Inelastic X-ray Scattering as a Novel Tool to Study the Electronic Strucutre of Mott Insulators

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than half-century of intense research efforts. The unexpected discovery of high-temperature superconductivity in doped Mott insulators is a highlight of this intellectual crisis. Well-developed momentum-resolved spectroscopies such as photoemission and neutron scattering cannot address problems associated with the Mott gap as angle-resolved photoemission can only probe the occupied states and neutrons do not couple to the electron's charge directly and (thermal) neuron energy is typically too low. We report observation of highly dispersive (up to 1.3 eV) charge excitations across the Mott gap in the parent cuprate insulator Ca sub 2 CuO sub 2 C1 sub 2 using very high resolution resonant inelastic x-ray scattering (RIXS) near the Cu K-edge shedding light on the anisotropy of full Mott-gap. These results are consistent with theoretical calculations based on the generalized Hubbard model providing the first direct support for the model to describe anisotropic excitations across the Mott-gap. Given its deeply bulk-sensitive and weak-coupling nature and the ability to prove he dispersive behavior of the unoccupied bands over several Brillouin zones, RIXS has the promise to become an important experimental tool to study the electronic structure of novel insulators.