ICTS

I will discuss special features of quantum light-matter interactions inside structured waveguides due to their finite bandwidth, band edges, and nontrivial topological properties. For linear waveguides with infinite bandwidth, the transmission and reflection coefficients of a side-coupled two-level emitter (2LE) are the same as the reflection and transmission coefficients of a direct-coupled 2LE. I'll show how this transport analogy breaks down for structured waveguides due to the appearance of Lamb shift only for the direct-coupled 2LE.

Ref: Quantum light-matter interactions in structured waveguides, Rupak Bag and Dibyendu Roy, Phys. Rev. A 108, 053717

We study 1D stochastic models with two conservation laws. One of the models is the coupled continuum stochastic Burgers equation, for which each current is a sum of quadratic non-linearities, linear diffusion, and spacetime white noise. The second model is a two-lane stochastic lattice gas. As distinct from previous studies, the two conserved densities are tuned such that the flux Jacobian, a 2 × 2 matrix, has coinciding eigenvalues. In the steady state, we investigate spacetime correlations of the conserved fields and the time-integrated currents at the origin. For a particular choice of couplings the dynamical exponent 3/2 is confirmed. Furthermore, at these couplings, continuum stochastic Burgers equation and lattice gas are demonstrated to be in the same universality class. (This is based on the work reported in arXiv:2401.06399.)

"Over the past several decades, nanopore technology has risen as a highly promising approach for biomolecule sequencing. Solid-state nanopores have emerged as one of the most versatile tools for single biomolecule detection and characterization [1]. Nanopore sensing is based on the measurement of variations in ionic current as biomolecules translocate through nanometer-sized channels, in response to an external voltage applied across the membrane. The passage of a biomolecule through a pore yields information about its structure and chemical properties[2].

In our investigation, we employed comprehensive all-atom Molecular Dynamics (MD) simulations to explore the electric field-driven translocation of a homopeptide sequence through double-layer graphene nanopores. Application of an external electric field facilitated the observation of ionic current (blockade current) generated by ion passage through the graphene nanopore, alongside the assessment of residence time (dwell time) for individual amino acids during the peptide translocation process.
Our findings suggest that dwell time may offer a more nuanced perspective compared to blockade current. Our analytical approach revealed a discernible influence of amino acid charges on the translocation of homopeptides. Furthermore, our study successfully identified post-translational modifications at the single-molecule level within homopeptide sequences as they traversed the sensing region of the nanopore.
This capability allowed us to discriminate between distinct peptide sequences based on the specific charges present within the peptides.
Reference:
1. Wang, Y., Zhao, Y., Bollas, A. et al. Nanopore sequencing technology, bioinformatics and applications. Nat Biotechnol 39, 1348–1365 (2021)
2. Aksimentiev A, Schulten K. Imaging alpha-hemolysin with molecular dynamics: ionic conductance, osmotic permeability, and the electrostatic potential map. Biophys J. Jun; 88(6), 3745-61 (2005)"

One of the key predictions of Parisi’s broken replica symmetry theory of spin glasses is the existence of a phase transition in an applied field to a state with broken replica symmetry. This transition takes place at the de Almeida-Thouless (AT) line in the h − T plane. We have studied this line in the power-law diluted Heisenberg spin glass in which the probability that two spins separated by a distance r interact with each other falls as a power-law. Tuning the exponent σ is equivalent to changing the dimension d of the short-range system, with the relation being d = 2/(2σ − 1) for σ < 2/3. We have found by numerical simulations that the AT line does not exist for σ > 2/3 (i.e below 6 dimensions). Therefore, the Parisi scheme is not appropriate for spin glasses in three dimensions.

Several simulation codes, including GTC, GYRO, ORB5, GENE, etc., have been created to investigate microturbulence within the linear and nonlinear domains of tokamak and stellarator cores. These codes employ flux coordinates to simplify computations affected by anisotropy from confinement magnetic fields. However, flux coordinates encounter mathematical singularities at the magnetic separatrix surface. To address this challenge, we developed the Global Gyrokinetic Code using Cylindrical Coordinates (G2C3), assisted by neural networks, for studying electrostatic microturbulence in realistic tokamak geometries. G2C3 utilizes a cylindrical coordinate system for particle dynamics, enabling motion within arbitrarily shaped flux surfaces, including the tokamak's magnetic separatrix. The code employs an efficient particle locating hybrid scheme, utilizing a neural network and iterative local search algorithm for charge deposition and field scattering. G2C3 uses numerically integrated field lines to train the neural network as a universal function approximator, accelerating subroutines related to gathering and scattering operations in self-consistent gyrokinetic simulation. Notably, G2C3 is the sole code integrating a multilayer neural network with field line geometry for self-consistent simulation in the world fusion program. To demonstrate the new code's capabilities, we present results from self-consistent simulations of ion temperature gradient modes in the tokamak's core region.

