Shubnikov-de Haas oscillations reaching the quantum limit in two-dimensional electron systems at SrTiO3 (111) interfaces

Transition metal oxides in the (111) orientation have been predicted to harbor topological phases and unconventional quantum states because of their hexagonal crystal symmetry and strong interactions between charge, spin, and orbital degrees of freedom. We report Shubnikov-de Haas oscillations into the quantum limit at magnetic fields up to 35 T in high-mobility (>20 000 cm² V^-1 s^-1) two-dimensional electron liquids at (111)-oriented SrTiO₃ interfaces with controllable carrier densities. Spin splitting is observed at low Landau levels, which is attributed to the interplay between the Zeeman splitting and Rashba spin-orbit coupling according to our theoretical modeling, yielding Landé factor g = 0.29 and Rashba coefficient α = 0.6 meV nm. At high magnetic field after the system reaches the lowest Landau level, the temperature dependence of the resistance shows a metallic to insulating state transition with the magnetoresistance changing significantly from a primarily quadratic to a large linear field dependence.