arXiv subjects

M. Ghasemi Nezhadhaghighi

Publications and source records attributed to M. Ghasemi Nezhadhaghighi.

11 recordsLinked to original sources

arXiv2021

Fractional Stochastic Loewner Evolution and Scaling Curves

The Stochastic Loewner equation, introduced by Schramm, gives us a powerful way to study and classify critical random curves and interfaces in two-dimensional statistical mechanics. New kind of stochastic Loewner equation, called fractional stochastic Loewner evolution (FSLE), has been proposed for the first time. Using the fractional time series as the driving function of the Loewner equation and local fractional integrodifferential operators, we introduce a large class of fractal curves. We argue that the FSLE curves, besides the fractal dimension calculations, have crucial differences which caused by the Hurst index of the driving function. This extension opens a new way to classify different types of scaling curves based on the Hurst index of the corresponding driving function. Such formalism appear to be suitable to deal with the study of a wide range of two-dimensional curves appearing in statistical mechanics or natural phenomena.

cond-mat.stat-mech↗

arXiv2016

Crossing Statistics of Anisotropic Stochastic Surface

In this paper, we propose crossing statistics and its generalization, as a new framework to characterize the anisotropy in a 2D field, e.g. height on a surface, extendable to higher dimensions. By measuring $ν^+$, the number of up-crossing (crossing points with positive slope at a given threshold of height ($α$)), and $N_{tot}$ (the generalized roughness function), it is possible to distinguish the nature of anisotropy, rotational invariance and Gaussianity of any given surface. For the case of anisotropic correlated self- or multi-affine surfaces (even with different correlation lengths in various directions and/or directional scaling exponents), we analytically derive some relations between $ν^+$ and $N_{tot}$ with corresponding scaling parameters. The method systematically distinguishes the directions of anisotropy, at $3σ$ confidence interval using P-value statistics. After applying a typical method in determining the corresponding scaling exponents in identified anisotropic directions, we are able to determine the kind and ratio of correlation length anisotropy. To demonstrate capability and accuracy of the method, as well validity of analytical relations, our proposed measures are calculated on synthetic stochastic rough interfaces and rough interfaces generated from simulation of ion etching. There are good consistencies between analytical and numerical computations. The proposed algorithm can be mounted with a simple software on various instruments for surface analysis and characterization, such as AFM, STM and etc.

physics.comp-ph↗

arXiv2016

Conformal symmetry of electron-hole puddles in ungated graphene

In this paper the mono-layer graphene at the charge neutrality point is considered whithin Thomas-Fermi-Dirac theory, treating inhomogeneous external potentials and electron-electron interactions on equal footing. We present some general considerations concerning the probability measure of the ground state charge density. The system shows degrees of self-similarity. By analyzing the ground state carrier density profile, we show that although it is not Gaussian, the critical exponents are consistent with Kondev hyper-scaling relations. Using Schramm-Loewner (SLE) evolution we show that the ungated graphene has conformal invariance and the random zero-charge density contours are SLE$_κ$ with $κ=1.8\pm 0.2$.

cond-mat.stat-mech↗

arXiv2016

Scale-invariant puddles in Graphene: Geometric properties of electron-hole distribution at the Dirac point

We characterize the carrier density profile of the ground state of graphene in the presence of particle-particle interaction and random charged impurity for zero gate voltage. We provide detailed analysis on the resulting spatially inhomogeneous electron gas taking into account the particle-particle interaction and the remote coulomb disorder on an equal footing within the Thomas-Fermi-Dirac theory. We present some general features of the carrier density probability measure of the graphene sheet. We also show that, when viewed as a random surface, the resulting electron-hole puddles at zero chemical potential show peculiar self-similar statistical properties. Although the disorder potential is chosen to be Gaussian, we show that the charge field is non-Gaussian with unusual Kondev relations which can be regarded as a new class of two-dimensional (2D) random-field surfaces.

cond-mat.mes-hall↗

arXiv2015

Controlling Anderson localization in disordered heterostructures with Lévy-type distribution

In this paper, we propose a disordered heterostructure in which the distribution of refractive index of one of its constituents follows a Lévy-type distribution characterized by the exponent $α$. For the normal and oblique incidences, the effect of $α$ variation on the localization length is investigated in different frequency ranges. As a result, the controllability of Anderson localization can be achieved by changing the exponent $α$ in the disordered structure having heavy tailed distribution.

