Long-range Correlated Disorder Research Articles

Critical exponents of the Ising model in three dimensions with long-range power-law correlated site disorder: A Monte Carlo study

The critical behavior of the Ising model in three dimensions on a lattice with site disorder is studied by applying Monte Carlo simulation techniques. Two cases for the site disorder are considered: uncorrelated disorder and long-range correlated disorder with a spatial correlation function that decays according to a power law ${r}^{\ensuremath{-}a}$. The critical exponents $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ as well as updated results for the critical exponent $\ensuremath{\nu}$ and confluent correction exponent $\ensuremath{\omega}$ are provided for a variety of different correlation exponents $a$ and disorder concentrations ${p}_{d}$. The estimation is done by using finite-size scaling analyses and a global fit procedure which combines the results obtained for different concentrations of defects. From the estimated critical exponents, the validity of hyperscaling relations is studied and finally the critical temperatures are provided for different $a$ and ${p}_{d}$.

Role of long-range correlations in high harmonic generation in disordered systems

The mechanism of high harmonic generation (HHG) from condensed matter driven by ultrafast lasers is far from being understood due to the structural diversity and complexity. One of the long-standing open questions is the effect of random disorder in noncrystal HHG. In this work, we identify the critical role of correlations in disordered semiconductor systems and propose them as the criterion to classify the HHG processes. The HHG spectra can be classified into two different categories corresponding to different correlations: short-range correlation and long-range correlation (LRC). Thus correlations build up a solid relationship between the HHG emission and statistical characteristics of a disordered system. In particular, the HHG spectrum from the disordered system with LRC exhibits remarkable consistency with the spectrum from the periodic lattice, revealing that the LRC links HHG processes in disordered and periodic systems. Besides, the mobility edge induced by the long-range correlated disorder enables us to study the behavior of localized- and delocalized-state HHG in one system.

Ordering behavior of the two-dimensional Ising spin glass with long-range correlated disorder.

The standard short-range two-dimensional Ising spin glass is numerically well accessible, in particular, because there are polynomial-time ground-state algorithms. On the other hand, in contrast to higher dimensional spin glasses, it does not exhibit a rich behavior, i.e., no ordered phase at finite temperature. Here, we investigate whether long-range correlated bonds change this behavior. This would still keep the model numerically well accessible while exhibiting a more interesting behavior. The bonds are drawn from a Gaussian distribution with a two-point correlation for bonds at distance r that decays as (1+r^{2})^{-a/2}, a≥0. We study numerically with exact algorithms the ground-state and domain-wall excitations. Our results indicate that the inclusion of bond correlations still does not lead to a spin-glass order at any finite temperature. A further analysis reveals that bond correlations have a strong effect at local length scales, inducing ferro- and antiferromagnetic domains into the system. The length scale of ferro- and antiferromagnetic order diverges exponentially as the correlation exponent approaches a critical value, a→a_{crit}=0. Thus, our results suggest that the system becomes a ferro- or antiferromagnet only in the limit a→0.

Critical exponent ν of the Ising model in three dimensions with long-range correlated site disorder analyzed with Monte Carlo techniques

We study the critical behavior of the Ising model in three dimensions on a lattice with site disorder by using Monte Carlo simulations. The disorder is either uncorrelated or long-range correlated with correlation function that decays according to a power-law $r^{-a}$. We derive the critical exponent of the correlation length $\nu$ and the confluent correction exponent $\omega$ in dependence of $a$ by combining different concentrations of defects $0.05 \leq p_d \leq 0.4$ into one global fit ansatz and applying finite-size scaling techniques. We simulate and study a wide range of different correlation exponents $1.5 \leq a \leq 3.5$ as well as the uncorrelated case $a = \infty$ and are able to provide a global picture not yet known from previous works. Additionally, we perform a dedicated analysis of our long-range correlated disorder ensembles and provide estimates for the critical temperatures of the system in dependence of the correlation exponent $a$ and the concentrations of defects $p_d$. We compare our results to known results from other works and to the conjecture of Weinrib and Halperin: $\nu = 2/a$ and discuss the occurring deviations.

