Condensed Matter, Statistical, AMO and Nonlinear Physics Events


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Universal Stress Correlations in Crystalline and Amorphous Packings

We present a universal characterization of stress correlations in athermal systems, across crystalline as well as amorphous packings. We present numerical results for static configurations of particles interacting through harmonic as well as Lennard-Jones potentials, for a variety of preparation protocols and ranges of microscopic disorder. We show that the properties of the stress correlations at large lengthscales are surprisingly universal across all situations, independent of structural correlations, or the correlations in orientational order. In the near-crystalline limit, we present exact results for the stress correlations for both models, which work surprisingly well at large lengthscales, even in the amorphous phase. Finally, we study the differences in stress fluctuations across the amorphization transition, where stress correlations reveal the loss of periodicity with increasing disorder.

Past events

2023 | 2022 | 2020 | 2019


Physics of Strongly Correlated Electron Systems Conference


Active Brownian motion in a harmonic well: Studying statistical physics with Janus colloids and optical trap

Active Brownian dynamics are inherently asymmetric because of the persistence in motion up to a characteristic timescale, the persistent time (τR), governed by the rotational diffusion. Thus, active Brownian particles undergo exotic dynamical variations when they interact with complex environment possessing inherent asymmetry, and timescale such as relaxation time. Many of these dynamical behaviors and their governing parameters remain to be properly investigated and understood. Since naturally occurring micro swimmers and the application-oriented-synthesized-self-propelled particles often need to pass through complex fluids and confinements, it is of paramount importance to understand how their dynamics adapt to these environments.
In this talk, I will discuss our recent studies on the dynamics of self-propelled active Brownian particles confined to a harmonic potential, thus introducing them to a radially symmetric restoring force field with a characteristic equilibration time τ. In experiments, we have investigated dynamical behaviors of self-thermophoretic, as well as self-diffusiophoretic active motions of half-platinum-coated Janus colloids in an optical trap. We have also performed Brownian dynamics simulations and analytical calculations to have a better understanding of this system. Emphasizing on the technical details of our experiments, I will discuss a novel dynamical transition that the system undergoes. A competition between the characteristic length scale of the active Brownian motion and the harmonic well fully governs this transition. Time permitting, I will show that we can even characterize this as a weak to strong ergodicity breaking transition.

Physics of Strongly Correlated Electron Systems Conference


Physics of Strongly Correlated Electron Systems Conference


Topological transports in Weyl semi-metals

In recent years, the electronic and thermal transport properties of various topological systems, specially Weyl semimetal (WSMs) have attracted tremendous attention due to their Fermi arc surface states. For example, in the absence (presence) of the magnetic field, the non-trivial Berry curvatures (chiral anomaly and chiral magnetic effect) result in anomalous Hall conductivity (negative magneto-resistance). We show the universal scaling of magneto-Hall conductivity and thermo-electric conductivity, respectively, assisted by the chiral anomaly and chiral magnetic effect, for multi-WSMs (mWSMs) in planar Hall set up [1]. We investigate the effect of tilt and intriguing Fermi surface properties of mWSMs in the ballistic limit through the Magnus Hall conductivity [2,3]. We further extend our analysis to the quantum limit with Landau levels where we study the three-dimensional quantum Hall effect [4]. Apart from the above first-order transport signatures, we show that the topological charge of the activated Weyl nodes can be directly captured by the quantized circular photogalvanic effect which is a second-order response [5,6]. We then connect our predictions, based on models, with the recent theoretical as well as experimental findings obtained from real materials. Finally, we summarize the possible applicability of our framework to analyze various response properties in more realistic and experimentally viable platforms.
[1] Magneto-transport phenomena of type-I multi-Weyl semimetals in co-planar setups, T. Nag and S. Nandy, J. Phys.: Condens. Matter 33 (2021) 075504.
[2] Topological Magnus responses in two and three-dimensional systems, S. K. Das, T. Nag, and S. Nandy, Phys. Rev. B 104, 115420 (2021).
[3] Distinct signatures of particle-hole symmetry breaking in transport coefficients for generic multi-Weyl semimetals, T. Nag, and D. M. Kennes, Phys. Rev. B 105, 214307 (2022)
[4] Understanding the three-dimensional quantum Hall effect in generic multi-Weyl semimetals, F. Xiong, C. Honerkamp, D. M. Kennes, and T. Nag, Phys. Rev. B 106, 045424 (2022).
[5] Electronic structure and unconventional non-linear response in double Weyl semimetal SrSi_2, B. Sadhukhan and T. Nag, Phys. Rev. B 104, 245122 (2021).
[6] Role of time-reversal symmetry and tilting in circular photogalvanic responses, B. Sadhukhan and T. Nag, Phys. Rev. B 103, 144308 (2021).

Rod like proteins on membranes: patterns, shapes, and flows

Soft-matter like polymers, membranes, liquid-crystals provide the basic building blocks for biological systems, for example, a cell. Association of proteins to biological membranes is well known. An ensemble of rod like proteins attached to a membrane can deform it locally and thereby change the shape of a closed membrane vesicle. The rods themselves often create interesting patterns on membranes and this can be understood using physics of nematic liquid crystals. Often these proteins can move and generate flows beneath the membrane. Such flows are responsible for the change of cell-shape during cell division. I will focus on two examples, a) spontaneous formation of tubes from vesicles, and b) non-equilibrum, active flow of material in the growing intra-cellular partition during cell division.

1) Tubulation pattern of membrane vesicles coated with biofilaments. G. Kumar, N. Ramakrishnan, and A. Sain, Phys. Rev. E 99, 022414 (2019).
2) Dynamics and Stability of the Contractile Actomyosin Ring in the Cell. M.Chatterjee, A.Chatterjee, A.Nandi, and A.Sain Phys. Rev. Lett. 128, 068102 (2022)

Active liquid droplets


Colloquium: Quantum Technologies with ultra-cold Rydberg atoms, atomic spin ensembles and Quantum mixtures

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.

