Nonmonotonically tunable Rashba spin-orbit coupling by multiple-band filling control in SrTiO3-based interfacial d-electron gases

Spin-orbit coupling (SOC) for d-electron gas can be substantially enriched compared with the sp-electron gas due to the delicate ordering of the multiple d subbands. Here, we demonstrate the nontrivial Rashba SOC effect at SrTiO3-based interfaces (LaAlO3/SrTiO3 and LaVO3/SrTiO3) directly related to the Ti 3d subband ordering via magnetotransport characterizations. Unusual k-cubic Rashba SOC contributed from the d(xz/yz) states is revealed. More strikingly, when a gate voltage is swept to tune the band filling, the SOC strength initially increases and then decreases to form a dome feature, accompanied by an apparent single- to two-carrier transition. These two concomitant effects strongly indicate that the SOC behavior is largely determined by the Ti 3d subbands regardless of the overlayer boundary conditions, with the SOC strength peaked at the d(xy)-d(xz/yz) crossings due to the band hybridization effect as predicted. The present findings offer new insights into exploration of oxide-based quantum phases and spintronic devices.