Pure spin currents induced by spin-dependent scattering processes in SiGe quantum well structures

Abstract

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 ...

Wide-range optical spin orientation in Ge from near-infrared to visible light

“…In fact, thanks to the inversion symmetry of the crystal and the related absence of the D Yakonov-Perel spin scattering mechanism, Ge presents a longer spin coherence time than the one of GaAs. Spin manipulation [1], spin transport [2], spin optical pumping in the infrared [3][4][5][6][7], and electrical spin injection [8] in Ge have been reported. In our previous works [9,10], we demonstrated the room temperature operation of spin-PDs based on fully epitaxial Fe/MgO/Ge(001) heterostructures, working at 0.95 eV [11,12].…”

“…1 For λ L and P S , we used the values reported in Fig. 3(b), while l SF is left as a free parameter.…”

Interplay of spin and orbital magnetogyrotropic photogalvanic effects in InSb/(Al,In)Sb quantum well structures

“…The current perpendicular to the magnetic field is dominated by the polarization-independent contribution (LMPGE) and is therefore driven by relaxation processes. 28,41 At the same time, the longitudinal magnetic-field-induced photocurrent excited by circularly polarized radiation is solely governed by the photon angular momentum (CMPGE). 27 Comparison of our data on the magnetic-field-induced photocurrents with that reported earlier for GaAs-and InAs-based QW structures (for review, see Ref.…”

“…25,28,41 The latter is due to the spin-orbit interaction in gyrotropic media, such as InSb-and GaAs-based low-dimensional structures, which yields a scattering matrix element being proportional to [σ × (k + k )]. Here, k and k are the initial and the scattered wave vectors and σ is the vector composed of the Pauli matrices, and only structural inversion asymmetry is assumed.…”

Pure spin photocurrents

“…[8,35,38]). In such a relaxational mechanism, the spin current is generated in the process of energy relaxation of electrons no matter how the thermal equilibrium between the electron and phonon subsystems was initially disturbed.…”