ICTS

Fragility is the inability of a material to elas2cally support some infinite simal loads. In the linear elas2c response, the fragile regimes lie at the thresholds of material rigidity where certain combina2ons of the elas2c moduli vanish. While over the past decades, many sta2c realiza2ons of fragile elas2city have appeared in the field of mechanical metamaterials, in this talk, we discuss how the cytoskeleton of the living cell offers a unique dynamic realiza2on of fragile, ac2ve, renewable maBer. Ac2ve myosin motors undergoing turnover, in a strain dependent manner, on an overdamped ac2n nemato-elastomer dynamically renormalize its elas2c moduli. In this self-organiza2on process, fragile bulk and surface
modes may dynamically appear or disappear. The material, further, endogenously generates ac2ve nonlineari2es to stabilize the linear fragile response. In addi2on to this unconven2onal elas2city, the material displays ac2ve segrega2on and excitability, while ‘odd’ contribu2ons to the ac2ve stress may give rise to propaga2ng ‘odd’ waves.

The coupling of the active actin-myosin cortex and the cell membrane allows the cell to deform, move, and undergo division. We consider deformations of spherical membranes under the active drive of motor proteins. Using numerical calculations complemented by analytical theory, we show the emergence of several morphological phases and phase transitions. A spherical cell membrane coupled to curvature-inducing activator proteins and active forces from polymerizing actin and myosin show instabilities towards pattern formation, localized pulsation, and pole-to-pole running pulsations.

Experiments performed using micro-patterned 1 dimensional collision assays has allowed for precise quantitative analysis of the collective manifestation of the phenomenon of contact inhibition locomotion (CIL) wherein, individual migrating cells reorient their direction of movement when they come in contact with other cells. Inspired by these set of experiments, we present a discrete minimal 1D driven lattice gas model which mimics the CIL interaction between cells and active movement of the cell in 1-d channel. We analyze the emergent collective behaviour of migrating cells in such confined geometry.

We consider the collective dynamics of self-propelling particles in two dimensions. They can align themselves according to the direction of propulsion of their neighbours, together with small rotational fluctuations. They also interact with each
other via soft, isotropic, repulsive potentials. The particles are confined in a circular trap. The steepness of the trap is tuneable. When the trap is steep, particles flock along its boundary. They form a polar cluster that spreads over the boundary. The cluster is not spatially ordered. When the steepness is decreased beyond a threshold value, the cluster becomes round, compact and eventually spatial order (hexagonal) emerges in addition to the pre-established polar order. First we
investigate the kinetics of such ordering with varying steepness. Next we investigate the information flow within the ordered cluster while colliding with the boundary of the trap.

Amorphous materials display seemingly random behavior at short-length scales, with elasticity emerging at larger length scales. On the other hand, crystals, with a periodic spatial profile, also display similar elastic behavior at large length scales. Gradually introducing disorder into athermal crystalline packings can therefore be used to build a relation between the well-established physics of crystals and that of amorphous solids. We present a universal characterization of stress correlations in athermal systems, across crystalline as well as amorphous packings. We present exact results for the stress correlations in energy-minimized configurations of near-crystalline packings. We present numerical as well as theoretical results for the correlations in the components of the stress tensor across a wide variety of situations, including crystalline, as well as fully amorphous packings. We find that properties of the stress correlations at large lengthscales are display universal behaviour and are independent of the underlying structure.

Active matter refers to systems driven out of equilibrium through the uptake and dissipation of energy at the level of individual constituents. As a result, they display anomalous features that closely resemble numerous biological and natural phenomena, such as flocking animals and collective cell migration. Yet, a detailed understanding of these systems remains an open challenge. Over the past few decades, experimentalists have developed various systems to explore the physical
principles governing active matter. However, these systems are often very different from each other, thus, limiting the scope of drawing systematic comparisons between them. In this talk, I will present a novel robotic active matter system developed in our lab. It consists of a collection of centimeters-long programmable self-propelled robots to mimic various active matter systems. The robots have  hardware components like IR sensors and light intensity sensors that enable them to detect obstacles and react to external optical signals. As an initial step, at a single robot level, we have successfully programmed it to exhibit various scalar active particle models with an excellent agreement with theoretical models. In addition to revealing rich physics, these experiments offer potential applications for bio-inspired and nature-inspired robotics.

