ICTS

Presenter: Reza Farhadifar (Simons Foundation)
Authors: Reza Farhadifar (Simons Foundation), Sayantan Dutta (Princeton University), Gokberk Kabacaoglu (Simons Foundation), Wen Lu (Northwestern University), Vladimir I Gelfand (Northwestern University), Stanislav Y Shvartsman (Princeton University, Simons Foundation), Michael J Shelley (Courant Inst. (NYU), Flatiron Inst. (SF))

Cytoplasmic streaming is essential for transporting and mixing nutrients, proteins, and organelles within large plant and animal cells. The large ~200um Drosophila oocyte has recently gained attention for experimental and theoretical studies of this phenomenon. We present a quantitative study of streaming in Drosophila oocytes that combines PI of 3D time-lapse movies, with biophysical modeling and simulation. We observe a diverse family of 3D vortical flows across different oocytes, which differ in position and orientation, and which last tens of minutes. We show that a model of cytoskeletal activity at the periphery, organized by its interaction with interior fluid, explains the observed streaming structures. The emerging picture sheds light on a class of intracellular flows in large cells and highlights the wealth of questions at the interface of geometry, active matter, and basic biology.

The application of quantitative methods to biological problems faces the choice of how much detail to include and the generality of the conclusions. Both routine data analysis and airy pronouncements that have nothing to say about everything are to be avoided. The middle ground entails some use phenomenology, a well-used approach in both high and low energy physics. A sampling of examples will be presented from my work in the area of developmental biology, to give a flavor of what is possible. They include experiments in synthetic embryology where human stem cells are coaxed into making patterns and structures similar to real embyos, use of modern (ie post 1960) mathematics to enumerate categories of dynamical behaviors, and a bit of computational evolution to address the question of what pattern forming systems can be evolved by incremental changes.

We have performed a joint theoretical and experimental study of the assembly formed when BPP-COOH ( 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid) is deposited on Ag(111). Three-fourths of the molecules form a rigid Kagome 'host' network. The cavities of this network are occupied by the remaining 'guest' molecules, which display a punctuated rotation between positions corresponding to global minima in the rotational energy landscape. Calculations show that the topography of this landscape can be explained by the making and breaking of hydrogen bonds between the guest molecules and the host network. The height of the rotational barrier computed theoretically is in excellent agreement with that extracted from temperature-dependent experiments. The host network also bestows enantiospecificity on the system, due to the twist between the host network and the underlying Ag(111) surface. ref. https://doi.org/10.1002/ange.202107708

The self-organisation of cells into complex tissue relies on the tight regulation of cellular behavior. Typically, the regulation of cell decisions is attributed to pathways controlling the concentration of molecular species in response to intrinsic or extrinsic signals, such as in gene regulatory networks. Here, by contrast, we show in the paradigmatic example of cell death that cells manipulate how fluctuations propagate across spatial scales to regulate cellular behavior. Specifically, we find that the feedback between molecular and mesoscopic organelle fluctuations gives rise to a quasi-particle degree of freedom whose intriguing kinetic properties construct a kinetic low-pass filter of time- dependent concentrations of signaling molecules. This allows cells to distinguish
between fast fluctuations and slow, biologically relevant, changes in environmental signals. We demonstrate an order of magnitude effect of this phenomenon on the quality of the cell death decision and validate our predictions experimentally by dynamically perturbing the intrinsic apoptosis pathway. Our work reveals a new mechanism of cell fate decision making.

The physics of behavior seeks simple descriptions of animal behavior. The field has advanced rapidly by using techniques in low dimensional dynamics distilled from computer vision. Yet, we still do not generally understand the rules which shape these emergent behavioral manifolds in the face of complicated neuro-construction --- even in the simplest of animals. In this work, we introduce a non-neuromuscular model system which is complex enough to teach us something new but also simple enough for us to understand. We discover manifolds underlying the governing dynamics shaped and stabilized by a physical mechanism: an active-elastic, inverse-energy cascade. We explore the formulation of the governing dynamics of a polarized active elastic sheet in terms of the normal modes of an elastic structure decorated by a polarized activity at every node. By incorporating a torque mediated coupling physics, we show that the power is pumped from the shortest length scale up to longer length scale modes via a combination of direct mode coupling and preferential dissipation of higher frequency modes. We use this result to motivate the study of organismal locomotion as an emergent simplicity governing organism-scale behavior. To master the low dimensional dynamics on this manifold, we present a zero-transients limit study of the dynamics of +1 or vortex like defects in the ciliary field (which is experimentally supported for small organisms). We show, experimentally, numerically and analytically that these defects arise from this energy cascade to generate long-lived, stable modes of locomotive behavior. Using a geometric model, we show how the defect undergoes unbinding. We extend this framework as a tool for studying larger organisms with non-circular shape and introduce local activity modulation for defect steering. We expect this work to inform the foundations of organismal control of distributed actuation without muscles or neurons.

