Emergence of the persistent spin helix in semiconductor quantum wells

TL;DR

This paper reports the experimental observation of the persistent spin helix in GaAs quantum wells, demonstrating a significant increase in spin lifetime when Rashba and Dresselhaus spin-orbit couplings are balanced, confirming theoretical predictions.

Contribution

The study provides the first experimental evidence of the persistent spin helix in semiconductor quantum wells by tuning spin-orbit interactions independently.

Findings

Spin lifetime increases by two orders of magnitude near the symmetry point.

Persistent spin helix forms when Rashba and linear Dresselhaus couplings are equal.

Spin information propagation length diverges at the symmetry condition.

Abstract

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 \alpha) and linear Dresselhaus (\beta_1), 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 (\beta_3) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as \alpha approaches \beta_1. Here we observe experimentally the emergence of the persistent spin helix in GaAs quantum wells by independently tuning \alpha and \beta_1. Using transient spin-grating spectroscopy, we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point.........