arXiv subjects

Punyabrata Pradhan

Publications and source records attributed to Punyabrata Pradhan.

At least 37 records · Page 2Linked to original sources

arXiv2017

Center of mass perturbation as a normalizable estimate of dynamic balance in gait: an application comparing typically developing children with spastic cerebral palsy

It is well recognized that the relative position of the center of mass (pCOM) with respect to the base of support (BOS) is a determining factor in the maintenance of balance. However, during gait the dynamic nature of the BOS is not well defined and in most studies is completely ignored. Prior work tends to focus on the variability in the position of the center of mass (COM) with respect to the laboratory reference frame to attempt to quantify dynamic balance. We propose a modified method whereby the position of the COM in an end-effector based coordinate system may be used as an improved estimate of threat to balance during gait. The distance (DN) of the projection of the COM (pCOM) from the reference axis, normalized by half the foot length for inter-subject comparison, is an estimate of the gravitational moment arm. It can be shown that in concordance with theory, balance impaired subjects with spastic cerebral palsy (CP) have a larger DN than their typically developing peers. Furthermore, when compared with variational methods, DN shows better discriminative ability in characterizing groups.

physics.med-ph↗

arXiv2016

Spatial correlations, additivity and fluctuations in conserved-mass transport processes

We exactly calculate two-point spatial correlation functions in steady state in a broad class of conserved-mass transport processes, which are governed by chipping, diffusion and coalescence of masses. We find that the spatial correlations are in general short-ranged and consequently, on a large scale, these conserved-mass transport processes possess a remarkable thermodynamic structure in the steady state. That is, the processes have an equilibriumlike additivity property, and a corresponding fluctuation-response relation, which helps us to obtain subsystem mass distributions in the limit of subsystem size large.

cond-mat.stat-mech↗

arXiv2016

Symmetric exclusion processes on a ring with moving defects

We study symmetric simple exclusion processes (SSEP) on a ring in the presence of uniformly moving multiple defects or disorders - a generalization of the model proposed earlier [Phys. Rev. E 89, 022138 (2014)]. The defects move with uniform velocity and change the particle hopping rates locally. We explore the collective effects of the defects on the spatial structure and transport properties of the system. We also introduce an SSEP with ordered sequential (sitewise) update and elucidate the close connection with our model.

cond-mat.stat-mech↗

arXiv2016

Additivity, density fluctuations, and nonequilibrium thermodynamics for active Brownian particles

Using an additivity property, we study particle-number fluctuations in a system of interacting self-propelled particles, called active Brownian particles (ABPs), which consists of repulsive disks with random self-propulsion velocities. From a fluctuation-response relation - a direct consequence of additivity, we formulate a thermodynamic theory which captures the previously observed features of nonequilibrium phase transition in the ABPs from a homogeneous fluid phase to an inhomogeneous phase of coexisting gas and liquid. We substantiate the predictions of additivity by analytically calculating the subsystem particle-number distributions in the homogeneous fluid phase away from criticality where analytically obtained distributions are compatible with simulations in the ABPs.

cond-mat.stat-mech↗

arXiv2015

Additivity property and emergence of power laws in nonequilibrium steady states

We show that an equilibriumlike additivity property can remarkably lead to power-law distributions observed frequently in a wide class of out-of-equilibrium systems. The additivity property can determine the full scaling form of the distribution functions and the associated exponents. The asymptotic behavior of these distributions is solely governed by branch-cut singularity in the variance of subsystem mass. To substantiate these claims, we explicitly calculate, using the additivity property, subsystem mass distributions in a wide class of previously studied mass aggregation models as well as in their variants. These results could help in the thermodynamic characterization of nonequilibrium critical phenomena.

cond-mat.stat-mech↗

arXiv2015

Cluster-factorized steady states in finite range processes

We study a class of nonequilibrium lattice models on a ring where particles hop in a particular direction, from a site to one of its (say, right) nearest neighbours, with a rate that depends on the occupation of all the neighbouring sites within a range R. This finite range process (FRP) for R=0 reduces to the well known zero-range process (ZRP), giving rise to a factorized steady state (FSS) for any arbitrary hop rate. We show that, provided the hop rates satisfy a specific condition, the steady state of FRP can be written as a product of cluster-weight function of (R+1) occupation variables. We show that, for a large class of cluster-weight functions, the cluster-factorized steady state admits a finite dimensional transfer-matrix formulation, which helps in calculating the spatial correlation functions and subsystem mass distributions exactly. We also discuss a criterion for which the FRP undergoes a condensation transition.

