Condensed Matter, Statistical, AMO and Nonlinear Physics Events


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Cavity-coupled nitrogen-vacancy centers for quantum technological applications

In this talk, we discuss the optical properties of quantum emitters like NV centers and devise the methods that can enhance their utility in various applications ranging from tailored emission, quantum sensing, and bio-imaging. We have photonic cavities to control the spontaneous emission from the NV centers. The light transport properties of these structures are analyzed which shows the suppressed and enhanced photon density of states at the respective resonant modes. The modified emission properties of NV centers in frequency and time scale would be elaborated. The indigenous developments of a confocal microscope to map and isolate single NV centers and their emission characteristics would be discussed. I conclude the talk with a discussion on a few nanophotonics structure designs that can deterministically tune NV center emission properties.

Electrochemo-mechanics in Solid-state Batteries

Interfacial stability, viz., pore formation in the lithium metal anode (LMA) during discharge leading to current constriction dominated high impedance, current focusing induced solid-electrolyte (SE) fracture during charging, solid-electrolyte interphase (SEI) formation/behaviour, at the anode, is one of the major hurdles towards the development of solid-state batteries (SSBs). In addition, understanding cell polarization behaviour at high current density is critical to achieving the fast-charging battery and electric vehicle (EV) goal. In our discussion, we will address the origin of the LiǀSE interfacial instability. Wherein the role of charge-transfer and mass-transport kinetics at the non-equilibrium 1D and 2D defects in LMA will be discussed. We will then see, how the microstructural dependence of creep (i.e., strain rates) can be exploited to overcome this via appropriate thermomechanical processing. As a corollary, we will see that the control of LMA microstructure can reduce stack pressure requirements in SSBs. Next, we will establish a correlation between SE microstructure and stress mitigation to enhance the so-called the critical current density (CCD); the maximum current density prior to failure or cell shorting. And finally we will discuss the causes of failure in Li6PS5Cl, and the origin of non-linearity of LiǀLi6PS5Cl interfacial kinetics. References: 1. Singh, D.K.,* Fuchs, T., Krempaszky, C., Mogwitz, B., and Janek, J.* (2023). Non-Linear Kinetics of The Lithium Metal Anode on Li6PS5Cl at High Current Density: Dendrite Growth and the Role of Lithium Microstructure on Creep. Adv. Sci. n/a, 2302521. 2. Singh, D.K.,* Fuchs, T., Krempaszky, C., Schweitzer, P., Lerch, C., Richter, F.H., and Janek, J.* (2023). Origin of the lithium metal anode instability in solid-state batteries during discharge. Matter. 3. Singh, D.K.,* Fuchs, T., Krempaszky, C., Mogwitz, B., Burkhardt, S., Richter, F.H., and Janek, J.* (2022). Overcoming Anode Instability in Solid-State Batteries through Control of the Lithium Metal Microstructure. Adv. Funct. Mater. n/a, 2211067. 4. Singh, D.K.,* Henss, A., Mogwitz, B., Gautam, A., Horn, J., Krauskopf, T., Burkhardt, S., Sann, J., Richter, F.H., and Janek, J. (2022). Li6PS5Cl microstructure and influence on dendrite growth in solid-state batteries with lithium metal anode. Cell Reports Phys. Sci., 101043. 5. Krauskopf, T., Mogwitz, B., Hartmann, H., Singh, D.K., Zeier, W.G., and Janek, J. (2020). The Fast Charge Transfer Kinetics of the Lithium Metal Anode on the Garnet-Type Solid Electrolyte Li6.25Al0.25La3Zr2O12. Adv. Energy Mater. 2000945.

