Nonresonant inelastic x-ray scattering measurements on NiO and CoO show that strong dipole-forbidden $d\mathrm{\text{−}}d$ excitations appear within the Mott gap at large wave vectors. These dominant excitations are highly anisotropic, and have [001] nodal directions for NiO. Theoretical analyses based on a novel, energy-resolved Wannier function (within the $\mathrm{\text{local density approximation}}+\mathrm{\text{Hubbard}}U$) show that the anisotropy reflects the local exciton wave functions and local point-group symmetry. The sensitivity to weak symmetry breaking in particle-hole wave functions suggests a wide application to strongly correlated systems.

• Received 19 January 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.026401