Dynamic conductivity of ac–dc-driven graphene superlattice

The dynamic conductivity of graphene superlattice in the presence of ac electric field and dc electric field with longitudinal and transversal components with respect to superlattice axis was calculated. In the case of strong transversal component of dc field conductivity of graphene superlattice was shown to be such as if the electrons had got the effective mass. In the case of weak transversal component of dc field conductivity was shown to change its sign if the frequency of ac field was an integer multiple of half of Bloch frequency.     

Metal particle polarization

The effect of electron collisions with lattice atoms in metals on metal particle polarization in an ac electric field was analyzed. It was shown that, in contrast to collisionless (free) electron gas, an increase in the negative electronic permittivity with decreasing electric field frequency in relaxation metal particles is limited by the conductivity relaxation time. It was shown that the plasma frequency appears in relaxation metal only if the dielectric relaxation time is less than the free path time in metal.     

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.     

Is a Fermi-Dirac distribution function for a non-equilibrium electron gas in high crossed electric and magnetic fields possible?

We generalize our investigation of the electron distribution function of anelectron gas interacting with phonons and moving in high crossed electric and magnetic fields. We consider the case of a degenerate gas: we look for conditions under which the distribution function is of a Fermi-Dirac type with an electric and magnetic field dependent temperature.     

Oscillations of the kinetic coefficients in a two-dimensional electron system with the spin-orbit interaction in an alternating magnetic field

The response of a two-dimensional electron system with the spin-orbit interaction to a measuring direct-current electric field is investigated for the case in which the system is disturbed from the equilibrium by a microwave alternating magnetic field. Point nonmagnetic impurities are considered as scattering sources. It is demonstrated that the combined transitions occurring under the action of an alternating magnetic field lead to new oscillations of the diagonal components of the electrical conductivity tensor of the two-dimensional electron gas.     

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.     

Frequency dependent conductivity for asymmetric hopping in one dimension

The frequency dependent conductivity for one dimensional disordered classical systems is considered in the presence of an external static electric field which acts as a bias for the hopping rates. In the high frequency limit one obtains an expansion in inverse frequencies for the conductivity, whereas for low frequencies an expansion in the inverse moments of the distribution of the random transition rates is derived. The lowest order contributions of these expansions are explicitely evaluated and compared with recent calculations by Derrida and Orbach. The method is restricted to systems with nonsingular distributions of the transfer rates.     

Electric-field dependence of the intersubband optical absorption in a semiconductor quantum well

We present theoretical results of intersubband linear optical absorption in the conduction band of a GaAsAlGaAs quantum well with an applied electric field taking into account the field dependent linewidth. Our analysis is based on the one electron density matrix formulation with intrasubband relaxation processes due to polar optical phonon scattering and tunneling of electrons. We show that (a) for an increasing electric field the absorption peak corresponding to the transition of states 1 → 2 is shifted higher in energy and (b) the peak amplitude increases if the Fermi level is fixed and decreases if the electron density in the well is fixed when an increasing electric field is applied. The linewidth broadening also reduces the peak absorption amplitude.     

Free electron gas spectrum parameters of noble gases in dc electric field

Free electron gas is present in every gas, whether it is of atomic or molecular structure. Since the Maxwell spectrum type is the consequence of only thermal motion of constitutive gas particles; therefore, the presence of electric field leads to change the spectrum of charged particles due to their directed motion. However, it has been shown that in the case of occurrence only of elastic interactions between electrons and neutral gas particles (a condition that has been met in the case of weakly ionized noble gases of a relatively huge volume) the deviation of the gas spectrum of free electrons in the electric field from the Maxwell type is negligible. In such a case, the gas spectrum of free electrons is either of Maxwell type (if the frequency collision value is energy-independent) or of Druyvesteyn type (if the mean free electron path value is energy-independent). The Maxwell and Druyvesteyn distribution types are very similar. The only noticeable difference is that the tail of the Maxwell distribution decreases with the energy exponent to the first degree of energy, and the tail of Druyvesteyn distribution with the energy exponent to the second degree of energy. The aim of this paper is to determine whether the gas spectrum of free electrons in weakly ionized noble gases at small values of the product pd (pressure and inter-electrode distance) follows either the Maxwell's or Druyvesteyn's type, as well as to determine the dependence of spectrum parameters on the product pd. It has been established that better results are obtained on the assumption that the mean value of collision frequency is energy-independent.     

