Theoretical study of the cubic Rashba effect at the SrTiO3(001) surfaces

The origin of Rashba spin splitting in the two-dimensional electron gas at the (001) surface of SrTiO sub(3) is studied using first-principles calculations and tight-binding model. Calculations of oxygen vacancies under virtual crystal approximation reveal a two-dimensional electron-gas subband structure similar to polar materials, consistent with observations on SrTiO sub(3). Our studies also confirm that k dependence of the spin splitting is predominantly cubic in the surface Ti-t sub(2g) states, even though structural relaxations diminish the effect in d sub(xy) bands. A tight-binding model, explicitly including Ti-d and O-p states as well as next-nearest-neighbor interactions, is derived to understand the first-principles results. Effective Rashba Hamiltonians for the surface bands are derived using quasidegenerate perturbation theory and scenarios in which linear k contribution may be suppressed are discussed. However, the cubic terms in the Hamiltonian are found to be different from the model derived using k times p theory, leading to different pseudospin symmetry in the Brillouin zone.