cond-mat.stat-mech↗

arXiv2015

Zeroth law of thermodynamics for nonequilibrium steady states in contact

We ask what happens when two systems having a nonequilibrium steady state are kept in contact and allowed to exchange a quantity, say mass, which is conserved in the combined system. Will the systems eventually evolve to a new stationary state where certain intensive thermodynamic variable, like equilibrium chemical potential, equalizes following zeroth law of thermodynamics and, if so, under what conditions is it possible? We argue that the zeroth law would hold, provided both systems have short-ranged spatial correlations and they {\it interact weakly} to exchange mass with rates satisfying a balance condition - reminiscent of detailed balance in equilibrium. This proposition is proved for driven systems in general in the limit of small exchange rates (i.e., weak interaction) and is demonstrated} in various conserved-mass transport processes having nonzero spatial correlations.

cond-mat.stat-mech↗

arXiv2014

Interacting particles in a periodically moving potential: Traveling wave and transport

We study a system of interacting particles in a periodically moving external potential, within the simplest possible description of paradigmatic symmetric exclusion process on a ring. The model describes diffusion of hardcore particles where the diffusion dynamics is locally modified at a uniformly moving defect site, mimicking the effect of the periodically moving external potential. The model, though simple, exhibits remarkably rich features in particle transport, such as polarity reversal and double peaks in particle current upon variation of defect velocity and particle density. By tuning these variables, the most efficient transport can be achieved in either direction along the ring. These features can be understood in terms of a traveling density wave propagating in the system. Our results could be experimentally tested, e.g., in a system of colloidal particles driven by a moving optical tweezer.

cond-mat.stat-mech↗

arXiv2014

Gammalike mass distributions and mass fluctuations in conserved-mass transport processes

We show that, in conserved-mass transport processes, the steady-state distribution of mass in a subsystem is uniquely determined from the functional dependence of variance of the subsystem mass on its mean, provided that joint mass distribution of subsystems is factorized in the thermodynamic limit. The factorization condition is not too restrictive as it would hold in systems with short-ranged spatial correlations. To demonstrate the result, we revisit a broad class of mass transport models and its generic variants, and show that the variance of subsystem mass in these models is proportional to square of its mean. This particular functional form of the variance constrains the subsystem mass distribution to be a gamma distribution irrespective of the dynamical rules.

cond-mat.stat-mech↗

arXiv2011

Thermodynamic Theory of Phase Transitions in Driven Lattice Gases

We formulate an approximate thermodynamic theory of the phase transition in driven lattice gases with attractive nearest-neighbor interactions. We construct the van der Waals equation of state for a driven system where a nonequilibrium chemical potential can be expressed as a function of density and driving field. A Maxwell's construction leads to the phase transition from a homogeneous fluid phase to the coexisting phases of gas and liquid.

cond-mat.stat-mech↗

arXiv2011

Approximate thermodynamic structure for driven lattice gases in contact

For a class of nonequilibrium systems, called driven lattice gases, we study what happens when two systems are kept in contact and allowed to exchange particles with the total number of particles conserved. Both for attractive and repulsive nearest-neighbor interactions among particles and for a wide range of parameter values, we find that, to a good approximation, one could define an intensive thermodynamic variable, like equilibrium chemical potential, which determines the final steady state for two initially separated driven lattice gases brought into contact. However, due to nontrivial contact dynamics, there are also observable deviations from this simple thermodynamic law. To illustrate the role of the contact dynamics, we study a variant of the zero range process and discuss how the deviations could be explained by a modified large deviation principle. We identify an additional contribution to the large deviation function, which we call the excess chemical potential, for the variant of the zero range process as well as the driven lattice gases. The excess chemical potential depends on the specifics of the contact dynamics and is in general a-priori unknown. A contact dependence implies that even though an intensive variable may equalize, the zeroth law could still be violated.

cond-mat.stat-mech↗

arXiv2010

Nonequilibrium steady states in contact: Approximate thermodynamic structure and zero-th law for driven lattice gases

We explore driven lattice gases for the existence of an intensive thermodynamic variable which could determine "equilibration" between two nonequilibrium steady-state systems kept in weak contact. In simulations, we find that these systems satisfy surprisingly simple thermodynamic laws, such as the zero-th law and the fluctuation-response relation between the particle-number fluctuation and the corresponding susceptibility remarkably well. However at higher densities, small but observable deviations from these laws occur due to nontrivial contact dynamics and the presence of long-range spatial correlations.