Hexagonal boron nitride (hBN), the inorganic analogue of graphene, is currently being explored for several applications, including membrane separation devices and sensors. In these applications, wettability and friction are crucial interfacial phenomena and understanding them is vital for designing devices for seawater desalination and osmotic power harvesting. In this regard, the water contact angle (WCA) and water slip length (WSL) are the fundamental properties measured in experimental investigations of hBN–water interfaces. Although, in reality, 2D materials contain defects such as vacancies and exposed edges, so far, studies have not considered the effect of defects on the WCA and WSL on hBN surfaces. In this talk, we discuss the wetting and frictional behavior of monolayer and bulk hBN with atomic-scale defects and roughness. To this end, first-principles density functional theory (DFT) and classical-mechanical molecular dynamics simulations are used to calculate the charge distribution inside the hBN nanosheets and to characterize wetting and frictional properties of hBN, respectively. We consider six different topologies of defects in hBN – the B, N, BN, B2N, and B3N vacancy defects, as well as exposed zigzag edges – and also study the effect of the defect concentration on the WCA and WSL to investigate more realistically the interfacial properties of defective hBN. We find that defects at a lower concentration of 0.082 nm-2 no longer affect the wetting properties of hBN surfaces, although they do affect the frictional properties of the surface. On the contrary, exposed edges have a significant effect on the WCA and WSL even at this lower concentration, leading to a notable drop in both quantities, and leading to excellent agreement with experimental data. In summary, monolayer roughness, but not defects, can explain the experimentally observed water contact angle of 66° measured on freshly cleaved, uncontaminated hBN. Not only that, monolayer roughness can explain the measured water slip length of ~1 nm on hBN. Overall, our results indicate the importance of considering realistic models of hBN nanosheets with defects and surface roughness in simulations of water purifcation and energy harvesting applications.

Recent developments in the thermoelectric application of Ag2Se, which simultaneously shows high mobility and low thermal conductivity, are motivated by innovative thermoelectric theories. However, the relatively narrow bandgap prevents it from achieving a high thermoelectric figure of merit, zT, at room temperature. In this regard, the Rashba effect, spin-dependent band splitting, shows a new direction toward the enhancement of thermoelectric performance. Herein, we investigate the Rashba effect in Ag2Se, which originated mainly due to Te-doping, which provides a unique mechanism for tuning thermoelectric power factors. Additionally, the amorphous limit of lattice thermal conductivity can be achieved via the engineering of configurational entropy dominated by point defect scattering in the pseudo-ternary phase. Density functional theory calculations also reveal that sulfur atoms are locally off-centered, resulting in localized soft optical phonons coupled with acoustics phonons that reduce lattice thermal conductivity. Finally, a benchmark zT~2.1 at 400 K, originated from a combination of Rashba effect along with entropic effect, can provide a new area of research in the thermoelectrics domain. Since, Ag2Se, Ag2Se0.5Te0.5 have already been synthesized, we believe the phase Ag2Se0.5Te0.25S0.25 system will soon be realized experimentally.

References:
1. R. K. Biswas, and S. K. Pati, Modeling Pseudo-ternary Phase of Ag2Se For Realization of n-type Thermoelectrics with High zT (communicated in Appl. Phys. Lett.)
2. R. K. Biswas, and S. K. Pati, Exploring a Superlattice of SnO-PbO: A New Material for Thermoelectric Applications. ACS Appl. Energy Mater. 4, 2081 (2021).

