Theoretical study of spin currents in semiconductors

Abstract

Spintronics in semiconductors (SCs) offers a promising avenue for future information technologies. At the very heart of this technology is the widely known spin-orbit coupling (SOC) effect, which affords us the attractive prospect of spintronics without magnetism. The value of SOC is quickly realized through its ubiquity in nearly all aspects of SC spintronics, from the generation of spin polarized currents to the all-electric spin manipulation it permits in SC spintronic devices. It also drives the remarkable spin-Hall effect (SHE) which is a promising source of dissipationless spin currents. In this Thesis, we theoretically study several critical aspects of SC spintronics, with a focus on spin currents in the presence of SOC. These aspects include spin current generation, spin manipulation, and spin-dependent transport.

Firstly, methods to generate spin currents in SCs are proposed. These range from purely nonmagnetic, SOC-based systems to those which utilize external magnetic fields. Generally, nonmagnetic approaches are preferred as stray magnetic fields can adversely affect spins. Highly spin polarized currents (approaching 100% polarization) are predicted under certain conditions in both nonmagnetic and magnetic approaches.

Next, two spintronic transistor devices are proposed, which exploit the electronic tunability of the SOC in SC heterostructures. The first modifies the seminal Datta-Das device by including the effect of external magnetic fields. This is found to considerably relax transport constraints (namely single channeled transport) in the original model. The second device exhibits a gate bias modulation of spin current through the action of two spin-dependent gauge fields. Generally, such fields can be physically interpreted as effective magnetic fields, which affect the trajectory of carriers in a spin-dependent manner. These inevitably drive spin currents and are therefore of great importance to spintronics research.

An in-depth studyof gauge fields constitutes the second-half of this Thesis. In particular, we closely examine the intrinsic spin-Hall effect (SHE), in which dissipationless spin currents flow (these transport zero net charge) normal to an applied charge current in generic SOC systems. First, we propose a SHE of collimated conduction electrons in zincblende crystals. Important issues including calculation of the spin current and its robustness to impurities are discussed. Next, motivated by open questions, we divert our attention to the physical mechanisms which drive the SHE. Two mechanisms are known, but their relationship (if any) has hitherto been unclarified. One mechanism arises from the spin-dependent trajectory of carriers due to gauge fields in momentum space. The second results from a momentum-dependent polarization of spins. We succeed in formulating a gauge field description (in time space) of the latter mechanism. Moreover, we show that the two mechanisms are simply distinct manifestations of a common time-resolved process in SOC systems. Lastly, we discuss the ubiquity of the latter mechanism in SC spintronic and optical systems, and propose an analogous flow of pseudospin current in bilayer graphene.