Novel Electronic Properties of Topological insulators and 3D Dirac/Weyl semimetals
The electronic properties of graphene have opened up a new window in condensed matter physics research. The surface states of several materials such as B1-xSbx, Bi2Se3, Sb2Te3 and Bi2Te3 have striking similarities with graphene. In these materials, known as topological insulators, the bulk has an energy gap at the Fermi level, whereas the surface possesses gapless highly conducting states. Following the discovery of topological insulators, another new class of materials, 3D Dirac/Weyl semimetal, which is 3D analog of graphene has been reported recently. Several interesting phenomena predicted in high energy particle physics can be demonstrated in a table-top experiment in laboratory. In this seminar, I present some of our recent works on electronic transport properties of high quality single crystals of topological insulators and 3D Dirac/Weyl semimetals such as Bi1.5Sb0.5Te1.7Se1.3, Cd3As2, ZrSiS, VAl3, TbPtBi, etc.
Phase transitions and critical phenomena are characteristic features of many condensed matter systems. Phase transitions have been extensively studied in systems both in and out of equilibrium. A recent focus of research activity is on "critical" behaviour in the nonequilibrium in living systems constituting soft condensed matter. In this talk, an introduction will be given to the concept of criticality in the living cell. The concept will be illustrated with the examples of gene expression and cell differentiation, two key processes associated with the living cell. Experimental signatures of critical-like behaviour will be discussed and a theoretical framework for obtaining such signatures described. The utility of obtaining such signatures will also be highlighted.
Pattern Formation in the Kinetics of Phase Transitions
Consider a system which is rendered thermodynamically unstable by a sudden change of parameters, e.g., temperature, pressure, etc. The system evolves towards its new equilibrium state via the emergence and growth of domains enriched in the preferred phases. Problems in this area of "kinetics of phase transitions" have received much research attention, and arise in many areas of physics. In this talk, we review our understanding of this area. We conclude by discussing the important problem of surface-directed spinodal decomposition (SDSD), i.e., the interplay of phase separation and wetting at a surface.
February 21, 2018
Prof. Rajesh Gopakumar
International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore
String theory has evolved into a powerful framework which is capable of providing fresh insights into well established frameworks like quantum field theory (QFT). One such set of new perspectives have been on conformal field theories (CFTs) which are central to our understanding of QFTs. This talk will be centered on some very "down-to-earth CFTs" such as the Wilson-Fisher fixed point (and its analogues and generalisations) that governs critical phenomena in statistical mechanics. We will aim to bring out the utility as well as beauty of the new angles that string theory methods bring to these well studied systems.
February 14, 2018
Prof. Pushan Ayyub
Department of Condensed Matter Physics & Materials Science, Tata Institute of Fundamental Research, Mumbai
Probing uncharted regions in the structure-property landscape via size confinement
We often overlook the delicate balance that prevails between crystal structure and properties in condensed matter physics: a tiny variation in the local symmetry and interatomic spacing may cause massive changes in the way a solid behaves. We know this from the response of a solid to changes in the state variables such as temperature and pressure. However, sincea decrease in particle size causes small but regular changes in the unit cell dimensions,it canalso be a very effective parameter in exploring the property landscape of a solid. Size-driven lattice distortions may affect physical properties more drastically than temperature and pressure, and are less likely to cause â€˜co-lateral damagesâ€™.I will show that size-induced changes in the crystal structure play a crucial role in a variety of interesting situations, such as: (a) persistence of superconductivity down to unexpectedly small sizes, (b) appearance of a magnetic moment in isolated Fe atoms embedded in a nanocrystalline metals, (c) destruction of ferroelectricity in nanocrystallineoxides, and (d) stabilization of novel crystal structures.
January 31, 2018
Prof. Chandan Dasgupta
Department of Physics, Indian Institute of Science, Bangalore
Glass transition in dense systems of self-propelled particlesfont>
In several biological systems, such as bacterial cytoplasm, cytoskeleton-motor complexes and cell nuclei, self-propulsion or "activity" is found to fluidize a glass-like state that exhibits characteristic glassy features in the absence of activity. To develop a theoretical understanding of this activity-induced non-equilibrium glass transition, we have studied, using molecular dynamics and Brownian dynamics simulations, the effects of activity in two model glass forming liquids: the Kob-Andersen binary mixture in three dimensions and a two-dimensional system of two kinds of dumbbells. Activity is introduced by assuming that some of the particles in the system experience a random active force characterized by its magnitude and persistence time. We find that the introduction of activity dramatically reduces the glass transition temperature and the glass transition disappears beyond a threshold value of the activity. Some of the effects of activity on the dynamics in the liquid state are determined by an "active temperature" that adds to the bath temperature. However, several properties of the "active" supercooled liquid, obtained as the glass transition is approached by reducing the activity at a low temperature, are found to be qualitatively different from those of the "thermal" supercooled liquid obtained as the glass transition is approached by lowering the temperature at low activity. We present simple analytic arguments, based on a heuristic Langevin description of the dynamics of a particle in the cage formed by its neighbors, and a hydrodynamic description for the system of active dumbbells, which provide a rationalization of some of the features of the dynamics observed in our simulations. This work was carried out in collaboration with R. Mandal, P. J. Bhuyan, M. Rao and P. Chaudhuri.
January 17, 2018
Prof. Naba K Mondal
DAE Raja Ramanna Fellow, Saha Institute of Nuclear Physics, Kolkata
Neutrino Physics- Current Status & Future Prospects
Discovery of neutrino oscillation and the corresponding implied inference that neutrino has mass brought neutrino physics on the centre stage of particle physics research. Physics Nobel Prize in 2015 was awarded for this very important result. Discovery of neutrino oscillation is however only the beginning of an exciting era of intense experimental activities in neutrino physics. A new set of experiments are currently operational and another set of experiments are at different stages of construction or planning. Results from these experiments will take neutrino physics from discovery to precision measurement era. Some of these latest results and physics potentials of the planned experiments will be discussed in this presentation.