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Stretchable Persistent Spin Helices in GaAs Quantum Wells

BaC6

Wednesday, June 21, 2017

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 α and Dresselhaus β couplings to equal fixed strengths α = β, the total SO field becomes unidirectional thus rendering the electron spins immune to dephasing due to momentum scattering. A robust persistent spin helix (PSH), i.e., a helical spin-density wave excitation with constant pitch P = 2π/Q, Q = 4mα/¯ h2, has already been experimentally realized at this singular point α = β. Here we employ the suppression of weak antilocalization as a sensitive detector for matched SO fields together with a technique that allows for independent electrical control over the SO couplings via top gate voltage VT and back gate voltage VB, to extract all SO couplings as functions of VT and VB when combined with detailed numerical simulations. We demonstrate for the first time the gate control of β and the continuous locking of the SO fields at α = β, i.e., we are able to vary both α and β controllably and continuously with VT and VB, 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. This further protects spins from decay when electrically controlling the spin precession. We also quantify the detrimental effect of the cubic Dresselhaus term, which breaks the unidirectionality of the total SO field and causes spin decay at higher electron densities. The extracted spin-diffusion lengths and decay times as a function of α/β show a significant enhancement near α/β = 1. 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 ∼ 8−25 µm between solid-state spin qubits, where the spin diffusion length for α 6= β is an order of magnitude smaller.

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