Condensed Matter, Statistical, AMO and Nonlinear Physics Events


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Inducing organization in intrinsically disordered polymeric systems: Lessons from bacterial chromosome organization

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.
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









Past events

2022 | 2020 | 2019


Role of thermodynamic cost in information processing by cell signaling systems

Cell signaling systems are involved in sensing the changes in environment and enable the cell to elicit appropriate phenotypic responses. In most cases, the environmental ques ultimately lead to changes in gene expression triggered by a set of transcription factors. These transcription factors activated by the signaling pathway typically diffuse inside the nucleus to activate transcription of the required genes. However, the inherent stochasticity of the biochemical pathways associated with signaling processes severely limit the accuracy of estimating the environmental input by the transcription factors. To improve the accuracy, cells utilize multiple strategies namely reduction of noise, amplify the output or measurement of the input at several time points and multiple outputs. These strategies often impose extra thermodynamic cost through enhanced entropy production. Thermodynamic uncertainty relation (TUR) provides a general trade-off between fluctuation and entropy production rate. We explore this trade-off in various cell signaling systems using mathematical models coupled with single cell dynamics of transcription factors. Experimental observations coupled with TUR based theoretical models demonstrate the role of fluctuation and entropy production on cellular information processing.

Introduction to the abelian sandpile model of self-organized criticality

These lectures will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Quantum Clocks for the Fundamental Science

Quantum clocks have reached an unprecedented level of accuracy (few parts in 1020), which are realized either by probing forbidden optical transitions of neutral atoms localized in an optical lattice or using a single atomic ion trapped in an electrodynamic trap. The tick-rate of such clocks are influenced by any unimaginably tiny perturbations of the energy states associated with their clock transitions, which are even caused by variations of the fundamental constants, breaking of fundamental symmetries, gravitational redshifts, propagation of the gravitational waves, cosmic microwave background and so on. Thus, intercomparison of the geographically distributed ultra-stable, highly accurate quantum clocks is expected to act like “distributed & networked quantum sensors” to test the foundations of science and the general theory of relativity. The seminar shall cover some of these scientific motivations to build a Ytterbium-ion based optical clock at the Precision & Quantum Measurement laboratory at IUCAA, Pune.

Introduction to the abelian sandpile model of self-organized criticality

These lectures will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Levitated Optomechanics with Optical Tweezers

This talk will address our recent theoretical work on optical tweezers in vacuum and its experimental confirmation in the group of our collaborator N. Vamivakas. Specifically, the realization of a mechanical laser using the center-of-mass oscillations of an optically levitated nanoparticle will be described. The discussion will include threshold behavior, coherence, subthermal number squeezing, time dynamics, phase space characterization, higher order correlations and the role of stimulated emission in our single mode phonon laser. Based on this discussion, I will conclude that our device provides a pathway for engineering a coherent source of phonons on the mesoscale that can be applied to both fundamental problems in quantum mechanics as well as tasks of precision metrology.

Pre-thermalization and thermalization of isolated quantum systems

An isolated quantum many-body system is expected to evolve under its own unitary dynamics to attain thermal equilibrium predicted by statistical mechanics. Cold atom experiments have revealed an interesting scenario where some systems far from equilibrium relax quickly to an intermediate state (called the pre-thermal state) before slowly relaxing into their true thermal equilibrium. While prethermalization is conventionally discussed in the context of weakly perturbed integrable models, in this talk, I will discuss a very general yet conceptually simple theoretical framework to understand prethermalization in generic -possibly nonintegrable- systems. We show that prethermalization is ubiquitous when a perturbation breaks at least one local conservation law of the unperturbed system. I will discuss the prethermalization scenario in isolated and periodically driven quantum systems using a novel numerical technique called numerical linked cluster expansion. [1] K Mallayya, M Rigol, W De Roeck - PRX 2019 [2] K Mallayya, M Rigol - PRL 2019 [3] K Mallayya, M Rigol - PRB, 2021

Introduction to the abelian sandpile model of self-organized criticality

These lectures will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Controlling Nanoparticle Ordering by Directional Polymer Crystallization

