Experimental and Theoretical Investigations of Spin Transport in Semiconductors

Joseph Orenstein, Principle Investigator

Motivation: Energy dissipation is the number one problem facing continued evolution of electronic information technology. Recently developed theories predict that the flow of spin can, in principle, proceed with less dissipation of energy than the flow of charge. Furthermore, new research has indicated that it is possible to control the flow of electron spin purely via electric field. In this project, we investigate the fundamental physics that underlies the potential for electric-field gated spin-based devices. We believe the information obtained through our research will play an essential role in enabling the development of this new technological paradigm.

Theoretical prediction of “persistent spin helix:” Electric-field gating of spin current is possible because of spin-orbit coupling (SOC). Unfortunately, SOC is a double-edged sword: the same interaction that couples external E-fields to spin drives loss of spin memory. However, it has recently been predicted that spin conservation can be recovered in a two-dimensional electron gas, despite the presence of SO coupling. This can occur when the strength of two dominant SO interactions, the Rashba (α) and linear Dresselhaus (β1), are tuned to be equal, resulting in the conservation of a helical spin density wave known as “the persistent spin helix” (PSH). As a consequence of the stability of the PSH, the distance over which spin information can propagate is predicted to diverge as α ® β1. As the relative strength of the Rashba and Dresselhaus interactions can be modulated by external electric fields, the PSH effect provides a mechanism for rapid gate control of spin dynamics.

Transient grating spectroscopy: We have used transient spin-grating spectroscopy (TSG) to verify the existence of thePSH. In TSG, two pump beams (shown in green in Fig. 1) interfere at the sample surface. When the beams are cross-polarized, optical interference creates a standing wave of photon helicity in the region where the two beams overlap. Because of the optical orientation effect in GaAs, the helicity standing wave generates a spin polarization wave (SPW). The wavevector, q, of the SPW can be varied simply by changing the relative angle between the two pump beams. The lifetime of the photogenerated SPW is probed by diffracting another beam (shown in red) from the transient grating. The SPW lifetime as a function of q is shown in Fig. 2. The lifetime peaks sharply at the PSH wavevector, where we find an enhancement of two orders of magnitude relative to the lifetime in the absence of SO coupling. Excellent quantitative agreement with theory across a wide range of sample parameters allows us to obtain an absolute measure of all relevant SO terms in the Hamiltonan. The tunable suppression of spin-relaxation demonstrated in this project is well-suited for application to spintronics.