Electrically controlled spin dynamics in disordered semiconductors

The spin of electrons in a semiconductor environment couples not only to magnetic fields, but also to the orbital motion of the electron. As a consequence, transport in semiconductors includes a class of phenomena in which electrically induced charge motion influences the electron spin. The intricate interplay of spin and charge makes this type of effects a diverse research field of fundamental interest, but is also of practical relevance: Spin-orbit interaction (SOI) provides a mechanism to control the spin with electric fields. Being available in tailored materials, that are routinely used in microelectronics, SOI has therefore attracted intense interest for its potential in applications to use the electron spin alternatively to the charge in new types of electronic devices. In this thesis we investigate the interplay of spin and charge transport in disordered electron systems, where random impurities not only determine the electrical resistance but also the spin dynamics through spin-orbit interaction. A focus of this work is electric-dipole-induced spin resonance (EDSR), a versatile scheme of spin control using electric fields. Similar to standard paramagnetic resonance where a combination of static and ac magnetic fields drive spin rotations, in EDSR ac electric fields couple resonantly to the spin. Appropriately chosen pulses of these electric fields, which can be generated easier on-chip than ac magnetic fields, allow to achieve arbitrary spin rotations. In a diagrammatic analysis we find that the presence of disorder broadens the line-shape of EDSR and determines the maximal achievable polarization. We identify random internal magnetic fields as the origin of this line-broadening, which limits the efficiency of EDSR, and show that these limitations can be overcome in an optimal geometry where the internal fields are suppressed by the interference of different spin-orbit mechanisms. This leads to a substantial enhancement of the spin polarization at resonance. We moreover link these findings to spin currents giving rise to the spin-Hall effect. We interpret these spin currents in terms of spin polarization components. The behavior of the spin depends sensitively on whether the orbital motion is diffusive or phase coherent. Indeed, qualitatively different – mesoscopic – effects occur in the spin when electrons flow through phase-coherent systems. Whereas in charge transport such mesoscopic effects are well known, for the spin they have attracted interest only more recently in the context of spintronics. We find that the spin polarization, that arises due to dc transport and SOI, shows large mesoscopic fluctuations that exceed the polarization in incoherent samples with self-averaging. Since this average polarization has been successfully measured we expect this mesoscopic fluctuations to be within experimental reach as well.