cond-mat.stat-mech↗

arXiv2010

Nonequilibrium fluctuation theorems in the presence of a time-reversal symmetry-breaking field and nonconservative forces

We study nonequilibrium fluctuation theorems in the presence of a time-reversal symmetry-breaking field and nonconservative forces, in a stochastic as well as a deterministic set up. We consider a system and a heat bath, called the combined system, and show that the fluctuation theorems are valid even when the heat bath goes out of equilibrium during driving. The only requirement for the validity is that, when the driving is switched off, the combined system relaxes to a state having a uniform probability measure on a constant energy surface, consistent with microcanonical ensemble of an isolated system.

cond-mat.stat-mech↗

arXiv2010

Nonexistence of classical diamagnetism and nonequilibrium fluctuation theorems for charged particles on a curved surface

We show that the classical Langevin dynamics for a charged particle on a closed curved surface in a time-independent magnetic field leads to the canonical distribution in the long time limit. Thus the Bohr-van Leeuwen theorem holds even for a finite system without any boundary and the average magnetic moment is zero. This is contrary to the recent claim by Kumar and Kumar (EPL, {\bf 86} (2009) 17001), obtained from numerical analysis of Langevin dynamics, that a classical charged particle on the surface of a sphere in the presence of a magnetic field has a nonzero average diamagnetic moment. We extend our analysis to a many-particle system on a curved surface and show that the nonequilibrium fluctuation theorems also hold in this geometry.

cond-mat.stat-mech↗

arXiv2009

Ion transport through confined ion channels in the presence of immobile charges

We study charge transport in an ionic solution in a confined nanoscale geometry in the presence of an externally applied electric field and immobile background charges. For a range of parameters, the ion current shows non-monotonic behavior as a function of the external ion concentration. For small applied electric field, the ion transport can be understood from simple analytic arguments, which are supported by Monte Carlo simulation. The results qualitatively explain measurements of ion current seen in a recent experiment on ion transport through a DNA-threaded nanopore (D. J. Bonthuis et. al., Phys. Rev. Lett, vol. 97, 128104 (2006)).

cond-mat.stat-mech↗

arXiv2008

Non-equilibrium fluctuation theorems in the presence of local heating

We study two non-equilibrium work fluctuation theorems, the Crooks' theorem and the Jarzynski equality, for a test system coupled to a spatially extended heat reservoir whose degrees of freedom are explicitly modeled. The sufficient conditions for the validity of the theorems are discussed in detail and compared to the case of classical Hamiltonian dynamics. When the conditions are met the fluctuation theorems are shown to hold despite the fact that the immediate vicinity of the test system goes out of equilibrium during an irreversible process. We also study the effect of the coupling to the heat reservoir on the convergence of $<\exp(-βW)>$ to its theoretical mean value, where $W$ is the work done on the test system and $β$ is the inverse temperature. It is shown that the larger the local heating, the slower the convergence.

cond-mat.stat-mech↗

arXiv2006

Sampling rare fluctuations of height in the Oslo ricepile model

We have studied large deviations of the height of the pile from its mean value in the Oslo ricepile model. We sampled these very rare events with probabilities of order $10^{-100}$ by Monte Carlo simulations using importance sampling. These simulations check our qualitative arguement [Phys. Rev. E, {\bf 73}, 021303, 2006] that in steady state of the Oslo ricepile model, the probability of large negative height fluctuations $Δh=-αL$ about the mean varies as $\exp(-κα^4 L^3)$ as $L \to \infty$ with $α$ held fixed, and $κ> 0$.

cond-mat.stat-mech↗

arXiv2005

Probability distribution of residence times of grains in models of ricepiles

We study the probability distribution of residence time of a grain at a site, and its total residence time inside a pile, in different ricepile models. The tails of these distributions are dominated by the grains that get deeply buried in the pile. We show that, for a pile of size $L$, the probabilities that the residence time at a site or the total residence time is greater than $t$, both decay as $1/t(\ln t)^x$ for $L^ω \ll t \ll \exp(L^γ)$ where $γ$ is an exponent $ \ge 1$, and values of $x$ and $ω$ in the two cases are different. In the Oslo ricepile model we find that the probability that the residence time $T_i$ at a site $i$ being greater than or equal to $t$, is a non-monotonic function of $L$ for a fixed $t$ and does not obey simple scaling. For model in $d$ dimensions, we show that the probability of minimum slope configuration in the steady state, for large $L$, varies as $\exp(-κL^{d+2})$ where $κ$ is a constant, and hence $ γ= d+2$.

cond-mat.stat-mech↗