arXiv subjects

Vikas Malik

Publications and source records attributed to Vikas Malik.

12 recordsLinked to original sources

arXiv2026

Relaxation of the Random Site Coulomb glass Model in Two Dimensions

This study investigates the influence of material density, disorder in onsite energies, and localization length on relaxation dynamics within a two-dimensional random site Coulomb glass model at half-filling. To explore relaxation laws, we calculate the eigenvalue distribution of the linear dynamical matrix using mean-field approximations. Our findings indicate that the system initially undergoes rapid relaxation through energy-lowering transitions. The depletion of the single-particle density of states (DOS) near the Fermi level leads to slow relaxation, with fluctuations diminishing according to a power law. Subsequently, the system adheres to an exponential decay law after a specific period, defined as the relaxation time, which is inversely related to the minimum eigenvalue of the dynamical matrix. As the density of the system decreases, the relaxation rate slows down, resulting in an increase in the relaxation time. For a constant density and localization length, an increase in the disorder of onsite energies results in a longer relaxation time. A significant portion of the eigenvalue spectrum remains unaffected, suggesting that a reduction in localization length concurrent with increased disorder may play an equally vital role in the slow dynamics observed.

cond-mat.dis-nn↗

arXiv2023

Phase Ordering Kinetics of the Asymmetric Coulomb Glass Model

We present results for phase ordering kinetics in the {\it Coulomb glass} (CG) model, which describes electrons on a lattice with unscreened Coulombic repulsion. The filling factor is denoted by $K \in [0,1]$. For a square lattice with $K=0.5$ (symmetric CG), the ground state is a checkerboard with alternating electrons and holes. In this paper, we focus on the asymmetric CG where $K \lesssim 0.5$, i.e., the ground state is checkerboard-like with excess holes distributed uniformly. There is no explicit quenched disorder in our system, though the Coulombic interaction gives rise to frustration. We find that the evolution morphology is in the same dynamical universality class as the ordering ferromagnet. Further, the domain growth law is slightly slower than the {\it Lifshitz-Cahn-Allen} law, $L(t) \sim t^{1/2}$, i.e., the growth exponent is underestimated. We speculate that this could be a signature of logarithmic growth in the asymptotic regime.

cond-mat.stat-mech↗

arXiv2023

The effect of screening on the relaxation dynamics in the Coulomb glass

This paper examines the relaxation dynamics of a two-dimensional Coulomb glass lattice model with high disorders. The study aims to investigate the effects of disorder and Coulomb interactions on glassy dynamics by computing the eigenvalue distribution of the linear dynamical matrix using mean-field approximations. The findings highlight the significance of the single-particle density of states (DOS) as the main controlling parameter affecting the relaxation at intermediate and long times. For the model with unscreened Coulomb interactions, our results indicate that the depletion of the DOS near the Fermi level leads to logarithmic decay at intermediate times. As the relaxation progresses to longer times, a power-law decay emerges, with the exponent approaching zero as the disorder strength increases, suggesting the manifestation of logarithmic decay at high disorders. The effects of screening of interactions on the dynamics are also studied at various screening and disorder strengths. The findings reveal that screening leads to the filling of the gap in the density of states, causing deviation from logarithmic decay at intermediate disorders. Moreover, in the strong disorder regime, the relaxation dynamics are dominated by disorder, and even with screened Coulomb interactions, the electronic relaxation remains similar to the unscreened case. The time at which crossover to exponential decay occurs increases with increasing disorder and interaction strength.

cond-mat.dis-nn↗

arXiv2023

Variable range hopping in a non-equilibrium steady state

We propose a Monte Carlo simulation to understand electron transport in a non-equilibrium steady state (\textit{NESS}) for the lattice Coulomb Glass model, created by continuous excitation of single electrons to high energies followed by relaxation of the system. Around the Fermi level, the \textit{NESS} state approximately obeys the Fermi-Dirac statistics, with an effective temperature ($T_{eff}$) greater than the system's bath temperature ($T$). $T_{eff}$ is a function of $T$ and the rate of photon absorption by the system. Furthermore, we find that the change in conductivity is only a function of relaxation times and is almost independent of the bath temperature. Our results indicate that the conductivity of the \textit{NESS} state can still be characterized by the Efros-Shklovskii law with an effective temperature $T_{eff}>T$. Additionally, the dominance of phonon-less hopping over phonon-assisted hopping is used to explain the relevance of the hot-electron model to the conductivity of the \textit{NESS} state.