We present our recent works on model microswimmers under confinements, shear flow, and complex environments. We incorporate detailed hydrodynamic interactions and elastic properties of the flexible parts of the swimmers. Through two different models, suitable, respectively, for resolving the swimmers with remarkable details and at intermediate resolutions, we investigate the role of hydrodynamic flows. We show that fully resolved hydrodynamics and elastic properties are not just details but are crucial to understanding some of the open questions, such as those related to the hydrodynamic trapping of bacterial on a substrate and flagellar polymorphic dynamics of E. coli during its tumbling motion. 

Dense bacterial suspensions display collective motion exhibiting coherent flow structures reminiscent of turbulent flows. However, understanding the microscopic dynamics of bacterial fluid elements undergoing such turbulent motion is in its incipient stages. In this talk, I will discuss our experiments revealing correlations between microscopic dynamics and the emergence of collective motion in bacterial suspensions. Our detailed analysis of the trajectories of the passive tracers and the velocity field of the bacterial suspensions allowed us to systematically correlate the Lagrangian and the Eulerian perspectives. Bacteria within the collective dynamics followed initial ballistic dynamics followed by intermittent Lévy walk before the eventual decay to random Gaussian fluctuations. Intriguingly, the fluid correlation time decreased linearly with an increase in the effective activity, while the flow correlation length did not vary. The fluid correlation time defined the transition from Lévy to Gaussian fluctuations demonstrating the microscopic reason underlying the observation. Our results reveal transitions in microscopic dynamics underlying the bacterial turbulence and provide a lever to control the transitions between the microscopic regimes.

Re-normalization group theory allows continuous variation of critical exponents along a marginal direction (when there is one), keeping the scaling relations invariant. We propose a super-universality hypothesis (SUH) suggesting that, up to constant scale factors, the scaling functions along the critical line must be identical to that of the base universality class even when all the critical exponents vary continuously. We demonstrate this in the Ashkin-Teller (AT) model on a two-dimensional square lattice where two different phase transitions occur across the self-dual critical line: while magnetic transition obeys the weak-universality hypothesis where exponent ratios remain fixed, the polarization exhibits a continuous variation of all critical exponents. The SUH not only explains both kinds of variations observed in the AT model, it also provides a unified picture of continuous variation of critical exponents observed in several other contexts.

Exploring the intricate dynamics of phase separation in a confined cylindrical pore with wetting influence, this work delves into the fascinating phenomenon of binary fluid phase separation and its implications for various industrial and scientific applications. Unlike existing studies that suggested complete phase separation is unattainable within cylindrical porous media, our realistic model incorporates fluid-wall particle interactions. By implementing our model, we successfully achieved complete phase separation, presenting a significant advancement in understanding this phenomenon. Our numerical analysis explores the phase separation process under varying wetting strengths, pinpointing the critical threshold where partial wetting transitions to complete wetting. Predominantly we investigate the growth of the domain length scale and determine the growth exponent for all the cases, shedding light on the underlying dynamics of this intriguing phenomenon, which shows different behavior with different strength of interactions. Distinct growth exponents are exhibited by various particle types once transitioned to complete wetting regime. To provide further justification and explanation for these findings, we employ the structure factor, leveraging the Porod law and Super-universality principle.
Ref: D. Davis and B.S. Gupta, Phys. Rev. E, 108, 064607 (2023)

Macromolecules or polymers are generally found in crowded environments. The interior of a cell has a high concentration of macromolecules, typically constituting 20-30% of the total volume of a cell. Crowders play an important role in what is known as the coil-to-globule (C-G) transition of macromolecules, which is crucial for the functioning of biomacromolecules such as RNA, DNA, and proteins. The C-G transition can occur due to various reasons and phenomena, including solvent quality, co-non-solvency, temperature-mediated changes, depletion effect, etc. In this work, we studied the C-G transition occurring due to bridging interactions, where crowders act as bridges or glues between monomers to induce collapse. We performed extensive coarse-grained molecular dynamics simulations to investigate the phase diagram of both neutral and charged polymers in the presence of attractive crowders. We also shed light on the effects of crowder-crowder interactions, density, valency of counterions, and the size of crowders on such transitions.

