Screw dislocation-induced influence of transverse modes on Hall conductivity

The Hall conductivity of an electron gas on an interface showing a topological defectcalled screw dislocation is investigated. This kind of defect induces a singular torsionon the medium which in turn induces transverse modes in the quantum Hall effect. It isshown that this topology decreases the plateaus’ widths and shifts the steps in the Hallconductivity to lower magnetic fields. The Hall conductivity is neither enhanced nordiminished by the presence of this kind of defect alone. We also consider the presence oftwo defects on a sample, a screw dislocation together with a disclination. For a specificvalue of deficit angle, there is a reduction in the Hall conductivity. For an excess ofangle, the steps shift to higher magnetic fields and the Hall conductivity is enhanced.Our work could be tested only in common semiconductors but we think it opens a road to theinvestigation on how topological defects can influence other classes of Hall effect.     

Integer quantum Hall effect on an interface with disclinations

The Hall conductivity of an electron gas on an interface showing topological defects as disclinations in the presence of an orthogonal constant magnetic field is investigated. This kind of defect induces either positive or negative singular curvature in the medium. It is shown that the positive curvature decreases the quantum Hall plateau widths and shifts the steps in the Hall conductivity to lower magnetic fields. In contrast, the negative one leaves to the existence of two types of plateaus, one with higher widths and the other one with lower widths in comparison to the flat case. In this case, the shift in the steps of the Hall conductivity goes to higher magnetic fields. We also investigate the Hall conductivity for electrons around a cylindrically symmetric distribution of disclinations and it turns out that it is the same as that corresponding to a single effective disclination.     

Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.     

Hall Conductivity in the Cosmic Defect and Dislocation Spacetime

Influences of topological defect and dislocation on conductivity behavior of charge carriers in external electromagnetic fields are studied.Particularly the quantum Hall effect is investigated in detail.It is found that the nontrivial deformations of spacetime due to topological defect and dislocation produce an eiectric current at the leading order of perturbation theory.This current then induces a deformation on the Hall conductivity.The corrections on the Hall conductivity depend on the external electric fields,the size of the sample and the momentum of the particle.     

Electric- and magnetic-field-tuned Landau levels and Hall conductivity in AA-stacked bilayer graphene

We theoretically study the combined effect of magnetic and electric fields on the Landau levels and Hall conductivity in AA-stacked bilayer graphene. From the analytic expressions derived, we obtain explicit criterions for determining the zero-energy Landau level and different level crossings in the graphene bilayer. For providing a scheme of experimental verification, we further explore the quantum Hall effect in such a biased bilayer. It is found that the zero-conductance Hall plateau in this system can vanish at certain specific combinations of magnetic and electric fields, accompanying with the occurrence of resonance Hall conductivity steps.     

Gauge potential formulations of the spin Hall effect in graphene

Two different gauge potential methods are engaged to calculate explicitly the spin Hall conductivity in graphene. The graphene Hamiltonian with spin-orbit interaction is expressed in terms of kinematic momenta by introducing a gauge potential. A formulation of the spin Hall conductivity is established by requiring that the time evolution of this kinematic momentum vector vanishes. We then calculated the conductivity employing the Berry gauge fields. We show that both of the gauge fields can be deduced from the pure gauge field arising from the Foldy-Wouthuysen transformations.     

Transport coefficients and flux motion in Bi2Sr2CaCu2Ox single crystals

We present measurements of the electrical resistivity, thermal conductivity, and Hall, Nernst, and Seebeck effects in the mixed state of single crystalline Bi₂Sr₂CaCu₂O_x. It is shown that the sign of the Hall voltage changes twice as temperature decreases below T_c. From the Nernst effect we estimate the transport entropy S_φ to be about 10⁻¹⁰ erg/K cm. S_φ is equal to zero in the normal state, increases and passes through a maximum at the mixed state as expected. The temperature dependences of the thermoelectric power in magnetic fields are analogous to the resistive transition curves. These phenomena are discussed in terms of flux flow. The contribution of the flux flow to the thermal conductivity is estimated to be negligible. Lowering of the thermal conductivity at temperatures below T_c by a magnetic field is attributed to phonon scattering by the vortex lines.     

An Extended Extrinsic Mechanism for Anomalous Hall Effect

The extrinsic mechanism for anomalous Hall effect in ferromagnets is extended to include the contributions both from spin-orbit-dependent impurity scattering and from the spin-orbit coupling induced by external electric fields. The results obtained suggest that, within the framework of the extrinsic mechanisms, the anomalous Hall current in a ferromagnet may also contain asubstantial amount of dissipationless contribution independent of impurity scattering. After the contribution from the spin-orbit coupling induced by external electric fields is included, the total anomalous Hall conductivity is about two times larger than that due to spin-orbit dependent impurity scatterings.     