Adding nanoparticles (NPs) to a polymer is a well-established means to improve the organic materials’ thermomechanical properties. To date much interest has been focused on amorphous polymer hosts – instead, here, we consider semicrystalline polymers which constitue more that 65% of all polymers sold. We first revisit our previous finding that nanoparticles (NPs), which are initially miscible with a polymer melt, can be ordered by polymer crystallization under ppropriate conditions. The key variable here is a Peclet number (Pe) that compares the time scale of NP motion to the rate of polymer crystallization. For large Pe the NPs are “frozen” in place during polymer crystallization. However, for Pe values less than 1 the NPs are moved by the growing crystals and selectively placed in the interlamellar, interfibrillar and interspherulitic regions. These results are in line with the Keith and Padden’s findings, expanded on by Russell et al., on the spatial dispersion of amorphous polymer defects in a semicrystalline morphology. The resulting samples have Young’s moduli that are increased by a factor of ~2 relative to ones where the NPs are randomly ordered. We believe that additional synergies in properties can be obtained if we could move from spherulitic polymer morphologies (which are locally anisotropic but spatially isotropic) to more directionally crystallized samples with a preferential orientation. To this end we employ the zone annealing methodology, a directional crystallization protocol popular in metallurgy and first applied to semicrystalline polymers by Lovinger and Gryte nearly 50 years ago. Using this method, we show that we can order and orient the NP assemblies into desired directions. In addition to these novel experimental findings, we propose a new theoretical framework that allows us to model the directional crystallization of polymers and the key variables that control them.

The Spectrum of Wind Power Fluctuations

Wind is a variable energy source whose fluctuations threaten electrical grid stability and complicate dynamical load balancing. The power generated by a wind turbine fluctuates due to the variable wind speed that blows past the turbine. Indeed, the spectrum of wind power fluctuations is widely believed to reflect the Kolmogorov spectrum of atmospheric turbulence; both vary with frequency $f$ as $f^{-5/3}$. This variability decreases when aggregate power fluctuations from geographically distributed wind plants are averaged at the grid {\it via} a mechanism known as {\it geographic smoothing}. Neither the $f^{-5/3}$ wind power fluctuation spectrum nor the mechanism of geographic smoothing is understood. In this talk, I will chart out the non-equilibrium the character of wind power fluctuations, and explain the wind power fluctuation spectrum from the turbine through the grid scales. The $f^{-5/3}$ wind power fluctuation spectrum results from the largest length scales of atmospheric turbulence of order 200 km influencing the small scales where individual turbines operate. This long-range influence correlates outputs from geographically distributed wind plants over a range of frequencies that decreases with increasing inter-farm distance. Consequently, aggregate grid-scale power fluctuations remain correlated and are smoothed until they reach a limiting $f^{-7/3}$ spectrum, which is confirmed with field data. I will discuss the engineering and policy implications of these results and chart out the future directions I intend to pursue in the broad area of energy research.

Hybrid Quantum Registers: Efficient Control and Protection against Environmental Noise

Nuclear and electronic spins are attractive objects not only for spectroscopic studies, but also for emergent technologies like quantum information processing and sensing. The combination of different types of qubits allows one to take advantage of the favourable properties of each type. Examples include the nitrogen-vacancy (NV) center in diamond, where the electron spin interacts with 14N and 13C nuclear spins, or Si vacancies in SiC. One of the challenges for controlling hybrid systems is that the speed at which different spin species react to external control operations can differ by many orders of magnitude. It is therefore essential to develop techniques that result in sufficiently fast controls for the nuclear spins, whose gyromagnetic ratio is 3-4 orders of magnitude smaller than that of the electron spin. For this purpose, we have developed indirect control techniques that are based on sequences of microwave pulses applied to the electron spin in such a way that the motion of the electron spin generates a time dependent hyperfine interaction acting on the nuclear spin. Suitable sequences of microwave pulses can thus drive arbitrary evolutions of the nuclear spin, at rates that are far higher than those that can be achieved by radio-frequency pulses. In addition, these sequences can be optimised to protect the electron spin coherence against degradation by environmental noise.

