Charge-spin-orbital Fluctuations in Mixed Valence Spinels: Comparative Study of AlV2O4 and LiV2O4

Mixed valence spinels provide a fertile playground for the interplay between charge, spin, and orbital degrees of freedom in strongly correlated electrons on a geometrically frustrated lattice. Among them, AlV$_2$O$_4$ and LiV$_2$O$_4$ exhibit contrasting and puzzling behavior: self-organization of seven-site clusters and heavy fermion behavior. We theoretically perform a comparative study of charge-spin-orbital fluctuations in these two compounds, on the basis of the multiband Hubbard models constructed by using the maximally-localized Wannier functions obtained from the ab initio band calculations. Performing the eigenmode analysis of the generalized susceptibility, we find that, in AlV$_2$O$_4$, the relevant fluctuation appears in the charge sector in $\sigma$-bonding type orbitals. In contrast, in LiV$_2$O$_4$, optical-type spin fluctuations in the $a_{\rm 1g}$ orbital are enhanced at an incommensurate wave number at low temperature. Implications from the comparative study are discussed for the contrasting behavior, including the metal-insulator transition under pressure in LiV$_2$O$_4$.