Comment on ``Superconducting PrBa_2Cu_3O_x''

TL;DR

This paper critiques a recent claim of bulk superconductivity in PrBa2Cu3O_x, arguing that the magnetic data are inconsistent and suggesting that Ba substitution might explain the observed superconductivity.

Contribution

It provides a critical analysis of magnetic susceptibility data, questioning prior claims and proposing a possible explanation involving Ba substitution for Pr in the crystal structure.

Findings

Effective magnetic moment ff=2.09 from data, smaller than reported

Pr likely occupies only half of the RE sites, with Ba occupying others

Superconductivity may be due to Ba substitution, not Pr

Abstract

Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of this Comment is to show the inconsistency of the value of effective magnetic moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic susceptibility data. The estimation made directly from their data points gives a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This suggests that Pr occupies only about a half of the RE sites in TSFZ crystal. The other half of the RE sites is occupied most probably by the nonmagnetic Ba. Noteworthy, SC with T_c=43 K was observed earlier for Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius than Pr^{3+} and so the substitution of Ba for Pr could give a natural explanation not only for the SC in TSFZ Pr123 but also for the elongation of the distance between the CuO_2 planes observed by Zou et al.