Resonant inelastic x-ray scattering in electronically quasi-zero- dimensional CuB₂ O₄

We present a resonant inelastic scattering (RIXS) study using of CuB ₂ O₄, which contains a lattice of CuO₄ plaquettes electronically isolated by B+3 ions. The observed CuK -edge spectra show a small number of well-separated features, and the simple electronic structure of CuB₂ O₄ allows us to explore RIXS phenomenology. We find a low-energy feature that cannot be attributed to the same charge-transfer excitation discussed in other cuprates and is likely a d→d transition thought to be forbidden under common considerations of K -edge RIXS.