IISER Pune Physics Colloquium
At present, all the talks are held at 4 p.m every Monday at Physics Seminar Room 31
Colloquia during Spring 2023
February 06, 2023: Dr. Sanjukta Roy, RRI Bangalore
Title: Quantum Technologies with ultra-cold Rydberg atoms, atomic spin ensembles and Quantum mixtures
Abstract: Atoms excited to Rydberg states with high principal quantum numbers have exaggerated properties such as strong dipole-dipole interaction, large values of polarisabilities and long lifetimes. These exotic characteristics and a high degree of controllability make ultra-cold Rydberg atoms versatile atomic building blocks for a variety of quantum technologies, such as scalable Quantum Information networks, precise Quantum Sensing as well as singlephoton sources for secure Quantum Communications.
Stochastic fluctuations are ubiquitous in all physical systems and can provide valuable information about the characteristic nature and internal structure of the system. Spin correlation spectroscopy enables non-invasive detection of spin coherences of atomic spin ensembles and enables Quantum Sensing via precision magnetometry.
Quantum gas mixtures with dual atomic species offer a wealth of novel possibilities for Quantum Simulation of interacting many-body systems exploring the interplay between inter-species and intra-species interaction hitherto inaccessible in single-species experiments.
In this talk, I will give an overview of Quantum Technologies with ultra-cold Rydberg atoms, atomic spin ensembles and Quantum mixtures and present our recent results from our experiments on Rydberg atoms and Quantum mixtures at RRI.
January 23, 2023: Prof. Ravindra Bhatt, Princeton University
Title: Composite fermions and Fermi surfaces
Abstract: The Nobel prize winning discoveries of the integer and fractional quantum Hall effects (IQHE/FQHE) triggered intensive research on electrons in two dimensions in a strong perpendicular magnetic field. Detailed investigations uncovered a rich phase diagram of a seemingly very simple system and led to a comprehensive understanding of various phases, and exotic phenomena associated with them. These include charge fractionalization, Abelian and nonAbelian quantum states, topological spin excitations, and charge-density-wave phases, to name a few. This body of work paved the way for the new field of topological materials in the 21st century.
The composite fermion picture developed by Jain provides a natural way to understand the sequence of FQH phases. It also naturally predicts the existence of certain gapless phases at even denominator filling fractions of a Landau level in the midst of the more common gapped FQH phases with odd denominator filling fractions and quantized Hall conductance. In particular, the phase for a half-filled lowest Landau level (filling factor n = 1/2) is seen as a Fermi liquid of composite fermions formed out of electrons bound to two vortices, in the absence of a magnetic field.
After briefly reviewing the arguments for various fractional quantum Hall phases following the picture of composite fermions, we concentrate on the gapless phase at filling factor n = 1/2 and explore the nature of its Fermi surface. We will compare its behavior with that of Fermi surfaces of familiar metals with weak electron-electron interactions, which are known to depend sensitively on the electronic structure of the material. We ask questions such as - What is the relationship between the Fermi surface of electrons at zero magnetic field and the composite fermion Fermi surface? How sensitive is the latter to perturbations of the zero-field Hamiltonian? What happens when the system does not have rotational symmetry with a circular Fermi surface at zero magnetic field? Using a combination of analytic and numerical techniques, we show that the answer is both surprising and amenable to a parameter free experimental test, which it passes with surprising accuracy.
January 9, 2023: Prof. Sunil Mukhi, IISER Pune
Title: String Theory - Status Report on a Model and a Framework
Abstract: I will present a broad survey of the subject called String Theory -- the proposal that elementary particle physics can be unified into a theory of fundamental relativistic strings. I will first explain the proposal, the hopes that it raised and the status as seen from today's perspective. Then I will discuss the broader impact of the string framework, which is influential in different research areas from condensed matter to chaotic dynamics to particle physics to pure mathematics.
Colloquia during Fall 2022
November 14, 2022: Prof. Akshay Naik, IISC Bangalore
Title: 2D Suspended Structures
Abstract: Vibrating membranes fabricated using 2D materials are extremely sensitive to various stimuli. In this talk, I’ll present our efforts towards using 2D materials as sensors. One of these is a strain sensor fabricated using graphene on a thin silicon diaphragm. These tiny structures vibrate at very high frequencies (10-100MHz) and are exquisitely sensitive to changes in the strain. However, nonlinearities in these devices constraint their use as a linear sensor. In the talk, I’ll demonstrate methods for controlling and manipulating these nonlinearities. I’ll show how our inability to produce perfect devices leads to nonlinear effects and our ability to control and cancel out nonlinearities leads to enhancement of signal to noise ratio. This regime of near cancellation of the two strongest nonlinearities is not only useful for applications but also to observe higher order nonlinearities and nonlinear damping.
