Spin-current generation and detection in the presence of an ac gate

Abstract

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...

Spin current injection by intersubband transitions in quantum wells

“…Under excitation of a 1% fraction of electrons, achieved at S ≈ 10 kW/cm 2 and τ ≈ 1 ps, the corresponding effective current density e∆J z y /h ≈ 1 mAmp/cm. This is of the same magnitude as would be generated by the ac spin pumping in the n = 1 subband, as proposed by Mal'shukov et al 2 , but the effect would operate on a nanosecond time scale, as opposed to the picosecond time scale relevant here.…”

“…The anomalous spin-conserving term is of the order of α [2] D − α [1] D N 2 , while the other terms depend on the difference of the ratio α…”

“…The application of an external electric field has been suggested as a strategy for driving the system out of equilibrium and inducing a spin current exhibiting transport effects. Mal'shukov et al 2 and Governale et al 3 suggested applying a time-dependent bias across a semiconducting heterostructure, thus modulating the strength of the SO coupling and generating a spin current. Murakami et al 4 and Sinova et al 5 have shown that an in-plane electric field can cause a spin current, leading to the "intrinsic spin-Hall effect".…”

Spintronics: Fundamentals and applications

“…There is a wide range of recent theoretical proposals for devices that would give rise to a spin electromotive force (Ž utić et al, 2001a, 2001bBrataas et al, 2002;Governale et al, 2003;Long et al, 2003;Mal'shukov et al, 2003;Ting and Cartoixà , 2003), often referred to as spin(-polarized) pumps, cells, or batteries. However, even when it is feasible to generate pure spin current, this does not directly imply that it would be dissipationless.…”

Spin Dynamics and Spin Transport

“…It sounds tempting to apply to this problem a notion of spin currents defined similarly to electric currents. E.g., electrical spin injection and spin polarization produced by SO mechanisms and spin transport in such systems have been discussed recently in terms of spin currents driven by ac [18,19] and dc [20,21] electric fields. However, this appealing approach meets serious problems.…”