Anomalous Magnetoconductance Research Articles

Anomalous magnetoconductivity and relaxation times in holography

We study the magnetoconductivity induced by the axial anomaly via the chiral magnetic effect in strongly coupled holographic models. An important ingredient in our models is that the axial charge is non-conserved beyond the axial anomaly. We achieve this either by explicit symmetry breaking via a non-vanishing non-normalisable mode of an axially charged scalar or using a Stuckelberg field to make the AdS-bulk gauge field massive. The DC magnetoconductivites can be calculated analytically. They take a universal form in terms of gauge field mass at the horizon and quadratic dependence on the magnetic field. The axial charge relaxation time grows linearly with magnetic field in the large B regime. Most strikingly positive magnetoconductivity is still present even when the relaxation times are short τ5 ≈ 1/(πT) and the axial charge can not be thought of as an approximate symmetry. In the U(1) A explicit breaking model, we also observe that the chiral separation conductivity and the axial magnetic conductivity for the consistent axial current vanish in the limit of strong symmetry breaking.

Weak localization in low-symmetry quantum wells

Theory of weak localization is developed for electrons in semiconductor quantum wells grown along [110] and [111] crystallographic axes. Anomalous conductivity correction caused by weak localization is calculated for symmetrically doped quantum wells. The theory is valid for both ballistic and diffusion regimes of weak localization in the whole range of classically weak magnetic fields. We demonstrate that in the presence of bulk inversion asymmetry the magnetoresistance is negative: The linear in the electron momentum spin-orbit interaction has no effect on the conductivity while the cubic in momentum coupling suppresses weak localization without a change of the correction sign. Random positions of impurities in the doping layers in symmetrically doped quantum wells produce electric fields which result in position-dependent Rashba coupling. This random spin-orbit interaction leads to spin relaxation which changes the sign of the anomalous magnetoconductivity. The obtained expressions allow determination of electron spin relaxation times in (110) and (111) quantum wells from transport measurements.

Analysis of negative magnetoresistance: Statistics of closed paths. I. Theory

Statistics of closed paths in two-dimensional (2D) systems, which just determines the interference quantum correction to conductivity and anomalous magnetoconductance, has been studied by computer simulation of a particle motion over the plane with randomly distributed scatterers. Both ballistic and diffusion regimes have been considered. The results of simulation have been analyzed in the framework of diffusion approximation. They are used for calculation of the magnetic-field dependence of magnetoconductance in the model 2D system. It is shown that the anomalous magnetoconductance can be, in principle, described by the well-known expression, obtained in the diffusion approximation, but with the prefactor less than unity and phase breaking, which differs from true value.

Nonuniversal Magnetic-Field Correlation of Conductance Fluctuations in Quantum Cavities Attached to a Superconductor

We investigate the magnetic-field-induced transmission resonance in two-dimensional-electron-gas–superconductor junctions. The magnetic-field correlation of the conductance fluctuations is nonuniversal at low magnetic fields provided the normal region has a certain kind of rectangular-shape. The anomalous magnetoconductance fluctuations are robust against the imperfection of the quantum cavity geometry and the occupation of multiple modes in the normal lead. However, the anomalous fluctuations are suppressed if the cavity size is not sufficiently large in comparison to the lead width. This indicates the importance of multiple Andreev reflection for the anomalous behavior. The wave function at the resonance in the anomalous fluctuations regime reveals scars by regular classical orbits. Hierarchical nature of the anomalous conductance fluctuations is evidenced by the distributions of the current and the charge.

Anomalous magnetoconductance beyond the diffusion limit

The weak localization contribution to the conductance depends sensitively on the applied magnetic field. This anomalous magnetoconductance provides a unique tool for the identification of weak localization and other phenomena associated with it. The standard theory of the anomalous magnetoconductance is based on the diffusion approximation, which applies to the case where the number of elastic scattering events is very large during a time of phase coherence in the electronic wave. For a strictly two-dimensional film, an arbitrary number of scattering events is considered and marked deviations are found from the standard theory if this number is not large.

Magnetoconductance due to parallel magnetic fields in silicon inversion layers.

A new anomalous magnetoconductance in parallel magnetic fields has been observed in a two-dimensional electron system formed in n-channel silicon field-effect transistors in fields up to 1.5 T and in a temperature range 1--4.2 K. The magnetoconductance is positive at electron concentrations ${N}_{S}$g2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ electrons/${\mathrm{m}}^{2}$ and has been interpreted as an effect of suppression of the weak-localization correction to the conductivity. The experiment deduces the presence of an electron random walk in the direction perpendicular to the Si-${\mathrm{SiO}}_{2}$ interface with the rms value \ensuremath{\delta}z\ensuremath{\le}2.1\ifmmode\pm\else\textpm\fi{}0.15 A\r{}. At lower electron densities the magnetoconductance changes sign and has a magnitude much larger than predicted by the current electron-electron interaction theory. The enhancement has been explained by including the spin fluctuations induced by disorder.

Evidence for an Anderson transition in granular Sn films.

Conductivity and density-of-states measurements have been performed on granular Sn films with room-temperature conductivities \ensuremath{\sigma}(300 K) between 1 and ${10}^{4}$ ${\ensuremath{\Omega}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Our conductivity measurements indicate that superconductivity as well as normal metallic behavior disappear at the metal-insulator transition (${\ensuremath{\sigma}}_{c}$\ensuremath{\simeq}40 ${\ensuremath{\Omega}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). The anomalous magnetoconductivity in the metallic phase is consistent with the scaling picture for the Anderson transition. The temperature dependence of the conductivity is more complex, probably due to the influence of the granular film structure. The importance of the enhanced Coulomb repulsion near the Anderson transition is directly reflected by the appearance of a square-root anomaly in the density of states around the Fermi level. The granularity of the Sn films causes this anomaly to be considerably smaller than for homogeneous amorphous metals.

Anomalous Magnetoconductivity in Semimagnetic Semiconductors Showing Metallic Conduction

Anomalous magnetoconductivity in three demensions as a precursor effect of the Anderson localization is discussed for metallic samples of semimagnetic semiconductors such as Cd 1- x Mn x Se. Due to the large effective g -factor (of the order of 10 2 ) arising from rather strong exchange interaction between electrons and magnetic ions (Mn ions), the spin flipping contribution to the quantum correction of the condtuctivity saturates at weak magnetic fields (∼1 kG). Parameters appropriate for Cd 0.95 Mn 0.05 Se gives a relative quantum correction of the conductivity of the order of 10 -2 .