arXiv subjects

Abhishek Dhar

Publications and source records attributed to Abhishek Dhar.

At least 55 records · Page 3Linked to original sources

arXiv2022

Finite temperature equilibrium density profiles of integrable systems in confining potentials

We study the equilibrium density profile of particles in two one-dimensional classical integrable models, namely hard rods and the hyperbolic Calogero model, placed in confining potentials. For both of these models the inter-particle repulsion is strong enough to prevent particle trajectories from intersecting. We use field theoretic techniques to compute the density profile and their scaling with system size and temperature, and compare them with results from Monte-Carlo simulations. In both cases we find good agreement between the field theory and simulations. We also consider the case of the Toda model in which inter-particle repulsion is weak and particle trajectories can cross. In this case, we find that a field theoretic description is ill-suited due to the lack of a thermodynamic length scale. The density profiles for the Toda model obtained from Monte-Carlo simulations can be understood by studying the analytically tractable harmonic chain model (Hessian approximation of the Toda model). For the harmonic chain model one can derive an exact expression for the density that shines light on some of the qualitative features of the Toda model in a quadratic trap. Our work provides an analytical approach towards understanding the equilibrium properties for interacting integrable systems in confining traps.

cond-mat.stat-mech↗

arXiv2022

Filling an empty lattice by local injection of quantum particles

We study the quantum dynamics of filling an empty lattice of size $L$, by connecting it locally with an equilibrium thermal bath that injects non-interacting bosons or fermions. We adopt four different approaches, namely (i) direct exact numerics, (ii) Redfield equation, (iii) Lindblad equation, and (iv) quantum Langevin equation -- which are unique in their ways for solving the time dynamics and the steady-state. Our setup offers a simplistic platform to understand fundamental aspects of dynamics and approach to thermalization. The quantities of interest that we consider are the spatial density profile and the total number of bosons/fermions in the lattice. The spatial spread is ballistic in nature and the local occupation eventually settles down owing to equilibration. The ballistic spread of local density admits a universal scaling form. We show that this universality is only seen when the condition of detailed balance is satisfied by the baths. The difference between bosons and fermions shows up in the early time growth rate and the saturation values of the profile. The techniques developed here are applicable to systems in arbitrary dimensions and for arbitrary geometries.

quant-ph↗

arXiv2022

Entropy growth during free expansion of an ideal gas

To illustrate Boltzmann's construction of an entropy function that is defined for a microstate of a macroscopic system, we present here the simple example of the free expansion of a one dimensional gas of non-interacting point particles. The construction requires one to define macrostates, corresponding to macroscopic variables. We define a macrostate $M$ by specifying the fraction of particles in rectangular boxes $Δx Δv$ of the single particle position-velocity space $\{x,v\}$. We verify that when the number of particles is large the Boltzmann entropy, $S_B(t)$, of a typical microstate of a nonequilibrium ensemble coincides with the Gibbs entropy of the coarse-grained time-evolved one-particle distribution associated with this ensemble. $S_B(t)$ approaches its maximum possible value for the dynamical evolution of the given initial state. The rate of approach depends on the size of $Δv$ in the definition of the macrostate, going to zero at any fixed time $t$ when $Δv \to 0$. Surprisingly the different curves $S_B(t)$ collapse when time is scaled with $Δv$ as: $t \sim τ/Δv$. We find an explicit expression for $S_B(τ)$ in the limit $Δv \to 0$. We also consider a different, more hydrodynamical, definition of macrostates for which $S_B(t)$ is monotone increasing, unlike the previous one which has small decaying oscillations near its maximum value. Our system is non-ergodic, non-chaotic and non-interacting; our results thus illustrate that these concepts are not as relevant as sometimes claimed, for observing macroscopic irreversibility and entropy increase. Rather, the notions of initial conditions, typicality, large numbers and coarse-graining are the important factors. We demonstrate these ideas through extensive simulations as well as analytic results.

