Gian Salis, IBM Research Zurich

Direct mapping of the formation of a persistent spin helix

co-authors: M. P. Walser (1), C. Reichl (2),  W. Wegscheider (2)

(1) IBM Research--Zurich, Säumerstrasse 4, 8803 Ruüschlikon, Switzerland
(2) Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland 

Spin-orbit interaction (SOI) is a major reason for the decay of spin polarization in semiconductors. At the same time it is the basis for electrical spin control in spin-based information-processing schemes. Thanks to the presence of two separately tuneable contributions, the SOI in zincblende semiconductor quantum wells can be set to a symmetry point, in which spin decay is strongly suppressed for a helical spin mode. Signatures of such a persistent spin helix (PSH) have been probed using the transient spin grating technique and the inverse spin-Hall effect, but it has not yet been possible to observe the formation and the helical nature of a PSH. Here we directly map the diffusive evolution of a local spin excitation into a helical spin mode by a time- and spatially resolved magneto-optical Kerr rotation technique.  Depending on its in-plane direction, an external magnetic field interacts differently with the spin mode and either highlights the helical nature of the mode or destroys the SU(2) symmetry of the SOI and thus decreases the spin lifetime. All relevant SOI coefficients are experimentally determined and confirmed with a numerical simulation of spin diffusion in the presence of SOI.