cond-mat.stat-mech↗

arXiv2014

Entanglement dynamics in short and long-range harmonic oscillators

We study the time evolution of the entanglement entropy in the short and long-range coupled harmonic oscillators that have well-defined continuum limit field theories. We first introduce a method to calculate the entanglement evolution in generic coupled harmonic oscillators after quantum quench. Then we study the entanglement evolution after quantum quench in harmonic systems that the couplings decay effectively as $1/r^{d+α}$ with the distance $r$. After quenching the mass from non-zero value to zero we calculate numerically the time evolution of von Neumann and Rényi entropies. We show that for $1<α<2$ we have a linear growth of entanglement and then saturation independent of the initial state. For $0<α<1$ depending on the initial state we can have logarithmic growth or just fluctuation of entanglement. We also calculate the mutual information dynamics of two separated individual harmonic oscillators. Our findings suggest that in our system there is no particular connection between having a linear growth of entanglement after quantum quench and having a maximum group velocity or generalized Lieb-Robinson bound.

cond-mat.stat-mech↗

arXiv2013

Quantum entanglement entropy and classical mutual information in long-range harmonic oscillators

We study different aspects of quantum von Neumann and Rényi entanglement entropy of one dimensional long-range harmonic oscillators that can be described by well-defined non-local field theories. We show that the entanglement entropy of one interval with respect to the rest changes logarithmically with the number of oscillators inside the subsystem. This is true also in the presence of different boundary conditions. We show that the coefficients of the logarithms coming from different boundary conditions can be reduced to just two different universal coefficients. We also study the effect of the mass and temperature on the entanglement entropy of the system in different situations. The universality of our results is also confirmed by changing different parameters in the coupled harmonic oscillators. We also show that more general interactions coming from general singular Toeplitz matrices can be decomposed to our long-range harmonic oscillators. Despite the long-range nature of the couplings we show that the area law is valid in two dimensions and the universal logarithmic terms appear if we consider subregions with sharp corners. Finally we study analytically different aspects of the mutual information such as its logarithmic dependence to the subsystem, effect of mass and influence of the boundary. We also generalize our results in this case to general singular Toeplitz matrices and higher dimensions.

cond-mat.stat-mech↗

arXiv2013

Entanglement entropy in long-range harmonic oscillators

We study the Von Neumann and Rényi entanglement entropy of long-range harmonic oscillators (LRHO) by both theoretical and numerical means. We show that the entanglement entropy in massless harmonic oscillators increases logarithmically with the sub-system size as $S=\frac{c_{eff}}{3}\log l$. Although the entanglement entropy of LRHO's shares some similarities with the entanglement entropy at conformal critical points we show that the Rényi entanglement entropy presents some deviations from the expected conformal behavior. In the massive case we demonstrate that the behavior of the entanglement entropy with respect to the correlation length is also logarithmic as the short range case.

cond-mat.stat-mech↗

arXiv2011

First passage time processes and subordinated SLE

We study the first passage time processes of anomalous diffusion on self similar curves in two dimensions. The scaling properties of the mean square displacement and mean first passage time of the ballistic motion, fractional Brownian motion and subordinated walk on different fractal curves (loop erased random walk, harmonic explorer and percolation front) are derived. We also define natural parametrized subordinated Schramm Loewner evolution (NS-SLE) as a mathematical tool that can model diffusion on fractal curves. The scaling properties of the mean square displacement and mean first passage time for NS-SLE are obtained by numerical means.

cond-mat.stat-mech↗

arXiv2011

Contour lines of the discrete scale invariant rough surfaces

We study the fractal properties of the 2d discrete scale invariant (DSI) rough surfaces. The contour lines of these rough surfaces show clear DSI. In the appropriate limit the DSI surfaces converge to the scale invariant rough surfaces. The fractal properties of the 2d DSI rough surfaces apart from possessing the discrete scale invariance property follow the properties of the contour lines of the corresponding scale invariant rough surfaces. We check this hypothesis by calculating numerous fractal exponents of the contour lines by using numerical calculations. Apart from calculating the known scaling exponents some other new fractal exponents are also calculated.

cond-mat.stat-mech↗

arXiv2010

Discrete scale invariance and stochastic Loewner evolution

In complex systems with fractal properties the scale invariance has an important rule to classify different statistical properties. In two dimensions the Loewner equation can classify all the fractal curves. Using the Weierstrass-Mandelbrot (WM) function as the drift of the Loewner equation we introduce a large class of fractal curves with discrete scale invariance (DSI). We show that the fractal dimension of the curves can be extracted from the diffusion coefficient of the trend of the variance of the WM function. We argue that, up to the fractal dimension calculations, all the WM functions follow the behavior of the corresponding Brownian motion. Our study opens a way to classify all the fractal curves with DSI. In particular, we investigate the contour lines of 2D WM function as a physical candidate for our new stochastic curves.

cond-mat.stat-mech↗