Stacking and Twisting van der Waals Materials

Transition metal dichalcogenides (TMDs) form an interesting class of layered materials that holds promise for future 2D nanoelectronics. One of the key attributes of layered materials is the polytypism, arising from different stacking sequences of monolayers with same structure. The electronic behavior of the various polytypes are dependent on the interlayer interactions and the stacking sequence. These materials show correlated structural and electronic phases such that triggering structural transitions to other metastable phases also result in tunable metal-insulator/semiconductor electronic phase transitions. In this talk, I will discuss various strategies to tailor such structural phase transitions in 2D materials. Alloying/doping modulated structural instabilities during growth phase to realize composition tunable structural phases is one way of achieving this phase transition. Another potential pathway to induce structural phase transitions is through photo-induced non-radiative energy dissipation. I will describe the strong lattice response to optical excitations which has important implications for light-induced structural-phase transition in transition metal dichalcogenides. Twisted 2D bilayers of van der Waals materials, a new class of quantum materials, offer pioneering advances in field of nanoelectronics and photonics. As these layered materials can have various preferential stacking configurations with varying electronic behaviour, it is important to have a characterization technique that can unambiguously probe the stacking order and interlayer interactions in the 2D materials and twisted 2D bilayers. In this talk, I will discuss how to measure the variations in the interlayer coupling of bilayer 2D materials (graphene, TMDs) stacked at different twist angles using spectroscopic techniques of Raman and photoluminescence. Also I will demonstrate Raman spectroscopy as a quick imaging tool for clear distinction of stacking sequence in bilayer TMDs and the capabilities of Raman mapping technique to study various kirigami structures, bilayer nucleation centres and bilayer grain boundaries commonly observed during chemical vapor deposition based growth of TMDs. These spectroscopic signatures will be useful for studying the electronic correlations in twisted 2D homo/heterolayers as well as in differentiating the stacking order in CVD-grown ultrathin TMD films. Finally I will also address our recent efforts in the domain of CVD-based growth of novel 2D materials and 2D heterostructures.

SQUID-on-tip imaging in magic-angle graphene

The emergence of flat bands in twisted bilayer graphene leads to strongly correlated and superconducting phases. Charge carriers come in eight flavors described by a combination of their spin, valley, and sublattice polarizations. When the inversion and time reversal symmetries are broken by the substrate or by strong interactions, the degeneracy of the flavors can be lifted and their corresponding bands can be filled sequentially. Due to their non-trivial band topology and Berry curvature, each of the bands is classified by a topological Chern number, leading to the quantum anomalous Hall and Chern insulator states.
A nanoscale SQUID fabricated on the apex of a quartz pipette (a ‘SQUID-on-tip’) can be used as an ultra-sensitive scanning probe to measure local magnetic fields. Utilizing such a scanning SQUID-on-tip, I will describe two experiments for imaging equilibrium currents and equilibrium orbital magnetization in magic angle graphene.
Using a scanning SQUID-on-tip, we image the equilibrium currents in the quantum Hall regime and obtain tomographic imaging of Landau levels and map their local evolution with carrier density. This renders a nanoscale high precision map of the local twist-angle and reveals substantial twist-angle gradients and a network of jumps [1]. We show that the twist-angle gradients generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which drastically alter the quantum Hall state by forming edge channels in the bulk of the samples. The correlated states are found to be particularly fragile with respect to twist-angle disorder. We establish the twist-angle disorder as a fundamentally new kind of disorder, which alters the local band structure and may significantly affect the correlated and superconducting states.
In a separate experiment, we probe a twisted bilayer graphene device that exhibits the anomalous Hall effect. Using a SQUID-on-tip, we image the nanoscale Berry-curvature-induced equilibrium orbital magnetism [2] and show that the Chern number C, rather than being a global topological invariant, becomes position dependent, governing the polarity of the orbital magnetism. We detect the two constituent components of the orbital magnetization associated with the drift and the self-rotation of the electronic wave packets. At filling factor ν=1, we observe local zero-field valley-polarized Chern insulators forming a mosaic of microscopic patches with C=-1, 0, or 1. Upon further filling, we find a first-order phase transition due to recondensation of electrons from valley K to K', which leads to irreversible flips of the local Chern number and the magnetization, and the formation of valley domain walls giving rise to a hysteretic anomalous Hall resistance. The findings shed new light on the structure and dynamics of topological phases and call for exploration of the controllable formation of flavor domain walls and their utilization in twistronic devices.

[1] A. Uri, S. Grover, Y. Cao, J. A. Crosse, K. Bagani, D. Rodan-Legrain, Y. Myasoedov, K. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. Jarillo-Herrero, and E. Zeldov, Nature 581, 47 (2020)
[2] S. Grover, M. Bocarsly, A. Uri, P. Stepanov, G. Di Battista, I. Roy, J. Xiao, A. Y. Meltzer, Y. Myasoedov, K. Pareek, K. Watanabe, T. Taniguchi, B. Yan, A. Stern, E. Berg, D. K. Efetov and E. Zeldov, Nat. Phys. 18 885 (2022)

Colloquium: Kink Solutions With Power Law Tail

Most of the kink solutions that have been discussed in the last 50 years have exponential tail. However, in the last few years, kink solutions with power law tail have received considerable attention. I will compare and contrast the properties of these two different kind of kink solutions. In particular, while the stability equation for kinks with exponential tail (which is a Schrodinger-like equation) always has a finite gap between the zero mode and the beginning of the continuum, for kinks with power law tail, there is no gap between the zero mode and the beginning of the continuum. Further, while kink-kink and kink-antikink forces are exponentially small and their magnitude is same, in the case of kinks with exponential tail, the force has power law fall off and the kink-antikink forces is much weaker compared to the kink-kink force. I will also enumerate various models for two kinks with various possible combinations of power law and exponential tails.

