TB DOPING OF YBA2CU3O7-DELTA

An explanation is provided of the Tb doping anomaly: the superconducti ng transition temperature of Y1-yTbyBa2Cu3O7-delta remains constant at 92.5 K with increasing Tb content y (up to y almost-equal-to 0.5), an d shows no evidence of degradation by Cooper pair-breaking. This behav ior is considered anomalous because Tb has been determined to be in th e Tb+4 charge state, which is supposed to be deleterious to supercondu ctivity: comparable doping by Ce and Pr (also thought to be +4 ions), unlike Tb-doping, leads to pair-breaking and the quenching of supercon ductivity. This Tb anomaly is explained as normal behavior, within the context of the strained-layer superlattice and confined metallic oxyg en model of high-temperature superconductivity: it is due to Tb occupy ing rare-earth sites almost exclusively, and the low solubility of Tb on Ba sites. The superconductivity is rooted in the chain layers, and rare-earth-site magnetic Tb, Pr, and Ce, whatever their charge states, do not break pairs, because they are remote from the superconducting chain layers. In contrast, Ba-site Tb (or another magnetic rare-earth ion) would break pairs and quench superconductivity, being adjacent to chain-oxygen. Therefore, Tb's so-called anomalous behavior of not deg rading the transition temperature is actually normal and characteristi c of rare-earth ions on the rare-earth sites, while the pair-breaking of Ce and Pr is attributable to rare-earths on (anomalous) Ba sites. T his normal behavior appears to be anomalous only in the context of cup rate-plane models of superconductivity, and so casts doubt on the vali dity of those models, which are also in conflict with pair-breaking da ta for La2-betaSrbetaCuO4-delta, Nd2-zCezCuO4-delta, and Nd2-u-zCezGdu CuO4-delta, as well as for compounds of the YBa2Cu3O7-delta class.