Symmetry breaking at the (111) interfaces of SrTiO${}_3$ hosting a 2D-electron system

Abstract

We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO${}_3$/SrTiO${}_3$ and LaAlO${}_3$/EuTiO${}_3$/SrTiO${}_3$ heterostructures, hosting a quasi two-dimensional electron system (q2DES), and of a Ti-terminated (111) SrTiO${}_3$ single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D${}_4$${}_h$ crystal field is turned into trigonal D${}_3$${}_d$ crystal field in all cases. The symmetry adapted a${}_1$${}_g$ and e${}^{\ddot{I}}{€}_g$ orbitals are non-degenerate in energy and their splitting, Δ, is positive at the bare STO surface but negative in the heterostructures, where the a${}_1$${}_g$ orbital is lowest in energy. These results demonstrate that the interfacial symmetry breaking induced by epitaxial engineering of oxide interfaces has a dramatic effect on their electronic properties, and it can be used to manipulate the ground state of the q2DES.

6 pages article, plus 5 pages supplementary information