Soft amorphous materials are diversely used in many applications and hence understanding their mechanical properties from a microscopic perspective is essential, specially in the context of sustaining against failures that
would occur while under use. Recently, the failure via spontaneous cavitation during expansion processes has gained a lot of interest, even for soft glassy materials. The initial microcavities that form can eventually lead
to catastropic failure via large scale fractures. From a thermodynamic perspective, cavitation is understood to happen in cohesive glass-forming systems, when the solidgas coexistence is accessed during the material’s expansion. Understanding this phenomenon via the lens of the potential energy landscape framework has turned out to be an interesting exercise. Using extensive numerical simulations, we study the failure of model soft amorphous solids while undergoing
athermal quasi-static expansion, starting from a highdensity state where the material is spatially homogeneous. We observe that plastic instabilities occur while the solid is under expansion, and these events get expressed as sudden jumps in macroscopic pressure and energy. As the pressure starts building up inside the material, it eventually yields via cavitation. Subsequently, cavities merge, if the expansion is continued, and this leads to the final fracture, i.e. disintegration of the solid.Thus, cavitation acts as a precursor to fracture. We show that all plastic events, be it before yielding or afterwards, are characterized by saddle-node bifurcation, during which the smallest non-zero eigenvalue of the Hessian matrix vanishes via a square-root singularity. We also find that in the post-yielding regime, the statistics of pressure or energy jumps corresponding to the plastic events show sub-extensive system-size scaling, similar to the case of simple shear but with different exponents Thus, overall, this study reveals universal features in the transformation within the potential energy landscape, during mechanical failure in amorphous solids under any quasi-static deformation protocol. More recently, we have extended the scope of our investigations to other expansion related protocols to further probe the stability of the amorphous solid to diverse kinds of mechanical deformations.

It is a central question in the amorphous world whether structural inhomogeneities in the initial undeformed glassy samples determine the location of a shear band nucleation or whether the formation of a shear band is linked to stochasticity and the details of the shear protocol. We numerically investigate this by studying the detailed failure pathway in glasses with spatial inhomogeneity, generated using two different protocols. The first protocol uses a temperature gradient pulse [1] to introduce a spatial inhomogeneity dominated by density and potential energy, while the second protocol uses inhomogeneous annealing to generate glasses with inhomogeneities dominated by potential energy. Subsequently, we study the shear response of such inhomogeneous glassy samples over a range of shear-rates. We observe, the emergence of non-equilibrium steady states and the formation of shear-bands in the transient regime strongly depends on the level of spatial heterogeneity in glassy samples. Further, we show, stochasticity in the location of shearband nucleation is important if inhomogeneity is relatively small [2].

[1]V. Vaibhav, J. Horbach, and P. Chaudhuri, Response of glassy liquids to thermal gradients, Physical Review E 101, 022605 (2020).
[2]V. Vaibhav, J. Horbach, and P. Chaudhuri, Controlling the mechanical failure in glasses by designed spatial inhomogeneity, under review (2023).

While the mechanical disruption of microscopic structures in complex fluids by large shear flows has been studied extensively, the effects of applied strains on the dielectric properties of macromolecular aggregates have received far less attention. Simultaneous rheology and dielectric experiments can be employed to study the dynamics of sheared colloidal suspensions over spatiotemporal scales spanning several decades. Using a precision impedance analyzer, we
study the dielectric behavior of strongly sheared aqueous suspensions of thermoreversible microgel poly(N-isopropylacrylamide) (PNIPAM) particles at different temperatures. We also perform stress relaxation experiments to uncover the influence of large deformations on the bulk mechanical moduli of these suspensions. While we note a counter-intuitive slowdown of the dielectric relaxation dynamics, our bulk rheology experiments, performed under identical
conditions, reveal shear-thinning dynamics with increasing strain amplitudes. We propose the shear-induced rupture of fragile clusters of swollen PNIPAM particles to explain our observations. Our work illustrates that rheo-dielectric studies have enormous potential for providing deep insights into the length scale-dependent dynamical properties of complex systems such as dense suspensions and soft glasses.