Effects of Hall Currents on the Self-Gravitational Instability of a Finitely Conducting Plasma of Variable Density

The effects of Hall currents have been studied on the hydromagnetic stability of a self-gravitating, incompressible, viscous and finitely conducting plasma of variable density. For a uniform and horizontal magnetic field which is present, it is shown that the problem is characterized by a variational principle. Making use of this, proper solutions have been obtained for a semi-infinite plasma in which the density varies one-dimensionally (exponentially) along the vertical. The dispersion relation has been solved numerically for the different values of the parameters involved. It is found that the growth rate increases with both the Hall currents and resistivity, showing thereby the destabilizing character of these effects. However, the influence of viscosity is found to be stabilizing as the growth rate decreases with viscosity.     

自旋轨道耦合系统中的自旋流与自旋霍尔效应

作为自旋电子学的重要研究内容,如何在固态系统中产生、操控以及探测自旋流引起了研究人员的广泛兴趣。基于自旋轨道耦合的自旋霍尔效应为在非磁性半导体中产生自旋流提供了一种有效途径。然而,在具有自旋轨道耦合的系统中,自旋流并不守恒。如何理解这点并恰当地表述相应的连续性方程,成为自旋输运研究的基本问题之一。本文主要综述自旋轨道耦合系统中自旋流与自旋霍尔效应方面的研究进展。引入SU(2)规范势后,自旋流满足协变形式的连续性方程,该方程保证了SU(2)Kubo公式在不同规范固定下的自洽性。利用SU(2)场强张量,可以直接得到自旋密度和自旋流在SU(2)外场中受到的自旋力,该力在只有U(1)磁场时对应于Stern-Gerlach力。由于依赖杂质散射的外在自旋霍尔效应很难被利用,内在自旋霍尔效应的概念被提出:在非磁半导体中,U(1)电场会诱导出自旋流并导致系统边缘处的自旋积累。自旋霍尔效应已经在半导体和金属材料中被观察到。虽然在干净的二维电子气中自旋霍尔电导率是一普适常数e/8π,但杂质对它的影响却引起了人们的高度关注。通过引入退相干效应,自旋霍尔效应中杂质效应的一些令人困惑的理论结果,则得到清晰的解释。此外,本文还将介绍具有层间隧穿的双层二维电子气中的自旋输运现象。在能量简并点附近,自旋霍尔电导率和隧穿自旋电导率均会出现共振现象。当两层间的杂质势强度存在差异时,隧穿自旋电导率随门压的变化曲线呈现出非对称性,显示出自旋二极管效应。     

自旋轨道耦合系统中的自旋流与自旋霍尔效应

作为自旋电子学的重要研究内容,如何在固态系统中产生、操控以及探测自旋流引起了研究人员的广泛兴趣.基于自旋轨道耦合的自旋霍尔效应为在非磁性半导体中产生自旋流提供了一种有效途径.然而,在具有自旋轨道耦合的系统中,自旋流并不守恒.如何理解这点并恰当地表述相应的连续性方程,成为自旋输运研究的基本问题之一.本文主要综述自旋轨道耦合系统中自旋流与自旋霍尔效应方面的研究进展.引入SU(2)规范势后,自旋流满足协变形式的连续性方程,该方程保证了SU(2)Kubo公式在不同规范固定下的自洽性.利用SU(2)场强张量,可以直接得到自旋密度和自旋流在SU(2)外场中受到的白旋力,该力在只有U(1)磁场时对应于Stern-Gerlach力.由于依赖杂质散射的外在自旋霍尔效应很难被利用,内在自旋霍尔效应的概念被提出:在非磁半导体中,U(1)电场会诱导出自旋流并导致系统边缘处的自旋积累.自旋霍尔效应已经在半导体和金属材料中被观察到.虽然在干净的二维电子气中自旋霍尔电导率是一普适常数e/8π,但杂质对它的影响却引起了人们的高度关注.通过引入退相干效应,自旋霍尔效应中杂质效应的一些令人困惑的理论结果,则得到清晰的解释.此外,本文还将介绍具有层间隧穿的双层二维电子气中的自旋输运现象.在能量简并点附近,自旋霍尔电导率和隧穿白旋电导率均会出现共振现象.当两层间的杂质势强度存在差异时,隧穿自旋电导率随门压的变化曲线呈现出非对称性,显示出自旋二极管效应.     

Gap opening in the zeroth Landau level in gapped graphene: pseudo-Zeeman splitting in an angular magnetic field

We present a theoretical study of gap opening in the zeroth Landau level in gapped graphene as a result of pseudo-Zeeman interaction. The applied magnetic field couples with the valley pseudospin degree of freedom of the charge carriers leading to the pseudo-Zeeman interaction. To investigate its role in transport at the charge neutrality point (CNP), we study the integer quantum Hall effect in gapped graphene in an angular magnetic field in the presence of pseudo-Zeeman interaction. Analytical expressions are derived for the Hall conductivity using the Kubo-Greenwood formula. We also determine the longitudinal conductivity for elastic impurity scattering in the first Born approximation. We show that pseudo-Zeeman splitting leads to a minimum in the collisional conductivity at high magnetic fields and a zero plateau in the Hall conductivity. Evidence for activated transport at CNP is found from the temperature dependence of the collisional conductivity.     

