Change of the optical conductivity spectra with orbital and spin ordering in spinel MnV₂O₄

Spinel MnV₂O₄ exhibits orbital ordering at T _c=57 K accompanied by a structural phase transition from cubic to tetragonal and ferrimagnetic ordering. We measured the temperature dependence of the optical reflectivity spectra on the cleaved surface of a MnV ₂O₄ single crystal with controlled twin structures below T_c. We found that there are two peaks at 1.6 and 2.3 eV, corresponding to the Mott excitations, in the optical conductivity spectra obtained by the Kramers-Kronig transformation of the reflectivity, and that the two peaks exhibit a similar anisotropic change below T_c. Furthermore, we found that these two peaks exhibit a different temperature dependence above T_c. We analyzed these results by a model in which the multiplet structures of the d state for the Mott excitation are fully taken into account, and discussed the occupation of the triply degenerate t_2g orbitals both below and above T_c.