A many-body interacting system of classical kicked rotor serves as a prototypical model for stldying Floqlet heating dynamics. Having established the fact that this system exhibits a long-lived prethermal phase with a qlasiconserved average Hamiltonian before entering into the chaotic heating regime, we lse spatiotemporal flctlation correlation of kinetic energy as a two-point observable to probe the above dynamic phases. We remarkably fnd the diflsive transport of flctlation in the prethermal regime slggesting an lnderlying hydrodynamic pictlre in a generalized Gibbs ensemble with a defnite temperatlre that depends on the driving parameter and the initial conditions. On the other hand, the heating regime is characterized by a diflsive growth of kinetic energy where the correlation is sharply localized arolnd the flctlation center for all time. Conseqlently, we attriblte nondiflsive and nonlocalized strlctlre of correlation to the crossover regime, connecting the prethermal phase to the heating phase, where the kinetic energy displays a complicated growth strlctlre. We lnderstand these nlmerical fndings lsing the notion of relative phase matching where the prethermal phase (heating regime) refers to an
efectively colpled (isolated) natlre of the rotors. We exploit the statistical lncorrelated natlre of the angles of the rotors in the heating regime to fnd the analytical form of the correlator that mimics olr nlmerical resllts in a convincing way.

We explore the persistence properties of a class of worm algorithms in the vicinity of equilibrium critical points, finding evidence for universal behavior characterized by a nontrivial Hurst exponent.
(with Sabyasachi Choudhuri [TIFR] and Anoop Raj [IITB])

Recent experiments show that periodic drives on dipolar systems lead to long-lived prethermal states. These systems are weakly coupled to the environment and reach prethermal states in a timescale much shorter than the timescale for thermalization. Such nearly-closed systems have previously been analyzed using Floquet formalism, which shows the emergence of a prethermal plateau. We use a fluctuation-regulated quantum master equation (FRQME) to describe these
systems. In addition to the system-environment coupling, FRQME successfully captures the dissipative effect from the various local interactions in the system. Our investigation reveals a cascaded journey of the system to a final steady state. The cascade involves a set of prethermal or arrested states characterized by a set of quasi-conserved quantities. We show that these prethermal states emerge in a timescale much shorter than the relaxation timescale. We also find and report the
existence of a critical limit beyond which the prethermal plateau ceases to exist.

Stochastic resetting is a strategy for boosting speed of target-searching process. After proposed a decade ago, most studies have been carried out under the assumption that resetting takes place instantaneously. However, as thermodynamic cost must be incurred for resetting process due to its irreversible nature, instantaneous resetting is not physically allowed unless infinite cost is provided. Here, we take into full consideration on both cost and first passage time (FPT) for stochastic resetting process, where reset is implemented by using a trapping potential during a finite time. Introducing iterative generating function method and counting functional, we calculate average work and FPT in this process. From this result, we obtain the explicit form of the time-cost trade- off relation, which gives the lower bound of mean FPT for a given work, when the trapping potential is linear in position. This trade-off relation clearly shows that instantaneous resetting is achievable only when infinite amount of work is provided. More surprisingly, this trade-off relation calculated from the linear potential seems to be valid for wide range of trapping potentials.

An extensively addressed question in the context of driven dilute inelastic systems is regarding the non-Maxwellian form of their velocity statistics. Experiments, numerical studies and phenomenological models have looked at the problem, yet not been able to provide a consistent answer. In this talk, I will present our results on a microscopic model of the system illustrating an exact universal form for their statistics, along with some recent developments

Active matter is a special kind of condensed matter system, that is inherently driven far away from equilibrium. The constituent of such systems are capable of self propeling by their own, by consuming energy from the environment, hence termed as active particles. These particles are different from the passive Brownian particle and expected to exhibit some interesting features in the dynamics. In this talk I will mainly focus on the dynamics of an inertial active Ornstein-Uhlenbeck particle and discuss some interesting features in the dynamical beahaviour of such particle in various circumstances.