November 7, 2022: Prof. Varun Bhalerao, IIT Bombay
Title: Daksha: Finding High Energy Emissions from Gravitational Wave sources
Abstract: What happens when two neutron stars collide? Are these really the sources of most of the gold, platinum, and other heavy elements in the universe? After decades of theoretical speculation, we finally found the answer in August 2017. Advanced gravitational wave detectors - LIGO and Virgo - discovered the coalescence of two neutron stars 130 million light years away from Earth. At nearly the same time, Earth-orbiting satellites saw a flash of high energy radiation from the same direction. What followed was arguably the most frantic period of activity in modern astronomy: with over 3500 individuals from 950 institutes joining forces to pinpoint the source in the sky and uncover it's secrets. But despite several neutron star mergers being discovered since we never caught any more photons from them - the current satellite network is just not sensitive enough. Enter Daksha: a highly ambitious Indian mission to create two satellites that are several times more sensitive to these bursts than anything ever flown. I will give a brief introduction to the electromagnetic counterparts of gravitational wave source. Then I will describe the proposed mission and expected science returns.
October 31, 2022: Prof. Nirmalaya Ghosh, IISER Kolkata
Title: Weak measurements on Spin optical effects
Abstract: The weak value amplification (WVA) concept, introduced by Aharonov, Albert, and Vaidman, has proven to be fundamentally important and extremely useful for numerous metrological applications. This quantum mechanical concept can be understood using the wave interference phenomena and can therefore be realized in classical optical settings also. In this talk, I shall illustrate how the WVA concept can be formulated within the realm of classical electromagnetic theory of light and discuss its use for the amplification of tiny spin orbit interaction effects of classical light beam. I shall present our recent experimental work on the realization of WVA in standard path interference by introducing a weak coupling between the path degree of freedom of an interferometer and the polarization degree of freedom of light. Taking example of Fano resonance, it will be shown how WVA of an appropriate weak interaction parameter may naturally evolve in a rich variety of non-trivial wave phenomena that originate from fine interference effects. In this regard, our recent work on extending weak measurements into the domain of plasmonics, on demonstrating weak measurements using spectral line shape of resonance as pointer in precisely designed metamaterials, observing natural WVA of Faraday effect in Fano resonances from hybrid magneto-plasmonic systems will be highlighted.
October 17, 2022: Prof. Madan Rao, NCBS-TIFR, Bangalore
Title: Decoding and Encoding of Molecular Information in Distributed Cellular Systems
Abstract: It is often useful to think about Cells and Tissues as Distributed Computing Systems, especially in the context of the processing of noisy molecular information. I will illustrate this in two parts. In the first, I will talk about cellular compartmentalization and receptor promiscuity as a strategy for accurate inference of position during Morphogenesis. In the second, I will discuss the synthesis of a complex Glycan code in the Golgi cisternae, and how cisternal number and enzyme promiscuity achieves the target distribution with high fidelity.
October 10, 2022: Prof. Dilip Angom, Manipur University
Title: Quantum phases of ultracold atoms in optical lattices
Abstract: Optical lattices filled with atoms at very low temperatures, few nano Kelvins, exhibit a rich variety of quantum phases. The phases depend on the parameters like the number of atoms, strength of interaction, nature of the interaction, etc. Another class of quantum phases emerge when disorder or impurities are introduced. The beauty of exploring quantum phases and the physics of quantum many-body systems with optical lattices are, the lattice structure is near ideal and the parameters of the system are tunable. As a result, these systems have emerged as simulators of condensed matter systems and potential candidates of realizing quantum information devices. In general, the quantum phases can be grouped into two types depending on the atoms being either bosons or fermions. In the
talk we shall discuss the case of bosons.
September 12, 2022: Prof. Ranjini Bandyopadhyay, RRI Bangalore
Title: Settling dynamics and flow instabilities in shear-thinning soft glassy suspensions
Abstract: I shall describe some of our recent table-top experiments. In the first experiment, millimeter-sized steel balls were dropped in a soft glassy clay suspension. We observed that the balls of larger diameters failed to achieve terminal velocity over the entire duration of the experiment. I shall propose a toy model that correctly predicts the time-dependence of the ball velocity for a range of ball sizes and clay concentrations. In another experiment, we observed the emergence of a rich variety of patterns: dense viscous fingers, dendritic patterns, viscoelastic fractures, flowers and jagged interfaces, in miscible and immiscible displacements of a soft glassy clay suspension by a Newtonian fluid in a quasi two-dimensional radial geometry. I shall show that each pattern morphology can be distinguished by its areal ratio: the area of the fully developed pattern, normalized by the area of the smallest circle enclosing it. Besides demonstrating the range of interfacial patterns that can be generated with a fluid pair comprising a Newtonian and a non-Newtonian fluid, these results are relevant in the understanding of geological processes involving mud displacements.
