Resonant inelastic X-ray scattering study of quasi-zero-dimensional copper metaborate

CuB$_2$O$_4$ consists of many CuO$_4$ plaquettes separated by B ions. We report a study of the electronic excitation spectra of this system in order to explore the relationship between excitation symmetry and the resonant inelastic X-ray scattering (RIXS) technique. We find a small number of well separated features in the experimentally accessible range of 0.5-15 eV energy transfer, and weak dispersion is suggestive of the quasi-zero-dimensional nature of this system. Systematic trends in the data are suggestive of a composite nature to one of the observed features. Using a cluster model, we describe these unexpected trends and clarify how the choice of experimental geometry selectively influences the sensitivity to particular excitation symmetries in the RIXS experimental technique.