Mott gap excitations in twin-free YBa2Cu3O7-d (Tc = 93 K) studied by RIXS

Mott gap excitations in the high- T c superconductor of the optimal doped YBa 2 Cu 3 O 7 − δ ( T c = 93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab -plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b ∗ direction, while the excitation from the CuO 2 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 model reproduces the observed spectra by taking the different parameters of the on-site Coulomb energy. The fact of the Mott gap of the CuO chain site is much smaller than that of CuO 2 plane site is observed for the first time.