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 intrinsic spin-Hall effect on spin accumulation and electric conductance in a diffusive regime of a 2D electron gas has been studied for a 2D strip of a finite width. It is shown that the spin polarization near the flanks of the strip, as well as the electric current in the longitudinal direction exhibit damped oscillations as a function of the width and strength of the Dresselhaus spin-orbit interaction. Cubic terms of this interaction are crucial for spin accumulation near the edges. As expected, no effect on the spin accumulation and electric conductance have been found in case of Rashba spin-orbit interaction.PACS numbers: 72.25. Dc, 71.70.Ej, 73.40.Lq Spintronics is a fast developing area to use electron spin degrees of freedom in electronic devices [1]. One of its most challenging goals is to find a method for manipulating electron spins by electric fields. The spin-orbit interaction (SOI), which couples the electron momentum and spin, can be a mediator between the charge and spin degrees of freedom. Such a coupling gives rise to the so called spin-Hall effect (SHE) which attracted much interest recently. Due to SOI the spin flow can be induced perpendicular to the DC electric field, as has been predicted for systems containing spin-orbit impurity scatterers [2]. Later, similar phenomenon was predicted for noncentrosymmetric semiconductors with spin split electron and hole energy bands [3]. It was called the intrinsic spin-Hall effect, in contrast to the extrinsic impurity induced effect, because in the former case it originates from the electronic band structure of a semiconductor sample. Since the spin current carries the spin polarization, one would expect a buildup of the spin density near the sample boundaries. In fact, this accumulated polarization is a first signature of SHE which has been detected experimentally, confirming thus the extrinsic SHE [4] in semiconductor films and intrinsic SHE in a 2D hole gas [5]. On the other hand, there were still no experimental evidence of intrinsic SHE in 2D electron gases. The possibility of such an effect in macroscopic samples with a finite elastic mean free path of electrons caused recently much debates. It has been shown analytically [6,7,8,9,10,11] and numerically [12] that in such systems SHE vanishes at arbitrary weak disorder in DC limit, for isotropic, as well as anisotropic [10] impurity scattering, when SOI is represented by the so called Rashba interaction [13]. As one can expect in this case, there is no spin accumulation at the sample boundaries, except for the pockets near the electric contacts [7]. At the same time, the Dresselhaus SOI [14], which dominates in symmetric quantum wells, gives a finite spin-Hall conductivity [11]. The latter can be of the order of its universal value e/8π . The same has been shown for the cubic Rashba interaction in hole systems [12,15]. In this connection an important question is what sort of the spin accumulation could Dresselhaus SOI induce near sample boundaries. Another problem which, as far as we know,...

We have shown that in narrow 2D semiconductor channels the D'yakonov-Perel' spin relaxation rate is strongly reduced. This relaxation slowdown appears in special waveguide diffusion modes which determine the propagation of spin density in long channels. Experiments are suggested to detect the theoretically predicted effects. A possible application is a field effect transistor operated with injected spin current.

We have investigated the weak-localization effects on the magnetoresistance of a two-dimensional electron gas in a quantum well of a semiconductor with a zinc-blende crystal structure under a sufficiently weak magnetic field H applied parallel to interfaces, so it interacts only with the electron spins due to the Zeeman term in the Hamiltonian. We found a positive magnetoresistance, which depends strongly on the crystal orientation of the well, and varies with the direction of the field depending on the relative strengths of the Rashba and Dresselhaus terms in the spin-orbit coupling. For a ͓001͔-oriented well we found that the magnetic field destroys antilocalization when the Zeeman energy gH is larger than ( so ) Ϫ1/2 , where is the inelastic dephasing time and so the spin-orbit relaxation time. On the other hand, in a symmetric ͓011͔oriented quantum well, the Zeeman interaction leads to a weak localization of electrons.

We performed path integral simulations of spin evolution controlled by the Rashba spin-orbit interaction in the semiclassical regime for chaotic and regular quantum dots. The spin polarization dynamics have been found to be strikingly different from the D'yakonov-Perel' (DP) spin relaxation in bulk systems. Also an important distinction have been found between long time spin evolutions in classically chaotic and regular systems. In the former case the spin polarization relaxes to zero within relaxation time much larger than the DP relaxation, while in the latter case it evolves to a time independent residual value. The quantum mechanical analysis of the spin evolution based on the exact solution of the Schrödinger equation with Rashba SOI has confirmed the results of the classical simulations for the circular dot, which is expected to be valid in general regular systems. In contrast, the spin relaxation down to zero in chaotic dots contradicts to what have to be expected from quantum mechanics. This signals on importance at long time of the mesoscopic echo effect missed in the semiclassical simulations. I INTRODUCTIONSpin relaxation in semiconductors is an important physical phenomenon being actively studied recently in connection with various spintronics applications [1]. In doped bulk samples and quantum wells (QW) of III-V semiconductors at low temperatures spin relaxation is mostly due to the DP mechanism [2]. This mechanism does not involve any inelastic processes, so that the exponential decay of the spin polarization is determined entirely by the spin-orbit interaction (SOI) and elastic scattering of electrons on the impurities. However, in case of confined systems such as quantum dots (QD) with atomic-like eigenstates, the SOI has been incorporated into the structure of the wave functions of the discrete energy levels. Without inelastic interactions, an initial wave packet with a given spin polarization will evolve in time as a coherent superposition of these discrete eigenstates. Therefore, the corresponding expectation value of the spin polarization will oscillate in time without any decay. To obtain a polarization decay in the QD's, extra effects have to be introduced into the system, e.g., the inelastic interactions between electrons and phonons mediated by the spin-orbit [3,4] and nuclear hyperfine effects [3,5,6]. Accordingly, a spin relaxation in QD's induced by these effects is a real dephasing process.Unlike such an inelastic relaxation in QD's, the DP spin relaxation in unbounded systems seems to be a quite different phenomenon, because the scattering on impurities is elastic and there is no dephasing of the electron wave functions in the systems. However, the spin polarization does decay in time exponentially, as if it would be a true dephasing process. To explain this phenomenon, let us consider an electron moving diffusively through an unbounded system with random elastic scatters. This electron is described by a wave packet represented by a superposition of continuum eigenstates. During a ...