Microwave-radiation induced giant magnetoresistance oscillations recently discovered in highmobility two-dimensional electron systems are analyzed theoretically. Multiphoton-assisted impurity scatterings are shown to be the primary origin of the oscillation. Based on a theory which considers the interaction of electrons with electromagnetic fields and the effect of the cyclotron resonance in Faraday geometry, we are able not only to reproduce the correct period, phase and the negative resistivity of the main oscillation, but also to predict the secondary peaks and additional maxima and minima observed in the experiments. These peak-valley structures are identified to relate respectively to single-, double-and triple-photon processes.PACS numbers: 73.50. Jt, 78.20.Ls The discovery of a new type of giant magnetorersistance oscillations in a high mobility two-dimensional (2D) electron gas (EG) subject to crossed microwave (MW) radiation field and a magnetic field, 1,2,3,4,5 especially the observation of "zero-resistance" states developed from the oscillation minima, 4,5,6,7 has revived tremendous interest in magneto-transport in 2D electron systems. 8,9,10,11,12,13 These radiation-induced oscillations of longitudinal resistivity R xx are accurately periodical in inverse magnetic field 1/B with period determined by the MW frequency ω rather than the electron density N e . The observed R xx oscillations exhibit a smooth magneticfield variation with resistivity maxima at ω/ω c = j − δ − and minima at ω/ω c = j + δ + (ω c is the cyclotron frequency, j = 1, 2, 3...) having positive δ ± around 1 4 .4 The resistivity minimum goes downward with increasing sample mobility and/or increasing radiation intensity until a "zero-resistance" state shows up, while the Hall resistivity keeps the classical form R xy = B/N e e with no sign of quantum Hall plateau over the whole magnetic field range exhibiting R xx oscillation.To explore the origin of the peculiar "zeroresistance" states, different mechanisms have been suggested. 8,9,10,11,12,13 It is understood that the appearance of negative longitudinal resistivity or conductivity in a uniform model suffices to explain the observed vanishing resistance.9 The possibility of absolute negative photoconductance in a 2DEG subject to a perpendicular magnetic field was first explored 30 years ago by Ryzhii.14,15 Recent works 8,10,11 indicated that the periodical structure of the density of states (DOS) of the 2DEG in a magnetic field and the photon-excited electron scatterings are the origin of the magnetoresistance oscillations. Durst et al.8 presented a microscopic analysis for the conductivity assuming a δ-correlated disorder and a simple form of the 2D electron self-energy oscillating with the magnetic field, obtaining the correct period, phase and the possible negative resistivity. Shi and Xie 11 reported a similar result using the Tien and Gorden current formula 16 for photon-assisted coherent tunneling. In these studies, however, the magnetic field is to provide an oscillatory DO...

Considering electron-impurity, electron-acoustic-phonon and electron-optical-phonon scatterings, the nonlinear steady-state transport properties of graphene are studied theoretically by means of the balance equation approach. We find that the conductivity as a function of electric field strength, E, exhibits strongly nonlinear behavior for E larger than a critical value, E(c)≈0.1 kV cm(-1). With the increase of E from zero, the conductivity first decreases slowly and then it falls rapidly when E>E(c). The dependence of electron temperature on E is also demonstrated.

This paper is concerned with the derivation of the retarded Green's function for a two-dimensional graphene layer in a perpendicular magnetic field in two explicit, analytic forms, which we employ in obtaining a closed-form solution for the Green's function of a tightly confined magnetized graphene quantum dot. The dot is represented by a δ(2)(r)-potential well and the system is subject to Landau quantization in the normal magnetic field.

Using a nonequilibrium Green function approach, we systematically investigate the current induced spin polarization (CISP) in a two-dimensional heavy-hole system with cubic Rashba spin-orbit coupling, driven by in-plane electric and magnetic fields. We find that when a magnetic field is applied along the direction of electric field, the longitudinal conductivity drops monotonously with an increase of magnetic-field strength or of hole density. The spin polarization along the electric-field direction is just the Pauli paramagnetism and it quadratically increases with an increase of hole density. The nonvanishing out-of-plane component of spin polarization emerges for both short-range and long-range disorders, and it changes sign with the variation of magnetic field, especially for long-range hole-impurity scattering. In the magnetic-field dependences of this out-of-plane CISP and of the in-plane CISP perpendicular to the electric field, there are singular magnetic fields, below or above which the effects of magnetic field are completely different.

In this paper, we determine the spectrum and density of states of a graphene quantum dot in a normal quantizing magnetic field. To accomplish this, we employ the retarded Green function for a magnetized, infinite-sheet graphene layer to describe the dynamics of a tightly confined graphene quantum dot subject to Landau quantization. Considering a δ((2))(r) potential well that supports just one subband state in the well in the absence of a magnetic field, the effect of Landau quantization is to 'splinter' this single energy level into a proliferation of many Landau-quantized states within the well. Treating the graphene sheet and dot as a closed system subject to a fully Hermitian Hamiltonian (including boundary conditions), there is no indication of decay of the Landau-quantized graphene dot states into the quantized states of the host graphene sheet for 'tight' confinement by the δ((2))(r) potential well, notwithstanding extension of the dot Green function (and eigenfunctions) outside the δ((2))(r) potential well.