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have drawn immense attention from the scientific community because of their spin, valley, optoelectronics, and catalytic applications, making them unique from their bulk counterparts. Recently, among all the TMDs, rhenium disulfide (ReS 2 ) has begun to attract much attention due to its extremely weak interlayer coupling strength and anisotropic properties. The weaker interlayer coupling strength leads to persisting the monolayer behavior of ReS 2 till its bulk phase and resilient its electronic and optical properties with applied external pressure. However, on the other hand, weaker interlayer coupling strength makes it challenging to determine the stacking order of multilayer or bulk structure of ReS 2 . Using first principles density functional theory (DFT) calculations, we have identified two distinct stacking orders (AA & AB) of bilayer ReS 2 , which correspond to two local energy minima of the potential energy surface, resolving the prevailing discrepancies in the literature. Further, we have revealed the stacking-order-driven differences in vibrational and optical properties of bilayer ReS 2 using density functional perturbation theory (DFPT) and GW-Bethe-Salpeter equation (BSE) simulations, respectively. In addition to weak interlayer coupling strength, another uniqueness of ReS 2 lies in its distorted 1T triclinic crystal structure where the additional d valence electrons of Re atoms form zigzag Re chains parallel to the b-axis, drastically reducing its symmetry and making its electronic and optical properties anisotropic. The low triclinic crystal symmetry and in-plane anisotropy in the structure make it challenging to resolve its absolute characteristics, like the origin of extra phonon modes Raman spectra beyond three layer of ReS 2 . We have demonstrated that the observed additional phonon modes are driven by unique layer-parity-dependent splitting of Eg 1 mode and inversion symmetry breaking. The findings underscore the stacking-order driven electronic, vibrational and optical properties of ReS 2 , mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

In electrical insulators, heat is carried by the quantized collective lattice vibrations called phonons. Resistance to heat flow in these materials is caused by phonon scattering processes. Thermal phonon transport in these materials is governed by the semi-classical Boltzmann Transport Equation (BTE). Solutions of the BTE are commonly derived assuming the validity of relaxation time approximation (RTA), where all phonon scattering events are assumed to be momentum-dissipative in nature. While the RTA-based BTE solution describes the heat flow in several materials reasonably well, it fails to capture the ultrahigh thermal conductivity and the exceptional phonon transport properties of materials like diamond and boron nitride. Here we present the solutions of the BTE without the RTA for phonon transport through these ultrahigh thermal conductivity materials, and demonstrate that accurately distinguishing momentum-conserving (Normal) and momentum-dissipative (Umklapp) scattering events in our formulation is crucial to correctly predict their thermal transport properties.

This work is supported by the DST-SERB Core Research Grant (CRG/2020/006166) and Prime Minister Research Fellowship (192002-2069).

To determine the topological quantum numbers of fractional quantum Hall (FQH) states hosting counter-propagating (CP) downstream (Nd) and upstream (Nu) edge modes, it is pivotal to study quantized transport both in the presence and absence of edge mode equilibration. While reaching the non-equilibrated regime is challenging for charge transport, we target here the thermal Hall conductance GQ, which is purely governed by edge quantum numbers Nd and Nu. Our experimental setup is realized with a hBN encapsulated graphite gated monolayer graphene device. For temperatures up to 35mK, our measured GQ at nu = 2/3 and 3/5 (with CP modes) match the quantized values of non-equilibrated regime (Nd + Nu)k0T, where k0T is a quantum of GQ. With increasing temperature, GQ decreases and eventually takes the value of equilibrated regime |Nd - Nu|k0T. By contrast, at nu =1/3 and 2/5 (without CP modes), GQ remains robustly quantized at Ndk0T independent of the temperature. Thus, measuring the quantized values of GQ at two regimes, we determine the edge quantum numbers, which opens a new route for finding the topological order of exotic non-Abelian FQH states.

A magnetic system is normally expected to show long range magnetic order at a temperature scale comparable to the interaction strength between the spins on different sites. Such magnetic orderings are suppressed with reduction in the dimensionality as determined by the spin-spin interactions between magnetic sites along the three different directions. Theoretically it is realized that 1D spin chain systems are ideal to host rich variety of quantum many body ground states depending on the type and strength of the exchange interactions. For example, a uniform spin-1/2 Heisenberg antiferromagnetic chain should show disordered Quantum Spin Liquid (QSL) like state at T = 0 K, with gapless excitations through the creation of spin-1/2 spinons in pair. However, in real materials, in most of the cases inevitable interactions between the 1D chains lead to 3-dimensional long range magnetic ordering. In this work, we investigated low temperature magnetic properties of a 1D spin-1/2 chain system: A copper based Metal Organic Framework (MOF), which shows no signs of ordering and an interesting QSL like state down to 270 mK with indications of gapless excitations as evident from the thermodynamic measurements as well as muon spin relaxation (μSR) experiments.