Microwave-induced magnetic field state with zero conductivity in GaAs/AlAs Corbino disks and hall bars

The microwave photoconductivity of the 2D electron gas in GaAs/AlAs heterostructures has been investigated at a temperature of 4.2 K in magnetic fields up to 1.5 T. It has been found that the magnetic field state with zero conductivity appears in GaAs/AlAs Corbino disks irradiated by 130.70-GHz microwave radiation. This state was previously observed only in GaAs/AlGaAs Corbino disks with much higher electron mobility and lower density. It has been shown that the microwave photoconductivity measured in high magnetic fields on Corbino disks can significantly differ from the value calculated from the results of the measurements on Hall bars. This difference is explained by the fact that the conditions of the appearing magnetoplasmons that affect the magnitude and character of the microwave photoconductivity (photoresistance) in the Corbino disks are nonequivalent to those in the Hall bars.     

Bound values for Hall conductivity and percolation under quantum Hall effect conditions

The percolation under Quantum Hall Effect conditions in inhomogeneous medium has been studied. The lower and upper bound possible values for effective Hall conductivity values have been established. It has been shown that these bound values for Hall conductivity differ from bound values for metal conductivity. It comes from unusual character of current percolation under Quantum Hall Effect conditions. The physical sense of obtained results has been discussed.     

Bound values for Hall conductivity of heterogeneous medium under quantum Hall effect conditions

Bound values for Hall conductivity under quantum Hall effect (QHE) conditions in inhomogeneous medium has been studied. It is shown that bound values for Hall conductivity differ from bound values for metallic conductivity. This is due to the unusual character of current percolation under quantum Hall effect conditions.      

Electron polarizability of crystalline solids in quantizing magnetic fields and topological gap numbers

A theory of the static electron polarizability of crystals whose energy spectrum is modified by quantizing magnetic fields is presented. The polarizability is strongly affected by nondissipative Hall currents induced by the presence of crossed electric and magnetic fields: these can even change its sign. Results are illustrated in detail for a two-dimensional square lattice. The polarizability and the Hall conductivity are, respectively, linked to the two topological quantum numbers entering the so-called Diophantine equation. These numbers could in principle be detected in actual experiments.     

Quantum hall effect in inhomogeneous media: Effective characteristics and local current distribution

An analysis is made of the problem of current flow in heterophase inhomogeneous media in the quantum Hall effect regime. Duality relations are derived and expressions are obtained for the effective conductivity of inhomogeneous media over the entire range of concentrations. Local current distributions (fields) are determined in the quantum Hall effect regime.     

Lorenz number determination of the dissipationless nature of the anomalous Hall effect in itinerant ferromagnets

We have measured the thermal Hall conductivity for ferromagnetic Ni and Ni0.97Cu0.03. In the low temperature region ( less, similar 100 K), we show for the first time that the Wiedemann-Franz law is satisfied even for the anomalous Hall current. While the Hall Lorenz number for the normal part decreases rapidly with temperature, that for the anomalous part shows much less deviation from the free-electron Lorenz number. This evidences the dissipationless nature of the anomalous Hall effect.     

Magnetotransport in a semconductor superlattice with transverse magnetic field

Magnetotransport in a semiconductor superlattice (SL) under transverse magnetic field has been investigated. It is shown that in weak magnetic and electric fields there is negative magnetoresistivity along the SL layers and positive magnetoresistivity along the SL axis. The Hall resistivity is much less than the usual semiconductor value. With an increase of electric field, there appears a negative differential conductivity (NDC) along the SL layers, and the Hall voltage depends nonlinearly on current density. In higher electric field, destroying the miniband structure, the magnetoresistivity along the SL axis is negative. The magnetoresistivity along the SL axis in strong magnetic field is positive for any current density. The Hall resistivity in strong magnetic (electric) field equals the classical value.     

Quantum galvanomagnetic properties of n-type inversion layers on Si(100) MOSFET

Transverse magnetoconductivity σ_xx and Hall effect in n-type inversion layers of Si(100) MOSFET are measured for various source-drain fields between 0.08 and 40 V/cm under magnetic fields up to 150 kOe at 1.4 K. Conductivity peaks in low Landau levels are in good agreement with theory. Effect of the source-drain field in the magnetoconductivity is found to be very important in higher Landau levels as well as in the appearance of the lowest Landau level peak. Immobile electrons are clearly observed in conductivity bottoms. Electrode geometry effect for Hall effect measurement under strong magnetic fields is discussed.