University of Paris, Sud
Topological phases at SrTiO3 surfaces and LaAlO3-SrTiO3 interfaces
Two-dimenional electron fluids (2DES) have been uncovered at surfaces and interfaces of several transition metal oxide perovskites. These 2DES are endowed with properties which are usually not present in the host material(s). In this respect, the LaAlO3-SrTiO3 (LAO-STO) heterostructure is a paradigmatic compound which exhibits gate tunable metallicity, superconductivity, spin textures stemming from a Rashba spin-orbit contribution, possibly magnetic fluctuations as well.
In the case of bare surfaces, the existence of the 2DES has been linked to the presence of oxygen vacancies which are confined near the boundary. For (001) oriented STO, we recently predicted that the metallic state is in fact a topological phase which should give rise to one-dimensional edge states. The multi-orbital character of the conduction band electrons compounded with confinement and bulk spin-orbit was instrumental in promoting a band inversion leading to the topological regime. A similar scenario is expected for (001) LAO-STO and we'll review these two cases, first.
We'll then turn to the case of (111) oriented LAO-STO where signatures of a 2DES have been found in transport. For this particular orientation, confinement impacts all orbitals equally and one does not expect the same type of crossing points in the Brillouin zone (BZ) as were found for the (001) orientation. Remarkably, we predict two topological regimes in the (111) case. One is linked to a parity inversion at the M points of the honeycomb BZ. This is similar to what we found for (111) STO and KTaO3. In addition, we uncover a second topological regime coexisting with the previous one. In the presence of Coulomb interactions, an inversion in the ordering of the sub-bands caused by confinement takes place. This inversion is evidenced in Hall effect measurements. This inversion leads to three-band crossings and in the presence of spin-orbit interactions gives rise to a Z2 topological state, with its own set of edge states. Our findings are based on a tight-binding modeling of the band structure derived from transport and from ARPES (in analogy with STO111) as well as Poisson-Schroedinger calculations including Coulomb effects.
M. Vivek, M.O. Goerbig, and M. Gabay, Phys. Rev. B 95, 165117 (2017)
A. Monteiro, M. Vivek, D.J. Groenendijk, P. Bruneel, I. Leermakers, U. Zeitler, M. Gabay, A.D. Caviglia, arXiv:1808.03063 (2018)