Single phase multiferroics are interesting because they exhibit a strong coupling between ferroelectricity and magnetism because the ferroelectric ordering is induced by magnetic ordering. Such materials are important due to their potential for applications in spintronic devices. I will be presenting the ferroelectricity induced by polar incommensurate and commensurate spin orders in the well-known green phase compound Gd 2 BaCuO 5 , which crystallizes in a centrosymmetric orthorhombic structure (Pnma). This compound undergoes a long-range antiferromagnetic ordering at = 11.8 K, where both Gd 3+ and Cu 2+ spins order in an elliptical cycloidal configuration with magnetic super space group P2 1 ma1'(0,0,g)0s0s associated to incommensurate modulation vector (0, 0, g), which is accompanied by the emergence of ferroelectric polarization. With decreasing temperature, it undergoes a lock-in transition at ~ 6 K, below which the magnetic structure becomes commensurate with k c = (0, 0, ½) and strongly noncollinear, which causes an additional contribution to the electric polarization resulting from the polar magnetic space group (P a ca2 1 ). Based on the symmetry analysis of magnetoelectric interactions, I discuss that the ferroelectricity in both commensurate and incommensurate phases is driven by a complex interplay of two spins and single-spin contributions from magnetic ions located in noncentrosymmetric environments.

References
[1] P. Yanda, I. V Golosovsky, I. Mirebeau, N. V Ter-Oganessian, J. Rodríguez-Carvajal, and A. Sundaresan, Phys. Rev. Res. 2, 23271 (2020).

Nematode behavior is extensively explored and understood by studying them on soft 2D surfaces or inside homogeneous liquid media. However, in their natural habitat, many nematodes navigate a much complex three-dimensional self-healing environment such as soft soil, rotten fruit, and plant stem. Maintaining and exploring nematodes directly in such self- healing environments is critical for understanding their behavior in most natural settings. Hence, to mimic the natural habitat of nematodes, we have designed a transparent, self-healing soil-like material from jammed granular microgel systems. The transparent nature of this platform enables direct visualization of nematodes whereas self-healing nature allows them to move in 3D. Using the canonical example of model C. elegans, we investigate how they navigate through the jammed microgel system. Further, we explore how the stiffness of their microenvironment affects their locomotion. Together, our results will present an approach to maintain and image nematodes in 3D and harvest them from this medium for further processing. Preliminary data will be presented.

Recently, single crystals of tin selenide (SnSe) have drawn immense attention in the field of thermoelectrics owing to its anisotropic layered crystal structure and ultra-low lattice thermal conductivity. Layered SnSe attains an orthorhombic crystal structure (Pnma) at ambient conditions. However, the cubic rock-salt phase (Fm-3m) of SnSe can only be stabilized at very high pressure and thus experimental realization of the cubic phase remains elusive. Herein, we have successfully stabilized the high-pressure cubic rock-salt phase of SnSe by alloying with AgBiSe2 (0.30 ≤ x ≤ 0.80) at ambient temperature and pressure. Orthorhombic polycrystalline phase is stable in (SnSe)1-x(AgBiSe2)x for composition range of 0.00 ≤ x < 0.28 and these are measured to be narrow band gap semiconductors, whereas the band gap closes upon increasing the concentration of AgBiSe2 (0.30 ≤ x < 0.70) with the cubic rock-salt structure. We confirm stabilization of cubic structure at x = 0.30 and associated changes in electronic structure using first-principles theoretical calculations. Pristine cubic SnSe exhibits topological crystalline insulator (TCI) quantum phase, but the cubic (SnSe)1-x(AgBiSe2)x (x = 0.33) possesses semi-metallic electronic structure with overlapping conduction and valence bands. Cubic polycrystalline (SnSe)1-x(AgBiSe2)x (x = 0.30) sample shows n-type conduction at room temperature while the orthorhombic (SnSe)1-x(AgBiSe2)x (0.00 ≤ x < 0.28) samples retain its p-type character. Thus, by optimizing the electronic structure and the thermoelectric properties of polycrystalline SnSe, a high zT of 1.3 at 823 K has been achieved in (SnSe)0.78(AgBiSe2)0.22.