Levitodynamics: Levitated nanoparticles in the study of stochastic thermodynamics, aerosols, materials science

Micro and nanoparticles can be individually levitated by different trapping mechanisms, among which optical tweezers and Paul traps are the most extended approaches. Even if they do not diffuse away due to the presence of the trap, trapped particles are still subjected to Brownian motion due to collisions with gas molecules. The analysis of the dynamics of trapped particles provides very useful information about the properties of the particles and the environment, allowing one to perform a variety of experiments in which a particle is used as an ultrasensitive probe. In this talk, I will present recent works that demonstrate the versatility and applicability of the technique in a large variety of fields, going from non-equilibrium dynamics to materials science to aerosol physics.

Magnetic resonance: spins as quantum sensors

When spin was established as a fundamental property of elementary particles, the goal of excitation and detection of transitions between different spin states was considered to be of purely academic interest. Successful experiments became possible thanks to advances of microwave and radio-frequency technology. As the instrumental capabilities improved, it became apparent that spins are an extremely versatile window into our environment, since they can report on a vast variety of chemical and physical parameters like molecular composition, temperature or motional processes. This led to a vast array of applications, first in physics, then chemistry, biology, materials science and medicine. While some fundamental physical principles limit the sensitivity of the technique, these limitations are constantly being pushed, sometimes to the limit of experiments with individual spins. Current developments in quantum information and quantum sensing continue to improve the sensitivity, spectral resolution and information content for an increasingly diverse range of applications.

External pressure tuning of strongly correlated materials

Exotic phenomena and unconventional phases often occur in regions of competing energy scales in correlated electron materials. The ground state of such materials can be tuned using non-thermal control parameters such as magnetic field, chemical substitution, and external pressure. In this regard, external pressure is an excellent tool to tune the ground state of materials without introducing additional disorders. Here, I will discuss various physical property measurement techniques under hydrostatic pressure. As an example, I will present our results on LuFe4Ge2, an itinerant antiferromagnet with a frustrated quasi-one-dimensional structure. Results from electrical transport, magnetic susceptibility, AC calorimetry, muon-spin resonance, Mössbauer spectroscopy, and x-ray diffraction experiments under hydrostatic pressure provide a detailed temperature—pressure phase diagram and elucidate the interplay of structure and magnetism in LuFe4Ge2.

Thermoelectric materials, applied strategies, and future directions

Thermoelectric (TE) materials are attractive due to their ability to convert waste heat into electric power. Significant developments in TE technology have been recorded over the past two decades. They comprise two key aspects: (i) basic understanding of electronic and thermal transport, facilitated by combination between theory and experiments; and (ii) optimizing the performance of various TE materials by integrating these interrelated transport parameters synergistically. An overview of used strategies for improving the TE performance of materials with exciting materials physics is covered in this talk. Interstitial engineering, nanocomposite, spacer engineering, charge carrier energy filtering, topological surface state contribution in TE materials, microstructural and surface modification engineering are some of the approaches examined. An intriguing aspect of the effects of microstructure on the TE transport properties of nanostructured Bi2Te3-based compounds is addressed, which is coupled with their topologically insulating (TI) nature. We create effective signals of TI states to increase TE performance by fine-tuning of grain sizes/misorientation. The structure-property correlation is strengthened by advanced characterization tools and a first principle calculation. Aside from this study, there is also discussion of other fascinating work that has been done in the before. These studies are significant for comprehending electron/phonon transport and developing high-performance thermoelectric materials.

Introduction to the abelian sandpile model of self-organized criticality

This lecture will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Strongly correlated phases in models with random interactions

Strong interactions between electrons gives rise to many fascinating emergent phases of matter. In certain situations it may result in a complete breakdown of quasiparticle picture leading to anomalous behavior and stark deviations from conventional theories. We will discuss how random models in the Sachdev-Ye-Kitaev (SYK) class have enhanced our understanding of such entangled phases of matter. As an important example, we will present a SYK-type random model of electrons for finite hole doping away from a Mott insulator. Using renormalization-group technique I will show that it hosts a deconfined critical point accompanied with a sharp change in charge-carrier density. In this approach we can calculate some exponents exactly. This model successfully captures the key aspects of high-Tc cuprates. I will also briefly discuss other examples such as an anomalous metal found in disordered superconducting films.