cond-mat.stat-mech↗

arXiv2022

A splash in a one-dimensional cold gas

We consider a set of hard point particles distributed uniformly with a specified density on the positive half-line and all initially at rest. The particle masses alternate between two values, $m$ and $M$. The particles interact via collisions that conserve energy and momentum. We study the cascade of activity that results when the left-most particle is given a positive velocity. At long times we find that this leads to two fascinating features in the observed dynamics. First, in the bulk of the gas, a shock front develops separating the cold gas from a thermalized region. The shock-front travels sub-ballistically, with the bulk described by self-similar solutions of Euler hydrodynamics. Second, there is a splash region formed by the recoiled particles which move ballistically with negative velocities. The splash region is completely non-hydrodynamic and we propose two conjectures for the long time particle dynamics in this region. We provide a detailed analytic understanding of these coexisting regimes. These are supported by the results of molecular dynamics simulations.

cond-mat.stat-mech↗

arXiv2022

Run-and-tumble particle in one-dimensional confining potential: Steady state, relaxation and first passage properties

We study the dynamics of a one-dimensional run and tumble particle subjected to confining potentials of the type $V(x) = α\, |x|^p$, with $p>0$. The noise that drives the particle dynamics is telegraphic and alternates between $\pm 1$ values. We show that the stationary probability density $P(x)$ has a rich behavior in the $(p, α)$-plane. For $p>1$, the distribution has a finite support in $[x_-,x_+]$ and there is a critical line $α_c(p)$ that separates an active-like phase for $α> α_c(p)$ where $P(x)$ diverges at $x_\pm$, from a passive-like phase for $α< α_c(p)$ where $P(x)$ vanishes at $x_\pm$. For $p<1$, the stationary density $P(x)$ collapses to a delta function at the origin, $P(x) = δ(x)$. In the marginal case $p=1$, we show that, for $α< α_c$, the stationary density $P(x)$ is a symmetric exponential, while for $α> α_c$, it again is a delta function $P(x) = δ(x)$. For the special cases $p=2$ and $p=1$, we obtain exactly the full time-dependent distribution $P(x,t)$, that allows us to study how the system relaxes to its stationary state. In addition, in these two cases, we also study analytically the full distribution of the first-passage time to the origin. Numerical simulations are in complete agreement with our analytical predictions.

cond-mat.stat-mech↗

arXiv2022

Gap Statistics for Confined Particles with Power-Law Interactions

We consider the $N$ particle classical Riesz gas confined in a one-dimensional external harmonic potential with power law interaction of the form $1/r^k$ where $r$ is the separation between particles. As special limits it contains several systems such as Dyson's log-gas ($k\to 0^+$), Calogero-Moser model ($k=2$), 1d one component plasma ($k=-1$) and the hard-rod gas ($k\to \infty$). Despite its growing importance, only large-$N$ field theory and average density profile are known for general $k$. In this Letter, we study the fluctuations in the system by looking at the statistics of the gap between successive particles. This quantity is analogous to the well-known level spacing statistics which is ubiquitous in several branches of physics. We show that the variance goes as $N^{-b_k}$ and we find the $k$ dependence of $b_k$ via direct Monte Carlo simulations. We provide supporting arguments based on microscopic Hessian calculation and a quadratic field theory approach. We compute the gap distribution and study its system size scaling. Except in the range $-1 -2$ with both Gaussian and non-Gaussian scaling forms.

cond-mat.stat-mech↗

arXiv2022

Robustness of Kardar-Parisi-Zhang scaling in a classical integrable spin chain with broken integrability

Recent investigations have observed superdiffusion in integrable classical and quantum spin chains. An intriguing connection between these spin chains and Kardar-Parisi-Zhang (KPZ) universality class has emerged. Theoretical developments (e.g. generalized hydrodynamics) have highlighted the role of integrability as well as spin-symmetry in KPZ behaviour. However understanding their precise role on superdiffusive transport still remains a challenging task. The widely used quantum spin chain platform comes with severe numerical limitations. To circumvent this barrier, we focus on a classical integrable spin chain which was shown to have deep analogy with the quantum spin-$\frac{1}{2}$ Heisenberg chain. Remarkably, we find that KPZ behaviour prevails even when one considers integrability-breaking but spin-symmetry preserving terms, strongly indicating that spin-symmetry plays a central role even in the non-perturbative regime. On the other hand, in the non-perturbative regime, we find that energy correlations exhibit clear diffusive behaviour. We also study the classical analog of out-of-time-ordered correlator (OTOC) and Lyapunov exponents. We find significant presence of chaos for the integrability-broken cases even though KPZ behaviour remains robust. The robustness of KPZ behaviour is demonstrated for a wide class of spin-symmetry preserving integrability-breaking terms.