Quantum for Space

Space industry and space exploration are incredibly complex and require precise, dynamic decision-making in order to be successful. Quantum computing has the potential to make a significant impact in the field of space operations from maximizing the utilization of Earth Observation Satellites (EOS) through quantum-based scheduling algorithms, to optimizing space debris removal missions, and even fine-tuning satellite placement for optimal coverage and functionality.
In addition, the use of quantum computing in space exploration can lead to more cost-effective and efficient methods of carrying out space-related tasks, while also enabling new discoveries and opportunities that were previously not possible. The seminar will cover quantum use cases for space.

1/f Spectrum of the Stress Dynamics with the Bak-Tang-Wiesenfeld Sandpile

With the original Bak--Tang--Wisenefeld (BTW) sandpile we uncover the 1/f noise in the mechanism maintaining self-organized criticality (SOC) - the question raised together with the concept of SOC. We posit that the dynamics of stress in the BTW sandpile follows quasi-cycles of graduate stress accumulation that end up with an abrupt stress-release and the drop of the system to subcritical state. In thermodynamic limit, the intra-cycle dynamics exhibits the 1/f spectrum that extends infinitely and corresponds to the stress-release within the critical state. This is the joint work with Prof. Shnirman (Institute of Earthquake Prediction Theory and Mathematical Geophysics).

Fluctuation correlations in the periodically driven many-body systems

A many-body system driven externally will absorb heat and effectively heat up to an infinite temperature state. In some systems, the heating is preceded by an effective Prethermal state, where the system can be described by an effective GGE at an effective temperature. This heating dynamics has been studied in quantum systems but limited results are available in the classical counterpart. In this talk, I will discuss the dynamics and characterisation of heating in interacting arrays of periodically kicked rotors which are paradigmatic models of many-body chaos theory. Specifically, we go beyond average temperature to study spatio-temporal temperature fluctuations to characterise the different phases. We describe the pre-thermal fluctuations by an effective hydrodynamic theory, while the fluctuations in heating regime using independent rotor approximation. Characterisation through fluctuation allows us to identify intermediate cross-over phase with distinct properties.

Composite fermions and their Fermi surfaces

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.

Fluctuations, precisions, and strong coupling effects in quantum Otto cycle

In the age of miniaturization of technologies, one of the most important applications of quantum thermodynamics is designing efficient thermal machines. In this regards, it is important to analyze the fluctuations of relevant thermodynamic quantities like efficiency and power as they determine the quality of the output. On the other hand, conventional approaches to study quantum thermal machines are typically restricted to weak coupling (and Markovian) scenario which is actually an idealization. New experimental techniques and recent theoretical progresses have now opened avenues to consider the performance of thermal machines beyond weak coupling scenario including non-Markovian effects, a crucial step towards understanding the practical setups. In this talk, precision and fluctuations in periodically modulated continuous quantum thermal machines will be discussed using Floquet techniques and counting field statistics. We present a generic theory for such machines, followed by specific examples of continuous machines driven by sinusoidal and circular modulations. We demonstrate the validity of thermodynamic uncertainty relations and present the bounds on the fluctuations in efficiency of such machines. In the second part of the talk, we present a model an Otto cycle connected to a single qubit bath and study its thermodynamic properties. Our analysis goes beyond the conventional weak coupling scenario and illustrates the effects of finite baths, including non-Markovianity. We find closed form expressions for efficiency (coefficient of performance), power (cooling power) for the heat engine regime (refrigerator regime).

FriQuant Seminar: Quantum Simulations and precisions measurements with ultracold atoms

​The invention of techniques to cool atoms down to nano-Kelvin temperatures has opened up enormous possibilities to manipulate and use them for a variety of scientific studies and for technological applications. A plethora of physics can be studied using the collections of atoms in a thermal as well as quantum state. The seminar covers, the ongoing work at IISER Pune and I-HUB QTF in using ultracold thermal samples of 87Rb atoms in 1-D optical lattices for simulating physics of classically chaotic quantum systems and as a test bed to understand the physics of Anderson localization. By further cooling, the collection of these atoms goes through a Bose-Einstein condensation (BEC) phase transition — a quantum state of matter. Atoms in the BEC state act like a giant coherent matter wave which can be used as an equivalent of light waves in performing interferometry — The Atom Interferometry (AI). Using AI, a gravimeter is demonstrated to measure local gravitational acceleration (‘g’) with a very high degree of precision. This sensor has huge practical applications, ranging from underground resource mapping to detection of tunnels.

Capillarity and wetting in particulate suspensions

The soft materials community has enormous knowledge about the structure and rheology of suspensions comprising particles dispersed in a liquid. Much less is known about suspensions composed of particles suspended in two immiscible fluids – systems in which capillary forces between particles play a major role. Such two-fluid particle suspensions can show a rich diversity of microstructures: particle networks aggregated by fluid menisci, compact capillary aggregates, Pickering emulsions, and bicontinuous morphologies. These microstructures result from a coupling between interfacial tension between the fluids, particle wettability, and viscous forces during mixing. Mixtures across a wide range of particle sizes and fluids show similar behavior suggesting that such coupling has some universal features.
Our experimental studies have explored this coupling in particles dispersed in two immiscible polymeric liquids. This talk will catalog the transitions between various microstructures, and show how a non-equilibrium state diagram can be constructed for ternary mixtures of particles and two fluids. Clear understanding of such a non-equilibrium state diagram can guide new approaches for materials processing. We will provide examples of bicontinuous materials or conductive polymer composites, whose development was guided by these insights.