Unusual electronic structure of ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

A model for the electronic structure of ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is proposed with three independent components: insulating ${\mathrm{CuO}}_2$ planes with oxidation state ${\mathrm{Cu}}^{\mathrm{II}}$, mixed valent Pr ions with almost equal numbers of ${\mathrm{Pr}}^{\mathrm{III}}$ and ${\mathrm{Pr}}^{\mathrm{IV}}$, and ${\mathrm{CuO}}_3$ chains described by a t-J model with filling ≊1/2. The difference from the high-${\mathit{T}}_{\mathit{c}}$ superconductors comes from an enhanced stability of the ${\mathrm{Pr}}^{\mathrm{IV}}$ state due to hybridization with O neighbors, and involves a transfer of holes from primarily planar O 2${\mathit{p}}_{\mathrm{\sigma }}$ to 2${\mathit{p}}_{\mathrm{\pi }}$ states. The insulating nature of ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is ascribed to the extreme sensitivity of the dc conductivity of the mixed valent Pr ions and the ${\mathrm{CuO}}_3$ chains to vacancies and impurities.