Increase Of Disorder Strength Research Articles

The Localization Effect in Kondo Insulators with Disorder

The localization effect in Kondo insulators with f -electron disorder is investigated by the typical-medium theory based on the self-consistent solution of the dynamical mean-field theory combined with the coherence potential approximation. The localized states due to disorder are extracted from the difference between the local density of states computed by the coherent potential approximation and the typical-medium theory. As the disorder strength increases, the localized states increase near the gap edges. As a result, the insulating gap increases in the temperature dependence of the conductivity, and the peak of the optical conductivity shifts to higher energy depending on the disorder strength. These behaviors are quite different from those of the coherent potential approximation.

Role of the symmetry parameterβin the strongly localized regime

The generalization of the Dorokhov-Mello-Pereyra-Kumar equation for the description of transport in strongly disordered systems replaces the symmetry parameter $\beta$ by a new parameter $\gamma$, which decreases to zero when the disorder strength increases. We show numerically that although the value of $\gamma$ strongly influences the statistical properties of transport parameters $\Delta$ and of the energy level statistics, the form of their distributions always depends on the symmetry parameter $\beta$ even in the limit of strong disorder. In particular, the probability distribution is $p(\Delta)\sim \Delta^\beta$ when $\Delta\to 0$ and $p(\Delta) \sim \exp (-c\Delta^2)$ in the limit $\Delta\to\infty$.

Anomalous Quantum Transport in a Thin Film

We present a numerical study of electron transport in a thin film of varying disorder strength with the distance from its surface. A simple tight-binding model is used to describe the system, in which the film is attached to two metallic electrodes and the coupling of this film to the electrodes is illustrated by the Newns-Anderson chemisorption theory. Quite interestingly we observe that, in the smoothly varying disordered film current amplitude increases with the increase of disorder strength in the strong disorder regime, while it decreases in the weak disorder regime. This behavior is completely opposite from a conventional disordered film, where current amplitude always decreases with the increase of disorder strength.

Disorder-enhanced transmission of a quantum mechanical particle through a disordered tunneling barrier in one dimension: Exact calculation based on the invariant imbedding method

We revisit the problem of disorder-enhanced tunneling transmission of a quantum mechanical particle through a disordered tunneling barrier in one dimension. Using the invariant imbedding theory of wave propagation generalized to randomly stratified media, we calculate the disorder-averaged logarithmic transmittance in the thick barrier limit and the disorder-averaged transmittance in a numerically exact manner. We confirm that the tunneling decay length obtained from the mean logarithmic transmittance behaves nonmonotonically as a function of the disorder strength and takes its maximum value at some finite value of the disorder parameter. We find that this nonmonotonic dependence persists in the presence of weak inelastic scattering inside the tunneling barrier. When the system size is larger than some critical value, which is somewhat smaller than the wavelength of the incident matter wave, we observe that the disorder-averaged transmittance also shows a similar nonmonotonic dependence on the disorder strength. In other words, weak disorder enhances the transmission, while strong disorder suppresses it. When the system size is smaller than the critical value, the disorder-averaged transmittance decreases monotonically as the disorder strength increases.

Quantum transport through finite width mesoscopic ring: Enhancement of the current amplitude with the increase of the edge disorder strength in the limit of strong disorder

We investigate the effect of edge disorder on quantum transport through a finite width mesoscopic ring attached to two semi-infinite one-dimensional metallic electrodes by the use of Green's function technique. Parametric calculations are given based on the tight-binding formulation to describe the transport properties through such a bridge system. A novel transport phenomenon is observed which gives the enhancement of the current amplitude with the increase of the edge disorder strength in the strong disorder regime, while the amplitude decreases in the weak disorder regime. This feature is completely opposite to that of the bulk disordered ring. In this context we also study the effects of the radius and the width of the ring on such transport and see that the transport properties are significantly influenced by them.

Effect of disorder on the pairing properties of electron-doped high-Tcsuperconductors

The effect of disorder on pairing properties in the mixed ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+is$- and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$-wave pairing states in electron-doped high-${T}_{c}$ cuprates is investigated by self-consistently solving the Bogoliubov--de Gennes equations. It is found that the dominant ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave component will give way to the $s$-wave component, while the ${d}_{xy}$-wave component will have the same magnitude as the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave component with the increase of disorder strength. In real space, the ${d}_{xy}$-wave component competes locally with the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave component, while the $s$-wave component does not seem to show such competition with the latter. Both transitions from the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave pairing to the mixed ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+is$-wave pairing and to the mixed ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$-wave pairing can result in the absence of zero-bias peaks in the local tunneling spectra, but it has a U-shaped form for the former and a V-shaped form for the latter. These results may serve to probe the pairing symmetry in the electron-doped cuprates.

Numerical Study of the Spin-Hall Conductance in the Luttinger Model

We present the first numerical studies of the disorder effect on the recently proposed intrinsic spin-Hall conductance in a three dimensional lattice Luttinger model. The results show that the spin-Hall conductance remains finite in a wide range of disorder strength, with large fluctuations. The disorder-configuration-averaged spin-Hall conductance monotonically decreases with the increase of disorder strength and vanishes before the Anderson localization takes place. The finite-size effect is also discussed.

LOCALIZED ENTANGLEMENT IN ONE-DIMENSIONAL ANDERSON MODEL

The entanglement in one-dimensional Anderson model is studied. The pairwise entanglement has a direct relation to the localization length and is reduced by disorder. Entanglement distribution displays the entanglement localization. The pairwise entanglements around localization center exhibit a maximum as the disorder strength increases. The dynamics of entanglement are also investigated.

Numerical study of spin quantum Hall transitions in superconductors with broken time-reversal symmetry

We present the results of numerical studies of spin quantum Hall transitions in disordered superconductors, in which the pairing order parameter breaks time-reversal symmetry. We focus mainly on $p$-wave superconductors in which one of the spin components is conserved. The transport properties of the system are studied by numerically diagonalizing pairing Hamiltonians on a lattice, and by calculating the Chern and Thouless numbers of the quasiparticle states. We find that in the presence of disorder, (spin-)current carrying states exist only at discrete critical energies in the thermodynamic limit, and the spin quantum Hall transition driven by an external Zeeman field has the same critical behavior as the usual integer quantum Hall transition of noninteracting electrons. These critical energies merge and disappear as disorder strength increases, in a manner similar to those in lattice models for integer quantum Hall transition.

Vortex dynamics and defects in simulated flux flow.

We present the results of molecular dynamic simulations of a two-dimensional vortex array driven by a uniform current through random pinning centers at zero temperature. We identify two types of flow of the driven array near the depinning threshold. For weak disorder the flux array contains few dislocation and moves via correlated displacements of patches of vortices in a {\it crinkle} motion. As the disorder strength increases, we observe a crossover to a spatially inhomogeneous regime of {\it plastic} flow, with a very defective vortex array and a channel-like structure of the flowing regions. The two regimes are characterized by qualitatively different spatial distribution of vortex velocities. In the crinkle regime the distribution of vortex velocities near threshold has a single maximum that shifts to larger velocities as the driving force is increased. In the plastic regime the distribution of vortex velocities near threshold has a clear bimodal structure that persists upon time-averaging the individual velocities. The bimodal structure of the velocity distribution reflects the coexistence of pinned and flowing regions and is proposed as a quantitative signature of plastic flow.