Universal Rashba spin precession of two-dimensional electrons and holes

– We study spin precession due to Rashba spin splitting of electrons and holes in semiconductor quantum wells. Based on a simple analytical expression that we derive for the current modulation in a broad class of experimental situations of ferromagnet/nonmagnetic semiconductor/ferromagnet hybrid structures, we conclude that the Datta-Das spin transistor (i) is feasible with holes and (ii) its functionality is not affected by integration over injection angles. The current modulation shows a universal oscillation period, irrespective of the different forms of the Rashba Hamiltonian for electrons and holes. The analytic formulas approximate extremely well exact numerical calculations of a more elaborate Kohn–Luttinger model. Transport effects based on coherent manipulation of the spin degree of freedom in low– dimensional semiconductors are currently attracting a lot of attention [1]. These studies, enabled by recent progress in nanofabrication technology to create high–quality samples, are motivated by both their interesting fundamental physics and their potential for future device applications [2]. A lot of progress in the field has been stimulated by the exploitation of spin precession due to Rashba spin–orbit (SO) coupling in 2D systems both for electrons [3–5] and for holes [6]. A prominent example is the spin–controlled field–effect transistor (spin FET) introduced by Datta and Das [7], followed by more recent proposals for novel devices utilizing Rashba SO coupling [8]. Both in the original [7] and most subsequent [9–14] works, a quasi– onedimensional (1D) confinement was considered essential for proper spin–FET action. Spin precession in truly 2D electron systems was studied numerically in a number of works [15,16]. On the other hand, ever-present spin relaxation will reduce the spin polarization of currents, preventing the realization of gate-controlled modulation. Such processes arise, e.g., from magnetic impurities but most importantly from elastic impurity scattering that randomizes the direction of the effective Rashba field. Stronger spin-orbit coupling and band mixing phenomena imply a shorter spin relaxation time for the holes respect to the electrons, that can be compensated by shorter precession length. A nice proposal, which exploits tunability of Rashba SO coupling, to overcome the detrimental effects due to scattering processes is presented in the last paper of Ref. [8].