Stretchable Persistent Spin Helices in GaAs Quantum Wells

The Rashba and Dresselhaus spin-orbit (SO) interactions in 2D electron gases act as effective magnetic fields with momentum-dependent directions, which cause spin decay as the spins undergo arbitrary precessions about these randomly oriented SO fields due to momentum scattering. Theoretically and experimentally, it has been established that by fine-tuning the Rashba alpha and renormalized Dresselhaus beta couplings to equal fixed strengths alpha = beta, the total SO field becomes unidirectional, thus rendering the electron spins immune to decay due to momentum scattering. A robust persistent spin helix (PSH), i.e., a helical spin-density wave excitation with constant pitch P = 2 pi/Q, Q = 4ma/h(2), has already been experimentally realized at this singular point alpha = beta, enhancing the spin lifetime by up to 2 orders of magnitude. Here, we employ the suppression of weak antilocalization as a sensitive detector for matched SO fields together with independent electrical control over the SO couplings via top gate voltage V-T and back gate voltage V-B to extract all SO couplings when combined with detailed numerical simulations. We demonstrate for the first time the gate control of the renormalized beta and the continuous locking of the SO fields at alpha = beta; i.e., we are able to vary both alpha and beta controllably and continuously with V-T and V-B, while keeping them locked at equal strengths. This makes possible a new concept: stretchable PSHs, i.e., helical spin patterns with continuously variable pitches P over a wide parameter range Stretching the PSH, i.e., gate controlling P while staying locked in the PSH regime, provides protection from spin decay at the symmetry point alpha = beta, thus offering an important advantage over other methods. This protection is limited mainly by the cubic Dresselhaus term, which breaks the unidirectionality of the total SO field and causes spin decay at higher electron densities. We quantify the cubic term, and find it to be sufficiently weak so that the extracted spin-diffusion lengths and decay times show a significant enhancement near alpha = beta. Since within the continuous-locking regime quantum transport is diffusive (2D) for charge while ballistic (1D) for spin and thus amenable to coherent spin control, stretchable PSHs could provide the platform for the much heralded long-distance communication similar to 8-25 mu m between solid-state spin qubits, where the spin diffusion length for alpha not equal beta is an order of magnitude smaller.