Colloquium: Monomer Percolation

Recent work has identified an interesting connection between topologically protected ground-state degeneracies in a class of random quantum systems, and unmatched sites in the classic combinatorial problem of maximum matchings of the corresponding random lattices. This connection suggests a mechanism for the formation of localized magnetic moments in spin-liquid insulators. A computational study of such maximum matching problems has also led to the identification of interesting variants of the classical percolation transition. In this talk, I will introduce the main ideas involved from scratch and provide a synopsis of these recent developments.

Quantum Hamiltonian Complexity in thermal equilibrium

Interacting quantum many-body systems can exhibit a rich variety of interesting physical phenomena. But studying the properties of many-body systems can be computationally difficult. Tools from theoretical computer science enable us to characterize the computational complexity of physical problems. The application of these tools to quantum many-body systems is the domain of Hamiltonian complexity. A seminal result due to Kitaev showed that approximating the ground-state energy of general quantum spin Hamiltonians within a given error is hard even for quantum computers. Since then, a long line of research has established similar hardness results for various families of Hamiltonians and also for estimating other ground-state properties. However, surprisingly little is known about the problem of approximating thermal equilibrium properties of quantum many-body systems. In this talk, I will present results regarding the computational complexity of approximately computing the partition function of quantum many-body systems. Firstly, I will show that the problem of approximating quantum partition functions is formally equivalent in computational difficulty to three other problems. These include the problem of estimating the density of states, estimating observables in the thermal state and lastly a quantum analog of an approximate counting task well-known in classical computer science. This gives evidence that the problem of approximating the quantum partition function defines a new computational complexity class.
Secondly, I will present a new efficient approximation algorithm for the free energy for a family of "dense" Hamiltonians e.g., systems of spins with all-to-all interactions. This algorithm is based on the variational characterization of the free energy: the main idea is to solve a "relaxed" convex optimization program over sets of reduced density matrices and then show that this in fact gives a good approximation to the free energy.

Nat. Phys. 18 (11) 1367–1370 (2022)

Friquant Seminar: Learning Distributions with Quantum-enhanced Generative Models

The development of novel algorithms that process information in ways that are classically intractable is one of the prime motivations in quantum information research. Machine learning is a rapidly advancing field with broad applications in the natural sciences where quantum-inspired algorithms may offer significant speedup or accuracy advantage. To date, several quantum algorithms for discriminative machine learning have been formulated, and lately, quantum-enhanced generative machine learning models have gained tremendous attention. However, the higher levels of noise, and lack of scalability of current quantum devices limit the depth and complexity of these algorithms. In this talk, Mr. Anantha will introduce the framework of generative learning, variational quantum algorithms and then proceed to describe a novel hybrid quantum-classical algorithm suitable for noisy-intermediate quantum devices. Mr. Rao will show how the algorithm outperforms conventional generative models and exhibits an advantage for certain datasets. Finally, Mr. Rao will discuss crucial directions for the improvement of the current method that will be key to developing more challenging quantum-inspired algorithms for learning complex distributions.

Non-collinear spin textures: a route towards single layer-based spintronics memory devices