We study the response of model amorphous solids to applied shear stress, and analyse the role of thermal fluctuations in determining the observed macro behaviour.

Non-Gaussian displacement distributions are universal predictors of dynamic heterogeneity in slowly varying environments. Here, we explore heterogeneous dynamics in supercooled liquid using molecular dynamics simulations and show the efficiency of the information-theoretic measure in quantifying dynamic heterogeneity over the widely used moment-based quantifications of non-Gaussianity. Our analysis shows that the heterogeneity quantified by the negentropy is significantly different from the one obtained using the conventional approach that considers deviation from Gaussianity up to lower-order moments. Further, we extract the timescales of dynamic heterogeneity using the two methods and show that the differential changes diverge as the system experiences strong intermittency near the glass transition.

The entropy of the fully packed limit of the covering of a lattice using rigid rods that cannot overlap is determined using upper and lower bounds.

We study the motion of an inertial microswimmer in a non-Newtonian environment with a finite memory and present the theoretical realization of an unexpected transition from its random self-propulsion to rotational (circular or elliptical) motion. Further, the rotational motion of the swimmer is followed by spontaneous local direction reversals yet with a steady state angular diffusion. Moreover, the advent of this behaviour is observed in the oscillatory regime of the inertia-memory parameter space of the dynamics. We quantify this unconventional rotational motion of microswimmer by measuring the time evolution of direction of its instantaneous velocity or orientation. By solving the generalized Langevin model of non-Markovian dynamics of an inertial active Ornstein-Uhlenbeck particle, we show that the emergence of the rotational (circular or elliptical) trajectory is due to the presence of inertia and memory in the environment or medium.

We study a system of $N$ noninteracting particles on a line in the presence of a harmonic trap, where the trap center undergoes a bounded stochastic modulation. We show that this stochastic modulation drives the system into a nonequilibrium stationary state, where the joint distribution of the positions of the particles is not factorizable. This indicates strong correlations between the positions of the particles that are not inbuilt, but rather get generated by the dynamics itself. Moreover, we show that the stationary joint distribution can be fully characterized and has a special conditionally independent and identically distributed (CIID) structure. This special structure allows us to compute several observables analytically even in such a strongly correlated system, for an arbitrary bounded drive. These observables include the average density profile, the correlations between particle positions, the order and gap statistics, as well as the full counting statistics. We then apply our general results to two specific examples where (i) the drive represents a dichotomous telegraphic noise, and (ii) the drive represents an Ornstein-Uhlenbeck process. Our analytical predictions are verified in numerical simulations, finding excellent agreement.

In this talk starting from a simple proof showing that for a Markovian system connected to two thermal reservoirs with temperatures T1 and T2, the transition rate between two energy states labelled by m and m′, W_{m,m′} cannot be written as W1_{m,m′}+W2_{m,m′}, where W1_{m,m′}, W2_{m,m′} are the transition rates between energy states for systems connected to the corresponding thermal reservoirs, we go on to show that the evolution of a system as a whole connected to two thermal reservoirs is non-Markovian by considering an example of a system made up of two points at different temperatures, each following a Markovian evolution.

Face masks are used to intercept respiratory droplets to prevent spreading of air- borne diseases. Designing face masks with better efficiency needs microscopic understanding on how respiratory droplets move through a mask. Here we study a simple model on the interception of droplets by a face mask. The mask is treated as a polymeric network in an asymmetric confinement, while the droplet is taken as a micrometer-sized tracer colloidal particle, subject to driving force that mimics the breathing. We study numerically, using the Langevin dynamics, the tracer particle permeation through the polymeric network. We show that the permeation is an activated process following an Arrhenius dependence on temperature. The potential energy profile responsible for the activation process increases with tracer size, tracer bead interaction, network rigidity and decreases with the driving force and confinement length. A deeper energy barrier led to better efficiency to intercept the tracer particles of a given size in the presence of driving force at room temperature. Our studies may help to design masks with better efficiency.

In this talk, I will be discussing about some of our recent results on exact moment calculations of a chiral active brownian particle.

