New features of molecular wires can be observed when they are irradiated by laser fields. These effects can be achieved by periodically oscillating fields but also by short laser pulses. The theoretical foundation used for these investigations is a density matrix formalism where the full system is partitioned into a relevant part and a thermal fermionic bath. The derivation of a quantum master equation, either based on a time-convolutionless or time-convolution projection-operator approach, incorporates the interaction with time-dependent laser fields non-perturbatively and is valid at low temperatures for weak system-bath coupling. From the population dynamics the electrical current through the molecular wire is determined. This theory including further extensions is used for the determination of electron transport through molecular wires. As examples, we show computations of coherent destruction of tunneling in asymmetric periodically driven quantum systems, alternating currents and the suppression of the directed current by using a short laser pulse.

We introduce the spin and momentum dependent force operator which is defined by the Hamiltonian of a clean semiconductor quantum wire with homogeneous Rashba spin-orbit (SO) coupling attached to two ideal (i.e., free of spin and charge interactions) leads. Its expectation value in the spin-polarized electronic wave packet injected through the leads explains why the center of the packet gets deflected in the transverse direction. Moreover, the corresponding spin density will be dragged along the transverse direction to generate an out-of-plane spin accumulation of opposite signs on the lateral edges of the wire, as expected in the phenomenology of the spin Hall effect, when spin-↑ and spin-↓ polarized packets (mimicking the injection of conventional unpolarized charge current) propagate simultaneously through the wire. We also demonstrate that spin coherence of the injected spin-polarized wave packet will gradually diminish (thereby diminishing the "force") along the SO coupled wire due to the entanglement of spin and orbital degrees of freedom of a single electron, even in the absence of any impurity scattering. The classical Hall effect 1 is one of the most widely known phenomena of condensed matter physics because it represents manifestation of the fundamental concepts of classical electrodynamics-such as the Lorentz forcein a complicated solid state environment. A perpendicular magnetic field B exerts the Lorentz force F = qv × B on current I flowing longitudinally through metallic or semiconductor wire, thereby separating charges in the transverse direction. The charges then accumulate on the lateral edges of the wire to produce a transverse "Hall voltage" in the direction qI × B. Thus, Hall-effect measurements reveal the nature of the current carriers.Recent optical detection 2,3 of the accumulation of spin-↑ and spin-↓ electrons on the opposite lateral edges of current carrying semiconductor wires opens new realm of the spin Hall effect. This phenomenon occurs in the absence of any external magnetic fields. Instead, it requires the presence of SO couplings, which are tiny relativistic corrections that can, nevertheless, be much stronger in semiconductors than in vacuum.4 Besides deepening our fundamental understanding of the role of SO couplings in solids, 4,5 the spin Hall effect offers new opportunities in the design of all-electrical semiconductor spintronic devices that do not require ferromagnetic elements or cumbersome-to-control external magnetic fields. 5While experimental detection of the strong signatures of the spin Hall effect brings to an end decades of theoretical speculation for its existence, it is still unclear what spin-dependent forces are responsible for the observed spin separation in different semiconductor systems. One potential mechanism-asymmetric scattering of spin-↑ and spin-↓ electrons off impurities with SO interactionwas invoked in the 1970s to predict the emergence of pure (i.e., not accompanied by charge transport) spin current, in the transverse direction to the flow of lon...

An accurate and efficient numerical approach is developed for the transient electronic dynamics of open quantum systems at low temperatures. The calculations are based on a formally exact hierarchical equations of motion quantum dissipation theory [J. S. Jin et al., J. Chem. Phys. 128, 234703 (2008)]. We propose a hybrid scheme that combines the Matsubara expansion technique and a frequency dispersion treatment to account for reservoir correlation functions. The new scheme not just admits various forms of reservoir spectral functions but also greatly reduces the computational cost of the resulting hierarchical equations, especially in the low temperature regime. Dynamical Kondo effects are obtained and the cotunneling induced Kondo transitions are resolved in the transient current in response to time-dependent external voltages.

We investigate effects of quantum interferences and Coulomb interaction on the counting statistics of electrons crossing a double quantum dot in a parallel geometry by using a generating function technique based on a quantum master equation approach. The skewness and the average residence time of electrons in the dots are shown to be the quantities most sensitive to interferences and Coulomb coupling. The joint probabilities of consecutive electron transfer processes show characteristic temporal oscillations due to interference. The steady-state fluctuation theorem that predicts a universal relation between the number of forward and backward transfer events is shown to hold even in the presence of the Coulomb coupling and interference.