Dominik Zumbühl Wednesday, 10:40 - 11:10
Spin-Orbit and Hyperfine Coupling in GaAs 2D and 0D Electron Systems
In a first part, I will present a novel concept: the stretchable persistent spin helix (PSH), which we have recently experimentally realized in GaAs quantum wells. We employ the suppression of weak antilocalization as a detector for matched spin-orbit (SO) fields in the PSH state. Using a top gate voltage VT and a back gate voltage VB, we demonstrate for the first time control of the renormalized Dresselhaus term b. Simultaneously and independently, we use the same gates to control the Rashba a term. Combined with numerical simulations, we extract all SO couplings as functions of VT and VB. We are able to vary both a and b controllably and continuously with VT and VB, while keeping them continuously locked together at equal strengths of the SO fields at a=b. This makes possible a new concept: the stretchable PSH, i.e., a helical spin pattern with continuously variable pitch, controllable over a wide parameter range. The stretchable PSH protects spins from decay while electrically controlling the spin precession. In this regime, quantum transport is diffusive (2D) for charge while ballistic (1D) for spin, thus amenable to coherent spin control. Thus, stretchable PSHs provide a platform for the much heralded long-distance communication overcoming ~8 –25 mm between spin qubits, where the spin diffusion length for a << b or a >> b is much shorter.
In a second part, I will present measurements of the spin-orbit and hyperfine mediated spin relaxation rate W in a gate defined single electron GaAs quantum dot at electron temperatures of ~60 mK as a function of both direction as well as strength of magnetic field, spanning an unprecedented range from 0.6 T to 14 T applied in the plane of the 2D electron gas. Due to the interplay of the Rashba and Dresselhaus SO contributions, W shows strong anisotropy when varying the direction of the applied in-plane magnetic field B with a piezoelectric rotator. Along the crystal axis where SO coupling is weak, a spin relaxation time T1 of 57±15 sec has been obtained at a magnetic field of 0.6 T. However, quite surprisingly, this is still more than one order of magnitude shorter than the expected value based on SO mediated spin relaxation. Further, W shows a B3 dependence and becomes isotropic at the lowest magnetic fields. These observations thus indicate hyperfine interaction mediated spin relaxation (non flip-flop) via phonons at the lowest magnetic fields used here. Command of the dot orbitals, control of the B-field direction and low-B-field measurements – made possible by a low electron temperature – reveal hyperfine spin relaxation and allow comprehensive modeling, giving excellent agreement between experiment and theory.