Archive

 

The Journal Club Archive

Date/DayTopicSpeakerPosition / Affiliation

May 7, 2018

 Sachin Dev VermaCavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, UK

December 22, 2017

 Girish ChandraDoctoral Student, SPS, JNU

December 21, 2017

 Rahul DeshmukhDoctoral Student, The Graduate Center, CUNY, NY, USA

December 18, 2017

 Neha TiwariDoctoral Student, SPS, JNU

December 15, 2017

 Dr. Ashwani Kumar TripathiSPS, JNU

November 24, 2017

 Noufal JaseemDoctoral Student, IISER Thiruvananthapuram

November 20, 2017

Dr. Pramit K ChowdhuryAssociate Professor @ Department of Chemistry, IIT-Delhi

November 16, 2017

Dr. Manidipa BanerjeeAssociate Professor @ School of Biological Sciences, IIT-Delhi

November 10, 2017

Ranjani SeshadriPhD Student @ Centre for High Energy Physics, IISc

November 6, 2017

Prof. R. RajaramanEmeritus Professor of Theoretical Physics @ SPS, JNU

November 3, 2017

Pratyay GhoshSRF @ SPS, JNU

October 13, 2017

Prasenjit DasPDF @ Weizmann Institute of Science, Israel

October 6, 2017

Prof. Patrick DasguptaProfessor, Dept. of Physics & Astrophysics, University of Delhi

February 26, 2016

Dr. Sudarshan AnanthAssociate Professor @ IISER Pune
March 2, 2016

1, 2  Professor, SPS, JNU

3 Associate Professor. SPS, JNU

September 19, 2016 Prof. R. RajaramanEmeritus Professor of Theoretical Phys, SPS, JNU
September 15, 2016 Prof. V. RavishankarProfessor, Dept. of Phys, IIT Delhi

 

Hot-carrier cooling in hybrid Pb-Sn halide perovskites

Abstract : Organic-inorganic hybrid perovskites have emerged as an excellent class of semiconductors owing to their exceptional optoelectronic properties. Enormous efforts are being made globally to understand their photophysical properties to improve their design and utilization in solar cells, light emitting diodes, photodetectors and lasers. However, a deep understanding about hot-carrier dynamics, crucial to make hot-carrier solar cells, in these class of semiconductors remain modest. Excitation of charges from valence band (VB) to higher energies in conduction band (CB) generates hot-carriers i.e. electrons (holes) in a Boltzmann distribution with kinetic energies above CB. Underlying mechanism of hot-carriercooling is of fundamental importance for enhancing the device functionalities. In this talk, I will take you on a tour of transient absorption spectroscopy (TAS) and microscopy (TAM) which employ ~10 fs pulses. I will show you how hot-carrier cooling dynamics can be observed from TAS and what makes these perovskites apotential candidate for hot-carrier solar cells. Brief overview will also be provided on how state-of- the-art TAM can enable us to directly monitor carrier transport in semiconductors.



 

Phase Behavior and Dielectric Relaxation of Binary Mixtures of Rotationally Disordered Crystals

Abstract : An especially challenging problem is the understanding of the glass transition with its tremendous but continuous slowdown of molecular dynamics when approaching the glass state. For that, the study of relaxation processes and phase behavior of orientationally disordered (or plastic) crystals is very advantageous since these materials involve only rotational degree of freedom. This makes much simpler to treat in theoretical and simulation approaches of the glass transition, and therefore these materials are often considered as model systems for structural glass formers. In this talk, we will discuss the Differential Scanning Calorimetry (DSC), Dielectric spectroscopy and X-ray results for binary mixtures of Neopentanol (NPOH) with tert-Butyl chloride (TBC) and Carbon tetrachloride (CTC), and try to understand the relaxation behavior of NPOH. The experimental results indicate the formation of the solid solutions. The crystalline solid thus formed is found to be orientationally disordered and supercools easily to form glassy crystal for mole fraction of NPOH in the range of 0.7 – 0.9. In the dielectric study, a primary α-process and two sub-T g processes are found for TBC-NPOH binary system, whereas for CTC-NPOH binary system only one sub-T g process is found.



 

Breaking the RET barrier using Metamaterials

Abstract : Resonance Energy Transfer (RET) between molecules is perhaps most famously known for its use in determining molecular structures. It is a direct interaction between two dipoles/molecules and its efficiency is governed by the distance between the molecules. This precise distance dependence makes it a very convenient ‘ruler’ for biologists but it’s useful in measuring distances of only up to 12-15 nm. Extending this range would be great, not only from the point of view of being able to study supermolecular assemblies, but also in photovoltaics where increased RET efficiency can greatly improve artificial light harvesting and energy storage. I will discuss our work in breaking this barrier using metamaterials – a class of composite man-made materials with novel properties that can be easily tuned to suit our requirements. We exploit this tunability and show RET between a donor-acceptor pair separated by a distance of 160 nm.


