretic regime. Heat evacuation and thermal stability in current carrying superconducting structures is of wider interest due to various applications. When a normal-superconductor interface is formed in a current carrying superconductor, its stability/instability is dictated by the efficiency of heat evacuation and this in turn determines the electrical transport properties and hysteretic behavior of different superconducting devices. In this talk I plan to review the phase slip and thermal instability in weak-links that control their current-voltage characteristics.
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Rajesh Kumar
12209068
Periodically patterned sculptured thin films for optical applications
20 October 2014 (Tuesday)
11 am
FB-382
Sculptured thin films (STFs) are three dimensional self-organized nanostructured materials that can be deposited to form well-defined structures by vapor deposition at large oblique angles. Periodically patterned STF (PP-STF) with spatially organized arrangements of the nano structures are made by deposition on pre-patterned substrates and show remarkable optical applications such as opticalactivity, surface enhanced spectroscopy, photonic crystal sensors etc. The large anisotropy and chirality in the optical response of STF make them potential candidates for incorporating in optical devices for control of polarization and directional emission/absorption. Large local field enhancements in metallic STF and the possibility of finely tuning their resonance through external means or fields makes them suitable candidates for surface enhance spectroscopic like surface enhance fluorescence (SEF) and surface enhance Raman scattering (SERS). In this seminar, I will begin by discussing the growth mechanisms of these PP-STF fabricated by glancing angle deposition technique and then describe some of the novel optical properties of these PP-STF. Finally, I will discuss about surface enhanced fluorescence and SERS from metallic sculptured thin films. |
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Rameshwari Naorem
12209873
Scattering of electromagnetic radiation from disordered systems
14 October 2014 (Tuesday)
3 pm
FB-382
In multicomponent metallic alloys (like equimolar AlCuCoCrNi, MoCrFeNiCu & CuMnCoCrFeNi), configurational entropy may offset the tendency for compound formation to give rise to disordered solid solution(s). These alloys have been christened as high entropy alloys (HEA). The effect of the addition of multiple alloying elements to form a concentrated disordered solid solution on the x-ray diffraction (XRD) pattern is poorly understood. The role of atomic disorder and strain at the atomic level on the XRD pattern is also poorly characterized. Analogous to HEA with positional disorder one may envisage systems with orientational disorder, wherein the enhanced configurational entropy due to multiple orientational variants can stabilize a high symmetry phase, in spite of the strain in the lattice. In the case of the cluster compounds (of type AM4X8, where A is a trivalent atom like Ga, Ge or Al, M is a divalent transition metal and X is a chalcogenide), the strain arises due to Jahn-Teller distortion of tetrahedral clusters of transition metal ions. Further, compounds with both positional and orientational disorder can be envisaged. The effect of this combined disorder in cluster compounds on the XRD patterns and on structural, magnetic and transport properties are yet to be studied critically. Systems with orientational disorder of multiple origins (structural and spin) have not been studied in detail either from a stability perspective or from a scattering perspective. On a larger length scale (optical wavelengths) chiral structures have been synthesized for many applications like antennas and arrays, antenna radome and waveguides. Scattering of coherent light waves from systems with chiral disorder offers interesting possibilities. |
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Mohammad Zaffar
12109873
Mueller matrix Imaging in human tissue
29 August, 2014 (Friday)
10:30 am
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Khun Sang Phukon
12109065
Gravitational Wave Detection And Parameter Estimation
3rd June, 2014 (Tuesday)
4 pm
FB-382
There has been a worldwide effort to detect gravitational waves using ground based laser interferometer detectors. Inspiralling compact binaries in close orbits are the most promising and well-understood astrophysical sources of gravitational waves in the frequency band which may observable at these detectors. Gravitational waves will be detected by the method called match filtering, i.e, by correlating detectors output with a set a theoretical waveform templates. I will discuss the geometric formalism used to generate templates in the parameter space. Post detection accurate estimation of parameter is essential. For estimation of parameters and the associated errors, Fisher matrix based analysis can be followed. I will address effects corresponding to detector sensitivity, detector bandwidth and implications of higher order PN phasing corrections to template in parameter estimation. |
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Pritam Kumar Roy
13109882
Harnessing Elastic Instabilities in Soft Matter
2nd June, 2014 (Monday)
4 pm
FB-382 |
