Coupling between Mott excitation and d-d transitions in CoV2 O4

We studied the optical properties of CoV2O4, in which a broad spectrum arising from the Mott excitation of the V d states and sharp peaks from the intra-atomic d-d transitions in the Co ions coexist. We found that the width of the d-d transition peaks decreases and that Fano resonance appears in the steady-state optical spectrum with decreasing temperature. We also found that the width of the d-d transition peaks increases less than 1 ps after a pump pulse is applied to the Mott excitation. These results indicate that the Mott excitation and the d-d transitions are strongly coupled in this compound.