August 29, 2022: Prof. Apratim Chatterji, IISER Pune
Title: Inducing organization in intrinsically disordered polymeric systems: Lessons from bacterial chromosome organization
Abstract: In recent times, researchers are using statistical physics to understand the emergent dynamics and organization in living systems, such as organization of the DNA within the living cell.
The mechanism and driving forces of chromosome segregation in the bacterial cell cycle of E. coli is one of the least understood events in its life cycle [1,2,3]. Using principles of entropic repulsion between polymer loops confined in a cylinder, we use Monte Carlo simulations to show that the segregation is spontaneously enhanced by the adoption of a certain DNApolymer architecture as replication progresses. Secondly, the chosen polymer-topology ensures its self-organization along the cell axis while segregation is in progress, such that various chromosomal segments (loci) get spatially localized as seen in vivo. The evolution of loci positions matches the corresponding experimentally reported results. Additionally, the contact map generated using our bead-spring model reproduces the macro-domains of the experimental Hi-C maps.
Thus we have proposed a framework [4] which reconciles many spatial organizational aspects of E. coli chromosomes as seen in-vivo and provides a consistent mechanistic understanding of the process underlying the segregation of bacterial chromosomes. Certain proteins are expected to contribute to changing the DNA-polymer architecture. We also studied other polymer architecture and saw that the same mechanism could be used to explain the experimental data of the C.crescentus chromosome [5].
[1] Jay K. Fisher, A. Bourniquel, G. Witz, B. Weiner, M. Prentiss, and N. Kleckner.
Four-dimensional imaging of e. coli nucleoid organization and dynamics in living cells.
Journal Cell, 153(4):882–895, 2013.
[2] A. Japaridze, C. Gogou, J. W. J. Kerssemakers, H. M. Nguyen, and Cees Dekker.
Direct observation of independently moving replisomes in Escherichia coli.
Nature Communications, 11(1), June 2020.
[3] Virginia S. Lioy, Ivan Junier, and Frédéric Boccard.
Multiscale dynamic structuring of bacterial chromosomes.
Annual Review of Microbiology, 75(1), August 2021.
[4] D.Mitra, S. Pande, Apratim Chatterji.
DNA-polymer architecture orchestrates the segregation and spatial organization of bacterial chromosomes during replication.
Soft Matter (2022)
[5] D.Mitra, S. Pande, Apratim Chatterji.
Modified topology of ring polymers in confinement leads to spatial organization
August 22, 2022: Prof. Basudeb Dasgupta, TIFR, Mumbai
Title: A New Kind of Black Hole
Abstract: In the last two years, gravitational wave observatories have detected mergers of compact astrophysical objects with low masses, tantalizingly close or potentially lower than the Chandrasekhar limit. These detections are beginning to motivate studies of their origins. Primordial black holes, created much before stars, are an oft-discussed possibility. We point out a new alternate possibility: neutron stars can catastrophically accumulate dark matter particles, collapse, and transmute to low-mass black holes. I will discuss the theory of dark matter accumulation in stars, subsequent collapse to low-mass black holes, and observational signatures of such transmuted black holes.
August 08, 2022: Prof. Arka Banerjee, IISER Pune
Title: Cosmology with nonlinear structure formation: Simulations and Statistics
Abstract: The formation, evolution, and clustering of structures in the Universe, such as galaxies, is a sensitive tool to probe some of the deepest questions in physics: the properties of Dark Energy and Dark Matter, the total mass and hierarchy of the Standard Model neutrinos, among many others. On large scales, perturbation theory approaches have been successfully applied to extract almost all the information from structure formation. On smaller scales, there is a vast amount of untapped information, but perturbation theory approaches break down due to nonlinearities driven by gravitational collapse. Harnessing this small scale information will be the next big challenge in observational cosmology. I will discuss how structure formation on small scales can be accurately modeled through numerical simulations. I will briefly describe how the modification of existing numerical techniques are allowing us to accurately model the nonlinear predictions of an expanding set of theoretical models, such as cosmologies with massive neutrinos, Dark Matter with additional interactions, and so on. In the second part of my talk, I will focus on statistical measures to characterize the clustering of structures on small scales. The widely used 2-point correlation function fails to capture all the details of the nonlinear clustering, motivating the exploration of other summary statistics. I will discuss one such set of summary statistics - the k-nearest neighbor distributions, which are easy to compute on a given dataset while being sensitive to higher order clustering.