Although many experimental and theoretical studies on natural selection have been carried out in a constant environment, as natural environments typically vary in time, it is important to ask if and how the results of these investigations are affected by a changing environment. We study the properties of the conditional fixation time defined as the time to fixation of a new mutant that is destined to fix in a finite, randomly mating diploid population with intermediate dominance that is evolving in a periodically changing environment. It is known that in a static environment, the conditional mean fixation time of a co-dominant beneficial mutant is equal to that of a deleterious mutant with the same magnitude of selection coefficient. We find that this symmetry is not preserved, even when the environment is changing slowly. For an initially deleterious mutant under moderate and slowly varying selection, the fixation time differs substantially from that in a constant environment when the mutant is recessive. As fixation times are intimately related to the levels and patterns of genetic diversity, our results suggest that for beneficial sweeps, these quantities are only mildly affected by temporal variation in environment. In contrast, environmental change is likely to impact the patterns due to recessive deleterious sweeps strongly.

The retina is a thin piece of neural tissue in the eye which encodes the image falling on it as electrical impulses (spikes) that are sent to higher brain regions. Visual perception begins in the retina with circuits already discovered for colour discrimination, motion detection among others. Such encoded representation is possible across a wide range of light levels due to various adaptation process in the retina. The process that adjusts the sensitivity of the neurons to the incident light intensity is also based on the light-history. We study the responses of neurons (retinal ganglion cells) from the neonatal chick retina to time varying flicker light-stimuli using a multi-electrode array recording system. Many features of avian vision have not been documented. We address some of these issues with our measurements. Reverse correlation analysis was used to estimate the linear temporal filter of the response. The study identified neurons that exhibited colour-opponent responses [1], which manifested as antagonistic behaviour to blue and green colour stimuli. These observations and interpretations may be useful in developing a retinal prosthetic for vision related disorders.

[1] C. S. Deepak, Abhijith Krishnan, and K. S. Narayan. Temporal characteristics of chick retinal ganglion cell responses, effects of luminance, contrast and colour. Manuscript to be communicated.

I will discuss how the competition between entangling unitary dynamics and disentangling projective measurements leads to a new kind of dynamical phase transition in terms of the quantum entanglement in certain tensor networks. Based on a mapping to a travelling wave equation, I will present some exact results for the transition in tree-tensor networks and all-to-all connected 2-local tensor networks.

Quantum entanglement has emerged as an important characterization of the fundamental quantum mechanical nature of non-trivial many-body states. In this work, we discuss an implementation of a new path integral method [1] to compute entanglement in Hubbard model within dynamical mean field theory (DMFT) in one (1d) and two (2d) dimensions. The complicated boundary conditions in the usual replica path integral formulation of subsystem Renyi entropy is replaced by a time-dependent self-energy “kick” in this approach. We show that an integration over the strength of
the “kick” term can be utilized to efficiently extract the Renyi entanglement entropy for interacting fermions. Using this method we compute the second Renyi entropy as a function of subsystem size in the Hubbard model within DMFT for both metallic and Mott insulating phases. We explore the thermal entropy to entanglement crossover in the subsystem Renyi entropy in the correlated metallic phase. We show that the subsystem-size scaling of second Renyi entropy is well described by the crossover formula which interpolates between the volume-law thermal Renyi entropy and the universal boundary-law Renyi entanglement entropy with logarithmic violation, as predicted by conformal field theory.

[1] Arijit Haldar, Surajit Bera and Sumilan Banerjee, Renyi entanglement entropy of Fermi and non-Fermi liquids: Sachdev-Ye-Kitaev model and dynamical mean field theories, Phys. Rev. Research 2, 033505 (2020).

In the world of low Reynolds number, the role of inertia is entirely negligible, which has resulted in the evolution of novel swimming strategies adopted by the microorganisms to overcome the fluidic drag. Inspired by such techniques, we model magnetically actuated helical swimmers as active particles 1 . The activity and density of this artificial system can be tuned externally to better understand the evolution of collective behaviour like the swarming/flocking phenomena. In the past, helical swimmers were designed for reciprocal swimming with motility in the form of back-and-forth motion and unspecified directionality. This represents a zero-force, zero-torque active matter system with enhanced diffusion 2,3 . Here for the first time 4 , we break the time-reversal symmetry by engineering a suitable magnetic field aided by thermal fluctuations in the surrounding medium. We derive the inspiration from a system of Brownian ratchet to design an asymmetric time-varying potential in doing so. The swimmers can exhibit non-reciprocal swimming with enhanced diffusivities, with activity as a function of the frequency of the external field and a two-parameter space. The experimental results and numerical simulations are in excellent agreement, establishing an all-magnetic active matter.

