Revisiting the theory of melting of three-sublattice order
In condensed matter systems with competing interactions, the low temperature behaviour often exhibits interesting patterns of ordering. One example is three-sublattice ordering of scalar degrees of freedom in a variety of quasi-two dimensional systems with triangular lattice symmetry. Examples include three-sublattice ordered monolayers of various adsorbates on graphite-like substrates, and three-sublattice ordered low temperature states in various frustrated easy-axis magnets. Standard Landau-Ginzburg theory treatments of the temperature-driven melting of three-sublattice order predict a two-step melting transition in such magnets in zero easy-axis field (or at half-filling for the monolayers), and a three-state Potts transition in a nonzero field. In this talk, we revisit these old and classical results, and show that such two-step melting is associated with an unusual thermodynamic signature in the easy-axis susceptibility. Additionally, we argue that further neighbour interactions can drive the system to a multicritical point that represents a new universality class of two-dimensional melting.
Investigating Cosmic string theories with Liquid Crystal Experiments
Liquid crystals provide a very convenient system where topological defects, such as strings, can be experimentally studied in a variety of physical conditions. We will discuss how the observations of string formation in a liquid crystal system can be used to test theories of cosmic string formation in the early universe. Main focus of these investigations is on various universal aspects of defect formation with which one can establish rigorous quantitative correspondence between these condensed matter experiments and elementary particle physics models of the early universe.
The discovery of the Higgs boson at the CERN Large Hadron Collider(LHC) has opened up the door for the next phase of investigation into the concept of spontaneous breaking of gauge symmetries. The Higgs boson has a unique property, not shared by other states of the Standard Model(SM), namely that it interacts with itself. The measurement of the self couplings of the Higgs boson is essential for the reconstruction of the Higgs potential, and thereby confirm the idea of spontaneous breaking of the gauge symmetry of the SM. I shall discuss the issue of the measurement of trilinear couplings of the Higgs boson in the SM as well as in the Minimal Supersymmetric Standard Model(MSSM) in detail. In the case of the MSSM we shall take into account all the current experimental constraints on the parameters of the model in order to assess the possibility of the measurement of the trilinear Higgs couplings at a high energy electron positron collider.
The Messiah of Masses---- An Answer That Becomes Questions
Starting at a level where one does not need to know any particle physics, the experimental and theoretical issues that led to the conjecture on, and discovery of, the Higgs boson will be briefly reviewed. It will be then emphasized that, rather than closing our book of accounts on elementary particles, the Higgs boson leads us to new questions, to which we have no answers yet. Thus we are currently facing fresh challenges related to fundamental physics.
New prospects for room temperature superconductivity: Molecular solids under high pressure:
Wigner and Huntington surmised, in 1935, that insulating molecular solid H2 will become a metal at a pressure ~25 GPa. However, H2 and other molecular solids have resisted becoming a metal even at a few 100 GPa. Instead they undergo covalent bond reorganization and several structural changes with pressure. In this background, in a recent experiment, molecular solid H2S has become a superconductor with a Tc ~205K, under a pressure of ~200 GPa. We present a theory where covalent bond reorganization and a resulting resonating valence bond state causes superconductivity and offers new hope for room temperature superconductivity. Does the old and wise nature realize this very high temperature superconductivity somewhere ?
Melting of the vortex lattice in a Type II superconductor: A story from images
In a Type II superconductor, magnetic flux lines interact among themselves to form a periodic structure, the "vortex lattice", which mimics a soft periodic solid. This system has long been used as a versatile model system to study the interplay between interaction and random pinning. Here, I will present real space images of the vortex lattice as it melts with temperature or magnetic field, acquired using the low temperature, high field scanning tunneling microscope built in our laboratory. These images reveal that the presence of a random pinning potential fundamentally alters the order-disorder transition, and gives rise to a variety of metastable states that can be accessed through thermomagnetic cycling.
1) Anand Kamlapure, Garima Saraswat, Somesh Chandra Ganguli, Vivas Bagwe, Pratap Raychaudhuri, and Subash P. Pai, Review of Scientific Instruments 84, 123905 (2013).
2) Somesh Chandra Ganguli, Harkirat Singh, Indranil Roy, Vivas Bagwe, Dibyendu Bala, Arumugam Thamizhavel, and Pratap Raychaudhuri Phys. Rev. B 93, 144503 (2016).
3) Somesh Chandra Ganguli, Harkirat Singh, Rini Ganguly, Vivas Bagwe, Arumugam Thamizhavel and Pratap Raychaudhuri, J. Phys.: Condens. Matter 28, 165701 (2016).
4) Somesh Chandra Ganguli, Harkirat Singh, Garima Saraswat, Rini Ganguly, Vivas Bagwe, Parasharam Shirage, Arumugam Thamizhavel &Pratap Raychaudhuri Scientific Reports 5, 10613 (2015).