Experiments on solids under high magnetic fields

Experimental physics is challenging and fun. Scientific and technological progress in the modern world could not be imagined without careful experiments on semiconductors in the last 7 - 8 decades. As a result, semiconductors form hearts and brains of modern computation, optoelectronic and light-wave technologies. In this talk, I will describe experiments on semiconductors using light and strong magnetic fields which have provided vital tests to the theories of quantum mechanics. Classic textbook quantum phenomena such as 'particle in a box' and confinement effects, quantum mechanics of artificial hydrogen atoms etc. are routinely studied in the laboratory through such experiments. We will discuss the strongest magnetic fields which have been created in a laboratory. Alongside, recent progress in the area of research on 2D materials will be described, where every year, many fascinating discoveries are taking place setting up foundations of future quantum computation and communication technologies [1].

Spin liquid behaviour in the candidate material Sr_3CuSb_2O_9

New states of matter that are driven by quantum fluctuations have captured the interest of a large fraction of condensed matter physicists. Spin liquids are an exciting example of such a state in the context of magnetism. I will start with a general introduction to where the search for spin liquids fit in the science of magnetism. I will give a flavour of how theoretical ideas play out in practice by using the example of the material Sr_3CuSb_2O_9 that has been synthesized and studied recently at IIT Bombay.

Interplay of symmetry and strong correlations in excitonic insulator candidates

In low electron density materials, interactions can lead to highly correlated q]uantum states of matter with intriguing properties. Ta2NiSe5, an excitonic insulator (EI) candidate, exists in a novel broken symmetry phase below 327K, characterized by both robust exchange interaction and strong electron-lattice coupling. We study this broken symmetry phase of Ta2NiSe5 using a new spectroscopic probe, the quadrupole circular photogalvanic effect (QCPGE). The coupling of light to electrons in Ta2NiSe5 mediated by electric quadrupole/magnetic dipole coupling produces helicity-dependent DC current response even in the presence of bulk inversion symmetry, making it particularly sensitive to certain symmetry breaking effects. We show that the electronic exchange interaction in Ta2NiSe5 can lead to a triclinic structure with a broken symmetry. Our results provide an incisive probe of the symmetries of the low temperature phase of Ta2NiSe5 and add new symmetry constraints to the identification of a strongly correlated excitonic insulator phase in this material. Furthermore, the high sensitivity of QCPGE to subtle symmetry breaking effects in systems with bulk inversion symmetry will enable its use in studying other detailed structure-function relationships in complex crystalline systems.

Manipulation of Time Reversal Symmetry Breaking Superconductivity in Sr2RuO4 by Uniaxial Stress

Although the normal-state electronic structure of Sr 2 RuO 4 is known with exceptional precision, even after two decades of research, the symmetry of its certainly unconventional superconducting state is under strong debate, e.g. the long-time favoured spin-triplet p x ± ip y state is ruled out by recent NMR experiments [1]. However, in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two-component order parameters. Probing Sr 2 RuO 4 under uniaxial stress offers the possibility to lift the degeneracy between such components [2]. One key prediction for Sr 2 RuO 4 , a splitting of the superconducting and TRSB transitions under uniaxial stress, has not been observed so far. I will show results of muon spin relaxation (μSR) measurements on Sr 2 RuO 4 placed under uniaxial stress, wherein a large stress-induced splitting between the onset temperatures of superconductivity and TRSB was observed [3]. Moreover, at high stress beyond the Van Hove singularity, a new spin density wave ordered phase was detected for the first time. In order to perform μSR measurements under uniaxial stress, a custom piezoelectric based pressure cell was developed [4]. This cell is going to be useful for a range of other materials, in which the Fermi surface or magnetic interaction strengths can be tuned leading to strong modifications of the electronic state.
[1] A. Pustogow, et al., Nature 574, 72 (2019)
[2] C. Hicks, et al., Science 344, 283 (2014)
[3] V. Grinenko*, S. Ghosh* et al., Nature Physics 17, 748–754 (2021)
[4] S. Ghosh et al., Review of Scientific Instruments 91, 103902 (2020)
[5] S Ghosh et al. APS March Meeting, Bulletin of the American Physical Society, Vol 66, 1, (2021)

