arXiv subjects

Reza Sepehrinia

Publications and source records attributed to Reza Sepehrinia.

16 recordsLinked to original sources

arXiv2024

Griffiths singularity in quasi-one dimensional restricted $\pm J$ Ising spin glass

We obtain the exact ground state energy of the quasi-one dimensional restricted $\pm J$ Ising spin glass in a uniform magnetic field using the transfer matrix method. Magnetic field dependence allows us to derive the magnetization as a function of concentration and magnetic field. It turns out that, in the limit of zero field, the magnetization tends to a nonzero value with a singular dependence on the magnetic field. We derive the explicit form of the singularity in thermodynamic quantities such as energy $E\simeq E_0+m_0 h + E_1e^{-h_0/h}$, which is an essential singularity known as Griffiths singularity. We confirm our analytical results using the numerical approach based on iterative equations for energy.

cond-mat.dis-nn↗

arXiv2024

AC conductivity and magnetic dichroism of two-dimensional antiferromagnetic Dirac semimetals

We investigate the magneto-optical properties of two-dimensional nonsymmorphic Dirac semimetals in the presence of antiferromagnetic order. Using the Kubo formula, we calculate the conductivity tensor of two-dimensional CuMnAs, a prototype antiferromagnetic Dirac material, as a function of light frequency. From the finite-frequency conductivity tensor, we derive the dynamic dielectric function and magnetic linear dichroism, demonstrating how they are influenced by the orientation of the N{é}el order parameter. Adjusting the N{é}el vector changes both the sign and amplitude of the system's magneto-optical response. We propose that magnetic linear dichroism spectroscopy is a powerful technique for determining the orientation of the N{é}el vector.

cond-mat.mes-hall↗

arXiv2023

Magnetic proximity in a coupled Ferromagnet-Spin glass system

We study the competition between ferromagnetic and spin glass phases using a system of coupled infinite-range Ising and Sherrington-Kirkpatrick models. We obtain the replica-symmetric solution for the free energy of this system in terms of magnetization and Edwards-Anderson order parameters in both subsystems. Using these order parameters we are able to identify different phases in the system and determine which phase is dominant for different strengths of the coupling between two subsystems. We observe that both subsystems are in the same phase although with different order parameters. The phase boundary between paramagnetic-ferromagnetic and paramagnetic-spin glass phases is more or less similar to that in the Sherrington-Kirkpatrick phase diagram. But the boundary between ferromagnetic-spin glass phases becomes qualitatively different as the coupling between the two subsystems changes. Remarkably, for intermediate values of the coupling, this phase boundary is such that there could be a reentrant transition between spin glass and ferromagnetic phases by increasing the temperature. We found that for some range of the coupling, the second order transition between these phases turns into first order at a tricritical point. Further, we carry out the stability analysis by considering deviations from the replica symmetric solution.

cond-mat.dis-nn↗

arXiv2022

Localization in a one-dimensional alloy with an arbitrary distribution of spacing between impurities: Application to Lévy glass

We have studied the localization of waves in a one-dimensional lattice consisting of impurities where the spacing between consecutive impurities can take certain values with given probabilities. In general, such a distribution of impurities induces correlations in the disorder. In particular with a power-law distribution of spacing, this system is used as a model for light propagation in Lévy glasses. We introduce a method of calculating the Lyapunov exponent which overcomes limitations in the previous studies and can be easily extended to higher orders of perturbation theory. We obtain the Lyapunov exponent up to fourth order of perturbation and discuss the range of validity of perturbation theory, transparent states, and anomalous energies which are characterized by divergences in different orders of the expansion. We also carry out numerical simulations which are in agreement with our analytical results.

cond-mat.dis-nn↗

arXiv2021

Localization and delocalization in one-dimensional systems with translation-invariant hopping

We present a theory of Anderson localization on a one-dimensional lattice with translation-invariant hopping. We find by analytical calculation, the localization length for arbitrary finite-range hopping in the single propagating channel regime. Then by examining the convergence of the localization length, in the limit of infinite hopping range, we revisit the problem of localization criteria in this model and investigate the conditions under which it can be violated. Our results reveal possibilities of having delocalized states by tuning the long-range hopping.

