X. L. Lei

Frequency limitations of negative differential mobility in vertical conduction in superlattices

The frequency‐dependence of the carrier drift velocity in superlattice miniband conduction in response to a small signal ac electric field is investigated for laterally unconfined and one‐dimensionally confined (2D) superlattices biased in the negative differential mobility regime. Numerical calculations show that in both systems there exists a mobility‐transition frequency of order of 100 GHz, above which the differential mobility becomes positive. This frequency in a 2D superlattice is about three times that in an unconfined system, owing to enhanced carrier scattering. In a domain‐driven microwave oscillator, the mobility‐transition frequency is expected to be relatively small compared to the oscillator frequency. Nonetheless, the higher mobility‐transition frequency in the 2D superlattice would render it more advantageous for use in negative differential mobility microwave oscillators.

Robust Negative Differential Conductance and Enhanced Shot Noise in Transport Through a Molecular Transistor With Vibration Assistance

In this paper, we analyze vibration-assisted sequential tunneling (including current-voltage characteristics and zero-frequency shot noise) through a molecular quantum dot with two electronic orbitals asymmetrically coupled to the internal vibration. We employ rate equations for the case of equilibrated phonons, and strong Coulomb blockade. We find that a system with a strongly phonon-coupled ground state orbital and weakly phonon-coupled excited state orbital exhibits strong negative differential conductance; and it also shows super-Poissonian current noise. We discuss in detail the reasons and conditions for the appearance of negative differential conductance.

Ac-cotunneling through an interacting quantum dot in a magnetic field

We analyze inelastic cotunneling through an interacting quantum dot subject to an ambient magnetic field in the weak tunneling regime under a non-adiabatic time-dependent bias-voltage. Our results clearly exhibit photon-assisted satellites and an overall suppression of differential conductance with increasing driving amplitude, which is consistent with experiments. We also predict a zero-anomaly in differential conductance under an appropriate driving frequency.

Qubit measurement by a quantum point contact: A quantum Langevin-equation approach

We employ a quantum Langevin equation approach to establish non-Markovian dynamical equations, on a fully microscopic basis, to investigate the measurement of the state of a coupled quantum dot qubit by a nearby quantum point contact. The ensuing Bloch equations allow us to examine qubit relaxation and decoherence induced by measurement, and also the noise spectrum of meter output current with the help of a quantum regression theorem, at arbitrary bias-voltage and temperature. Our analyses provide a clear resolution of a recent debate concerning the occurrence of a quantum oscillation peak in the noise spectrum.

THE SPIN-HALL EFFECT IN p-TYPE BULK SEMICONDUCTORS

The spin Hall effect provides a new possible way to effectively inject spins into paramagnetic semiconductors. Here, we investigate the spin-Hall effect in a p-type Luttinger semiconductor employing a two-band kinetic equation analysis. The long-range disorder effect on spin-Hall current (SHC) is considered within the self-consistent Born approximation. We find that in addition to the intrinsic SHC proposed previously, there is a nonvanishing SHC that originates from long-range electron-impurity scattering, but which is independent of impurity density in the diffusive regime. This SHC has an opposite sign from the intrinsic one, leading to a significant reduction of the total SHC. We also carry out a numerical analysis of the hole density dependencies of SHC and spin mobility, finding that with increasing hole density, the SHC first increases and then falls, while the spin mobility monotonically decreases.

Anomalous Hall effect in a two‐dimensional electron system: unified analysis of side‐jump and skew scattering mechanisms

We present a unified kinetic equation approach to analyze the side‐jump and skew scattering anomalous Hall contributions in a two‐dimensional electron system with extrinsic spin‐orbit coupling and also spin‐orbit coupling induced directly by the driving electric field. Considering long‐range electron‐impurity scattering up to the second Born approximation, we derive the side‐jump and skew scattering contributions to the anomalous Hall conductivity on an equal basis. Our numerical analysis shows that in a typical two‐dimensional semiconductor with magnetization, both the side‐jump and skew anomalous Hall conductivities are of the same order of magnitude. For an attractive electron‐impurity scattering potential, their signs are the same as that of the ordinary Hall conductivity. Also, we make it clear that the anomalous Hall effect is relatively small in comparison with the ordinary one in the diffusive regime.

Ferromagnetic Lead Effects on Inelastic Cotunneling Current and Shot Noise of an Interacting Quantum Dot

We analyze inelastic cotunneling through a strongly Coulomb‐blockaded quantum dot attached to two ferromagnetic leads in the weak coupling limit using a quantum Langevin equation approach, from which we derive a fully microscopic quantum based Bloch‐type equation to describe the cotunneling‐induced spin relaxation dynamics. Based on this, we then obtain explicit analytical expressions for the local magnetization, cotunneling current, and its fluctuations.

TEMPORAL DEVELOPMENT OF NONLINEAR BLOCH ELECTRON MINIBAND TRANSPORT

We present a balance-equation calculation of the transient response of Bloch electron transport in a superlattice miniband to an external time-dependent electric field parallel to the growth axis. Upon turning on a step electric field having strength within the range of negative differential mobility in the steady-state velocity-field curve, the drift velocity exhibits very pronounced overshoot. Immediately after the drift velocity reaches its maximum, the instantaneous inverse effective mass may fall within the range of negative values in consequence of the driving field. The rise of the electron temperature, however, is much slower, such that the behavior of the velocity response to an impulsive electric field is markedly dependent on the duration of the impulse.