Influence of two independent sources with long-range correlated disorder in the localization properties of mixed transmission lines

We study the localization properties of correlated disordered mixed transmission lines. These systems are formed by the repetition of a set of p successive direct cells followed by q successive dual cells. The spectrum shows a set of d = p + q sub-bands for any kind of disorder where we can find extended and localized states and even gaps. The correlated disorder is introduced only in the capacitances C j , y α , b and inductances L j , y β , b of the dual cells. Meanwhile, the capacitances C j , x and inductances L j , x of the direct cells are kept constant. b measures the amplitude of the disorder, and the correlation exponents α and β quantify the degree of long-range correlation imposed on the system, which is generated by the Fourier filtering method. For fixed p , q and b values it is possible to find extended states as a function of α and β in any of the d sub-bands. For each frequency corresponding to an extended state we can find an asymmetric phase diagram , so that for all α ≥ α c and β ≥ β c with α c ≥ β c , all states are extended. These critical values α c and β c were obtained studying the scaling behavior of the C ω average overlap amplitude. In addition, for fixed α and β , we also find a critical b c value, so for b ≥ b c all states are localized states because the large amplitude b of the fluctuation of the long-range correlated disorder destroys the extended states for all values of α and β . • Mixed transmission lines with p direct cells and q dual cells are studied. • Capacitances an inductances are distributed using two independent sources with long-range correlated disorder. • The localization properties shows an asymmetric phase diagram as a function of the correlation exponent α and β. • The scaling behavior of the C ω average overlap amplitude determines the existence of extended states.

Localization-delocalization transition in discrete-time quantum walks with long-range correlated disorder.

We study the effects of spatially long-range correlated phase disorder on the Hadamard quantum walk on a line. The shift operator is built to exhibit an intrinsic disorder distribution featuring long-range correlations. To impose such, we resort to fractional Brownian motion with power-law spectrum 1/k^{2α} with α≥0 being the exponent that controls the degree of correlations. We discuss the scaling behavior of the walker's wave packet and report a localization-delocalization transition controlled by α. We unveil two intermediate dynamical regimes between exponential localization and full delocalization.

Wavepacket dynamics in one-dimensional system with long-range correlated disorder

We numerically investigate dynamical property in the one-dimensional tight-binding model with long-range correlated disorder having power spectrum 1/fα (α: spectrum exponent) generated by Fourier filtering method. For relatively small α<αc(=2) time-dependence of mean square displacement (MSD) of the initially localized wavepacket shows ballistic spread and localizes as time elapses. It is shown that α-dependence of the dynamical localization length determined by the MSD exhibits a simple scaling law in the localization regime for the relatively weak disorder strength W. Furthermore, scaled MSD by the dynamical localization length almost obeys an universal function from the ballistic to the localization regime in the various combinations of the parameters α and W.

Scaling laws for random walks in long-range correlated disordered media

We study the scaling laws of diffusion in two-dimensional media with long-range correlated disorder through exact enumeration of random walks. The disordered medium is modelled by percolation clusters with correlations decaying with the distance as a power law, $r^{-a}$, generated with the improved Fourier filtering method. To characterize this type of disorder, we determine the percolation threshold $p_{\text c}$ by investigating cluster-wrapping probabilities. At $p_{\text c}$, we estimate the (sub-diffusive) walk dimension $d_{\text w}$ for different correlation exponents $a$. Above $p_{\text c}$, our results suggest a normal random walk behavior for weak correlations, whereas anomalous diffusion cannot be ruled out in the strongly correlated case, i.e., for small $a$.