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Suchita
12109876
Slow light: processes and applications
16 May 2014 (Friday)
11:30 pm
FB-382
Slow light is the propagation of light in a medium under reduced group velocity conditions. In earlier work, this has been studied in various media and structures. All these studies have one common feature, which is a sharp resonance peak, which leads to the modification of the group index and hence the group velocity. In this seminar, Stimulated Brillouin Scattering (SBS) based slow light is being presented. SBS, a nonlinear optical effect, is easily observed in optical fibers at very low input powers. SBS gain region gives the variation in refractive index of the medium and hence generates the slow light effect. SBS has both gain and loss regions depending upon the operating wavelength/frequency region which provides the positive as well as negative optical time delay. This property of SBS makes it more efficient than other sources of slow light generation in optical fibers. There are also certain limitations in SBS which requires a detailed study to improve the effect. Slow light has many potential applications in optical communication networks, quantum computing, information storage and ultrafast information processing. |
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Tanmay Maji
12209874
Transverse momentum Dependent Parton Distribution functions
09 May, 2014 (Friday)
12:30 pm
FB-382
Transverse Momentum Dependent(TMD) parton densities are the 3-dimensional generalizations of the standard Parton Distribution functions (PDFs). Many recent experiments on Semi-Inclusive Deep Inelastic Scattering (SIDIS) and Drell-Yan processes suggest that the collinear picture of fast moving hadrons is not sufficient and one equires to investigate the hadronic structure involving partons with transverse momentum distributions. The TMDs are function of lightcone momentum fraction, square of momentum transferred and the intrinsic transverse momentum of the parton. In leading twist, there are eight altogether eight TMD PDFs. Calculation of TMDs from first principle in QCD requires nonperturbative techniques. Till now, TMDs are generally evaluated in different models. In this talk, I'll discuss some recent theoretical results of TMDs. Our future plan is to investigate the TMDs using LightFront Wave Function Formalism as well as in AdS/QCD. |
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Kamalika Nath
12109872
Investigation of magnetic properties and their configurations in low dimensional magnetic structures
25 April, 2014 (Friday)
5:30 pm
FB-382
The talk will present an overview of recent results on the behaviour of magnetism in low-dimensional magnetic structures. Typical results related to different magnetic configurations in nano-structures,competition between different energy scales like magneto-crystalline and shape anisotropy and their effect on different quantities,viz., the coercive field, saturation fields, shape of hysteresis loops etc. in these nano-structures will be discussed. Some results relating to the manipulation of domain walls,domain wall pinning and experimental techniques used in these investigations, will be reviewed. Some results on developing low dimensional magnetic structures and studying their properties developed in the lab will also be presented in the seminar. |
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Shailendra Kumar Rathor
12109874
Fluid Instabilities and Long-term Evolution of Intracluster Medium
25th April, 2014 (Friday)
12 pm
FB-382
In the past two decades, instabilities in intracluster medium (ICM) permeating every galaxy cluster has become a topic of active research in astrophysics. The presence of a weak magnetic field in ICM changes the dynamical stability of the system dramatically. In this talk, ICM will be modeled as a thermally stratified weakly-collisional plasma layer. The curious role of the anisotropy due to weak magnetic field on the heat flux in ICM will be highlighted. Such an anisotropy is responsible for introducing some interesting instabilities e.g magnetothermal instability (MTI), and heat-flux driven buoyancy instability (HBI). It will be seen that further consideration of anisotropy in pressure tensor has unignorable effects. We shall also discuss the dynamical stability of dilute plasma in the light of simultaneous presence of the thermal and the compositional stratifications. Finally, I shall elaborate on my immediate and long-term plans towards the successful completion my intended Ph.D. work. |
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Mr. Anmol Thakur
12109063
Transport and many body effects in Low dimensional systems
10th April, 2014 (Thursday)
11 am
FB-382
We study the low energy collective modes for 2D systems with spin-orbit coupling.The electric transport and spin dynamics in such systems is related by an operator identity at lower energies.The identity relates the electric current to in-plane spin degree of freedom, which in the presence of electron electron interactions leads to an undamped spin plasmon mode whose dispersion is similar to that of a sound mode. We study the spin plasmon mode, in the presence of both - short ranged and long ranged interaction. We next want to explore the effect of static disorder on such modes.
