Momentum Dependence of Mott Gap Excitations in Optimally Doped YBa2Cu3O7−δ Studied by Resonant Inelastic X‐ray Scattering

Momentum dependence of Mott gap excitations in the twin‐free YBa2Cu33O7−δ at optimal doping has been studied by the resonant inelastic x‐ray scattering at the Cu K‐edge. We have succeeded to distinguish between the excitations from CuO chain and that from the CuO2 plane. The excitation from the chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the plane is broad at 1.5−4 eV and almost independent of the momentum transfer. Theoretical calculation based on the one‐dimensional and two‐dimensional Hubbard models reproduces these features when we assume a smaller value of on‐site Coulomb repulsion of the chain site than that of the plane site.