2009/03/28 by Jake D. Koralek, Chris Weber, Joe Orenstein +4 · 2 citations
Physics and Astronomy · #cond-mat.mtrl-sci #cond-mat.mes-hall
paper · pdf · doi:10.1038/nature07871
published as Nature 458, 610-613 (2009) · Will be published in Nature on April 2, 2009
arxiv created 2009/03/28 · arxiv updated 2009/12/01
According to Noethers theorem, for every symmetry in nature there is a corresponding conservation law. For example, invariance with respect to spatial translation corresponds to conservation of momentum. In another well-known example, invariance with respect to rotation of the electrons spin, or SU(2) symmetry, leads to conservation of spin polarization. For electrons in a solid, this symmetry is ordinarily broken by spin-orbit coupling, allowing spin angular momentum to flow to orbital angular momentum. However, it has recently been predicted that SU(2) can be achieved in a two-dimensional electron gas, despite the presence of spin-orbit coupling. The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the persistent spin helix. SU(2) is realized, in principle, when the strength of two dominant spin-orbit interactions, the Rashba (strength parameterized by α) and linear Dresselhaus (β1), are equal. This symmetry is predicted to be robust against all forms of spin-independent scattering, including electron-electron interactions, but is broken by the cubic Dresselhaus term (β3) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as αapproaches β1. Here we observe experimentally the emergence of the persistent spin helix in GaAs quantum wells by independently tuning αand β1. Using transient spin-grating spectroscopy, we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point.........