 

Biophysical insight into the heparin‐peptide interaction and its modulation by a small molecule

Abstract : The heparin‐protein interaction plays a vital role in numerous physiological and pathological processes. Not only is the binding mechanism of these interactions poorly understood, studies concerning their therapeutic targeting are also limited. Here, we have studied the interaction of the heparin interacting peptide (HIP) from Tat (which plays important role in HIV infections) with heparin. Isothermal titration calorimetry binding exhibits distinct biphasic isotherm with two different affinities in the HIP‐heparin complex formation. Overall, the binding was mainly driven by the nonionic interactions with a small contribution from ionic interactions. The stoichiometric analysis suggested that the minimal site for a single HIP molecule is a chain of 4 to 5 saccharide molecules, also supported by docking studies. The investigation was also focused on exploiting the possibility of using a small molecule as an inhibitor of the HIP‐heparin complex. Quinacrine, because of its ability to mimic the HIP interactions with heparin, was shown to successfully modulate the HIP‐heparin interactions. This result demonstrates the feasibility of inhibiting the disease relevant heparin‐protein interactions by a small molecule, which could be an effective strategy for the development of future therapeutic agents.


 

Dewetting of solid films: Triple line kinetics and mass shedding

Abstract : Dewetting of the solid films has been studied in a large number of experimental systems, such as SOI, or metal films on various substrates. Several theoretical approaches have been proposed in the past ten years to understand these phenomena, ranging from Kinetic Monte Carlo to phase field, and continuum Mullins-like models. We present a continuum model to study the so-called mass shedding effect, which leads to the breakup of the film at a finite distance from the triple-line. Our results show that mass shedding is accelerated by orders of magnitude due to wetting effects. We also address the question of the limit of validity of the usual approximation of a constant contact angle at the triple line between the solid, the film, and the vacuum (or vapor). Our results suggest that the Young-Dupré relation is subjected to systematic deviations, which could be measured in experiments.


 

Practical restrictions and environmental effects on quantum-limited measurements

Abstract : High precision measurements play an important role in the advancement of basic and applied sciences. In any metrological scheme, the ultimate precision is determined by the laws of nature governing the constituent particles of the measuring device. At the most fundamental level, these laws are the laws of quantum mechanics. By using quantum systems and harnessing their non-classical features like entanglement to probe physical systems it has been experimentally shown that it is possible to do better than classical protocols in precision measurement. There are still many challenges in implementing quantum-limited metrology. In this talk, we consider some of these challenges such as practical constraints and environmental effects and investigate how they affect the measurement uncertainty. After a brief introduction to quantum metrology, we first consider a practical restriction on the readout process. We study the limitations placed on the optimal initial state of the quantum probe made of qubits (two-level quantum systems) and on the achievable measurement uncertainty by the restriction on the readout procedure. Then we examine the adverse effects of photon loss on the attainable measurement precision in an optical interferometric setup. A two-mode Gaussian product state in an interferometer subjected to photon loss is studied. Finally, a central probe qubit coupled to an Ising ring of qubits is investigated as a potential quantum magnetometer. The possibility of exploiting the environmental effects to obtain quantum-enhanced sensitivity is explored here.


 

Protein Folding Basics: Structural Aspects and Driving Forces

Abstract : Proteins are one of the most important biological macromolecules present in the physiological interior. Two of the major questions that are frequently referred to in the field of protein folding are: (i) Given a sequence of amino acids, can one predict the final structure of the native state and (ii) Once the polypeptide chain is synthesized, what are the possible pathways that this unfolded/unstructured chain follow to reach the final native state, the latter generally having a well-defined three-dimensional structure. In this discussion, we will explore briefly the varied aspects of the folding of a protein, with the journey starting from its birth in the ribosome to its attainment of the final structure.


 

Recent advances in cryoelectron microscopy based structural analysis

Abstract : The last few years has seen unprecedented progress in the field of cryoelectron microscopy and image processing, with the resolution and size barriers being broken. Central to these efforts has been the development of equipment and methods such as direct electron detector, phase pate and in-column filters, in addition to the availability of powerful microscopes and better algorithms for data collection and processing. Currently, cryoelectron microscopy can routinely generate detailed structural information of proteins at resolutions of 2 - 3 Å, which was previously possible only through X-ray crystallography. In addition, although cryoEM was previously applicable for larger proteins complexes exclusively, high resolution structure of biomolecules less than 100 KD are frequently being resolved. Since cryoEM based structural analysis requires significantly less sample and time compared to other structural biology methods, it is expected to revolutionize the field of structural biology and forever alter the landscape of structure-based drug design. The current scenario international and national scenarios, in context of the recent Nobel Prize, will be discussed.