Colloquia during Spring 2022
April 18, 2022: Dr. Susmita Adhikari, IISER Pune
Title: Dark Matter halos and the Universe
Abstract: The evolution of galaxies is tied to the evolution of dark matter halos in the cosmic web. The galaxies we observe in the sky trace the underlying field of dark matter, laying out the pieces of a puzzle that must come together to reveal a map and timeline of our universe. Understanding the formation and evolution of non-linear structure in the universe can shed light on some of the most fundamental questions in Cosmology. In this talk I will introduce the directions of my research, in particular, focussing on how the distribution of matter in dark matter halos can help us understand the nature of gravity, dark matter and the evolution of galaxies. I will also discuss how we can explore the interconnections between gravitational wave science and structure formation in light of the new window to the universe provided by the gravitational wave detectors.
April 11, 2022: Dr. Sourabh Dube, IISER Pune
Title: Nonresonant multilepton probes of BSM phenomena
Abstract: The multilepton final state is a versatile tool to look for signs of beyond standard model phenomena in the LHC dataset. In this talk, I will start with a little background on why multileptons, and then describe our most recent work [1] - we performed a search in a multilepton final state with the complete Run 2 dataset collected by CMS at the LHC. The search was specifically for three models of BSM phenomena, vector-like tau leptons, type-III seesaw model fermions, and leptoquarks. I will describe our model-specific approach using boosted decision trees, and discuss the model-independent results as well. I hope to aim the talk towards all physicists (incl. students) rather than an expert audience.
March 28, 2022: Prof. Rudolf Bratschitsch, University of Münster, Münster, Germany
Title: Single-photon emitters in 2D materials
Abstract: Atomically thin materials serve as a promising new material class for optoelectronics. Monolayer semiconductors such as MoS2 or MoSe2 exhibit prominent photoluminescence. Recently, we have discovered bright and stable single-photon emitters in single layers of WSe2, which renders atomically thin semiconductors also interesting for quantum optics and quantum technologies. In my talk, I will show that these quantum light sources can be controlled by mechanical strain and demonstrate deterministic positioning of the emitters on the nanoscale. Furthermore, I will present single-photon emission from GaSe, and demonstrate that the photons can be routed in dielectric waveguides on a photonic chip. Finally, I will discuss the nature and prospects of single-photon emitters in the van der Waals insulator hexagonal boron nitride (hBN) by focusing on the role of phonons and how large emitter arrays can be created with commercially available hBN nanocrystals.
February 21, 2022: Prof. Subir Das, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru
Title: Kinetics of Phase Transitions: A few interesting exceptions to the traditional picture
Abstract: When a homogeneous or disordered system is quenched inside the miscibility gap or ordered region of the phase diagram, it falls unstable to fluctuations. The approach to the new equilibrium, specific to the final state point, consists of formation and growth of domains rich in one or the other species within the system. Such nonequilibrium processes are often described as phenomena parallel to the equilibrium critical phenomena where the inverse of time assumes the role of deviation of temperature from the critical value. After introducing the typical problems of interest in this domain and associated universality, like in standard critical phenomena, I will discuss some exceptions to the traditional understanding. The major part of the lecture will be devoted to the ``by now'' well-known Mpemba effect. This counterintuitive effect is historically related to the faster freezing of a hotter body of water than a colder one when these are quenched to a common subzero temperature. While the confirmation and understanding for this original system still remains a matter of debate, despite being talked about since ancient times, there began efforts to investigate it in other systems. In this context of generalized Mpemba effect, I will discuss recent results on the observation of it in a simple model system that exhibits para to ferromagnetic transition. These results, while temporarily adding to the puzzle, may motivate works that can help understand the effect via well-controlled studies.
February 7, 2022: Prof. Pratap Raychaudhary, Tata Institute of Fundamental Research (TIFR), Mumbai.
Title: Cooper pairing without superconductivity: Phase fluctuations, Pseudogap state, Superinsulator and Bose Metal
Abstract: Cooper pairing, where pairs of electrons with opposite momentum and opposite spins form a singlet bound state is usually associated with the onset of superconductivity. Within the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity spin-zero Cooper pairs condense into a phase coherent state, giving rise to the zero resistance. However, an increasing number of experiments suggest that superconductivity is just one of the exotic states made of Cooper pairs. For example, it is now observed that in many superconductors Cooper pairing continues to persist above Tc even though the zero resistance state is destroyed by phase fluctuations. On the other hand, even at very low temperatures in many systems under the application of magnetic field one observes transition to a “superinsulator” where the conductance instead of resistance appears to vanish at a finite temperature. It is believed that in a superinsulator the Cooper pairs are in an eigenstate of number instead of phase and hence localized. Even more interestingly, in some systems Cooper pairs appear to exist in a dissipative metallic state, called Bose metal. In this talk, I will talk about these various novel states in the backdrop of the work done over the last ten years in Superconductivity Lab at TIFR.
