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

Abstract

Spinel MnV${}_{2}$O${}_{4}$ 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${}_{2}$O${}_{4}$ 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 ${T}_{c}$.