Effect of dielectric polarizability on Rashba spin-orbit coupling in Al/Sr_1-xCa_xTiO₃ interfaces

SrTiO₃ (STO)-based two-dimensional electron gases (2DEGs) are promising candidates for energy efficient spintronic devices. In particular strong Rashba spin-orbit coupling (SOC) has been found at the interfaces between STO and LaAlO₃ or Al, where a 2DEG forms. Recently, first principles density-functional-theory calculations revealed that the electrical field associated with the broken mirror symmetry at the oxide 2DEG polarizes the atomic orbitals as well as induces a polar lattice displacement across the interface as origin of the Rashba SOC effects [1]. However, it is still unclear to what extent these two effects contribute and the exact theoretical understanding of the SOC effects in oxide 2DEGs.

Here, we investigate 2DEGs at the interface between the extremely polarizable material Sr_1−xCa_xTiO₃ and Al. The perovskite STO is a quantum paraelectric with a very large dielectric constant, however, a proper ferroelectric state can be introduced by a tiny substitution of Sr with Ca, which increases the dielectric constant and thus the polarizibility of the material. We have varied the carrier density within a certain range using electrostatic gating of the 2DEG and analyzed the gate-dependence of Rashba spin-orbit fields by analysis of weak-localization in magnetotransport. From this we infer a clear trend for a range of carrier densities: The increase in Ca doping, i.e. reinforcing the ferroelectricity of the substrate, significantly increases the Rashba coefficient compared to undoped STO [2].