PHYSICAL REVIEW B VOLUME 41, NUMBER 10 1 APRIL 1990 Suppression of superconductivity in PrBa2Cu307. . 4f and conduction-band hybridization effect G. Y. Guo and W. M. Temmerman (Received 30 October 1989) In order to understand the suppression of superconductivity in Y, „Pr„Ba&Cu307 (0. 6 x ~ 1. 0) and also the electronic properties of the high-T, superconductors RBa2Cu307 (R denoting the rare earths except Ce, Pr, Pm, and Tb), we have performed self-consistent, spin-polarized electronic structure and all-electron total-energy calculations for three representative materials: RBa2Cu307 (R = Y, Pr, and Gd). The salient results are that the band structures of the RBa2Cu307 compounds are nearly the same as that of YBa2Cu307 when R ions assume trivalency, and extra electrons donat- ed by the hypothetical tetravalent Pr ions would mainly fill the conduction bands in a rigid-band manner, thereby substantially reducing the important "hole" account. However, the tetravalent Pr compound is shown to be energetically less favorable than the trivalent Pr compound; finally, large spin splittings (up to 75 meV) are found in the conduction bands around the Fermi level in PrBa, Cu, O, when the 4f and conduction-band hybridization effects are taken into account. In con- trast, such spin splittings are ten to five times smaller in GdBa2Cu307. This result corroborates that the magnetic pair breaking is responsible for the suppression of superconductivity in the Pr com- pound. I. INTRODUCTION It has been found that substitution of the rare earths La-Lu for Y in the high-T, superconductor YBa2Cu307 &, with the exception of Ce, Pr, Pm, and Tb, has no significant effect on the superconductivity. ' For example, RBa2Cu307 (R denoting Nd, Sm, Gd, Dy, etc. ) compounds are also high-temperature superconductors with T, near 90 K. ' This was at first surprising as the presence of the magnetic ions would suppress the super- conductivity in the ordinary superconductors. However, it now appears from. numerous experiments that the essential structural ingredient for the high-T, supercon- ductivity is the two-dimensional Cu-Oz planes found in these copper oxides. Band structure calculations ' show that the relevant bands are the highly dispersive planar Cu-0 dp~ antibonding bands straddling the Fermi level. The electronic properties of these systems, such as elec- tronic transport and superconductivity, seem to be dom- inated by the carrier concentrations (or electronic hole counts) residing on these Cu-02 planes, which are con- trolled by, e. g. , the oxygen vacancy concentration 5. Rare-earth ions in these superconductor oxides show a valence of 3+, i.e. , like Y, each contributes three conduc- tion electrons. One thus expects that the electronic band structures of the superconducting compounds RBa2Cu307 would be similar to that of YBa2Cu307. The remaining f electrons are presumably localized on the rare-earth ion sites, giving rise to localized magnetic mo- ment behavior. Since the conduction electrons in the Cu-02 planes have almost zero density of states at the rare-earth ions sites, the presence of the magnetic mo- ments should have negligible effect on the superconduct- ing property. PrBa2Cu307 is isostructural to YBa2Cu307, while CeBa2Cu307 and TbBa2Cu307 could not be synthezied. No investigations on the Pm compound have been report- ed, presumably because the Pm nucleus is radioactively unstable. However, superconductivity is not detected in PrBa2Cu307. In the Pr„Y, Ba2Cu307 system, T, de- creases gradually to zero as x is increased up to 0. 6 where the system becomes nonsuperconducting and exhibits nonmetallic behavior. Nonsuperconductivity of PrBa2Cu3Cu7 has been a puzzle ever since the material was made. Several mechanisms have been proposed to explain the suppression of T, in PrBa2Cu 307 and Pr„Y& „Ba2Cu&07 (0. 6 ( x ( 1. 0). The Pr ion shows either a valence of four, as in praseo- dymium dioxides, or a valence of three, as in Pr2F3. ' Therefore, it was first proposed that the superconductivi- ty is destroyed by the presence of the tetravalent Pr ions. When Y is replaced by Pr, some of the electronic holes are neutralized by the extra conduction electrons donated by the tetravalent Pr ions, thereby reducing the electronic hole count believed to be crucial for the high-T, super- conductivity. " The effective magnetic moment of the Pr ion in Pr„Y, Ba2Cu307 derived from magnetic suscep- tibility measurements varies between 3. 6pz for Pr + and 2. 5pz for Pr +. This presumably suggested that some Pr ions are tetravalent or the Pr ion in Pr„Y, „Ba2Cu307 has a mixed valence. Evidence for the presence of the tetravalent Pr ions from electronic transport experiments was also reported. ' However, several x-ray adsorption spectroscopy measurements' show that Pr is essentially trivalent in the Pr Y, „Ba2Cu307 compounds, thereby disputing the plausible "tetravalency" mechanism for the depression of T, . Therefore, Neukirch et al. instead' speculated that T, is suppressed through changes in the band structure when Y is substituted by Pr in RBa2Cu3O7. That the Pr ion is trivalent in 41 6372 1990 The American Physical Society