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We show that spin-dependent electron-phonon interaction in the energy relaxation of a two-dimensional electron gas results in equal and oppositely directed currents in the spin-up and spin-down subbands yielding a pure spin current. In our experiments on SiGe heterostructures the pure spin current is converted into an electric current applying a magnetic field that lifts the cancellation of the two partial charge flows. A microscopic theory of this effect, taking account of the asymmetry of the relaxation process, is developed and is in good agreement with the experimental data. DOI: 10.1103/PhysRevB.75.155317 PACS number͑s͒: 73.21.Fg, 72.25.Fe, 78.67.De, 73.63.Hs Lately, there has been much interest in the use of the spin of carriers in semiconductor quantum well ͑QW͒ structures together with their charge to realize concepts such as spintronics and spin-optoelectronics. 1 The transport of the spin of charge carriers in semiconductor nanostructures is one of the key problems in this field. Among the necessary conditions to realize spintronics devices there are a high spin polarization in QWs and a large spin splitting of subbands. The latter is important to control spins by an external electric field via the Rashba effect. 2 While most of the investigations aimed at spintronics and spin-optoelectronics have been carried out on III-V compounds, some recent results obtained on nonmagnetic SiGe nanostructures applying electron spin resonance 3,4 ͑ESR͒ and the circular photogalvanic effect ͑CPGE͒, 5,6 demonstrated that this material may be a promising system for spin-based electronics. ESR and CPGE data show that spin relaxation times in SiGe QWs can be sufficiently long, 3,4,[7][8][9][10][11] that the spin degeneracy is lifted, 3,5,6,11,12 that the g factor is tunable by crystallographic direction, electron density, Ge content, kinetic energy of free carriers, and electric current 3,13-17 and that spin manipulation can be achieved by means of the spin-echo method. 4 Here we report on an electrically measured observation of pure spin currents causing spatial spin separation in SiGe quantum well structures, allowing manipulation of spins in this material which is attractive for high-speed electronics and spintronics. Spin currents recently attracted rapidly growing interest since they can provide new tools for the realization of all-electric nonmagnetic semiconductor spintronics. Various phenomena comprising charge photocurrents driven by the spin degree of freedom 18-26 and spin separation caused by pure spin currents 27-32 were reported. Most of these phenomena originate from the well known lifting of spin degeneracy. The latter causes the band structure to split into spin-up and spin-down branches described by linear in wave vector k terms in the Hamiltonian due to structure inversion asymmetry ͑SIA͒ or bulk inversion asymmetry ͑BIA͒. The pure spin currents reported here are caused by less known spin-dependent electron scattering processes 33,34 which generate a pure spin current, causing spin separation in a ...
We show that spin-dependent electron-phonon interaction in the energy relaxation of a two-dimensional electron gas results in equal and oppositely directed currents in the spin-up and spin-down subbands yielding a pure spin current. In our experiments on SiGe heterostructures the pure spin current is converted into an electric current applying a magnetic field that lifts the cancellation of the two partial charge flows. A microscopic theory of this effect, taking account of the asymmetry of the relaxation process, is developed and is in good agreement with the experimental data. DOI: 10.1103/PhysRevB.75.155317 PACS number͑s͒: 73.21.Fg, 72.25.Fe, 78.67.De, 73.63.Hs Lately, there has been much interest in the use of the spin of carriers in semiconductor quantum well ͑QW͒ structures together with their charge to realize concepts such as spintronics and spin-optoelectronics. 1 The transport of the spin of charge carriers in semiconductor nanostructures is one of the key problems in this field. Among the necessary conditions to realize spintronics devices there are a high spin polarization in QWs and a large spin splitting of subbands. The latter is important to control spins by an external electric field via the Rashba effect. 2 While most of the investigations aimed at spintronics and spin-optoelectronics have been carried out on III-V compounds, some recent results obtained on nonmagnetic SiGe nanostructures applying electron spin resonance 3,4 ͑ESR͒ and the circular photogalvanic effect ͑CPGE͒, 5,6 demonstrated that this material may be a promising system for spin-based electronics. ESR and CPGE data show that spin relaxation times in SiGe QWs can be sufficiently long, 3,4,[7][8][9][10][11] that the spin degeneracy is lifted, 3,5,6,11,12 that the g factor is tunable by crystallographic direction, electron density, Ge content, kinetic energy of free carriers, and electric current 3,13-17 and that spin manipulation can be achieved by means of the spin-echo method. 4 Here we report on an electrically measured observation of pure spin currents causing spatial spin separation in SiGe quantum well structures, allowing manipulation of spins in this material which is attractive for high-speed electronics and spintronics. Spin currents recently attracted rapidly growing interest since they can provide new tools for the realization of all-electric nonmagnetic semiconductor spintronics. Various phenomena comprising charge photocurrents driven by the spin degree of freedom 18-26 and spin separation caused by pure spin currents 27-32 were reported. Most of these phenomena originate from the well known lifting of spin degeneracy. The latter causes the band structure to split into spin-up and spin-down branches described by linear in wave vector k terms in the Hamiltonian due to structure inversion asymmetry ͑SIA͒ or bulk inversion asymmetry ͑BIA͒. The pure spin currents reported here are caused by less known spin-dependent electron scattering processes 33,34 which generate a pure spin current, causing spin separation in a ...
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