January 24, 2022: Prof. V Ravindran, Institute of Mathematical Sciences (IMSC), Chennai.
Title: Threshold resummation in QCD
Abstract: We discuss a framework that resumes threshold enhanced large logarithms to all orders in QCD perturbation theory for a variety of inclusive and differential observables at the LHC. We demonstrate that the diagonal partonic channels show remarkable factorization properties that allow us to beyond soft plus virtual approximation. Using collinear factorisation and renormalisation group invariance we define a Soft-Collinear (SC) function which encapsulates soft and collinear dynamics of the perturbative results to all orders in strong coupling constant. Resummed results for Higgs boson and Drell-Yan productions improve the predictions by reducing uncertainties resulting from the choice of unphysical scales.
Colloquia during Fall 2021
November 22, 2021: Prof. Gilad Perez, Weizmann Institute
Title: New Approaches to New Physics Discovery from Strong Field QED to Quantum Sensors.
Abstract: One of the primary objectives of physics is to decipher the fundamental laws of nature, and it is now facing one of its biggest challenges. On the one hand, observations are telling us that new forces and/or particles must exist, however, these cannot be linked to a definite length or energy scale. On the other hand, the conventional perception that Higgs and electroweak dynamics require new physics at the TeV scale is currently under considerable pressure by the null results from the Large Hadron Collider, and direct dark matter searches. As an alternative, we introduce a two-pronged program, consisting of new theoretical and experimental ideas in both the luminosity and precision frontiers. As for the luminosity front, we present a new concept, that combines knowledge from strong-field quantum-electrodynamics (QED) with that of high energy physics to propose: LUXE-NPOD - NP searches with an optical dump at the LUXE experiment, that would uncover an uncharted territory of new feebly interacting particles. Regarding the precision front, we present several new theoretical results with actual experimental efforts, that offer new ways to search for light scalars and ultra-light dark matter (ULDM). We show that equivalence-principle tests and direct ULDM search are independent of each others, and apply this concept to new efforts using atomic and molecular ensemble- experiments to search for ULDM.
October 25, 2021: Prof. Urbasi Sinha, Raman Research Institute, Bangalore
Title: Photonic Quantum Science and Technologies
Abstract: Quantum mechanics is a cornerstone of modern physics. Just as the 19th century was called the Machine Age and the 20th century the Information Age, the 21st century promises to go down in history as the Quantum Age. Quantum Computing promises unprecedented speed in solving certain classes of problems while Quantum Cryptography promises unconditional security in communications. In this talk, I will discuss the world of single and entangled photons and also discuss ongoing work towards quantum computing, quantum information, and quantum cryptography in our Quantum Information and Computing lab at the Raman Research Institute, Bengaluru.
I will end with our broad vision for the future, which includes establishment of long-distance secure quantum communications in India and beyond involving satellite based, fibre based as well as integrated photonics based approaches towards the global quantum internet.
September 20, 2021: Prof. Suvankar Chakraverty, Institute of Nano Science and Technology, Mohali
Title: Planar hall effect, Anomalus Hall effect, Shubnikov-de Haas oscillations, Berry’s phase, Chiral Anomaly(?): KTaO3 based interfaces rock!
Abstract: In recent times, momentum dependent splitting of spin-bands in an electronic system, the “Rashba effect”, has gained a lot of interest because of its applications in future generation spintronic devices. The Rashba effect is important not only because it has tremendous potential for technical applications, but also it is a hunting ground of emergent physical properties owing to the linear dispersion relation at the crossing point of the two spin bands.In this work, we present observation of emergent phenomena arising at the interface of two insulating perovskite oxides due to Rashba spin-band splitting. In our first work, we improvise a novel conducting interface by juxtaposing KTaO3 (KTO) with another insulator, namely LaVO3 (LVO). This heterointerface exhibits strong spin-orbit coupling which is the highest among perovskite oxide heterostructures reported so far. The system is also found to show signature of topological chiral anomaly via observation planar Hall effect (PHE) and anomalous inplane magnetoresistance (AMR) similar to that observed for topological systems. In our next work, we show the realization of conducting interface between ferromagnetic EuO and non-magnetic KTO. This heterostructure is found to exhibit Shubnikov-de Haas oscillations. The observed oscillations suggest the presence of two Fermi surfaces. For both the Fermi surfaces, we have seen the presence of a non-trivial “Berry phase” suggesting that the surfaces enclose a “Dirac point” and the Berry phase originates from the inner and outer Fermi surfaces of the Rashba spin-split bands. Analysing the SdH, Hall and magnetoresistance data, we have drawn a possible band diagram near the Fermi surface for EuO-KTO heterointerface. Our observations suggest that perovskite oxides with strong spin orbit coupling and relativistic conduction electrons could be a hunting ground not only for spintronic materials but also for emergent physics.