The catalytic action of invertase generates bilayer asymmetry that stabilises membrane curvature. The driving mechanism for the generation of membrane curvature by invertase is investigated using giant unilamellar vesicles
(GUVs). The invertase cleaves the sucrose in the exterior compartment, thereby creating a sugar asymmetry across the bilayer mem-brane that is measured for GUV membranes consisting of the lipid dioleoylphosphatidylcholine (DOPC). Finally, the advantage of this method to control membrane curvature and to stabilize multisphere morphologies is demonstrated. The GUV system in the presence of invertase is beneficial as a tool to generate multiple on-demand compartments with more extended stability after the enzymatic activity has established the asymmetry.

Reference: ChemSystemsChem, https://doi.org/10.1002/syst.202200027

TBA

We study a spin-1 Hamiltonian with Heisenberg (JH) and biquadratic (JB) exchanges supplemented further by a Q2-term made out of biquadratic terms. For JH = 0, this is equivalent to a SU(3)-symmetric Hamiltonian where deconfined critical behaviour has been found earlier by Lou et al [1] as JB/JQ2 is tuned. When Heisenberg exchange is turned on, the symmetry is now reduced to that of SU(2) and we are interested in understanding the nature of Antiferromagnetic-Valence Bond solid transition in the reduced symmetry case of SU(2), S = 1. A previous study [2] with JH/JQ2 as the tuning parameter had found a (weakly) first-order transition. Our results in the absence of JH perturbation shows a continuous transition with critical values (Q2/J = 0.169, νON = 0.64, ηON = 0.37) for the order parameter, ON) in agreement with the SU(3) study of Lou et al [1]. When a finite JH = 0.1 is present, we find the following exponents (ν = 0.44, η = 0.19) suggesting a different universality class than SU(3).

[1] J. Lou, A. W. Sandvik, and N. Kawashima, Antiferromagnetic to valence-bond-solid transitions in two-dimensional SU(n) heisenberg models with multispin interactions, Phys. Rev. B 80, 180414 (2009).
[2] J. Wildeboer, N. Desai, J. D’Emidio, and R. K. Kaul, First-order n´eel to columnar valence bond solid transition in a model square-lattice s = 1 antiferromagnet, Phys. Rev. B 101, 045111 (2020).

I will present a class of star-shaped networks, composed of identical spin-1/2 chains coupled to a central spin, that have two properties: (1) They reduce to free-fermion hopping on the graph, and (2) Incoherently driving the centre generates strong multipartite entanglement across the network. In particular, measuring the global parities of a subset of legs projects the outermost qubits onto a maximally-entangled W state. The manifold of W states can be controlled by one or more auxiliary qubits that mediate the coupling to the centre. This construction works for any number of legs and may be realisable in superconducting circuits.

The elliptic type of bursting in a neuronal system is a recurrent alternation between large amplitude oscillations and quiescent phases with small amplitude oscillations. This kind of rhythmic pattern can be found in many neuronal systems. This talk reviews the synchronization behaviour of two and three Bautin-type elliptic bursters with linear direct coupling schemes, and it can be extended to various network topologies. We will reveal various kinds of synchronization behaviour of connected neural burster networks and their importance, possibly effective information processing, transmission mechanisms, and neurobiological science for studying complex systems.
1. M. E. Izhikevich, ``Synchronization of Elliptic Bursters’’ SIAM Review 43, 315 – 44 (2001).
2. Abdul Kalam Azad, Peter Aswin, ``Within-Burst Synchrony Changes for Coupled Elliptic Bursters’’ SIAM Journal on Applied Dynamical Systems 10, 261-281 10, 2010. 