cond-mat.stat-mech↗

arXiv2022

Fundamental limitations in Lindblad descriptions of systems weakly coupled to baths

It is very common in the literature to write down a Markovian quantum master equation in Lindblad form to describe a system with multiple degrees of freedom and weakly connected to multiple thermal baths which can, in general, be at different temperatures and chemical potentials. However, the microscopically derived quantum master equation up to leading order in system-bath coupling is of the so-called Redfield form which is known to not preserve complete positivity in most cases. Additional approximations to the Redfield equation are required to obtain a Lindblad form. We lay down some fundamental requirements for any further approximations to Redfield equation, which, if violated, leads to physical inconsistencies like inaccuracies in the leading order populations and coherences in the energy eigenbasis, violation of thermalization, violation of local conservation laws at the non-equilibrium steady state (NESS). We argue that one or more of these conditions will generically be violated in all the weak system-bath-coupling Lindblad descriptions existing in literature to our knowledge. As an example, we study the recently derived Universal Lindblad Equation (ULE) and use these conditions to show violation of local conservation laws due to inaccurate coherences but accurate populations in energy eigenbasis. Finally, we exemplify our analytical results numerically in an interacting open quantum spin system.

quant-ph↗

arXiv2021

Heat transport in an ordered harmonic chain in presence of a uniform magnetic field

We consider heat transport across a harmonic chain of charged particles, with transverse degrees of freedom, in the presence of a uniform magnetic field. For an open chain connected to heat baths at the two ends we obtain the nonequilibrium Green's function expression for the heat current. This expression involves two different Green's functions which can be identified as corresponding respectively to scattering processes within or between the two transverse waves. The presence of the magnetic field leads to two phonon bands of the isolated system and we show that the net transmission can be written as a sum of two distinct terms attributable to the two bands. Exact expressions are obtained for the current in the thermodynamic limit, for the the cases of free and fixed boundary conditions. In this limit, we find that at small frequency $ω$, the effective transmission has the frequency-dependence $ω^{3/2}$ and $ω^{1/2}$ for fixed and free boundary conditions respectively. This is in contrast to the zero magnetic field case where the transmission has the dependence $ω^2$ and $ω^0$ for the two boundary conditions respectively, and can be understood as arising from the quadratic low frequency phonon dispersion.

cond-mat.stat-mech↗

arXiv2021

Localization effects due to a random magnetic field on heat transport in a harmonic chain

We consider a harmonic chain of $N$ oscillators in presence of a disordered magnetic field. The ends of the chain are connected to heat baths and we study the effects of the magnetic field randomness on the heat transport. The disorder, in general, causes localization of the normal modes due to which a system becomes insulating. However, for this system, the localization length diverges as the normal mode frequency approaches zero. Therefore, the low frequency modes contribute to the transmission, $\mathcal{T}_N(ω)$, and the heat current goes down as a power law with the system size, $N$. This power law is determined by the small frequency behaviour of some Lyapunov exponent, $λ(ω)$, and the transmission in the thermodynamic limit, $\mathcal{T}_\infty(ω)$. While it is known that in the presence of a constant magnetic field $\mathcal{T}_\infty(ω)\sim ω^{3/2},~ ω^{1/2}$ depending on the boundary conditions, we find that the Lyapunov exponent for the system behaves as $λ(ω)\simω$ for $\expval{B}\neq 0$ and $λ(ω)\simω^{2/3}$ for $\expval{B}=0$. Therefore, we obtain different power laws for current vs $N$ depending on $\expval{B}$ and the boundary conditions.

