S. Y. Liu

Diffusive transport in graphene: The role of interband correlation

We present a kinetic equation approach to investigate dc transport properties of graphene in the diffusive regime considering long-range electron-impurity scattering. In our study, the effects of interband correlation (or polarization) on conductivity are taken into account. We find that the conductivity contains not only the usual term inversely proportional to impurity density Ni but also an anomalous term that is linear in Ni. This leads to a minimum in the density dependence of conductivity when the electron density Ne is equal to a finite critical value Nc. The effects of various scattering potentials on the conductivity minimum are also analyzed. Using typical experimental parameters, we find that for random-phase-approximation–screened electron-impurity scattering, the minimum conductivity is about 4.42e2/h when Ne≈0.11Ni, and the conductivity varies almost linearly with the electron density for Ne>Ni.

Quantum computation with two-level trapped cold ions beyond Lamb-Dicke limit

We propose a simple scheme for implementing quantum logic gates with a string of two-level trapped cold ions outside the Lamb-Dicke limit. Two internal states of each ion are used as one computational qubit (CQ) and the collective vibration of ions acts as the information bus, i.e., the bus qubit (BQ). Using the quantum dynamics for the laser-ion interaction as described by a generalized Jaynes-Cummings model, we show that quantum entanglement between any one CQ and the BQ can be coherently manipulated by applying classical laser beams. As a result, universal quantum gates, i.e., the one-qubit rotation and two-qubit controlled gates, can he implemented exactly. The required experimental parameters for the implementation, including the Lamb-Dicke (LD) parameter and the durations of the applied laser pulses, are derived. Neither the LD approximation for the laser-ion interaction nor the auxiliary atomic level is needed in the present scheme.

Temperature dependence of microwave-induced magnetoresistance oscillation in two-dimensional electron systems

Microwave-induced magnetoresistance oscillations in two-dimensional electron systems are examined from a photon-assisted transport scheme with short-range impurity scatterings. Analytical results at high filling factors strongly support early considerations and the recent experimental observation that the temperature dependence of the oscillation amplitude is exponential and originates primarily from the single-particle lifetime.

Nonlinear magnetoresistance of an irradiated two-dimensional electron system

Nonlinear magnetotransport of a microwave-irradiated high-mobility two-dimensional electron system under a finite direct current excitation is analyzed using a dc-controlled scheme with photon-assisted transition mechanism. The predicted amplitudes, extrema, and nodes of the oscillatory differential resistance versus the magnetic field and the current density are in excellent agreement with the recent experimental observation [Hatke et al. Phys. Rev. B 77, 201304(R) (2008)].

Elementary operations for quantum logic with a single trapped two-level cold ion beyond Lamb-Dicke limit

Abstract A simple alternative scheme for implementing quantum gates with a single trapped cold two-level ion beyond the Lamb–Dicke (LD) limit is proposed. Based on the quantum dynamics for the laser–ion interaction described by a generalized Jaynes–Cummings model, one can introduce two kinds of elementary quantum operations i.e., the simple rotation on the bare atomic state, generated by applying a resonant pulse, and the joint operation on the internal and external degrees of the ion, performed by using an off-resonant pulse. Several typical quantum gates, including Hadamard gate, controlled-Z and controlled-NOT gates etc., can thus be implemented exactly by using these elementary operations. The experimental parameters including the LD parameter and the durations of the applied laser pulses, for these implementation are derived analytically and numerically. Neither the LD approximation for the laser–ion interaction nor the auxiliary atomic level is needed in the present scheme.

Inverse spin Hall effect by spin injection

Motivated by a recent experiment [S. O. Valenzuela and M. Tinkham, Nature (London) 442, 176 (2006)], the authors present a quantitative microscopic theory to investigate the inverse spin-Hall effect with spin injection into aluminum considering both intrinsic and extrinsic spin-orbit couplings using the orthogonalized-plane-wave method. Their theoretical results are in good agreement with the experimental data. It is also clear that the magnitude of the anomalous Hall resistivity is mainly due to contributions from extrinsic skew scattering.

Diffusive Transport in Graphene

Recently, a single-layer of carbon atoms, termed graphene, has attracted a great deal of interest due to its great potential for application in electronics. In experiments involving graphene, a finite residual conductivity was found at zero gate voltage in the density dependence of conductivity. However, the theoretical explanation of this observation has been confused, with derivations predicting differing values of residual conductivity. In this paper, considering electron-impurity scattering, we present a kinetic equation approach to investigate transport in graphene. The effect of interband polarization on conductivity is taken into account. We find that, in the density dependence of conductivity, there is a minimum (rather than residual) conductivity sensitively dependent on the carrier-impurity scattering potential. For higher electron density, the conductivity varies almost linearly with the electron density.

Anomalous Hall effect in two-dimensional semiconductors: The roles of electron-impurity and electron-phonon scatterings

Taking account of both the electron-impurity and electron-phonon scatterings, the anomalous Hall effect in two dimensional electron systems is investigated by means of a kinetic equation approach. The spin-orbit coupling directly induced by an external driving electric field and the extrinsic spin-orbit coupling (associated with electron-impurity and electron-phonon scatterings) are included. The side-jump and skew-scattering contributions arising from these spin-orbit couplings to anomalous Hall current are expressed in terms of the distribution function. Performing a numerical calculation for InSb/AlInSb quantum wells, we analyze the effects of both electron-impurity and electron-phonon scatterings on anomalous Hall current. The temperature dependence of anomalous Hall current is also studied in the regime 0<T<300 K.

Inhomogeneous 2D polariton radiation excited by a finite electromagnetic wave train

A non-stationary process of polariton-mode excitation in a two-dimensional excitonic layer (at z = 0) by a light-wave extinction front is analyzed here using the electromagnetic dyadic Greens function formalism. The electromagnetic response of the 2D layer includes inhomogeneous radiative exciton-polariton modes corresponding to complex poles of the matrix Greens function.