 

Graphene with Spin-Orbit Coupling: Transport and Wave packet dynamics in presence of barriers

Abstract : I will be discussing some interesting phenomena that arise when we consider Spin- Orbit (S-O) coupling (of two kinds - Kane-Mele and Rashba) in Graphene. An interface of such S- O coupled graphene with its pristine counterpart has robust chiral edge modes in spite of the gapless nature of the pristine graphene. Such junctions also have interesting transport and spin- conversion properties. I will also discuss the role played by periodic barriers in modifying the spectrum of S-O coupled graphene. Finally, I will talk about the combined effect of S-O couplings and periodic potentials on the propagation of wave packets, focusing on the special case when the strengths of the Kane-Mele and Rashba S-O couplings are equal.


 

Giant Accelerators: Why is Particle Physics = High Energy Physics ?

Abstract: After the Higgs Boson was discovered, the Large Hadron Collider (LHC) at the CERN Laboratory in Geneva, where this particle was detected, came into larger public prominence. While primarily designed for exploring very fundamental physics, LHC is also an extraordinary engineering marvel. But it is just the last in a series of large accelerators of increasing energy and varying designs that have been used by particle-physicists for decades to pursue their subject. This talk will be at an introduction to accelerators, their history, the basic physics behind their design and the reasons why they are needed for doing particle physics. The talk will be designed to be understandable to SPS students.


 

Nematic Order: a novel magnetic phase

Abstract : Frustration is a crucial feature that governs the modern day condensed matter physics. A spin system is frustrated when one fails to find a minimum energy configuration which satisfies all the bonds simultaneously. Such a system is known to disfavor any magnetic order in the ground state and produces excitingly new disordered phases. By definition, a disordered state obeys the spin-rotation and time-reversal symmetry. An ordered state respects neither. One might ask whether more such possibilities exist. Or, more precisely, if there exists such a state which retains the time-reversal invariance but breaks the spinrotation symmetry. The answer to that would be a state named ‘Spin Nematic order.' This talk explores ‘Spin Nematic order.' and also similar other states, which either break spinrotation symmetry, lattice-rotation symmetry, or a combination of both, while preserving the time-reversal symmetry. Combinedly, all such exotic states are called ‘Nematic order'


 

Granular Systems & Solid Friction

In this talk, I will describe the single particle Brownian motion under the influence of solid friction. The numerical results for mean-squared velocity and velocity autocorrelation function are in good agreement with the analytical results in all physical dimensions. Next, we apply the solid friction on a system of granular particles. We consider this frictional force only as the mechanism of dissipation of energy. The instantaneous temperature of the system decays with time in a power-law fashion. In the early stage of evolution, the decay exponent is 3/2 which crosses over to 1 at later times. The density and velocity fields show clustering which is characterized by equal-time correlation functions and structure factors show dynamical scaling. The average domain size of density and velocity fields grow as L∼t α with α=1/3.


 

Gravitational waves

Abstract : This lecture will provide a basic physics oriented introduction to gravitational wave (GW) sources and their detection as well as the mysterious astrophysical sources like fast radio-bursts (FRBs) and gamma-ray bursts (GRBs). Fundamental physics like electromagnetic theory, classical mechanics, quantum theory, particle physics, statistical mechanics and gravitational physics are routinely used to understand these sources that commonly involve pulsars, magnetars, black holes, massive helium stars that have lost their hydrogen envelope, etc. Tremendously high magnetic fields, rapid rotation and intense gravity are the hallmark of some these compact objects. The energy budget of GW sources and GRBs are humongous, running to 10^{51}--10^{54} erg, unlike the FRBs whose energy requirements are about 10 orders of magnitude lower. On the other hand, the time scales over which these transient sources make their presence felt are very tiny. LIGOs and AstroSat may play very significant roles in the future in unraveling the burst conundrum.


 

Spacetime and Quantum Mechanic

ABSTRACT: Elementary dimensional analysis predicts problems when attempting to combine gravity with quantum mechanics. In this context, I 'll highlight four 'important' questions about gravity. The importance of scattering amplitudes in understanding gravity and possibly theories with particles of higher spin is also discussed.


 

On the discovery of Gravitational Waves & LIGO Experiment

Abstract:On the discovery of Gravitational Waves & LIGO Experiment


 

What is inside a pulsar?

ABSTRACT: Pulsars (nickname for Pulsating Sources of Radio) were discovered in 1967 by Jocely n Bell and Anthony Hewish. They are remarkable objects in the sky from which blips (pulses) of radio waves are detected by radio telescopes. The pulses are so regular in frequency that it was wondered in the immediate aftermath of their discovery whether they were the much awaited signals from an alien civilization ! But through a combination of observation and theory it was soon realized that a Pulsar is in fact one giant rotating nucleus. After a brief general introduction to Pulsars, the talk will focus on what is likely to be their internal composition and why they are also called “Neutron Stars”. The talk will be very introductory , meant for students, and will be a simple exercise in quantum mechanics, Pauli Principle and nuclear basics.


 

Physics & Information

ABSTRACT: This talk includes, the basic ideas of quantum information and computation and show how its impact on Science and technology of Information Science. A knowledge of basic quantum mechanics is assumed.

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