Metallic state driven by frustrated electronic correlation in Cu-O double chain of PrBa2Cu4O8

A proposal is made that the metallic state observed in the Cu-O double chain unit of PrBa2Cu4O8 is a peculiar state driven by geometrical frustration. We treat one-dimensional 1/4-filled extended Hubbard models with competing intersite Coulomb interactions appropriate for this compound, and estimate the charge gap using Lanczos exact diagonalization method. It is argued that the charge ordered insulating states, stabilized in the absence of frustration. can be "melted" when the degree of frustration is large so that a metallic state is realized. In the limit of large on-site Coulomb interaction U, the systems can be mapped onto one-dimensional S= 1/2XXZ models, where our metallic state corresponds to a spin-liquid state due to the competing z component of the exchange couplings.