Archived Physics Colloquia

October 28, 2019: Arul LaxmiNarayan, IIT Madras

Title: Quantum physics and the butterfly effect

Abstract: The butterfly effect is a metaphor for the extreme sensitivity of nonlinear chaotic systems found generically when there is more than 1-degree of freedom, examples abound from the gravitational three-body problem to the weather. Chaos and quantum physics have had an uneasy relationship which has implications in explorations of the no-man's land of classical-quantum boundaries and goes to the heart of the foundations of statistical physics. The talk will introduce surprising connections of classical chaos to quantum entanglement and implications therein to many-body localization and thermalization of isolated systems. Recent proposals of measuring quantum chaos, such as out-of-time-ordered correlators (OTOC) and scrambling will be discussed briefly as they bring us to the latest avatar of the butterfly effect which seems to have reached black holes, now conjectured to be nature's most chaotic and fastest scramblers.

September 30, 2019: L Sriramkumar, IIT Madras

Title: The status of the inflationary paradigm

Abstract: Inflation is arguably the most compelling paradigm to describe the origin of perturbations in the early universe. The remarkable success of the inflationary paradigm can be attributed to its simplicity and efficiency. However, the efficacy of the inflationary scenario also seems to be responsible for an important drawback. Despite the strong constraints from Planck on primordial non-Gaussianities, a rather large set of models seems to remain consistent with the cosmological data, even as new models continue to be proposed. Aiming at a non-specialist audience, I shall begin my talk with an introduction to the hot big bang model and the need for an epoch of inflation. Thereafter, I shall discuss the inflationary observables and the performance of inflationary models against the cosmological data. I shall also touch upon the status of the possibility of features in the primordial spectrum, an aspect which I have investigated in some detail. I shall then discuss certain recent developments related to the formation of primordial black holes and constraints on the gravitational wave background from LIGO. Finally, I shall conclude with a few remarks concerning the status of the inflationary paradigm.

August 19, 2019: Prasant Panigrahi, IISER Kolkata

Title: Quantum photons from dipole coupled two-level atomic system.

Abstract: We demonstrate that light quanta of well-defined characteristics can be generated in a coupled system of three two-level atoms. The quantum nature of light is controlled by the entanglement structure, discord, and monogamy of the system, which leads to sub- and superradiant behavior, as well as sub-Poissonian statistics, at lower temperatures. Two distinct phases with different entanglement characteristics are observed with uniform radiation in one case and the other displaying highly focused and anisotropic radiation in the far-field regime. At higher temperatures, radiance witness is found to exhibit sub- and superradiant behavior of radiation intensity in the absence of entanglement albeit with non-zero quantum discord. This establishes the physical manifestation of quantum discord. It is also observed that the radiation intensity can be a precise estimator of the inter-atomic distance of a coupled system of two-level atomic systems. Our investigation shows, for the first time, the three body correlation in the form of a ‘monogamy score’ controlling the sub- and superradiant nature of radiation intensity.

March 25, 2019: Tanmoy Das, IISC Bangalore

Title: A New Superconducting Mechanism in the Market

Abstract: One of the fascinating facts about superconductivity is that despite more than 100 years of extensive research and 7 Nobel prizes, we are far behind our main goals. The race continues to both achieving room temperature superconductivity as well as to obtain a theoretical understanding of the mechanism. How can two electrons attract each other when they experience a repulsive Coulomb interaction? By now we know two mechanisms for attractive interaction between the same-charge fermions: Meson mediates an attraction between protons. Phonon mediates attractive potential between electrons with superconductivity. In this talk, I will present a new mechanism of attractive potential between electrons, forming superconductivity. In many intermetallics and heavy-fermion compounds, atoms can possess fractional valency due to valence fluctuation between conduction and localized electrons (such as f-orbitals or flat bands). In such localized f-orbitals, two f-electrons with opposite spins cannot be occupied on the same orbit since they have strong Coulomb repulsion. This means, in the field theory language, a singly occupied f-electron site is attached with an unoccupied f-state (which is a holon gauge field) whose job is to repel another f-electron. However, the unoccupied f-site can be occupied by a conduction electron since the presence of valence fluctuation channel allows mutation between the f- and conduction electrons. We show that the doubly occupied state with f- and conduction electrons condensates like a Cooper pair. I will present this theory along with detailed comparison with recent experimental surprises of conventional superconductivity in heavy-fermion materials where decades old studies predicted unconventional superconductivity.

Feb 04, 2019: Sudhir Jain, BARC, Mumbai

Title: Figures of Sound and Threads of Silence

Abstract: The quiescent parts of wave patterns created on vibrating drums are intriguing and beautiful. Similar patterns appear when we propagate a microwave pulse in a cavity, propagate water waves in different arrangements of circular scatterers, sound waves in solid blocks, antidot lattices mimicking Lorentz gas, electronic transport through ballistic microscopic semiconductor structures, quantum corrals, quantum well billiards, and of course in simple quantum billiards. Classification of the ``sound figures" of certain non-separable billiards leads to successful counting of the nodal domains. Several statistical measures will be shown to unfold the geometrical features of the nodal lines. Probability distributions of the amplitudes of the eigenfunctions and their intensity moments reveal non-analytic features in the neighbourhood of quiescent or silent or dark regions. In conclusion, Inverse problems will be mentioned where a useful connection is being established with neuroimaging.

January 7, 2019: Vivek Datar, TIFR Mumbai

Title: The India based Neutrino Observatory, mini-ICAL and a shallow ICAL

Abstract: The genesis of the India based Neutrino Observatory project is described briefly. The flagship experiment is based on a 51,000 ton magnetised iron calorimeter (ICAL) which aims to determine the mass ordering of the 3 tiny neutrino masses through a measurement of atmospheric muon neutrinos and muon anti-neutrinos. An 85 ton 4mx4mx10 layer mini- ICAL detector with 10 glass RPCs has been built and is presently taking data at the rented premises of INO at Madurai. The delay in starting construction at the preferred site in the Theni district in Tamil Nadu has encouraged us to look at other options for locating the ICAL detector including the possibility of a shallow depth ICAL together with an efficient cosmic veto shield. The first steps towards examining this possibility are outlined.