References
[1] Mandal, P., Patil, G., Kakoty, H., &amp; Ghosh, A. (2018). Magnetic Active Matter Based on Helical Propulsion. Accounts of chemical research, 51(11), 2689-2698.
[2] Mandal, Pranay, and Ambarish Ghosh. &quot;Observation of enhanced diffusivity in magnetically powered reciprocal swimmers.&quot; Physical review letters 111.24 (2013):
248101
[3] Patil, G., &amp; Ghosh, A. (2021). Anomalous Behavior of Highly Active Helical Swimmers. Frontiers in Physics, 8, 656.
[4]Manuscript under preparation.

Soft-nanocomposites comprising liquid crystals (LCs) are gaining significant interest in recent times and have become a hot topic of research owing to the complementarity of the properties of the constituents. The primary aim of the present work has been to employ such nanocomposites and investigate various aspects of photoluminescence (PL) and its modulation with the aim to fabricate switchable PL devices. A salient feature of these studies is a novel route of using the liquid crystalline orientational order to realize linear assemblies of the employed cesium lead halide perovskite (CsPbBr3) and gradient CdSeS quantum dots and generate temporal and spatial modulation of emission.

Quantum dots of full inorganic perovskites such as CsPbX3 (X = Cl, Br, I) are becoming popular due to their better stability, excellent photoelectric performance and high luminescence efficiency [1]. We have incorporated small amounts (2%) of CsPbBr3 QDs into a room temperature LC and found several novel features [2]. In the nematic medium, the QDs form linear self-assembly, whose direction is dictated and controlled by the nematic director. Transmission electron microscopy images show that there is an anisotropic arrangement even at the nanometre scale caused by surface interaction of the QDs at their body diagonal. Thin films of this unique and fascinating assembly exhibit attractive absorption and emission features, which include (i) retainment of the inherent high PL efficiency of QDs, and (ii) double PL anisotropy, the in-plane (X-Y) as well as out-of-plane directions (X-Z), where X denotes the director direction [3].

The gradient CdSeS QDs, on the other hand, address the issue of the trap states due to dangling bonds which are the prime cause of low quantum efficiency. When dispersed in LCs these nanocomposites have a non-existent anisotropy, but an imposed polymer network creates the linear assembly of QDs resulting in anisotropy of PL in addition to enhancing the base PL. Further, we have developed systems in which even a low magnitude (0.6 mW/cm2) actinic light results in a high (45%) PL modulation [4]. The formulated protocols being generic, hold promises for the development of light-switchable QD-based emissive displays and photonic devices.

References:
[1] D. Wang et al., Nanoscale, 8, 1565 (2016)
[2] P.Satapathy et al., Adv. Opt. Mat., 7, 1801408 (2019)
[3] P. Satapathy et al., Crystals. , 9, 378 (2019)
[4] P. Satapathy et al., Chem. Photo. Chem., 4, 413 (2020)

Jamming observed in biological systems has given much confidence to explore the microscopic onset of jamming, using the models of activity driven glasses. Such dense active matter, thus, has brought the complexity of active matter into the physics of glass, where self-propulsion forces at micro-scale dominate over the inter-particle interactions and the thermal fluctuations, suggesting a unique model to investigate the role of non-thermal fluctuations in manifesting dynamical arrest, unlike by mechanical means. In this work, we focus on the dynamics of jamming in the dense active matter of infinite persistence time using microscopic simulations. We analyse the transient dynamics when a dense active liquid approaches a force-balanced state upon decreasing self-propulsion magnitude. Our investigations reveal the presence of a three-step relaxation process by extracting the associated time scales that separate the qualitative behaviour in the different regimes.

A non-magnetic microsphere optically trapped in a dilute suspension of nanoparticles shows an enhancement in the trap stiffness owing to directed motion of the nanoparticles towards the trap centre. This is due to the response of the surface charge present on the nanoparticles to the electric field gradient associated with the optical trap. An optically trapped microsphere in combination with magnetic nanoparticles can thereby be used as an effective probe to sense changes in magnetic environments due to the ability of these nanoparticles to respond to such changes. In this talk I will present experimental proof of the response of this combination probe to changes in the environment like presence of external magnetic fields and field gradients as well as magnetic microbeads in the suspension and their agglomerated structures.