One of the most exciting quests in spintronics is the discovery of more efficient currentinduced magnetization switching for setting distinct magnetic states. Recently, non-collinear spin textures with antiferromagnetic ground state have received much attention because they display magneto-transport properties reminiscent of ferromagnets, even though they may have zero magnetization1-2. An important goal in antiferromagnetic spintronics, to realize the device potential of non-collinear antiferromagnets, is the setting and switching of the magnetic states.
In the first part of my talk, I will discuss the novel Seeded Spin-Orbit Torque, by which spin current from the proximal heavy metal can set the magnetic states of even thick layers of a non-collinear kagome antiferromagnet Mn3Sn where kagome planes are perpendicular to the film plane3. This mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface close to the heavy metal which seeds the resulting spin texture of the entire thickness. In the second part, using a proximal ferromagnetic layer permalloy, I will demonstrate that spin currents with both in-plane and out-of-plane polarizations are generated from non-collinear antiferromagnet Mn3Sn where kagome planes are parallel to the film plane4. Temperature and thickness dependence studies reveal that the in-plane polarized spin current is related to the antiferromagnetic order parameter whereas the out-of-plane polarized spin current originates from Mn3Sn/Permalloy interface.
The coexistence of anomalous Hall effect and spin Hall effect in a single non-collinear antiferromagnet makes it possible to think that spin currents from non-collinear antiferromagnet can switch their own magnetic configurations which will lead to a single layer based magnetic memory device for spintronics applications.
[1] S. Nakatsuji, et al., Nature 527, 212-215 (2015).
[2] V. Baltz et al., Rev. Mod. Phys. 90, 015005 (2018).
[3] B. Pal, B. K. Hazra et al., Science Advances 8, eabo5930 (2022).
[4] B. K. Hazra, B. Pal et al., arXiv:2211.12398 (2022).

Cellular decision-making at various levels — Insights from Mathematical Modeling

Biological cells are “living machines” capable of taking exceedingly complex decisions. From unicellular prokaryotic bacteria to a collection of mammalian cells, everyone needs to undertake various decisions — moving up the chemical gradient towards a favorable environment, healing a wound, differentiating to a different cell type, or metastasizing (for a cancer cell). Shedding light on the underlying detailed mechanisms of such processes through the lens of mathematical modeling is truly fascinating! In this talk, first I will give an overview of a systematic and rigorous modeling framework (based on multivariate Fokker-Planck equation, Gillespie simulation) we developed to explore how the intrinsic fluctuations in the chemotaxis network of an E. coli affect the motility of the organism. Collective cell migration, on the other hand, is of critical importance in nearly all stages of life, which involves numerous cells acting in a coordinated way. I shall talk about our explorations of the connections between cell-level adhesion and cluster-level dynamics of two-dimensional cell clusters via a realistic framework based on the cellular Potts model. We found that cells have an optimal adhesion strength that maximizes cluster migration speed. The optimum negotiates a tradeoff between maintaining intercellular contact and cluster fluidity. Spatiotemporal pattern formation plays a key role in various biological phenomena including Epithelial-Mesenchymal Transition (EMT) (during development and cancer initiation). Though there exists a plethora of studies on reaction-diffusion systems enabling pattern formation, the bio-molecular and mechanistic underpinnings of these processes have not been modeled in detail. I shall talk about the emergence of spatiotemporal dynamics due to transcriptional (cooperative) gene regulation, host-circuit interaction, and protein dimerization. We investigated the tissue-level patterns formed due to the coupling of inherent multistable and oscillatory behavior of emergent 2- and 3-node gene regulatory network motifs (GRNs) coupled with their molecular diffusion, with varying diffusion coefficients, in one- and two-dimensional space. These analyses offer valuable insights into the design principles of pattern formation facilitated by GRNs. The last part of the talk is to identify the overlaps, applications, extensions, and integrations of the above-mentioned modeling frameworks (analytical and computational), to unveil aspects of the underlying biophysical principles of multicellular invasion of a phenotypically heterogeneous tumor during cancer metastasis. Insights on these aspects will be a leap forward toward making predictions for experimental data on EMT, and reveal potential design principles for more effective therapeutic strategies that can prevent disease propagation.

Tensor network techniques for simulating real quantum materials

Tensor networks are quantum inspired techniques for studying quantum many-body systems. While these tools have become the natural choice for studying strongly interacting systems in 1d, its application to higher dimensions, specially for realistic systems remain challenging. In this talk, I will present a two dimensional tensor network algorithm to study finite temperature properties of frustrated quantum systems and real materials. We will use it to study the thermodynamic properties of the highly challenging spin-1/2 Heisenberg anti ferromagnet on the Kagome lattice and obtain competitive benchmarking results. We then use our method to study the finite temperature properties of the actual quantum material and a spin liquid candidate Ca_{10}Cr_7O_{28} and compare it with experimental data. Our technique incorporates both thermal fluctuations as well as quantum correlations in the study of this real material. Our work provides new insights towards settling the existing controversy between the experimental data and previous theoretical works on the magnetization process of this material.

Colloquium: 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.

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.

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.

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).

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.