Temperature dependence of crystal field excitations in CuO

We report a study on the temperature dependence of charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering spectroscopy. Thanks to a very high-energy resolution (ΔE = 60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. The spectra broaden considerably with increasing temperature, consistently with an enhancement of the coupling between crystal field excitations and the temperature-dependent continuum of states above the band gap. We discuss this and other mechanisms that may explain this temperature dependence.