We predict that in a narrow gap III-V semiconductor quantum well or quantum wire, an observable electron spin current can be generated with a time dependent gate to modify the Rashba spin-orbit coupling constant. Methods to rectify the so generated AC current are discussed. An all-electric method of spin current detection is suggested, which measures the voltage on the gate in the vicinity of a 2D electron gas carrying a time dependent spin current. Both the generation and detection do not involve any optical or magnetic mediator.PACS numbers: 71.70. Ej, 72.25.Dc, One key issue in spintronics based on semiconductor is the efficient control of the spin degrees of freedom. Datta and Das 1 suggested the use of gate voltage to control the strength of Rashba spin-orbit interaction (SOI) 2 which is strong in narrow gap semiconductor heterostructures. In InAs-based quantum wells a variation of 50% of the SOI coupling constant was observed experimentally. 3,4Consequently, much interest has been attracted to the realization of spin polarized transistors and other devices based on using electric gate to control the spin dependent transport. 5Besides using a static gate to control the SOI strength and so control the stationary spin transport, new physical phenomena can be observed in time dependent spin transport under the influence of a fast varying gate voltage. Along this line, in this article we will consider a mechanism of AC spin current generation using time dependent gate. This mechanism employs a simple fact that the time variation of Rashba SOI creates a force which acts on opposite spin electrons in opposite directions. Inversely, when a gate is coupled to a nearby electron gas, the spin current in this electron gas also induces a variation of the gate voltage, and hence affects the electric current in the gate circuit. We will use a simple model to clarify the principle of such a new detection mechanism without any optical or magnetic mediator. The systems to be studied will be 1D electron gas in a semiconductor quantum wire (QWR) and 2D electron gas in a semiconductor quantum well (QW).We consider a model in which the Rashba SOI is described by the time dependent Hamiltonian H so (t) = α(t)( k ×ν) · s, where k is the wave vector of an electron, s is the spin operator, andν is the unit vector. For a QWRν is perpendicular to the wire axis, and for a QW perpendicular to the interfaces. The time dependence of the coupling parameter α(t) is caused by a time dependent gate. 6 To explain clearly the physical mechanisms leading to the spin current generation, we will first consider the 1D electron gas in a QWR, and assume α(t) to be a constant α for t<0, and α(t)=0 for t>0. For the 1D system we choose the x direction as the QWR axis and y axis parallel toν, to write the SOI coupling in the form H so (t)= α(t)k x s z . For t<0 the spin degeneracy of conduction electrons is lifted by SOI, producing a splitting ∆= αk x between s z =1/2 and s z =−1/2 bands, as shown in Fig. 1 by solid curves together with the Fermi energy...

The authors apply the generalized master equation to analyze timedependent transport through a finite quantum wire with an embedded subsystem. The parabolic quantum wire and the leads with several subbands are described by a continuous model. We use an approach originally developed for a tight-binding description selecting the relevant states for transport around the bias-window defined around the values of the chemical potential in the left and right leads in order to capture the effects of the nontrivial geometry of the system in the transport. We observe a partial current reflection as a manifestation of a quasi-bound state in an embedded well and the formation of a resonance state between an off-set potential hill and the boundary of the system.

We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of an external magnetic field and the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using "exact numerical diagonalization". * vidar@hi.is †

Coherent electron transport through a quantum channel in the presence of a general extended scattering potential is investigated using a T-matrix Lippmann-Schwinger approach. The formalism is applied to a quantum wire with Gaussian type scattering potentials, which can be used to model a single impurity, a quantum dot or more complicated structures in the wire. The well known dips in the conductance in the presence of attractive impurities are reproduced. A resonant transmission peak in the conductance is seen as the energy of the incident electron coincides with an energy level in the quantum dot. The conductance through a quantum wire in the presence of an asymmetric potential is also shown. In the case of a narrow potential parallel to the wire we find that two dips appear in the same subband which we ascribe to two quasi bound states originating from the next evanescent mode.

We use a non-Markovian master equation to describe the transport of Coulomb interacting electrons through an electromagnetic cavity with one quantized photon mode. The central system is a finite parabolic quantum wire that is coupled weakly to external parabolic quasi-one-dimensional leads at $t=0$. With a stepwise introduction of complexity to the description of the system and a corresponding stepwise truncation of the ensuing many-body spaces we are able to describe the time-dependent transport of Coulomb-interacting electrons through a geometrically complex central system. We take into account the full electromagnetic interaction of electrons and cavity photons without resorting to the rotating wave approximation or reduction of the electron states to two levels. We observe that the number of initial cavity photons and their polarization can have important effects on the transport properties of the system. The quasiparticles formed in the central system have a lifetime limited by the coupling to the leads and radiation processes active on a much longer timescale.Comment: RevTeX (pdf-LaTeX) 11 pages with 12 jpg-figures include

We obtain and analyze the effect of electron-electron Coulomb interaction on the time-dependent current flowing through a mesoscopic system connected to biased semi-infinite leads. We assume the contact is gradually switched on in time and we calculate the time-dependent reduced density operator of the sample using the generalized master equation. The many-electron states ͑MES͒ of the isolated sample are derived with the exact-diagonalization method. The chemical potentials of the two leads create a bias window which determines which MES are relevant to the charging and discharging of the sample and to the currents, during the transient or steady states. We discuss the contribution of the MES with fixed number of electrons N and we find that in the transient regime there are excited states more active than the ground state even for N = 1. This is a dynamical signature of the Coulomb-blockade phenomenon. We discuss numerical results for three sample models: short one-dimensional chain, two-dimensional ͑2D͒ lattice, and 2D parabolic quantum wire.

Quantum transport in a narrow constriction, and in the presence of a finite-range time-modulated potential, is studied. The potential is taken the form V (x, t) = V0 θ(x)θ(a − x) cos(ωt), with a the range of the potential and x the transmission direction. As the chemical potential µ is increasing, the dc conductance G is found to exhibit dip, or peak, structures when µ is at nhω above the threshold energy of a subband. These structures in G are found in both the small a (a ≪ λF ) and the large a (a ≫ λF ) regime. The dips, which are associated with the formation of quasi-bound states, are narrower for smaller a, and for smaller V0. The locations of these dips are essentially fixed, with small shifts only in the case of large V0. Our results can be reduced to the limiting case of a delta-profile oscillating potential when both a and V0a are small. The assumed form of the time-modulated potential is expected to be realized in a gate-induced potential configuration.

We investigate the transport through a quantum ring, a dot and a barrier embedded in a nanowire in a homogeneous perpendicular magnetic field. To be able to treat scattering potentials of finite extent in magnetic field we use a mixed momentum-coordinate representation to obtain an integral equation for the multiband scattering matrix. For a large embedded quantum ring we are able to obtain Aharanov-Bohm type of oscillations with superimposed narrow resonances caused by interaction with quasi-bound states in the ring. We also employ scattering matrix approach to calculate the conductance through a semi-extended barrier or well in the wire. The numerical implementations we resort to in order to describe the cases of weak and intermediate magnetic field allow us to produce high resolution maps of the "near field" scattering wave functions, which are used to shed light on the underlying scattering processes.