Conversion of Spin into Directed Electric Current in Quantum Wells

Abstract

A nonequilibrium population of spin-up and spin-down states in quantum well structures has been achieved applying circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicular to the direction of light propagation. Because of the spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left handed. A microscopic model is presented which describes the origin of the photon helicity driven current. The model suggests that the system behaves as a battery which generates a spin polarized current. DOI: 10.1103/PhysRevLett.86.4358 PACS numbers: 73.50.Mx, 68.65.-k, 73.50.Pz, 78.30.Fs The spin of electrons and holes in solid state systems is an intensively studied quantum mechanical property as it is the crucial ingredient for spintronics [1,2] and several schemes of quantum computation [3][4][5]. Among others, current investigations involve the spin lifetime in semiconductor devices [6][7][8] as well as the injection of spin polarized electrons (or holes) from semimagnetic semiconductor materials into semiconductors [9][10][11] or from ferromagnetic into nonmagnetic metals [12,13].It is well known that spin polarized electrons can be generated by circularly polarized light [14,15] and, vice versa, that the recombination of spin polarized charged carriers results in the emission of circularly polarized light [10,11,14]. However, little is known about spin dependent photocurrents when a semiconductor is irradiated by circularly polarized light [15,16]. Helicity dependent photocurrents in semiconductors have been observed in bulk Te utilizing the peculiarities of the valence band structure ("camel back") at the first Brillouin zone boundary and in bulk GaAs subjected to an external magnetic field [15]. A first indication of such a photon helicity dependent photocurrent in semiconductor heterojunctions was found in recent far infrared experiments on p-type GaAs͞AlGaAs heterojunctions containing a two-dimensional hole gas [17]. This preliminary experiment was discussed in phenomenological terms and lacked the microscopic connection to the carriers' spin.The experiments on quantum wells (QWs) described below uncover a novel property of an unbalanced spin polarization: its ability to generate a directed current where the current's direction depends solely on the predominant spin orientation. This effect may be illustrated as an electron analog of mechanical systems where a rotational motion ("spin") is transmitted into a linear one ("current") like a rotating wheel on a hard surface. Below we point out that spin injection into quantum wells of zinc-blende-type material leads always to an electric current in the plane of the quantum well. The reduced dimensionality of quantum wells lowers the crystallographic symmetry and introduces k-linear terms in the Hamiltonian. These k-linear terms lift the spin degenerate of energy bands in k-space which, in...

Photogalvanic effects in quantum wells

“…Helicity-dependent photocurrents in semiconductors have been observed in bulk Te utilizing the peculiarities of the valence band structure ('camel back') at the ÿrst Brillouin zone boundary and in bulk GaAs subjected to an external magnetic ÿeld [5]. Recently, this e ect has also been observed in low-dimensional structures [6,7]. The symmetry of the second-rank pseudotensor S coincides with that of the tensor of gyrotropy.…”

Theory of current-induced spin polarization in an electron gas

“…The generation of a current from a non-equilibrium spin polarization goes under the equivalent names of spin galvanic effect (SGE) or inverse Edelstein effect (IEE), while the reciprocal phenomenon of current-induced spin polarizations is referred to as inverse spin galvanic effect (ISGE) or Edelstein effect (EE). The first experimental observation of the SGE/IEE was in quantum wells, by measuring the current produced by the absorption of polarized light [10][11][12]. More recently it has been shown that a non-equilibrium spin polarization injected by spin-pumping into an Ag|Bi [13] or Fe|GaAs [14] interface also yields an electrical current.…”

Polarization-dependent photocurrents in polar stacks of van der Waals solids

“…This value exceeds the helicity-dependent spin-galvanic signal in 2D GaAs 26 . The ratio of the net linear PGE and circular PGE is small as τ /τ sk .…”