Critical behavior of the two-dimensional Ising model with long-range correlated disorder

We study critical behavior of the diluted 2D Ising model in the presence of disorder correlations which decay algebraically with distance as $\sim r^{-a}$. Mapping the problem onto 2D Dirac fermions with correlated disorder we calculate the critical properties using renormalization group up to two-loop order. We show that beside the Gaussian fixed point the flow equations have a non trivial fixed point which is stable for $0.995<a<2$ and is characterized by the correlation length exponent $\nu= 2/a + O((2-a)^3)$. Using bosonization, we also calculate the averaged square of the spin-spin correlation function and find the corresponding critical exponent $\eta_2=1/2-(2-a)/4+O((2-a)^2)$.

Positive magnetoconductivity of Weyl semimetals in the ultraquantum limit

In this paper, we numerically study the magnetic transport properties of disordered Weyl semimetals (WSM) in the ultra-quantum limit. We find a positive magnetic conductivity for the long-range disorder, although the system tends to have negative magnetic conductivity for the weak short-range disorder. Remarkably, for long-range disorder, such a positive magnetic conductivity cannot be described by the semiclassical Boltzmann transport theory even in the weak disorder limit, and the back-scattering assisted by the high Landau levels is always important. Our results have significant implications for the positive magnetic conductivity recently discovered in the WSM systems, and point out two physical mechanisms: (i) the long-range correlated disorder suppresses the back-scattering among the zeroth Landau level modes; (ii) with increasing the magnetic field, the back-scattering assisted by the high Landau levels will also be suppressed.

Two-loop Feynman integrals for ϕ4 theory with long-range correlated disorder

Two-loop massive Feynman integrals for ϕ4 field-theoretical model with long-range correlated disorder are considered. Massive integrals for the vertex function Γ(4) including two or three massless propagators for generic space dimension and for any value of the correlation parameter are evaluated analytically applying Mellin-Barnes method as well as familiar representation for one-loop integrals. Obtained expressions are presented in the form of hypergeometric functions.

Enhanced rare-region effects in the contact process with long-range correlated disorder.

We investigate the nonequilibrium phase transition in the disordered contact process in the presence of long-range spatial disorder correlations. These correlations greatly increase the probability for finding rare regions that are locally in the active phase while the bulk system is still in the inactive phase. Specifically, if the correlations decay as a power of the distance, the rare-region probability is a stretched exponential of the rare-region size rather than a simple exponential as is the case for uncorrelated disorder. As a result, the Griffiths singularities are enhanced and take a non-power-law form. The critical point itself is of infinite-randomness type but with critical exponent values that differ from the uncorrelated case. We report large-scale Monte Carlo simulations that verify and illustrate our theory. We also discuss generalizations to higher dimensions and applications to other systems such as the random transverse-field Ising model, itinerant magnets, and the superconductor-metal transition.

Anderson localization and momentum-space entanglement

.We consider Anderson localization and the associated metal–insulator transition for non-interacting fermions in D = 1, 2 space dimensions in the presence of spatially correlated on-site random potentials. To assess the nature of the wave function, we follow a recent proposal to study momentum-space entanglement. For a D = 1 model with long-range disorder correlations, both the entanglement spectrum and the entanglement entropy allow us to clearly distinguish between extended and localized states based upon a single realization of disorder. However, for other models, including the D = 2 case with long-range correlated disorder, we find that the method is not similarly successful. We analyze the reasons for its failure, concluding that the much desired generalization to higher dimensions may be problematic.

Griffiths phase and critical behavior of the two-dimensional Potts models with long-range correlated disorder.

The q-state Potts model with long-range correlated disorder is studied by means of large-scale Monte Carlo simulations for q=2, 4, 8, and 16. Evidence is given of the existence of a Griffiths phase, where the thermodynamic quantities display an algebraic finite-size scaling, in a finite range of temperatures. The critical exponents are shown to depend on both the temperature and the exponent of the algebraic decay of disorder correlations, but not on the number of states of the Potts model. The mechanism leading to the violation of hyperscaling relations is observed in the entire Griffiths phase.

