Symmetry breaking at the (111) interfaces of SrTiO_3 hosting a 2D-electron system

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^π_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.