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

TL;DR

This study investigates how symmetry breaking at (111) oxide interfaces affects the electronic structure of 2D electron systems, revealing that epitaxial engineering can significantly alter orbital energies and ground states.

Contribution

It provides detailed spectroscopic evidence of symmetry-induced orbital splitting changes at oxide interfaces, highlighting a method to control electronic properties via interface engineering.

Findings

Orbital splitting  is positive at the bare STO surface but negative in heterostructures.

Symmetry breaking transforms the crystal field from tetragonal to trigonal, affecting orbital degeneracy.

Epitaxial interface engineering can manipulate the ground state of the quasi 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}$${_g}$ and e${^\pi_g}$ orbitals are non-degenerate in energy and their splitting, \Delta, is positive at the bare STO surface but negative in the heterostructures, where the a${_1}$${_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.