Talks

Motivated by recent debates around the many-body localization (MBL) problem, and in particular its stability against systemwide resonances, we investigate long-distance spin-spin correlations across the phase diagram of the random-field XXZ model, with a particular focus on the strong disorder regime. Building on state-of-the-art shift-invert diagonalization techniques, we study the high-energy behavior of transverse and longitudinal correlation functions, computed at the largest possible distance, for a broad range of disorder and interaction strengths. Our results show that while transverse correlations display a fairly stable exponential decay over the entire XXZ phase diagram, longitudinal correlations exhibit markedly different behavior, revealing distinct physical regimes. More precisely, we identify an intermediate disorder region where standard observables show well-converged MBL behavior [J. Colbois et al., Phys. Rev. Lett. 133, 116502 (2024)] while the distributions of longitudinal correlations reveal unexpected fat-tails towards large values. These rare events strongly influence the average decay of longitudinal correlations, which we find to be algebraic in a broad region inside the supposed MBL phase, whereas the typical decay remains mostly exponential. At stronger disorder and weaker interactions, this intermediate regime is replaced by a more conventional exponential decay with short correlation lengths for both typical and average correlators, as expected for standard localization. Our findings shed light on the systemwide instabilities and raise important questions about the impact of such rare but large long-range correlations on the stability of the MBL phase. Finally, we discuss the possible fate of the intermediate region in the context of recent perspectives in the field.

The far-field decay of the concentration of motile particles around a static inclusion is well studied in the case of “dry” active matter. We show here that the scenario is dramatically different when the viscous hydrodynamic interaction enters, especially if the object is polar in shape. Advection by fluid flow and diffusion enter on the same footing, a “marginality” that leads to a power-law decay exponent for the concentration varying continuously with a dimensionless measure of the force required to hold the inclusion in place, and a singular distinction between the axisymmetric and non-axisymmetric cases. We
hope our findings will inspire experimental studies on inclusions in swimmer suspensions. This work was done in collaboration with Thibaut Arnoulx de Pirey and Yariv Kafri.

We will exactly determine dynamical correlation functions for conformal field theories (CFTs) driven by conformal generators beyond the dilatation operator, in 1+1 as well as 3+1 dimensions. Under floquet dynamics the system falls into one of three universal phases which we try to explain geometrically.

The air we inhale brings along a variety of harmful aerosols, which if deposited on the wall of the airways can cause severe respiratory illness. The lung's primary defense against airborne particles is provided by a film of mucus which lines the airway walls. This film is continuously transported, upward and out of the lungs, by a carpet of wall-attached cilia; thus, harmful particles are evacuated provided they deposit on the mucus. In this talk, I will show that particles can manage to avoid the mucus and deposit on the exposed airway wall. The fate of particles depends on their size—which sets the strength of Brownian or inertial forces—as well as the coupled flow of air and mucus. The Rayleigh-Plateau instability of the annular inter-fluid interface plays a key role, by controlling the morphology of the mucus film, and ultimately leads to a counter-intuitive result: More mucus does not imply more trapping; instead, more particles—allergens, pathogens, and hopefully aerosolised drugs—are able to reach the wall.

In this talk, I shall discuss our results characterizing the spatiotemporal chaos in classical spin systems-- both magnetically ordered as well as frustrated with the latter showing a classical spin-liquid down to very low temperatures.

Covariant loop quantum gravity, commonly referred to as the spinfoam model, provides a regularization for the path integral formalism of quantum gravity. A 4-dimensional Lorentzian spinfoam model with a non-zero cosmological constant has been developed based on quantum SL(2,C) Chern-Simons theory on a graph-complement three-manifold, combined with loop quantum gravity techniques. In this talk, I will give an overview of this spinfoam model and highlight its inviting properties, namely (1) that it yields finite spinfoam amplitude for any spinfoam graph, (2) that it is consistent with general relativity with a non-zero cosmological constant at its classical regime and (3) that there exists a concrete, feasible and computable framework to calculate physical quantities and quantum corrections through stationary phase analysis. I will also discuss recent advancements in this spinfoam model and explore its potential applications.

We explore how self-organization in swarms of interacting self-propelled particles can be used to optimize their displacement in confined geometries. Using a discrete model with Vicsek-like interactions, we examine how channel geometry influence pattern formation and transport properties. Wall-induced particle accumulation leads to clogs and band formations that obstruct movement. This analysis enables us to develop global strategies for controlling particle alignment and optimizing displacement. We apply reinforcement learning techniques to devise policies that enhance transport efficiency.

Recent experiments have implemented resetting by means of a time-varying external trap whereby trap stiffnesses are changed from an initial to a final value in finite-time. Such setups have also been studied in the context of Landauer's erasure principle. We analyse the thermodynamic costs of such a setup in steady state.

he emergence of surprising collective behaviors in systems driven out of equilibrium by local energy injection at the particle level remains a central theme in the study of active matter. Recently, chaotic flows reminiscent of turbulence have garnered significant attention due to their appearance in diverse biological and physical active matter systems. In this talk, I will demonstrate how even the simplest model of active particles -— self-propelled point particles -— can exhibit mesoscale flows, characterized by streams and vortices, when very persistent active forces compete with crowding at high densities.

In the second part, I will introduce a minimal model of non-reciprocal interactions inspired by human crowds, which generates collective flows strikingly similar to those of the self-propelled particles. Interestingly, as the system approaches the equilibrium limit by reducing non-reciprocity, it undergoes an absorbing phase transition characterized by an infinite number of absorbing states and critical exponents consistent with the conserved directed percolation universality class.

Of special interest in aggregation processes is the occurrence of extremely large clusters, or condensates, which hold a finite fraction of the mass. We find that condensates coexist with a power law distribution in the Takayasu model of aggregation with input, even though the mass is not conserved. While approaching steady state, mini-condensates form on a growing length scale. There is a single mobile condensate in steady state, and its movement leads to a dynamic re-organization of the landscape on a macroscopic scale, along with an emergent power law which differs from the usual Takayasu power. In an open system, the exit of the condensate from the edges leads to intermittent fluctuations of the total mass in steady state, quantified through a divergence of the scaled kurtosis.

[1] A. Das and M. Barma, Indian J. Phys., Special issue on Nonequilibrium Statistical Physics (2024)
https://link.springer.com/article/10.1007/s12648-023-03030-1
[2] R. Negi, R. Pereira and M. Barma, arXiv:2407.09827 , to appear, Phys. Rev. E (2024)