cond-mat.dis-nn↗

arXiv2023

A possible phase and dynamical transition in a three-dimensional Electron Glass

Using mean-field approximations, this paper identifies a phase transition in a three-dimensional Electron Glass lattice model. The density of states of the eigenvalue distribution of the inverse susceptibility matrix is used to identify the possibility of a phase transition. In the thermodynamic limit, the eigenvalue spectrum appears to extend to zero as $T \approx T_{c}$. To determine the dynamical relaxation laws near the transition temperature, we use the eigenvalue distribution of the linear dynamical matrix. Our analysis distinguishes between the phenomenon of phase transition and slow dynamics.

cond-mat.dis-nn↗

arXiv2020

Relaxation dynamics of the three-dimensional Coulomb Glass model

In this paper, we analyze the dynamics of the Coulomb Glass lattice model in three dimensions near a local equilibrium state by using mean-field approximations. We specifically focus on understanding the role of localization length ($ξ$) and the temperature ($T$) in the regime where the system is not far from equilibrium. We use the eigenvalue distribution of the dynamical matrix to characterize relaxation laws as a function of localization length at low temperatures. The variation of the minimum eigenvalue of the dynamical matrix with temperature and localization length is discussed numerically and analytically. Our results demonstrate the dominant role played by the localization length on the relaxation laws. For very small localization lengths we find a crossover from exponential relaxation at long times to a logarithmic decay at intermediate times. No logarithmic decay at the intermediate times is observed for large localization lengths.

cond-mat.dis-nn↗

arXiv2020

Charge ordering in the three-dimensional Coulomb glass at finite temperatures and low disorders

In this paper, we have studied the three dimensional Coulomb glass lattice model at half-filling using Monte Carlo Simulations. Annealing of the system shows a second-order transition from paramagnetic to charge-ordered phase for zero as well as small disorders. We have also calculated the critical exponents and transition temperature using a finite sizing scaling approach. The Monte Carlo simulation is done using the Metropolis algorithm, which allowed us to study larger lattice sizes. The transition temperature and the critical exponents at zero disorder matched the previous studies within numerical error. We found that the transition temperature of the system decreased as the disorder is increased. The values of critical exponents $α$ and $γ$ were less and value of $ν$ more than the corresponding zero disorder values. The use of large system sizes led to the correct variation of critical exponents with the disorder.

cond-mat.dis-nn↗

arXiv2019

Finite temperature phase transition in the two-dimensional Coulomb glass at low disorders

We present numerical evidence using Monte Carlo simulations of finite-temperature phase transition in two dimensional Coulomb Glass lattice model with random site energies at half-filling. For the disorder strengths ($W$) studied in this paper, we find the existence of charge-ordered phase (COP) below the critical temperature ($T_{c}(W)$). Also, the probability distribution of staggered magnetization calculated at each W shows a two-peak structure at their respective critical temperature. Thus the phase transition from fluid to COP as a function of temperature is second order for all $W$. We find no evidence of a spin glass phase between a fluid and the COP. Further, we have used a finite-size scaling analysis to calculate the critical exponents. The critical exponents at zero disorder are different from the one found at finite disorders, which indicates that the disorder is a relevant parameter here. The critical exponent for correlation length of $ν$ increases and $T_{c}$ decreases with increasing disorder. Similar behaviour for $ν$ was seen in the work of Overlin et al for three dimensional Coulomb Glass model with a positional disorder. Our study also shows that other critical exponents are also a function of the disorder.