Percolation with long-range correlated disorder

Long-range power-law correlated percolation is investigated using Monte Carlo simulations. We obtain several static and dynamic critical exponents as functions of the Hurst exponent H, which characterizes the degree of spatial correlation among the occupation of sites. In particular, we study the fractal dimension of the largest cluster and the scaling behavior of the second moment of the cluster size distribution, as well as the complete and accessible perimeters of the largest cluster. Concerning the inner structure and transport properties of the largest cluster, we analyze its shortest path, backbone, red sites, and conductivity. Finally, bridge site growth is also considered. We propose expressions for the functional dependence of the critical exponents on H.

Effect of long-range correlated disorder on the transverse localization of light in 1D array of optical waveguides

In this paper, the effects of the long-range correlated diagonal disordered optical waveguide arrays in the presence and absence of the positive Kerr nonlinearity are analyzed numerically. The calculated inverse localization length shows that the long-range correlation in a disordered system causes a decrease in the transverse localization in linear optical waveguide arrays. In the presence of positive Kerr nonlinearity, the inverse localization length is increased by increasing the nonlinear parameters in long-range correlated disordered systems in comparison with the uniform distribution disordered systems. This means that the long range correlation causes an enhancement of transverse localization in nonlinear waveguides in contrast with linear waveguide arrays. The calculated participation ratio and effective beamwidth confirm these results for linear and nonlinear systems.

Hyperscaling violation in the 2D 8-state Potts model with long-range correlated disorder

The first-order phase transition of the two-dimensional eight-state Potts model is shown to be rounded when long-range correlated disorder is coupled to energy density. Critical exponents are estimated by means of large-scale Monte Carlo simulations. In contrast to uncorrelated disorder, a violation of the hyperscaling relation γ/ν = d − 2xσ is observed. Even though the system is not frustrated, disorder fluctuations are strong enough to cause this violation in the very same way as in the 3D random-field Ising model. In the thermal sector, too, evidence is given for such violation in the two hyperscaling relations α/ν = d − 2xε and 1/ν = d − xε. In contrast to the random field Ising model, at least two hyperscaling violation exponents are needed. The scaling dimension of energy is conjectured to be xε = a/2, where a is the exponent of the algebraic decay of disorder correlations.

Phase diagrams of two-dimensional and three-dimensional disordered Bose gases in the local density approximation

We study superfluid transitions in bidimensional (2D) and tridimensional (3D) disordered and interacting Bose gases. We work in the limit of long-range correlated disorder such that it can be treated in the local density approximation. We present superfluid transition curves in both the disorder-temperature plane and the disorder-entropy plane in 2D and 3D Bose gases. Surprisingly, we find that a small amount of disorder is always favorable to the apparition of a superfluid. Our results offer a quantitative comparison with recent experiments in 2D disordered ultracold gases, for which no exact theory exists.

Ground-state phases of interacting Fermi gases in disordered one-dimensional lattices

We numerically investigate a one-dimensional Anderson–Hubbard model under harmonic confinement. The effects of disorder on the ground-state properties are studied for Gaussian-correlated disorder and random impurities upon changing the amplitude of the disorder strength, the correlation length or the number of impurities. For a large disorder correlation length, both the band- and Mott-insulating phases re-enter naturally as a result of a smooth long-range correlated disorder. For the randomly distributed impurities in a system of composite Mott- and band-insulating phases, we find that the band-insulating region is rapidly destroyed while the Mott region is more robust against the increase of impurities. The fluid regions are less affected in this case.

Conductance of graphene superlattices with correlated disorder in velocity profiles

The propagation of massless Dirac fermion waves through a graphene system is studied in the presence of a long-range correlated disorder. The system consists of a graphene layer in which the Dirac fermions velocity is position-dependent. The velocity profile is multiform and assumed to be long-range correlated. The effect of disorder in the transmission probability through the system with different sizes is also studied. In addition, we show that the conductance of the system increases with increasing the correlation exponent values giving rise to a metallic phase. We obtain a phase transition diagram in which the critical correlation exponent depends strongly on disorder strength. We demonstrate that in the limit of large system size, the conductance fluctuations become independent of the correlation exponent and tend to a constant value.