cond-mat.dis-nn↗

arXiv2019

Random Walks on Intersecting Geometries

We present an analytical approach to study simple symmetric random walks (RWs) on a crossing geometry consisting of a plane square lattice crossed by $n_l$ number of lines that all meet each other at a single point (the origin) on the plane. The probability density to find the walker at a given distance from the origin either in a line or in the plane geometry is exactly calculated as a function of time t. We find that the large time asymptotic behavior of the walker for any arbitrary number $n_l$ of lines is eventually governed by the plane geometry after a crossover time approximately given by $t_c\propto n_l^2$. We show that this competition can be changed in favor of the line geometry by switching on an arbitrarily small perturbation of a drift term in which even a weak biased walk is able to drain the whole probability density into the line at long time limit. We also present the results of our extensive simulations of the model which perfectly support our analytical predictions. Our method can, however, be simply extended to other crossing geometries with a single common point.

cond-mat.stat-mech↗

arXiv2018

Ground state properties of Ising chain with random monomer-dimer couplings

We study analytically the one-dimensional Ising model with a random binary distribution of ferromagnetic and antiferromagnetic exchange couplings at zero temperature. We introduce correlations in the disorder by assigning a dimer of one type of coupling with probability $x$, and a monomer of the other type with probability $1-x$. We find that the magnetization behaves differently from the original binary model. In particular, depending on which type of coupling comes in dimers, magnetization jumps vanish at a certain set of critical fields. We explain the results based on the structure of ground state spin configuration.

cond-mat.dis-nn↗

arXiv2018

Ground-state magnetization of the Ising spin glass: A recursive numerical method and Chen-Ma scaling

The ground-state properties of quasi-one-dimensional (Q1D) Ising spin glass are investigated using an exact numerical approach and analytical arguments. A set of coupled recursive equations for the ground-state energy are introduced and solved numerically. For various types of coupling distribution, we obtain accurate results for magnetization, particularly in the presence of a weak external magnetic field. We show that in the weak magnetic field limit, similar to the 1D model, magnetization exhibits a singular power-law behavior with divergent susceptibility. Remarkably, the spectrum of magnetic exponents is markedly different from that of the 1D system even in the case of two coupled chains. The magnetic exponent makes a crossover from being dependent on the distribution function to a constant value independent of distribution. We provide an analytic theory for these observations by extending the Chen-Ma argument to the Q1D case. We derive an analytical formula for the exponent which is in perfect agreement with the numerical results.

cond-mat.dis-nn↗

arXiv2016

Is $n\sinθ$ conserved along light path?

Snell's law states that the quantity $n\sinθ$ is unchanged in refraction of light passing from one medium to another. We inquire whether this is true in the general case where the speed of light varies continuously within a medium. It turns out to be an instructive exercise in application of Snell's law and Fermat's principle. It also provides good pedagogical problems in calculus of variations to deal with the subtleties of a variable domain of integration and inclusion of constraints. The final result of these exercises is that, contrary to an initial expectation, the answer to the question in the title is negative.

physics.optics↗

arXiv2016

Quantum-classical equivalence and ground-state factorization

We have performed an analytical study of quantum-classical equivalence for quantum $XY$-spin chains with arbitrary interactions to explore the classical counterpart of the factorizing magnetic fields that drive the system into a separable ground state. We demonstrate that the factorizing line in parameter space of a quantum model is equivalent to the so-called natural boundary that emerges in mapping the quantum $XY$-model onto the two dimensional classical Ising model. As a result, we show that the quantum systems with the non-factorizable ground state could not be mapped onto the classical Ising model. Based on the presented correspondence we suggest a promising method for obtaining the factorizing field of quantum systems through the commutation of the quantum Hamiltonian and the transfer matrix of the classical model.

cond-mat.stat-mech↗

arXiv2015

Quantum diffusion of a relativistic particle in a time-dependent random potential

We present a rigorous study of quantum diffusion of a relativistic particle subjected to a time-dependent random potential with $δ$ correlation in time. We find that in the asymptotic time limit the particle wave packet spreads ballistically in contrast with the nonrelativistic case, which in the same situation exhibits superballistic diffusion. The relativistic suppression of wave packet diffusion is discussed in connection with statistical conservation laws that follow from relativistic dynamics.