Zeeman and spin–orbit effects on the spin-Hall conductance

We show that when a two-dimensional interacting electron gas is submitted to a perpendicular magnetic field, the application of an in-plane electric field E induces a spin current perpendicular to E whose conductivity is quantized. This current can lead to spin accumulation that might be detected by means of optical experiments. The appearance of this intrinsic spin-Hall effect is crucially based on the validity of Kohn's theorem and on the presence of the Zeeman term in the electron Hamiltonian. The possibility of resonant effects in the spin-Hall conductivity due to the combined effect of Rashba and Dresselhaus spin–orbit couplings is discussed.     

Origin of current instability in GaAs/AlGaAs heterostructures

We propose a mechanism to explain the electric instability often observed in modulation-doped heterostructures GaAs/AlGaAs when current is passed along the heterostructure layers. The instability is caused by hot electron transport in AlGaAs layer that is not only heavily doped, but also strongly compensated due to the presence of DX-centers. This layer contains a large-scale random potential of significant magnitude, which strongly affects electron transport. The heating of electrons in the percolation cluster net and electron transfer from the cluster into the random potential wells result in the appearance of latent negative differential conductivity causing the current instability. When the instability gives rise to the formation of a high electric field domain, one of the domain walls blocks the current flow through the two-dimensional electron gas. Experimental results supporting this mechanism are given.     

Deceleration and acceleration of fast electrons in a dense gas in an electric field

The propagation of electrons in a gas at energies higher than the excitation energy of the K shell of the gas atoms is simulated numerically. Calculations show that, without a field, the penetration depth of the electrons into a gas heavier than nitrogen is limited primarily by their elastic collisions with atomic nuclei. For electrons moving in an electric field, the effect of elastic collisions is that there is no definite electric field strength above which an electron with a given energy will be continuously accelerated. Even in an electric field much stronger than the critical one, only a fraction of electrons are accelerated. The remaining electrons turn back due to elastic collisions and lose their energy in deceleration by the field. In this case, the propagation velocity of the centroid of the electrons tends to a constant value.     

Direct calculation of time varying Aharonov-Bohm effect

The Aharonov-Bohm effect (ABE) for steady magnetic fields is a well known phenomenon. However, if the current in the infinite solenoid that creates the magnetic field is time-dependent, that is in the presence of both magnetic and electric fields, there is no agreement whether the effect would be present. In this note, we try to investigate time varying ABE by a direct calculation in a set-up with a weak time dependent magnetic field. We find that the electric field arising out of the time-varying magnetic field in the path of the electrons does not enter the action integral but only changes the path of the electron from the source to the slits and then on to the detector. We find a frequency dependent AB phase shift. At low frequencies the result smoothly approaches the one for a constant field as the frequency tends towards zero. On the other hand, for high frequencies such that the AB-phase induced in the path of the wave packet oscillates rapidly, the net effect will be very small which is borne out by our results.     

Novel electric field effects on Landau levels in graphene

A new effect in graphene in the presence of crossed uniform electric and magnetic fields is predicted. Landau levels are shown to be modified in an unexpected fashion by the electric field, leading to a collapse of the spectrum, when the value of electric to magnetic field ratio exceeds a certain critical value. Our theoretical results, strikingly different from the standard 2D electron gas, are explained using a "Lorentz boost," and as an "instability of a relativistic quantum field vacuum." It is a remarkable case of emergent relativistic type phenomena in nonrelativistic graphene. We also discuss few possible experimental consequence.     

Electron magnetotransport in carbon nanotubes and negative magnetoresistance: The density matrix method

A solution (stationary and linear in electric field) to the kinetic equation for a one-electron density matrix in an arbitrary magnetic field is found for deformation-potential scattering in the approximation of a small deviation of the electron gas from equilibrium. An expression for the nanotube conductivity is obtained in the form of a sum over magnetic-quantization states. In the absence of a magnetic field, this expression coincides with the corresponding classical relations. In weak magnetic fields, the magnetoresistance of a multilayer nanotube is positive for high electron mobilities and negative for low mobilities. In intermediate fields, it reverses sign with increasing field strength. The magnetoresistance of a one-layer nanotube is always positive.     