August 23, 2021: Prof. Diptiman Sen, IISC Bangalore
Title: Periodically driven systems: some interesting phenomena [slide]
Abstract: We discuss some interesting phenomena which can arise when a quantum system is driven periodically in time. The first one is the generation of modes localized at the ends of a one-dimensional tight-binding model when it is subjected to a periodically varying electric field. We show that this can be understood by looking at the Floquet Hamiltonian: for certain values of the driving parameters, the Floquet Hamiltonian has a significant on-site potential at the end sites and this gives rise to end modes. The end modes can be detected through peaks in the differential conductance across the system. Next, we show that periodic driving with a large amplitude of some operator can give rise to freezing and an emergent conservation law in which the Floquet eigenstates are close to being eigenstates of that operator even when it does not commute with the time-independent part of the Hamiltonian. This can be understood using a Floquet perturbation theory. One also finds the opposite phenomenon of resonances where the Floquet eigenstates deviate strongly from being eigenstates of that operator. Finally, there are some systems which have constraints on the allowed states such that the Hamiltonian has special eigenstates called scar states which form a small subset of the states. When such a system is periodically driven starting with an initial state which has a large overlap with the scar states, correlation functions can show persistent oscillations and the system does not thermalize for a long time. The last two cases show that periodically driven systems may not thermalize for a long time due to various reasons.
Colloquia during Spring 2021
April 19, 2021: Prof. Giovanni Villadoro, ICTP Trieste
Title: Axion Dark Matter
Abstract: The axion is an hypothetical particle introduced more than 40 years ago to explain the absence of large time-reversal symmetry violations in strong nuclear interactions. If it exists it could easily explain one of the present mysteries of our Universe, the nature of dark matter. While the axion dynamics is governed by extremely simple equations it gives rise to a plethora of fascinating and complex effects, some of which have important consequences both for experimental searches and for the evolution of the Universe. After introducing the basic notions of axion physics and the status of experimental searches I will discuss recent developments in the understanding of the dynamics and evolution of axion dark matter.
March 8, 2021: Prof. Manas Kulkarni, ICTS Bangalore
Title: Family of long-ranged models: Collective behavior and dynamics
Abstract: I will discuss fascinating aspects of a family of classical long-ranged models in terms of their collective behavior and dynamics. The model consists of particles interacting with each other with power-law repulsive interaction for arbitrary powers inside a harmonic confinement. These family of models contain in them some special points such as one component plasma, the Dyson's log-gas, and the integrable Calogero-Moser model that have themselves been subject to intense investigation in physics and mathematics. I will present a collective description (field theory) of this family of many particle systems, and also present certain aspects of dynamics such as spacio-temporal spread of perturbations. These findings are expected to play an important role in our understanding of interacting many body physics. The results obtained are also, in principle, measurable in experiments for a wide class of power-law models given recent cutting-edge technologies.
February 22, 2021: Prof. Sunil Mukhi, IISER Pune
Title: Quantum Gravity in Three Space-Time Dimension
Abstract: Quantising gravity in 2+1 dimensions with anti-deSitter boundary conditions is a relatively tractable problem on which much progress has been made. I will describe some of the key developments including the discovery of black hole solutions, the role of boundary degrees of freedom and the duality to conformal field theory. I will then focus on the partition function of AdS3 gravity and what it teaches us about the AdS/CFT correspondence. The talk will involve aspects of general relativity as well as number theory, but will be phrased in relatively non-technical language that should be accessible to a fairly broad physics audience.
February 15, 2021: Prof. Deepak Dhar, IISER Pune
Title: Chase Escape Percolation
Abstract: Chase-escape percolation is a variation of the standard epidemic spread models. In this model, each site can be in one of three states: unoccupied, occupied by a single prey, or occupied by a single predator. Prey particles spread to neighboring empty sites at rate p, and predator particles spread only to neighboring sites occupied by prey particles at rate 1, killing the prey particle that existed at that site. It was found that the prey can survive with non-zero probability, if p>p_c with p_c<1. Using Monte Carlo simulations on the square lattice, we estimate the value of p_c = 0.49451 ± 0.00001, and the critical exponents are consistent with the undirected percolation universality class. We define a discrete-time parallel-update version of the model, which brings out the relation between chase-escape and undirected bond percolation. For all p < p_c in D-dimensions, the probability that the number of sites in the absorbing configuration has a stretched -exponential distribution in contrast to the exponential distribution in the standard percolation theory. We also study the problem starting from the line initial condition with predator particles on all lattice points of the line y=0 and prey particles on the line y=1. In this case, for p_c
Colloquia in 2020
September 7, 2020: Prof. Paolo Creminelli, ICTP Trieste
Title: Dark Energy and Gravitational Wave Observations
Abstract: The acceleration of the Universe is one of the key open problems in cosmology. Dynamical dark energy, at variance with the cosmological constant, behaves like a medium which spontaneously breaks Lorentz invariance. Gravitational waves propagating in this medium may have a speed different from the speed of light and can decay into fluctuations of the medium. These effects are constrained by the recent experimental observations of gravitational waves and this sets severe bounds on dark energy models.