cond-mat.dis-nn↗

arXiv2017

Effect of increasing disorder on domains of the two-dimensional Coulomb glass

We have studied a two dimensional lattice model of Coulomb glass for a wide range of disorders at $T\sim 0$. The system was first annealed using Monte Carlo simulation. Further minimization of the total energy of the system was done using Baranovskii et al algorithm followed by cluster flipping to obtain the pseudo ground states. We have shown that the energy required to create a domain of linear size L in d dimensions is proportional to $L^{d-1}$. Using Imry-Ma arguments given for random field Ising model, one gets critical dimension $d_{c}\geq 2$ for Coulomb glass. The investigations of domains in the transition region shows a discontinuity in staggered magnetization which is an indication of a first-order type transition from charge-ordered phase to disordered phase. The structure and nature of Random field fluctuations of the second largest domain in Coulomb glass are inconsistent with the assumptions of Imry and Ma as was also reported for random field Ising model. The study of domains showed that in the transition region there were mostly two large domains and as disorder was increased, the two large domains remained but there were a large number of small domains. We have also studied the properties of the second largest domain as a function of disorder. We furthermore analysed the effect of disorder on the density of states and showed a transition from hard gap at low disorders to a soft gap at higher disorders. At $W=2$, we have analysed the soft gap in detail and found that the density of states deviates slightly ($δ\approx 1.293 \pm 0.027$) from the linear behaviour in two dimensions. Analysis of local minima show that the pseudo ground states have similar structure.

cond-mat.dis-nn↗

arXiv2017

Critical behaviour in two-dimensional Coulomb Glass at zero temperature

The lattice model of Coulomb Glass in two dimensions with box-type random field distribution is studied at zero temperature for system size upto $96^{2}$. To obtain the minimum energy state we annealed the system using Monte Carlo simulation followed by further minimization using cluster-flipping. The values of the critical exponents are determined using the standard finite size scaling. We found that the correlation length $ξ$ diverges with an exponent $ν=1.0$ at the critical disorder $W_{c} = 0.2253$ and that $χ_{dis} \approx ξ^{4-\barη}$ with $\barη=2$ for the disconnected susceptibility. The staggered magnetization behaves discontinuously around the transition and the critical exponent of magnetization $β=0$. The probability distribution of the staggered magnetization shows a three peak structure which is a characteristic feature for the phase coexistence at first-order phase transition. In addition to this, at the critical disorder we have also studied the properties of the domain for different system sizes. In contradiction with the Imry-Ma arguments, we found pinned and non-compact domains where most of the random field energy was contained in the domain wall. Our results are also inconsistent with Binder's roughening picture.

cond-mat.dis-nn↗

arXiv2017

Study of Domains in the Ground State of the Two Dimensional Coulomb Glass

We have annealed two dimensional lattice model of Coulomb glass using Monte Carlo simulations to obtain the ground state. We have shown that the energy required to create a domain of linear size L in d dimensions is proportional to $L^{d-1}$. Using Imry-Ma arguments given for RFIM, one gets $d_{c}\geq 2$ for Coulomb glass. The investigations in the transition region shows that the domain wall of the metastable state in the charge-ordered phase shifts as disorder is increased to give disordered ground state at higher disorder strength indicating phase coexistence. This coupled with discontinuity in magnetization is an indication of first-order type transition from charge-ordered phase to disordered phase. The structure and nature of Random field fluctuations of the domain in Coulomb glass are inconsistent with the assumptions of Imry and Ma as was also reported for RFIM.

cond-mat.dis-nn↗

arXiv2017

First Order Transition in Two Dimensional Coulomb Glass

We have studied the ground states of two dimensional lattice model of Coulomb Glass via Monte Carlo annealing. Our results show a possibility of existence of a critical disorder (Wc) below which the system is in the charge ordered phase and above it the system is in the disordered phase. We have used finite size scaling to calculate Wc = 0.2413, the critical exponent of magnetization β = 0 indicating discontinuity in magnetization and the critical exponent of correlation length ν = 1.0. The distribution of staggered magnetization for different disorder strengths shows a three peak structure. We thus predict that two dimensional Coulomb Glass shows a first order transition at T=0.

cond-mat.dis-nn↗