In the 1970s, it was argued by Bekenstein that the laws of thermodynamics need to be modified in the presence of black holes. Notably, he argued that one must associate an entropy that is proportional to the area of the horizon of the black hole. Hawking, using a semiclassical computation associated a temperature, called the Hawking temperature, to blackholes. A microscopic/statistical mechanical understanding of the origin of this entropy was lacking until the mid 1990s when Strominger and Vafa carried out the first such computation. In this talk, we will discuss their result and subsequent progress over the last 20 years. The microscopic counting turns out to be of an interdisciplinary nature involving string theory, statistical physics, number theory, combinatorics and theoretical computer science.
March 22, 2016
Prof. Arup Raychaudhuri, S.N. Bose National Centre in Basic Sciences JD Block, Salt lake Sector III
The physics of metal-insulator transition is one of the most fascinating phenomena in modern day solid state sciences. It is well researched for years and ill understood, mostly. The process of transition from a metal (which has extended states) to an insulator (that has localized states) is not explainable within the frame work of a comprehensive theory till date, where disorder (e.g, those introduced by substitution) and electron correlation effects can be present together . In these schemes of things oxides form a new class that can have very highly resistive metallic phase co-existing with insulating phase. This talk will present an account of experimental pursuits, some of them using new tools and techniques that allow us to get new perspectives of metal-insulator transition close to the critical region. The talk will have a pedagogic component that will explain some of the basic concepts in this field and some necessary data to elucidate the recent results . It will also have a component of personal accounts that contain the agony and ecstasy of doing experiments.
March 14, 2016
Amitava Raychaudhuri, Sir Tarak Nath Palit Professor of Physics, University of Calcutta
Burning of the Sun & Turning of the Earth: Probing Nature with Neutrinos
Fusion reactions in the sun produce heat and light – and neutrinos. Is the solar model correct? Are we receiving the expected number of neutrinos on earth? Neutrinos are also produced in the earth’s atmosphere by cosmic rays. We expect these to be of the same number from all directions. Is this what we see? Experiments to address these issues – involving very talented scientists and engineers – led to results which imply that neutrinos are massive particles, when it was commonly thought otherwise. The Physics Nobel Prize of 2015 was awarded to Takaaki Kajita and Arthur B. McDonald for their leadership of two of these experiments. We begin with a brief introduction to neutrinos and their properties. We next turn to the experimental results that establish they have non-zero mass. What other properties of neutrinos are known? What are the known unknowns? Where do we go from here? A brief overview is provided.
Models of Earthquake Dynamics: Theory, Simulations & Comparisons with Aftershock Data
Earthquake dynamics is still very poorly understood and its physics models so far are not very successful. After a very brief survey of earthquake statistics, I will briefly present one "geometric" model and another "elastic depinning" model developed by us and present also their relative merits/successes when compared with the available well-characterized aftershock statistics.
When a black hole forms, the final state is said to be independent of details of the initial state of the collapsing matter. This is reminiscent of the phenomenon of thermalization in non equilibrium systems. Indeed, these two phenomena are sometimes related by a duality. Yet, while we believe we know a lot about thermalization, there is an active debate about the nature of "information loss" in the black hole system. We will discuss both issues with a bit of history and some recent inter-related developments which yield insight into the black hole mystery.
One of the fundamental issues in Cell Biology is to understand how the living cell translates physical and chemical processes to the management of information''. I will discuss how a transdisciplinary approach might throw light on this problem and illustrate this with our many collaborative research programs dealing with the active composite cell surface, organelle morphogenesis in trafficking pathways and cellular homeostasis and control. In doing so, I hope to lay out a more nuanced and ramified cross current of ideas and a richer engagement between theory and experiment, including a more abstract face of theory and its experimental implications.
We have learnt a great deal about the physics of the Universe at length scales of the order of an atto-metre, i.e. a nano-nanometre. This talk will trace the history of these great discoveries from an experimental point of view (though the speaker is a theorist), mentioning the importance of new technologies in retrieving information about such tiny objects, and showing how new theoretical ideas went hand-in-hand with new and often unexpected experimental discoveries. Finally, mention will be made of the giant LHC machine running at CERN, Geneva and some of the challenges for the future.
January 13, 2016
Sankar Das Sarma, Univ of Maryland, College Park, USA
How does an isolated quantum system come to thermal equilibrium due to interaction between its constituent subsystems? Or does it? What underlies the condition for quantum ‘ergodicity’? These are some of the basic questions to be discussed in this talk. The topic is of fundamental importance since it deals with the applicability of thermodynamics and statistical mechanics to isolated quantum systems, and asks the extent to which an isolated (macroscopic) quantum system can ever be described by a temperature.