We study N qubits having infinite-range Ising interaction and subjected to a periodic pulse of an external magnetic field. We analytically solve the cases of N = 5 to 11 qubits, finding its eigensystem, the dynamics of the entanglement for various initial states, and the unitary evolution operator. These quantities shows signatures of quantum integrability. For the general case of N > 11 qubits, we provide a conjecture on quantum integrability based on the numerical evidence such as degenerate spectrum, and the exact periodic nature of the time-evolved unitary evolution operator and the entanglement dynamics. Using linear entropy, we show that for the class of initial unentangled state, the entanglement periodically displays maximum and zero values.

The thermodynamical behaviour of a three dimensional anisotropic XY ferromagnet driven by elliptically polarized propagating magnetic field wave has been studied by Monte Carlo simulation. Bilinear exchange and single site anisotropies are considered here. The time average magnetisation component was found to show a nonequilibrium phase transition. The order of the phase transition has been found to depend on the amplitude of the propagating polarized field wave and the strength of the anisotropy. The comprehensive phase diagram is drawn. The tricritical line, separating the regions of second order and first order transitions, is also shown.

We characterize collective diffusion in hardcore run-and-tumble particles (RTPs) by calculating the bulk-diffusion coefficient for arbitrary density and tumbling rate. We study two minimal models of RTPs: Model I is a standard version of hardcore RTPs, whereas model II is a long-ranged lattice gas (LLG) with hardcore exclusion - an analytically tractable variant of model I. We calculate the bulk-diffusion coefficient analytically for model II and numerically for model I through an efficient Monte Carlo algorithm; notably, both models have qualitatively similar features. In the strong-persistence regime, the fascinating interplay between persistence and interaction is quantified in terms of two length scales: Persistence length and a “mean free path”, which is simply a measure of the average gap (empty stretch) size in the hopping direction. Notably, for finite density and tumbling rate, we do not find any diffusive instability in the systems, implying that motility-induced phase separation (MIPS) is not possible in the conventional models of RTPs.

Dynamical phase transitions are singular changes in the distribution of dynamical observables and reflect as a non-analyticity in their large deviation functions. I shall present a class of stochastic processes where the distribution of time-integrated (empirical) observables is singular and it relates to phase transitions of dynamical trajectories. These illustrative simple examples include Brownian motion on a sticky surface or in the presence of an absorbing wall where we consider a set of empirical observables like the local time, average velocity, and area. I shall show how to obtain the large deviation function of these observables using a backward Fokker-Planck approach, in which singularities emerge from a competition between survival and diffusion. I shall also discuss an equivalent scenario of these dynamical phase transitions in terms of tilted operators, which is analogous to the familiar mechanism of phase transitions in terms of the degeneracy of the transfer matrix in the Ising model. At the singular point, effective dynamics undergo an abrupt transition. Extending on the tilted operator approach, I shall show that similar phase transitions may generically arise in non-ergodic Markov chains. This scenario is robust and generalizable for non-Markov processes and for many-body systems, which can even lead to a sequence of such dynamical transitions.

We investigate heat transport through a one-dimensional open coupled scalar field theory, depicted as a network of harmonic oscillators connected to thermal baths at the boundaries. The non-Hermitian dynamical matrix of the network undergoes a stability-to-instability transition at the exceptional points as the coupling strength between the scalar fields increases. The open network in the unstable regime, marked by the emergence of inverted oscillator modes, does not acquire a steady state, and the heat conduction is then unbounded for general bath couplings. In this work, we engineer a unique bath coupling where a single bath is connected to two fields at each edge with the same strength. This configuration leads to a finite steady-state heat conduction in the network, even in the unstable regime. We also study general bath couplings, e.g., connecting two fields to two separate baths at each boundary, which shows an exciting signature of approaching the unstable regime for massive fields. We derive analytical expressions for high-temperature classical heat current through the network for different bath couplings at the edges and compare them. Furthermore, we determine the temperature dependence of low-temperature quantum heat current in different cases. Our study will help to probe topological phases and phase transitions in various quadratic Hermitian bosonic models whose dynamical matrices resemble non-Hermitian Hamiltonians, hosting exciting topological phases.