Hybrid helicity and magneto-vorticity flux conservation in superdense npe-plasma

In this paper, it is shown that the magnetic helicity dissipation per unit volume, coupled with the longitudinal conductivity, causes enhancement of the kinematic rotation of the electric (and magnetic) lines if the npe-plasma vorticity vector aligns with the electric (or the magnetic) field. In the case of a rigidly rotating npe-plasma under the influence of a strong magnetic field, the electric lines are rotating faster than the magnetic lines. It is deduced that the orthogonality of the electric and magnetic fields is an essential condition for the conduction current to remain finite in the limit of infinite electric conductivity of the npe-plasma. In this case, the magnetic field is not frozen into the npe-plasma, but the magnetic flux in the magnetic tube is conserved. The hybrid helicity is conserved if the “magneto-vorticity” vector is tangent to the level surfaces of constant entropy per baryon. The “magneto-vorticity” lines are rotating on the level surfaces of constant entropy per baryon due to the electromagnetic energy flow in the direction of the npe-plasma vorticity and the chemical potential variation locked with the kinematic rotation of the npe-plasma flow lines. In the case of an isentropic npe-plasma flow, there exists a family of timelike 2-surfaces spanned by the “magneto-vorticity” lines and the npe-plasma flow lines. In this case, the electric field is normal to such a family of timelike 2-surfaces. Maxwell like equations satisfied by “magneto-vorticity” bivector field are solved in axially symmetric stationary case. It is shown that the npe-plasma is in differential rotation in such a way that its each plasma shell (i.e., plasma surface spanned by “magneto-vorticity” lines) is rotating differentially without continually winding up “magneto-vorticity” lines frozen into the npe-plasma. It is also found that gravitational isorotation and Ferraro’s law of isorotation are intimately connected to each other because of coexistence of both the plasma vorticity and the magnetic field due to interaction between the electromagnetic field and npe-plasma flows.     

Dynamics of polaron in a polymer chain with impurities

In a polymer chain, an extra electron or hole distorts the chain to form a charged polaron, which is the charge carrier being responsible for conductivity. When an intermediate-strength electric field is applied, the polaron will be accelerated for a short time and then move at a constant velocity. The dynamical process of polaron in a polymer chain with impurities is simulated within a non-adiabatic evolution method, in which the electron wave function is described by the time-dependent Schrödinger equation while the polymer lattice is treated classically by a Newtonian equation of motion. We have considered two kinds of dynamical processes, one is the field-induced depinning of a charged polaron, which is initially bound by an attractive impurity; and the other is the scattering of a polaron from an impurity. In the former process, the charged polaron will depart from the attractive impurity only for the applied field with strength over a threshold, otherwise, the polaron will oscillate around the impurity. In the latter process, the charged polaron moves through the impurity in the presence of an electric field while it will be bounced back for a repulsive impurity or trapped to oscillate around an attractive impurity in the case that the applied electric field is weak and just be present for the polaron acceleration.Received: 4 June 2004, Published online: 14 December 2004PACS: 71.38.-k Polarons and electron-phonon interactions - 72.80.Le Polymers; organic compounds (including organic semiconductors)     

Non-linear conductivity and quantum interference in disordered metals

We report on the non-linear electric field effect in the conductivity of disordered conductors. We find that the electron-electron interaction in the particle-hole triplet channel strongly affects the non-linear conductivity. The non-linear effect introduces a field dependent temperature scale T_E and provides a microscopic mechanism for electric field scaling at the metal-insulator transition. We also study the magnetic field dependence of the non-linear conductivity and suggest possible ways to experimentally verify our predictions. These effects offer a new probe to test the role of quantum interference at the metal-insulator transition in disordered conductors. Received 9 February 2000     

Influence of spin-orbit coupling induced by in-plane external electric field on the intrinsic spin-Hall effect in a Rashba two-dimensional electron gas

We study theoretically the influence of spin--orbit coupling induced by in-plane external electric field on the intrinsic spin-Hall effect in a two-dimensional electron gas with Rashba spin--orbit coupling. We show that, after such an influence is taken into account, the static intrinsic spin-Hall effect can be stabilized in a disordered Rashba two-dimensional electron gas, and the static intrinsic spin-Hall conductivity shall exhibit some interesting characteristics as conceived in some original theoretical proposals.