Electrical modulation of weak-antilocalization and spin-orbit interaction in dual gated Ge/Si core/shell nanowires

Magnetic transport of holes in Ge/Si core/shell nanowires (NWs) is investigated under the control of dual electrical gating. The strength of the spin-orbit interaction (SOI) is analyzed from the weak-antilocalization (WAL) of the magnetoconductance (MC) as a function of a perpendicular magnetic field. By superimposing a small alternating signal on the voltage offset of both gates the universal conductance fluctuations are largely removed from the averaged MC traces, enabling a good fitting to WAL theory models. The tuning of both spin lifetime and the SOI strength is observed in the NWs with dual gating while the carrier density is kept constant. We observe an enhancement of spin lifetime with the mean free path due to the effect of geometrical confinement. The measured SOI energy of 1-6 meV may arise from the dipole coupled Rashba SOI, which is predicted to be one order of magnitude larger than the conventional Rashba coefficient in the Ge/Si core/shell NW system. A clear electrostatic modulation of SOI strength by a factor of up to three implies that Ge/Si NWs are a promising platform for the study of helical states, Majorana fermions and spin-orbit qubits.