Abstract We report unexpected states of organization in experiments and simulations on mixtures of motile fore-aft asymmetric rods and spherical beads. We show in experiments that in a suitably engineered channel geometry, two small groups of rods segregate themselves and immobilise a large collection of beads by pinning it from opposite ends. The system achieves a phase-separated state of bead-rich and bead-poor regions with an interface decorated by rods pointing towards the bead-rich region. In 2D experiments, we observe a similar interface formation of rods that gives a phase-separated state of beads with different densities.

We investigate the dynamics of freely-joined active droplet chains, which emerge as a dynamic interplay between the active droplets' self-generated chemical and hydrodynamic fields. The slip velocity of an individual droplet microswimmer can be explicitly controlled by modulating the external surfactant medium that fuels the droplet motion. This tuning of the slip velocity results in a modification of self-generated chemical and hydrodynamic flow fields of the droplet swimmer. We posit that freely-jointed super-structures, such as chains, of the active droplets can result in emergent spatio-temporal dynamics. We then demonstrate emergent complex dynamics of the active chains, such as periodic oscillatory motion from the resultant dynamical interplay between the chemical and hydrodynamic flow fields.

Most studies on the problem of equilibration of the Fermi–Pasta–Ulam–Tsingou (FPUT) system have focused on equipartition of energy being attained amongst the normal modes of the corresponding harmonic system. In the present work, we instead discuss the equilibration problem in terms of local variables, and consider initial conditions corresponding to spatially localized energy. We estimate the time-scales for equipartition of space localized degrees of freedom and find significant differences with the times scales observed for normal modes. Measuring thermalization in classical systems necessarily requires some averaging, and this could involve one over initial conditions or over time or spatial averaging. Here we consider averaging over initial conditions chosen from a narrow distribution in phase space. We examine in detail the effect of the width of the initial phase space distribution, and of integrability and chaos, on the time scales for thermalization. We show how thermalization properties of the system, quantified by its equilibration time, defined in this work, can be related to chaos, given by the maximal Lyapunov exponent. Somewhat surprisingly we also find that the ensemble averaging can lead to thermalization of the integrable Toda chain, though on much longer time scales.

Active suspensions, like bacterial swarms, can self-organize into complex dynamical states like active turbulence – flows that are vortical, chaotic and multi-scale, reminiscent of inertial turbulence. Using a generalized hydrodynamics model of ``active fluids", we show how superdiffusion via Lévy walks can masquerade as a crossover from ballistic to diffusive scaling in measurements of mean-squared-displacements. In fact, at high levels of activity, the flow manifests a variety of Lagrangian anomalies ranging from enhanced first-passage time distributions to pair-dispersion. A close inspection reveals that these anomalous statistics emerge due to flow regions marked by hitherto undetected oscillatory ``streaks'', which tend to persistently propel tracers. The flow is hence marked by a truly dynamical heterogeneity. While laying the theoretical framework for how activity drives living systems to defy bounds on inanimate matter, our work allows us to underline the essential differences between active and inertial turbulence.

I will discuss quantum chaos and spectral correlations in periodically driven (Floquet) fermionic and bosonic chains with long-range two-particle interactions in the presence and absence of particle-number conservation [U(1)] symmetry. For intermediate-range interactions, random phase approximation can be used to rewrite the spectral form factor in terms of a bi-stochastic many-body process generated by effective spin or boson Hamiltonians. In the particle-number conserving case, the effective Hamiltonians have non-abelian SU(2) and SU(1,1) symmetry, respectively for fermions and bosons, resulting in universal quadratic scaling of the Thouless time with the system size, irrespective of the particle number. In the absence of particle-number conservation, while we find a nontrivial systematic system-size dependence of the Thouless time for the bosonic model, it is independent of system size for kicked fermionic chains.

We investigate the kinetics of ferromagnetic ordering via Monte Carlo simulations of the nearest neighbour Ising model. We probe the relaxation of systems that are suddenly quenched from various initial temperatures in the paramagnetic region to state points inside the ferromagnetic region. It appears that for quenches to a fixed final temperature, a system from higher starting temperature shows the tendency of reaching the new equilibrium faster than that is prepared at a lower starting temperature. This is a surprizing observation that suggests the presence of Mpemba-like effect in the nonequilibrium dynamics of this model. We will present results to demonstrate certain systematic nature of the effect.