The passage of different liquids or electrical currents through different media in the light of percolation is a much studied topic in recent years. This can as well be applied to electrical movement in neurons inside the heart and brain tissues, which can usually be called Semi Directed Percolation (SDP). In this work we study a 2D square grid of $nxn$ cells which can model forest fire and heart-like cellular automata models. Here initially the cells may be in two states, "Waiting" and "Inactive". We chose the system so that the cells are "Waiting" with probability $p$. We initiate an Action Potential through the system from one end. An "Waiting" cell can be activated with an action potential and then the cell will be an "Active" one. A Waiting cell transforms to an Active one in the next step, if one or more of its "nearest cell neighbors" was Active. A cell follows the transformation as Waiting $\rightarrow$ Active $\rightarrow$ Inactive. We introduced three probabilities $p_{act}$ (probability of an Waiting cell to become Active), $p_{switch}$ (probability of an Inactive cell to become a Waiting one), $p_{inact}$ (probability of an Active cell to become Inactive). With variation of probabilities the percolation threshold, tortuosity, cluster size distribution and other relevant quantities have been studied. The system is also observed with NNN type activation of Waiting cells. The number of arrivals is increased with increased NNN type activation. Several other interesting features have been observed. The simple system is seen to explain the actions of heart/brain like systems very well.

Glassy dynamics are crucial for several critical biological processes, such as wound healing, cancer progressions, embryogenesis, cytoplasmic transportation, Etc. As biological systems are inherently out of equilibrium, how activity affects the glassy dynamics becomes fundamentally important. It also extends the scope and extent of the as-yet mysterious physics of glass transition. Theories of equilibrium glassy dynamics have been extended for the active glasses where the constituent particles have a self-propulsion force, f0, and a persistence time, τp, of their motion. f0 always drives the system away from the glassy regime, but effects of τp is more complex and depends on the details of activity. The relaxation dynamics of active glassy systems seem qualitatively similar to that in an equilibrium system at an effective temperature. However, activity has nontrivial complex effects on the dynamical heterogeneity. In this talk, I will discuss some of these effects and show that mode-coupling theory, extended for active systems, can surprisingly capture both these aspects of activity.

After application of a step stimulus the receptor activity and tumbling bias of an E.coli cell changes sharply to reach an extremal value and the cell is far from adaptation at this time. Surprisingly, our simulations show that the extremal activity is related to the free energy via Boltzmann distribution, which is expected only for adapted states. We perform exact calculation to explain this striking effect. We also make experimentally verifiable prediction that there is an optimum size of the step stimulus at which the extremal response is reached in the shortest possible time.

Biological cells are apt in making accurate measurement of extracellular stimuli, despite the noisy environments they inhabit and their noisy intracellular information processing apparatus. In this talk we will illustrate examples highlighting the limitations and accuracies in cellular information processing of extracellular stimuli. We will see that Dictyostelium measures chemotatic gradients at an accuracy set by the physical limitations imposed on gradient measurements. We will next highlight the accuracy in measurement of glutamine concentration in the synaptic cleft by neuronal cells. We conclude by presenting our study on the limitations to positional measurements in calcium signal transduction.

We consider an anharmonic chain which is driven periodically at one end and can dissipate energy at both ends. This models light transmission through an array of coupled qubits and was recently shown to exhibit an interesting parameter regime where energy is carried by breather modes. Analytic results on the the breather mode and the effect of thermal noise on it will be discussed.

In this talk, we try to understand the dynamical stability of a coupled scalar field theory. By discretizing the field theory to a lattice, the Hamiltonian of the model becomes a combination of regular and inverted harmonic oscillators. Interestingly, we observed that the Out-of-Time-Order Correlator (OTOC) corresponding to this Hamiltonian is exponentially growing (in the inverted frequency regime) in time, which indicates the instabilities (chaotic nature) in the model. We also study the heat transport in the lattice model for both symmetric and asymmetric coupling with the baths. For symmetric coupling, we re-expressed the problem in terms of the normal modes to find an equivalence of this transport as a limiting case of a previously well-studied transport problem in ordered harmonic lattices. Moreover, we described the dynamics of the model in terms of a pseudo-hermitian matrix. Some of its properties coincide with Green's function's properties from the heat transport study.

I will present an extensive Monte Carlo investigation of the metastable lifetime through the reversal of the magnetization of spin-s Ising and Blume-Capel models, where s = {1/2, 1, 3/2, 2, 5/2, 3, 7/2}.
The mean metastable lifetime (or reversal time) is studied as a function of the applied magnetic field and for both models is found to obey the Becker-D ̈oring theory, as was initially developed for the case of s = 1/2 Ising
ferromagnet within the classical nucleation theory. Moreover, the decay of metastable volume fraction nicely follows the Avrami’s law for all values of s and for both models considered. The results are published in Phys. Rev. E,
104 (2021) 014107. 

