Manifestation of Pure Spin Currents Induced by Spin Dependent Electron-Phonon Interaction S.D. Ganichev 1 , S.N.~Danilov 1 , V.V. Bel’kov 2 , S. Giglberger 1 , E.L. Ivchenko 2 , S.A. Tarasenko 2 , D. Weiss 1 , W. Prettl 1 , W. Jantsch 3 , F. Schäffler 3 , D. Gruber 3 1 Faculty of Physics, University of Regensburg, D-93040 Regensburg, Germany, 2 A.F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia, 3 Institut für Halbleiter- und Festkörperphysik, Johannes-Kepler Universität Linz, e-mail: sergey.ganichev@physik.uni-regensburg.de Abstract It is shown that energy relaxation of infrared radiation heated electron gas results in equal and oppositely directed currents in spin-up and spin-down subbands caused by spin dependent electron-phonon interaction. In experiment on SiGe structures a pure spin current was converted into electric current by application of a magnetic field distorting the balance between the two partial currents. Introduction During last year spin polarized currents, spin separation like in the spin Hall effect and pure spin currents, which is not accomplished by the charge current, attracted a growing attention. It is caused by the search for tools needed for realization of the all-electric non-magnetic semiconductor spintronics. In the present paper we demonstrate that Drude absorption as well as electron ga heating due to spin dependent electron-phonon interaction results in a pure spin current. A non-equilibrium free carrier heating is accompanied by spin flow similar to spin currents induced in experiments with simultaneous one- and two-photon coherent excitation [1] or in the spin Hall effect [2,3]. While in our experiments we used THz radiation it is obvious that effects due to the Drude absorption may also be observed at excitation in the microwave range where the basic mechanism is free carrier absorption as well. This could link electronics to spin-optics. While most of the investigations aimed to spintronics and spin-optoelectronics have been carried out on III-V compounds, some very recent results obtained on SiGe nanostructures applying electron spin resonance (ESR) and the circular photogalvanic effect (CPGE), demonstrated that this material may be a promising system for spin-based electronics. CPGE and ESR data proves the removal of the spin degeneracy in subbands being crucial for spin manipulation. Furthermore ESR data show that spin relaxation times in SiGe QWs can be sufficiently long. Model It is well known that in zinc-blende lattice based quantum well structures spin degeneracy in the band structure is removed due to bulk inversion asymmetry (BIA) or structure inversion asymmetry (SIA). This feature is crucial for spintronic allowing spin manipulation by external electric field and giving rise to various spin dependent phenomena, like e.g. spin- galvanic effect, spin-Hall effect and circular photogalvanic effect. Formally removal of spin degeneracy is described by k- linear terms in the Hamiltonian. ε k 0 e1 (+1/2) W < W 1 2 j ε k 0 e1 (-1/2) j spin-up subband spin-down subband W > W 1 2 Fig. 1: Microscopic origin of the electric current caused by asymmetry of the excitation. While removal of spin degeneracy is well known practically unknown is the fact that in gyrotropic media also electron- phonon interaction becomes to be spin dependent. Indeed due to the same arguments electron-phonon interaction also have an asymmetric part of electron-phonon interaction described by k-linear terms. In contrast to the previous examples it does not modify the single-electron spectrum but can give rise to spin dependent effects, like spin currents [4]. Phenomenologically the asymmetric part of electron-phonon interaction is given by ∑ + ζ Ξ = − j j j c phon el , ) ( k , k' V ) [ ] ∑ + + ζ × − ξ Ξ + j j j j cv 2 , 1 ) ( σ k k' Here j j , ζ is the phonon-induced strain tensor dependent on the phonon wavevector q = k' - k, c Ξ and cv Ξ are the intra- and inter-band constants of the deformation potential. For zinc- blende-lattice QWs the coefficient ξ is given by: , ) ( 3 0 SO g g SO cv m p i ∆ + ε ε ∆ = ξ h where 0 m is the free-electron mass, g ε and SO ∆ are the band gap and the valence band spin-orbit splitting of the bulk semiconductor used in the QW layer, Z p S p z cv ) = is the interband matrix element of the momentum operator between the Bloch functions of the conduction and valence bands, S and Z. A spin dependent electron-phonon interaction described by the above equation provides a root for several mechanisms of pure spin currents. Two microscopic mechanisms caused by asymmetry of photoexcitation and asymmetry of relaxation are