Charge excitations in LiV${}_2$O${}_5$ and NaV${}_2$O${}_5$: similarities and differences

Abstract

We calculate the optical conductivity of LiV${}_2$O${}_5$ and NaV${}_2$O${}_5$ using exact numerical diagonalization of a quarter-filled extended Hubbard model on a system of coupled ladders. In particular, electronic correlations are treated exactly, and a quantitative agreement between calculated and experimentally observed optical conductivity of these two vanadium oxides is presented. Furthermore, it is found that LiV${}_2$O${}_5$ differs from NaV${}_2$O${}_5$ not only in the charge ordering pattern but also in the nature of the inter-ladder coupling: In contrast to LiV${}_2$O${}_5$, in NaV${}_2$O${}_5$ neighboring ladders are coupled by a strong Coulomb repulsion, and not by inter-ladder hopping.