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 L-2,L-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 (d(xz)/d(yz) < d(xy) < d(3z2-r2) < d(x2-y2) or d(xy) < d(xz)/d(yz) < d(x2-y2) < d(3z2-r2)) expected from compressive or tensile epitaxial strain, respectively, it is revealed that d(xz), d(yz), and d(xy) orbitals are nearly degenerate, whereas d(3z2-r2) and d(x2-y2) orbitals are split with an energy gap similar to 100 meV in the epitaxial BaTiO3 films. We find that the unexpected degenerate orbitals d(xz)/d(yz)/d(xy) 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. ©2020 American Physical Society11sciescopu