dd excitations in three-dimensional q-space: A nonresonant inelastic X-ray scattering study on NiO

We have studied the dd excitations in NiO over three-dimensional momentum (q) space using nonresonant inelastic X-ray scattering. In addition to the previously reported peaks at 1.7 and 3.0 eV, another peak is found at 1.0 eV, with a dramatically different intensity distribution in momentum space.Contrary to the other two peaks that form oval structures maximizing at [111] directions, the 1.0 eV peak displays appreciable intensity along low-symmetry axes near [311] and [210], but vanishes in the three principal axes [100], [110], and [111], indicating a significant difference in the exciton wave function.We find good agreement between the experimental data and two state-of-the-art theories, advocating investigating other strongly correlated materials with similar experimental/theoretical approaches