August 31, 2020: Prof. Rahul Pandit, IISC Bangalore
Title: Particles and Fields in Partial-differential-equation Models for Fluid and Superfluid Turbulence
Abstract: Many natural and laboratory flows and industrial processes involve the advection of particles, bubbles, and droplets by turbulent flows in fluids or superfluids. The first part of this talk gives an overview of our studies of the statistical properties of particles and fields in such flows. These studies use extensive direct numerical simulations (DNSs) of partial-differential-equation (PDE) models for fluids and superfluids and some stochastic PDEs; we augment our DNSs with theoretical analyses of simple models. In particular, we investigate the statistical properties of tracer and heavy inertial particles in turbulent fluids and superfluids by using, respectively, the Navier-Stokes (NS) and Hall-Vinen-Bekharevich-Khalatnikov (HVBK) PDEs. Our studies elucidate (a) the first-passage-time problem in such turbulent flows (with potential applications to the spreading of viruses), (b) the statistical properties of particle trajectories, such as their persistence-time statistics, (c) the characterization of the irreversibility of superfluid turbulence, and (d) multiscaling in statistically homogeneous and isotropic superfluid turbulence. The second part of this talk begins with the Gross-Pitaevskii description of a superfluid; we add Newtoniain gravity by considering the Gross-Pitaevskii-Poisson equation (GPPE), which we then solve by extensive DNSs to elucidate the phase transition from a tenuous Bose gas, at high temperatures, to a condensed bosonic compact object at low temperatures; we introduce temperature by using a Fourier-truncated GPPE. Furthermore, we show that, if we add a crust potential and rotation to this GPPE, we get a minimal model for a pulsar-type condensed object that shows dynamical glitches which exhibit self-organized-critical behaviour of the type that has been observed in several experimental studies of pulsar glitches
This talk is based principally on the PhD thesis of my student, Akhilesh Kumar Verma; The first-passage studies have been carried out in collaboration with Akshay Bhatnagar and Dhrubaditya Mitra (NORDITA Stockholm); the work on the HVBK equation have been carried out jointly with Vishwanath Shukla (IIT, Kharagpur) and Abhik Basu (SINP, Kolkata); the investigations of the GPPE have been conducted along with Marc Brachet (ENS, Paris).
August 3, 2020 : Prof. Udo Seifert, Stuttgurt University
Title: From stochastic thermodynamics to the cost of precision
Abstract: Stochastic thermodynamics provides a universal framework for analyzing nano- and micro-sized non-equilibrium systems. Prominent examples are single molecules, molecular machines, colloidal particles in time-dependent laser traps and biochemical networks. Thermodynamic notions like work, heat and entropy can be identified on the level of individual fluctuating trajectories. They obey universal relations like the fluctuation theorem.
Thermodynamic inference as a general strategy uses consistency constraints derived from stochastic thermodynamics to infer otherwise hidden properties of non-equilibrium systems. As a paradigm for thermodynamic inference, the thermodynamic uncertainty relation provides a lower bound on the entropy-production through measurements of the dispersion of any current in the system. Likewise, it provides a model-free bound on the thermodynamic efficiency of molecular motors and quantifies the cost of temporal precision at finite temperature.
June 8, 2020 : Prof. Elichiro Komatsu, Max Planck Institute for Astrophysics, Garching
Title: Critical Tests of Theory of the Early Universe using the Cosmic Microwave Background
Abstract: The Cosmic Microwave Background (CMB) gives a photographic image of the Universe when it was still an „infant“. Its detailed measurements have given us a wealth of information such as the composition and history of the Universe. We are now using it to test our ideas about the origin of the Universe. I will review the physics of CMB and the key results from the recent experiments, and discuss future prospects on our quest to find the cosmic origins.
