Suppression and restoration of superconductivity in PrBa2Cu3O7 (a mini-review)

One of the most exciting cases of superconductivity suppression in high-Tc cuprates is that of RE1−xPrxBa2Cu3O7, where RE stands for a rare earth (see Refs. [1,2] for reviews). Even more exciting are indications that conductivity and superconductivity can be restored in stoichiometric PrBa2Cu3O7 [3,4]. This is such an unexpected result [5] that it is still not generally accepted and further experimental confirmation is required. Nevertheless, this fact was reported by several independent groups, and it is time now to understand the theoretical consequences of this finding. The most important message, if this finding is true, is that at x = 1, and, presumably, at intermediate x’s, there are free carriers in RE1−xPrxBa2Cu3O7, and the suppression of metallic conductivity at sufficiently large x must be due to localization of those carriers. In this paper I review the principal experimental findings about superconductivity, suppression thereof, and related properties of RE1−xPrxBa2Cu3O7 (a comprehensive review of the data published before 1992 can be found in Ref. [1], and many of more recent references are discussed in Ref. [2]) in a decision-tree manner, eliminating the models substantially incompatible with established experimental facts, eventually focussing on the pfσ hybridization models and listing on a verbal level the interpretations of the existing experiments as they emerge from such models. The first three experimental facts that I want to emphasize are