The statement that warm water freezes faster than cold water is commonly known as the Mpemba effect. Named after Erasto Mpemba, he showed in 1969 [1], that for liquid water, two systems similar in every way except their initial temperature, when quenched into a sub-freezing temperature, will show that the system which was initially warmer, undergoes the freezing process faster. In this study, using classical MD simulations of TIP4P/ice water, the process of freezing from liquid water to ice was observed through the direct coexistence method, and for various starting temperatures of liquid water, the time for complete freezing was compared for these temperatures. The comparisons resulted in the observation that the liquid water that was initially hotter took less time to fully freeze as compared to the liquid water that was colder. To obtain a molecular level understanding of the underlying mechanism, two possibilities were considered: that the difference between liquids at high versus low temperatures is either structural or dynamical. We calculate several autocorrelation functions and other structural quantities to get a microscopic origin of the Mpemba effect.
References:
1. E. B. Mpemba and D. G. Osborne, Cool?, Physics Education 4, 172 (1969).

We consider a gas of non-interacting fermions that is released from a box into the vacuum. This provides a simple analytically tractable model that reproduces many features of the Page curve characterizing the evolution of entanglement entropy during evaporation of a black hole. Apart from the entropy we consider several other physical observables and show that generalized hydrodynamics provides a rather surprisingly accurate description of the quantum dynamics.
[Joint work with Madhumita Saha and Manas Kulkarni]

We consider a closed quantum system subjected to stochastic Poissonian resetting. Resetting drives the system to a nonequilibrium stationary state (NESS) with a mixed density matrix which has both classical and quantum correlations. We provide a general framework to study these NESS correlations for a closed quantum system with a general Hamiltonian. We then apply this framework to a simple model of a pair of ferromagnetically coupled spins. We compute exactly the NESS density matrix of the full system. This then provides access to three basic observables, namely, (i) the von Neumann entropy of a subsystem, (ii) the fidelity between the NESS and the initial density matrix, and (iii) the concurrence in the NESS (that provides a measure of the quantum entanglement in a mixed state), as a function of the two parameters: the resetting rate and the interaction strength. One of our main conclusions is that a nonzero resetting rate and a nonzero interaction strength generate quantum entanglement in the NESS (quantified by a nonzero concurrence) and moreover this concurrence can be maximized by appropriately choosing the two parameters. Our results show that quantum resetting provides a simple and effective mechanism to enhance entanglement between two parts of an interacting quantum system.

We will discuss some recent results obtained for the Vicsek model with randomly placed obtsacles.

This work was done with T A de Pirey and Y Kafri, see arXiv:2402.12996. We examine theoretically the flow and concentration fields created by an inclusion in an momentum-conserving active suspension. An inclusion of polar shape is of particular interest, as it is motile in an active medium and needs to be held still by an external force. Advection by the resulting Stokeslet flow proves to be marginal, in the renormalization-group sense, and competes delicately with diffusion. The outcome is an active-particle concentration whose power-law decay with distance from the inclusion is governed by an anomalous exponent determined by the symmetry of the inclusion and varying continuously with the dimensionless advection strength, which also governs the angular dependence of the profile. A pair of pinned inclusions interact non-reciprocally at long distances.

Furukawa predicted that at late times, the domain growth in binary fluids scales as $\ell(t)\sim t^{2/3}$, and the growth is driven by fluid inertia. The {\it inertial growth regime} has been highly elusive in molecular dynamics (MD) simulations. We perform coarsening studies of the Stockmayer (SM) model comprising of magnetic dipoles that interact via long-range dipolar interactions as well as the usual Lennard-Jones (LJ) potential. This fascinating polar fluid exhibits a gas-liquid phase coexistence, and magnetic order even in the absence of an external field. From comprehensive MD simulations, we observe the inertial scaling [$\ell(t)\sim t^{2/3}$] in the SM fluid for an extended time window. The equilibrium morphologies have density dependent shapes which are robust and non-volatile, and exhibit characteristic magnetic properties that have versatile applications.

We discuss new results on the scaling of the energy spectrum in active turbulence and its consequences.

Morphogenetic patterns result from a tight coupling between genetic programs, active mechanochemical processes, and the geometry of shape. While much is known about genetic circuits underlying many biophysical processes, the mechanisms by which this information controls the dynamical evolution of geometrical shapes during development driven by active mechanics is far less explored. We will discuss a hydrodynamic model for describing evolving shapes of tissues wherein the dynamics of the geometry is controlled by active stresses (arising from cell proliferation and topological rearrangements) within the changing geometry. Our results in 1D reveal two distinct ``phases'' with controlled and uncontrolled growth of the tissue size. In 2D, the same framework leads to the emergence of non-trivial shapes.