In addition, we have also studied the transport phenomena in low dimensional system using Landeur-Buttilker formulae and Non Equilibrium Green's function approach which is now a most commonly used tool in the transport study of nano-dimensional MOSFETS and other devices. We plan to apply such techniques to study transport properties of hybrid superconducting systems in the future. |
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Mr. Raghawendra Kumar
12109067
Controlling light using optical nano-antennas
9th April, 2014 (Wednesday)
4 pm
FB-3872
Confinement of light to small volumes is of great importance for many applications ranging from solar concentrators, fluorescent enhancements, subdiffraction imaging etc. Nano-antennas provide a convenient way to manipulate light and enhance a range of linear and nonlinear phenomena. Nano-antennas can create intense, localized field distributions or enable coupling to highly subwavelength objects, which is generally not possible by optical components such as mirrors, lenses and gratings. In this seminar, I will give a basic introduction of optical nano antanna, and how it is used to concentrate light into sub-diffraction volumes. I will also describe a two-photon polymerization technique to fabricate 3D micro and nanostructures, which will be used to fabricate nano-antennas . I will focus on the enhancement and coupling of fluorescence from single molecules using these antennas. |
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Anil Kumar Singh
12109062
Local electronic properties of graphene
9th April, 2014 (Wednesday)
3 pm
FB-382
Graphene, a single layer of graphite, has raised extensive interest in a wide scientific community for its extraordinary thermal, mechanical, electrical and other properties. In particular, due to its high mobility graphene has been considered as a candidate material for applications in post-silicon electronics. In this seminar, I shall discuss some of the experiments probing the local and bulk electronic properties of graphene and the factors affecting these properties. In particular, the effect of back-gate, disorder, proximity to different materials (such as normal metal, superconductor), chemical dopants will be reviewed. I shall also present our plan on investigating the local electronic properties of graphene using scanning tunneling microscopy and spectroscopy. |
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Shamik Ghosh
12109875
Beyond the Cosmological Principle
31st March, 2014 (Monday)
11 am
FB-382
Cosmological Principle is the fundamental assumption of homogeneity and isotropy of space in the Standard Model of Cosmology. In the talk I will first discuss the Cosmological Principle and justify its importance towards building the Standard Model of Cosmology. I will briefly discuss the Standard Model and explain its success before proceeding to show the persistent anomalies when fitting experimental data. These anomalies require us to check the validity of the Cosmological Principle. I will then present various partially successful attempts to explain the anomalies observed in the data. These include local scale modification of observables due to secondary effects, phenomenological models and inflationary models.
Finally I will discuss some work that I have done in this field and the possible avenues that can be taken towards understanding the phenomena and building an appropriate model. |
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Ritu Gupta
12109068
Proximity effect at Ferromagnet-Superconductor interface
7th March, 2014 (Friday)
12 pm
FB - 382 |
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Dipanwita Ghanti
12109871
Collective force generation by microtubules and molecular motors.
5th March, 2014
4 pm
FB - 382 |
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Anshuman Dey
11109063
AdS/CFT correspondence and a Generalized Holographic Superconductor with Higher Derivative Couplings.
5th March, 2014
3 pm
FB -382 |
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Biplab Dutta
11109862
Anisotropy in Turbulent Flows
19th February, 2014 (Wednesday)
12 pm
FB - 382 |
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Alestin Mawrie
12109061
Novel phenomena in spin-orbit coupled fermionic systems.
21st February, 2014
11 am
FB -382 |
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Pavan Kumar
11209067
Microtubule polymerization kinetics: effects of structure and energetics.
29 November, 2013 (Friday)
12 pm
FB - 382 |
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Abhishek Juyal
11209061
Electrical manipulation and detection of domain walls in nanomagnets
15th November, 2013
10 am
FB - 382 |
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Vinay Kumar Shukla
11209068
Interfaces: An alternative route to magnetoelectricity
15th November, 2013
9 am
FB-382 |
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Vijay Singh
Y10209067
Investigation of Electric and Magnetic Properties of Compositionally Modulated Gallium Ferrite.
12 November, 2013
5 pm
FB-382 |
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Alekha Chandra Nayak
11209062
ELKO fermions
8 November 2013 (Friday)
12 pm
FB-382 |
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Dipak Rout
11209064
Light-matter interaction in dielectric and metallo-dielectric photonic crystals.
18th October, 2014
10 am
FB - 382 |
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Dibya Jyoti Sivananda
11109867
Unusual vortex states in ferromagnetic - superconducting heterostructures.
4th June,2013 (Tuesday)
11 am - 12 pm
FB-382 |
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Sumeet KD
11109865
12 pm - 1 pm
30th April, 2013 (Tuesday)
12 pm - 1 pm
FB-382 |
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Chandan Mondal
11109864
Proton GPDs in light front holographic QCD.
18th April, 2013 (Thursday)
12 pm - 1 pm
FB-382 |
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Sayandip Ghosh
11109869
Magnetism and Superconductivity in Iron Pnictides and Chalcogenides.
17th April, 2013 (Wednesday)
4 pm - 5 pm
FB-382 |
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Rahul Kothari
11109068
Poincare Gauge Theory of Gravity
15th April, 2013
12 pm - 1 pm
FB-382 |
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Ankur Rastogi
Y7109063
Electronic Transport in 2–Dimensional Electron Gas at the Interface of Insulating Perovskite Oxides.
12th April, 2013 (Friday)
11 a - 12 am
FB-382 |
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A Kani Mohamed
11109861
Polarization rotation based magnetometry
25th March, 2013
4 pm - 5 pm
FB-382 |
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Abhishek Kumar
11109062
Turbulence in thermally induced flows
15th March, 2013 (Friday)
12 pm - 1pm
FB-382 |
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Ummer K.V.
10109070
Control of emission in crystalline photonic nanostructures.
14th March, 2013 (Thursday)
3:30 pm - 4:30 pm
FB-382 |
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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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Mr. Jitesh Barman
Y11109868
Electrowetting induced morphological transition on topographically structured substrates.
6th February, 2013 (Wednesday)
11 am - 12 pm
FB-382 |
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