cond-mat.stat-mech↗

arXiv2021

Blast in a One-Dimensional Cold Gas: From Newtonian Dynamics to Hydrodynamics

A gas composed of a large number of atoms evolving according to Newtonian dynamics is often described by continuum hydrodynamics. Proving this rigorously is an outstanding open problem, and precise numerical demonstrations of the equivalence of the hydrodynamic and microscopic descriptions are rare. We test this equivalence in the context of the evolution of a blast wave, a problem that is expected to be at the limit where hydrodynamics could work. We study a one-dimensional gas at rest with instantaneous localized release of energy for which the hydrodynamic Euler equations admit a self-similar scaling solution. Our microscopic model consists of hard point particles with alternating masses, which is a nonintegrable system with strong mixing dynamics. Our extensive microscopic simulations find a remarkable agreement with Euler hydrodynamics, with deviations in a small core region that are understood as arising due to heat conduction.

cond-mat.stat-mech↗

arXiv2021

Microscopic theory of the fluctuating hydrodynamics in nonlinear lattices

The theory of fluctuating hydrodynamics has been an important tool for analyzing macroscopic behavior in nonlinear lattices. However, despite its practical success, its microscopic derivation is still incomplete. In this work, we provide the microscopic derivation of fluctuating hydrodynamics, using the coarse-graining and projection technique; the equivalence of ensembles turns out to be critical. The Green-Kubo (GK) like formula for the bare transport coefficients is presented in a numerically computable form. Our numerical simulations show that the bare transport coefficients exist for a sufficiently large but finite coarse-graining length in the infinite lattice within the framework of the GK like formula. This demonstrates that the bare transport coefficients uniquely exist for each physical system.

cond-mat.stat-mech↗

arXiv2021

COVID-19: Analytic results for a modified SEIR model and comparison of different intervention strategies

The Susceptible-Exposed-Infected-Recovered (SEIR) epidemiological model is one of the standard models of disease spreading. Here we analyse an extended SEIR model that accounts for asymptomatic carriers, believed to play an important role in COVID-19 transmission. For this model we derive a number of analytic results for important quantities such as the peak number of infections, the time taken to reach the peak and the size of the final affected population. We also propose an accurate way of specifying initial conditions for the numerics (from insufficient data) using the fact that the early time exponential growth is well-described by the dominant eigenvector of the linearized equations. Secondly we explore the effect of different intervention strategies such as social distancing (SD) and testing-quarantining (TQ). The two intervention strategies (SD and TQ) try to reduce the disease reproductive number, $R_0$, to a target value $R^{\rm target}_0 < 1$, but in distinct ways, which we implement in our model equations. We find that for the same $R^{\rm target}_0 < 1$, TQ is more efficient in controlling the pandemic than SD. However, for TQ to be effective, it has to be based on contact tracing and our study quantifies the required ratio of tests-per-day to the number of new cases-per-day. Our analysis shows that the largest eigenvalue of the linearised dynamics provides a simple understanding of the disease progression, both pre- and post- intervention, and explains observed data for many countries. We apply our results to the COVID data for India to obtain heuristic projections for the course of the pandemic, and note that the predictions strongly depend on the assumed fraction of asymptomatic carriers.

q-bio.PE↗

arXiv2021

Revisiting the Mazur bound and the Suzuki equality

Among the few known rigorous results for time-dependent equilibrium correlations, important for understanding transport properties, are the Mazur bound and the Suzuki equality. The Mazur inequality gives a lower bound, on the long-time average of the time-dependent auto-correlation function of observables, in terms of equilibrium correlation functions involving conserved quantities. On the other hand, Suzuki proposes an exact equality for quantum systems. In this paper, we discuss the relation between the two results and in particular, look for the analogue of the Suzuki result for classical systems. This requires us to examine as to what constitutes a complete set of conserved quantities required to saturate the Mazur bound. We present analytic arguments as well as illustrative numerical results from a number of different systems. Our examples include systems with few degrees of freedom as well as many-particle integrable models, both free and interacting.

cond-mat.stat-mech↗

arXiv2021

The Taylor-von Neumann-Sedov blast-wave solution: comparisons with microscopic simulations of a one-dimensional gas

