Unconventional crystal-field splitting in noncentrosymmetric BaTiO3 thin films

Understanding the crystal-field splitting and orbital polarization in noncentrosymmetric systems such as ferroelectric materials is fundamentally important. In this study, taking BaTiO3 as a representative material, we investigate titanium crystal-field splitting and orbital polarization in noncentrosymmetric TiO6 octahedra with resonant x-ray linear dichroism at the Ti L2,3 edge. The high-quality BaTiO3 thin films were deposited on DyScO3 (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction and element specific x-ray absorption spectroscopy were performed to characterize the structural and electronic properties of the films. In sharp contrast to conventional crystal-field splitting and orbital configuration (dxz/dyz<dxy<d3z2-r2<dx2-y2 or dxy<dxz/dyz<dx2-y2<d3z2-r2) expected from compressive or tensile epitaxial strain, respectively, it is revealed that dxz, dyz, and dxy orbitals are nearly degenerate, whereas d3z2-r2 and dx2-y2 orbitals are split with an energy gap ∼100 meV in the epitaxial BaTiO3 films. We find that the unexpected degenerate orbitals dxz/dyz/dxy result from the competition between the orbital splitting induced by epitaxial strain and that induced by polar distortions of BaTiO3 films. Our results provide a route to manipulate orbital degree of freedom by switching electric polarization in ferroelectric materials.