Chromatin Loops and Random Walks: From biology to physics, and back again

Random walks arise naturally in many biological contexts. In this talk, I will focus on some examples of non-trivial random walks which are relevant for loops in chromatin. In the first part of the talk, we will look at how large loops are formed between distant base pairs. Recent experiments suggest that loop formation is mediated by Loop Extrusion Factor (LEF) proteins like cohesin. Experiments on cohesin have shown that cohesins walk diffusively on DNA, and that nucleosomes act as obstacles to the diffusion, lowering the permeability and hence reducing the effective diffusion constant. An estimation of the times required to form the large loops typically seen in Hi-C experiments using these low effective diffusion constants seems to predict unphysically large looping times. In this talk, I'll propose a simple answer to this puzzle, and show that nucleosomes may play an unexpected role in regulating loop formation times. Motivated by this example, in collaboration with experimentalists, we design and conduct simple table-top experiments to investigate the role of obstacles on the first passage times of random walks. We formulate and solve the equations for mean first passage times in such cases, which shows non-trivial behaviour for different classes of obstacles. Finally, we will briefly discuss ongoing work on how the looped structure of chromatin affects search processes and search time for proteins walking on these complex networks.

Newly observed collective behaviour in systems of active Brownian particles

The ability of Active Brownian particles to undergo motility-induced phase separation (MIPS) even in the absence of any adhesive forces is well known. Detailed studies of such systems in the past have concluded that their phase behaviour is relatively simple, with only a homogeneous state and a MIPS state. However, we show that the phase behaviour of such systems is richer than what was previously conceived. More specifically, we observe another transition at high motility, where the particles form a percolated cluster. This transition follows all the characteristics of a standard percolation transition. In the second part of the talk, we show that the aggregates of such particles on walls undergo a morphological transition with a change in wall interactions. This transition is strikingly similar to the wetting-dewetting transitions in equilibrium systems.

Quantum heat engines with Carnot efficiency at maximum power

Heat engines constitute the major building blocks of modern technologies. However, conventional heat engines with higher power yield lesser efficiency and vice versa and respect various power-efficiency trade-off relations. This is also assumed to be true for the engines operating in the quantum regime. Here we show that these relations are not fundamental. We introduce quantum heat engines that deliver maximum power with Carnot efficiency in the one-shot finite-size regime. These engines are composed of working systems with a finite number of quantum particles and are restricted to one-shot measurements. The engines operate in a one-step cycle by letting the working system simultaneously interact with hot and cold baths via semilocal thermal operations. By allowing quantum entanglement between its constituents and, thereby, a coherent transfer of heat from hot to cold baths, the engine implements the fastest possible reversible state transformation in each cycle, resulting in maximum power and Carnot efficiency. Finally, we propose a physically realizable engine using quantum optical systems. References:
(1) Mohit Lal Bera, Sergi Julià-Farré, Maciej Lewenstein, and Manabendra Nath Bera, Quantum heat engines with Carnot efficiency at maximum power, Physical Review Research 4, 013157 (2022).
(2) Mohit Lal Bera, Maciej Lewenstein, Manabendra Nath Bera, Attaining Carnot efficiency with quantum and nanoscale heat engines, npj Quantum Information 7, 1-7 (2021).

QTIH Seminar: Route to a universal photonic quantum computer design

Any quantum computing architecture has a few ingredients: (i) the choice of qubit, (ii) the platform, (iii) the type of computation model, and (iv) the quantum error correction module containing the encoding and decoding procedure. In this talk I will discuss some aspects of a photonic quantum computer design that I have been investigating in the recent few years. In particular, I will present some recent interest and case for a continuous-variable encoded bosonic qubit approach in photonics, using the measurement-based model implemented using 3D cluster states. I will also highlight some of the many challenges both from the theoretical and the experimental side, including the difficulty in preparing non-Gaussian photonic states required for universal quantum computation. While photonics hardware seems amenable to scaling, which of the different competing platforms will eventually succeed in realizing fault-tolerant quantum computation is still completely open.

Cooper pairing without superconductivity: Phase fluctuations, Pseudogap state, Superinsulator and Bose Metal

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.