bstract: The two-dimensional conducting gas that forms at the interface between the two insulators LaAlO3 (LAO) and SrTiO3 (STO) has garnered attention due to wide variety of physical phenomena, including strong spin-orbit coupling, superconductivity, magnetism, and localization effects, among others. Most of the experimental and theoretical work so far has been on LAO/STO interfaces grown in the (001) crystal orientation, in which the system has rectangular symmetry at the interface. More recently, interest has focused on LAO/STO interface grown in the (111) crystal orientation, in which the interface has hexagonal symmetry, raising the possibility of topological effects.
I will discuss our recent experiments on two (111) oriented STO based interfaces: (111) LAO/STO and (111) LSAT/STO. In (111) LAO/STO, as with (001) LAO/STO, we find that the system exhibits superconductivity and magnetism coexisting at the same interface. Unlike the (001) interface, the (111) interface is highly anisotropic, showing different characteristics along two mutually perpendicular crystal directions in all measured transport properties, including longitudinal resistivity, Hall effect, quantum capacitance, superconductivity, and magnetism. The anisotropy occurs at low temperatures (~20 K) and is not related to any known structural transition of the system. The origin of this nematicity is not known, but its temperature dependence suggests that it is electronic in nature.
I will also present results of transport measurements on (111) LSAT/STO interfaces, a much less studied interface, which is distinguished by its smaller lattice mismatch (1% as opposed to 3% for LAO/STO). This results in less disorder, which can be seen by magnetoresistance oscillations and a nonlinear Hall effect that occur at much lower magnetic fields in comparison with LAO/STO. Hall effect measurements show evidence of a Lifshitz transition as a function of gate voltage, corresponding to a depopulation of a higher energy, high-mobility band as the gate voltage is reduced. As has been shown with (001) LAO/STO, the spin-orbit interaction can be tuned by gate voltage; unlike (001) LAO/STO, the spin-orbit interaction shows a sharp increase at gate voltages below the Lifshitz transition. Surprisingly, the range of gate voltage corresponding to strong spin-orbit interactions correlates with the range of voltage over which signatures of magnetism are seen at millikelvin temperatures, a correlation that is not understood at present.