cond-mat.dis-nn↗

arXiv2014

Anomalies in conductance and localization length of disordered ladders

We discuss the conditions under which an anomaly occurs in conductance and localization length of Anderson model on a lattice. Using the ladder hamiltonian and analytical calculation of average conductance we find the set of resonance conditions which complements the $π$-coupling rule for anomalies. We identify those anomalies that might vanish due to the symmetry of the lattice or the distribution of the disorder. In terms of the dispersion relation it is known from strictly one-dimensional model that the lowest order (i.e., the most strong) anomalies satisfy the equation $E(k)=E(3k)$. We show that the anomalies of the generalized model studied here are also the solutions of the same equation with modified dispersion relation.

cond-mat.dis-nn↗

arXiv2013

Irrational anomalies in one-dimensional Anderson localization

We revisit the problem of one-dimensional Anderson localization, by providing perturbative expression for Lyapunov exponent of Anderson model with next-nearest-neighbor (nnn) hopping. By comparison with exact numerical results, we discuss the range of validity of the naive perturbation theory. The stability of band center anomaly is examined against the introduction of nnn hopping. New anomalies of Kappus-Wegner type emerge at nonuniversal values of wavelength when hopping to second neighbor is allowed. It is shown that covariances in the first order of perturbation theory, develop singularities at these resonant energies which enable us to locate them.

cond-mat.dis-nn↗

arXiv2013

Dynamic renormalization group analysis of propagation of elastic waves in two-dimensional heterogeneous media

We study localization of elastic waves in two-dimensional heterogeneous solids with randomly distributed Lamé coefficients, as well as those with long-range correlations with a power-law correlation function. The Matin-Siggia-Rose method is used, and the one-loop renormalization group (RG) equations for the the coupling constants are derived in the limit of long wavelengths. The various phases of the coupling constants space, which depend on the value $ρ$, the exponent that characterizes the power-law correlation function, are determined and described. Qualitatively different behaviors emerge for $ρ<1$ and $ρ>1$. The Gaussian fixed point (FP) is stable (unstable) for $ρ<1$ ($ρ>1$). For $ρ<1$ there is a region of the coupling constants space in which the RG flows are toward the Gaussian FP, implying that the disorder is irrelevant and the waves are delocalized. In the rest of the disorder space the elastic waves are localized. We compare the results with those obtained previously for acoustic wave propagation in the same type of heterogeneous media, and describe the similarities and differences between the two phenomena.

cond-mat.dis-nn↗

arXiv2013

Numerical simulation of the localization of elastic waves in two- and three-dimensional heterogeneous media

Localization of elastic waves in two-dimensional (2D) and three-dimensional (3D) media with random distributions of the Lamé coefficients (the shear and bulk moduli) is studied, using extensive numerical simulations. We compute the frequency-dependence of the minimum positive Lyapunov exponent $γ$ (the inverse of the localization length) using the transfer-matrix method, the density of states utilizing the force-oscillator method, and the energy-level statistics of the media. The results indicate that all the states may be localized in the 2D media, up to the disorder width and the smallest frequencies considered, although the numerical results also hint at the possibility that there might a small range of the allowed frequencies over which a mobility edge might exist. In the 3D media, however, most of the states are extended, with only a small part of the spectrum in the upper band tail that contains localized states, even if the Lamé coefficients are randomly distributed. Thus, the 3D heterogeneous media still possess a mobility edge. If both Lamé coefficients vary spatially in the 3D medium, the localization length $Λ$ follows a power law near the mobility edge, $Λ\sim(Ω-Ω_c)^{-ν}$, where $Ω_c$ is the critical frequency. The numerical simulation yields, $ν\simeq 1.89\pm 0.17$, significantly larger than the numerical estimate, $ν\simeq 1.57\pm 0.01$, and $ν=3/2$, which was recently derived by a semiclassical theory for the 3D Anderson model of electron localization...

cond-mat.dis-nn↗

arXiv2010

Universality of Anderson transition in two-dimensional systems of symplectic symmetry class

We investigate localization of noninteracting particles with spins higher than 1/2 in a two-dimensional random potential in presence of spin-orbit coupling. We consider an integer spin ($s=1$) and a half-integer spin ($s=3/2$) belonging to orthogonal and symplectic symmetry classes, respectively. We show that particles with integer spin are localized and those with half-integer spin exhibit Anderson transition. The transition belongs to universality class of conventional symplectic model for spin-1/2 particles.

cond-mat.dis-nn↗