We study the response of an infinite system of point particles on the line initially at rest on the instantaneous release of energy in a localized region. We make a detailed comparison of the hydrodynamic variables predicted by Euler equations for non-dissipative ideal compressible gas and the results of direct microscopic simulations. At long times the profiles of the three conserved variables evolve to self-similar scaling forms, with a scaling exponent as predicted by the Taylor-von Neumann-Sedov (TvNS) blast-wave solution. The scaling functions obtained from the microscopic dynamics show a remarkable agreement with the TvNS predictions, except at the blast core, where the TvNS solution predicts a diverging temperature which is not observed in simulations. We show that the effect of heat conduction becomes important and present results from a numerical solution of the full Navier-Stokes-Fourier equations. A different scaling form is observed in the blast core and this is carefully analyzed. Our microscopic model is the one-dimensional alternate mass hard-particle gas which has the ideal gas equation of state but is non-integrable and known to display fast equilibration.

cond-mat.stat-mech↗

arXiv2021

Equivalence of NEGF and scattering approaches to electron transport in the Kitaev chain

We consider electron transport in a Kitaev chain connected at its two ends to normal metallic leads kept at different temperatures and chemical potentials. Transport in this set-up is usually studied using two frameworks -- the nonequilibrium Green's function (NEGF) approach or the scattering approach. In the NEGF approach the current and other steady state properties of a system are expressed in terms of Green's functions that involve the wire properties and self-energy corrections arising from the leads. In the scattering approach, transport is studied in terms of the scattering amplitudes of plane waves incident on the wire from the reservoirs. Here we show explicitly that these two approaches produce identical results for the conductance of the Kitaev chain. Further we show that the NEGF expression for conductance can be written in such a way that there is a one-to-one correspondence of the various terms in the NEGF expression to the amplitudes for normal transmission, Andreev transmission and Andreev reflection in the scattering approach. Thereby, we obtain closed form expressions for these. We obtain the wavefunctions of zero energy Majorana bound states(MBS) of the wire connected to leads and prove that they are present in the same parameter regime in which they occur for an isolated wire. These bound states give rise to perfect Andreev reflection responsible for zero bias quantized conductance peak. We discuss the dependence of the width of this peak on different parameters of the Hamiltonian and relate it to the MBS wavefunction properties. We find that the peak broadens if the weight of the MBS in the reservoirs increases and vice versa.

cond-mat.supr-con↗

arXiv2020

Quantum Dynamics under continuous projective measurements: non-Hermitian description and the continuous space limit

The problem of the time of arrival of a quantum system in a specified state is considered in the framework of the repeated measurement protocol and in particular the limit of continuous measurements is discussed. It is shown that for a particular choice of system-detector coupling, the Zeno effect is avoided and the system can be described effectively by a non-Hermitian effective Hamiltonian. As a specific example we consider the evolution of a quantum particle on a one-dimensional lattice that is subjected to position measurements at a specific site. By solving the corresponding non-Hermitian wave function evolution equation, we present analytic closed-form results on the survival probability and the first arrival time distribution. Finally we discuss the limit of vanishing lattice spacing and show that this leads to a continuum description where the particle evolves via the free Schrodinger equation with complex Robin boundary conditions at the detector site. Several interesting physical results for this dynamics are presented.

quant-ph↗

arXiv2020

Transport in spinless superconducting wires

We consider electron transport in a model of a spinless superconductor described by a Kitaev type lattice Hamiltonian where the electron interactions are modelled through a superconducting pairing term. The superconductor is sandwiched between two normal metals kept at different temperatures and chemical potentials and are themselves modelled as non-interacting spinless fermions. For this set-up we compute the exact steady state properties of the system using the quantum Langevin equation approach. Closed form exact expressions for particle current, energy current and other two-point correlations are obtained in the Landauer-type forms and involve two nonequilibrium Green's functions. The current expressions are found out to be sum of three terms having simple physical interpretations. We then discuss a numerical approach where we construct the time-evolution of the two point correlators of the system from the eigenspectrum of the complete quadratic Hamiltonian describing the system and leads. By starting from an initial state corresponding to the leads in thermal equilibrium and the system in an arbitrary state, the long time solution for the correlations, before recurrence times, gives us steady state properties. We use this independent numerical method for verifying the results of the exact solution. We also investigate analytically the presence of high energy bound states and obtain expressions for their contributions to two point correlators. As applications of our general formalism we present results on thermal conductance and on the conductance of a Kitaev chain with next nearest neighbour interactions which allows topological phases with different winding numbers.

cond-mat.supr-con↗