May 11, 2020: Dr. G J Sreejith, IISER Pune
Title: Fractional quantum Hall effect: Perspectives from the numerical studies
Abstract: Fractional quantum Hall effect occurs in clean 2D electronic systems, in high magnetic fields, and low temperatures. The relative obscurity of the context belies its true significance in modern condensed matter physics. At the core, the effect occurs due to formation of highly correlated quantum electronic liquids. Numerical studies built on a theory of emergent particles called composite fermions have been central to our understanding of FQH physics. Analysis of these states reveals rich universal physics and mathematical structures that have intrigued theorists and experimentalists alike. In this talk, I will introduce the relevant aspects of the composite fermion theory, and some current themes in experiments, and present some of the key numerical techniques and tools that reveal universal aspects - such as quasiparticle properties, entanglement spectra, shift, edge structures, root partitions etc - embodied within the correlated states.
March 09, 2020: Prof. Debajyoti Choudhury, Delhi University
Title: Why should we be interested in the top?
Abstract: The top quark was discovered a little over 25 years ago. However, very little was known about it until recently. I review the history and discuss how a precision study of its properties could possibly open a window to the unknown.
February 17, 2020: Prof. Arindam Ghosh, IISC Bangalore
Title: Bottom-up engineering of new electronic and structural effects with atomic layers
Abstract: Van der Waals heterostructures represent a new paradigm of material design, where two atomic or molecular planes of different chemical origin are brought together within the sub-nanometer van der Waals distance. When two atomic layers are placed so close their electronic states may hybridize, and the physical properties are modified by the rules of momentum conservation and structural commensurability. In this talk I shall present several new physical phenomena, in multiple domains ranging from electronic, opto-electronic to thermoelectric properties, that emerge as a result of van der Waals heterostructuring of two-dimensional (2D) materials. Apart from achieving high carrier mobility and ultra-low noise in electrical transport, encapsulating graphene by boron nitride leads to manifestation of edge transport and trigonal warping at low energies. Optoelectronic properties are strongly enhanced on graphene and transition metal dichalcogenide heterostructures, that can be extended to single photon detection. I shall also show new phenomena in thermoelectric transport in twisted bilayer graphene, where the Seebeck coefficient is strongly determined by the angular misorientation between the graphene layers in the van der Waals stack.
February 3, 2020: Prof. Ravindra Pratap Singh, PRL, Ahmedabad
Title: Optical vortices and entanglement duality
Abstract: Optical vortices also called light whirlpools are structures in light very similar to water whirlpools. Due to the helical wavefront, just as in water whirlpools, the light and consequently photons acquire orbital angular momentum that is different from spin angular momentum associated with circularly polarized light. We will discuss methods to generate these structures in the lab followed by their properties which are finding a variety of applications including the ones in classical and quantum communication. Entanglement being a great resource for quantum information processing, we will see how these structures can help in achieving different kinds of entanglements leading to duality of entanglement.
January 20, 2020: Prof. Surajit Dhara, University of Hyderabad
Title: Colloids in complex and dynamic environments
Abstract: Topological defects have been objects of intense studies in various disciplines starting from cosmology to condensed matter, optics and more recently in active matter. In liquid crystals (LCs) they are produced during the symmetry breaking phase transition. Such defects can be induced by dispersing foreign nano- and micro-particles in LCs. The embedded particles create elastic distortions in the LC medium inducing topological defects, and interact via long-range anisotropic elastic forces so generated. These forces obviously have no analogues in regular colloidal systems in an isotropic dispersive medium. An interesting manifestation of such novel forces is the ability of the colloidal system to self-assemble. In an experiment, such a process can be conveniently guided to create 2D and 3D colloidal crystals, with complex architectures. In this talk, I will present some of our recent studies on particle induced defects and transformations of such defects across the phase transitions in liquid crystals. We show that the elastic properties and the emergence of smectic layering have profound effects on these defects, in terms of the colloidal pair-interactions and their resulting two-dimensional assemblies. Finally, I will present some recent results on the electric field driven transport properties of Janus particles. In a striking departure from conventional electrophoresis, we show that metal-dielectric Janus particles can be piloted at will through a nematic liquid crystal film, in the plane perpendicular to an imposed AC electric field. We achieve complete command over particle trajectories by varying field amplitude and frequency, exploiting the sensitivity of electro-osmotic flow to the asymmetries of particle and defect structure. We propose a new method for measuring the induced electrostatic dipole moment of the Janus particles, through competition between elastic and electrostatic interactions. These findings open unexplored directions for the use of colloids and liquid crystals in controlled transport, assembly and dynamical topology.
January 6, 2020: Prof. David Tong, Cambridge University
Title: Quantum Field Theory: Dualities and What They're Good For
Abstract: Quantum field theory is hard. It's especially hard when the interactions between particles become strong. I'll describe recent progress in understanding this issue, and show how various ideas from condensed matter physics, high energy physics, and string theory have converged to give us new and surprising insights into the behaviour of quantum fields.