In modern era MOCVD, MBE, FLL and other investigational method, low dimensional structures having quantum confinement in one, two and three dimensions such as ultra-thin films, inversion layers, quantum wires and dots, have involved much consideration, not only for their probable in detection new phenomena in nanostructured electronics but also for their stimulating devices submission in heterostructure based numerous materials, that are being presently calculated because of the improvement of carrier mobility and such quantum confined systems find ex-digital networks, optical modulators and also in other devices. In this paper, an effort is made to study the Fermi-Dirac distribution function in degenerate semiconductors forming band tails ( ) on the basis of a newly formulated electron dispersion law ( is the well Fermi-Dirac function) and it is more interested in carrier degeneracy condition Fermi-Dirac distribution function the investigation is , since it is my support will degenerate semiconductors and the electronic device, the transport coefficient and electron dynamics change either .

We define a new disordered asymmetric simple exclusion process (ASEP) with two species of particles, first-class and second-class, on a two-dimensional toroidal lattice. The dynamics is controlled by first-class particles, which only move horizontally, with forward and backward hopping rates $p_i$ and $q_i$ respectively if the particle is on row $i$. The motion of second-class particles depends on the relative position of these with respect to the first-class ones, and can be both
horizontal and vertical. We show that the stationary weight of any configuration is proportional to a monomial in the $p_i$'s and $q_i$'s. Our process projects to the disordered ASEP on a ring, and so explains combinatorially the stationary distribution of the latter first derived by Evans (Europhysics Letters, 1996). We compute the partition function, as well as densities and currents of all particles in the steady state. We observe a novel mechanism we call the Scott Russell
phenomenon: the current of second class particles in the vertical direction is the same as that of first-class particles in the horizontal direction. This is joint work with P. Nadeau (European Journal of Combinatorics, 2022).

The physics of quantum Ising model (qIm) plays an important role in quantum many body system. We study and present the results of qIm and longer range quantum Ising model (lqIm) in presence of strong correlation. We
do the quantum field theoretical renormalization group (RG) calculation to study the behaviour of RG flow lines for different couplings for different region of parameter space. We show how the strong correlation effect enrich
the quantum physics of these two systems. We show explicitly that the ordered ferromagnetic (FM) phase to the disorder quantum paramagnet (dqpI) quantum phase transition occurs only in the strongly correlated regime for
qIm otherwise only dqpI phase appears for non-interacting and attractive regime. We show explicitly for  lqIm that FM to dqpI transition occurs at the extremely correlated region and also the dqpI phase appears in correlated
regime. We show that short range FM coupling and longer range coupling are competiting with each other and also the effect of strong correlation in this competition. We also show the most interesting feature that the transverse field oppose the FM coupling of qIm but it is favour the longer range coupling of lqIm. We find the evidence of another disorder quantum paramagnetic (dqpII) phase due to the relevance of longer range coupling. We also present the existence of another quantum phase transition from dqpII phase to FM phase. We show explicitly that there is no phase transition from dqpI phase to dqpII phase rather they coexists. We also observe three different kind of coexistence phases depending on the correlated regime. This work provides a new perspective not only for the statistical physics of quantum Ising model but also for the quantum many body systems. 

We study a system of equal-sized circular discs each with an asymmetrically placed pivot. The pivots form a regular triangular lattice, and the discs can rotate freely about the pivots, with the constraint that no discs can overlap with each other. We investigate a particular limit where the distance between the pivot point and the center of the plates is infinitesimal, which further simplifies the hamiltonian of the system. Our Monte Carlo simulations show that the probability distribution of the orientation of a randomly chosen disc shows multiple cusp-like singularities. We determine the positions and qualitative behavior of these singularities. We find that in addition to the expected geometrical singularities, the system shows an intervening Kosterlitz-Thouless phase between the phases with broken orientational symmetry and the disordered phase.