  Manipulating the orbital occupation of valence electrons via epitaxial strain
in an effort to induce new functional properties requires considerations of how
changes in the local bonding environment affect the band structure at the Fermi
level. Using synchrotron radiation to measure the x-ray linear dichroism of
epitaxially strained films of the correlated oxide CaFeO3, we demonstrate that
the orbital polarization of the Fe valence electrons is opposite from
conventional understanding. Although the energetic ordering of the Fe 3d
orbitals is confirmed by multiplet ligand field theory analysis to be
consistent with previously reported strain-induced behavior, we find that the
nominally higher energy orbital is more populated than the lower. We ascribe
this inverted orbital polarization to an anisotropic bandwidth response to
strain in a compound with nearly filled bands. These findings provide an
important counterexample to the traditional understanding of strain-induced
orbital polarization and reveal a new method to engineer otherwise unachievable
orbital occupations in correlated oxides.
