Symmetry breaking at the (111) interfaces of SrTiO3 hosting a two-dimensional electron system

We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO3/SrTiO3 and LaAlO3/EuTiO3/SrTiO3 heterostructures, hosting a quasi-two-dimensional electron system (q2DES), and of a Ti-terminated (111) SrTiO3 single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D-4h crystal field is turned into a trigonal D-3d crystal field in all cases. The symmetry adapted a(1g) and e(g)(pi) orbitals are nondegenerate in energy and their splitting Delta is positive at the bare STO surface but negative in the heterostructures, where the a(1g) 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