Shubnikov-de Haas Oscillations into 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 and temperatures down to 300 mK in high mobility (>20,000 cm²V^-1s^-1) two-dimensional electron liquids at (111) oriented SrTiO₃ interfaces with controllable carrier densities. Angular dependent 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. After the system reaches the quantum limit, 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.