Pb-substitution effect on the electronic properties of Bi2201, Bi2212 and Bi2223 superconductors

Abstract

Although Pb²⁺ substitution for Bi³⁺ increases the hole concentration, p, in Pb-substituted Bi-based superconductors, the Pb substitution effect has yet to be studied in detail. Herein the Pb substitution effect on the electronic properties of Bi2201, 2212, and 2223 superconductors is investigated to elucidate the role of Pb on the correlation between p and formal valence, v. Consistent with Mott-Hubbard theory, p increases as v increases in the Pb-free Bi2201 and Pb-free Bi2212 phases. In contrast, p decreases with increasing v for the Pb-substituted Bi2201 phase, which contradicts Mott-Hubbard theory but can be explained by the increase in Pb⁴⁺. For the Pb-substituted Bi2212 phase, p increases with increasing v from 2.0 to 2.2, and then remains constant at p = 0.10–0.20 from v 2.2 to 2.63. When v increases from 2.0 to 2.2, the hole is doped at a Cu site, which is the same behavior as that of the Pb-free Bi2212 phase. As v increases from 2.2 to 2.63, the hole is doped in a Pb site and not a Cu one. For the Pb-substituted Bi2223 phase, p decreases with increasing v, and a mixed-valence state between Pb²⁺ and Pb⁴⁺ may coexist. As the v increases, the Pb valence increases and the Cu valence decreases. These results indicate two factors influence p. One is correlated with the substitution of Sr²⁺ for La³⁺ or Y³⁺ for Ca²⁺, which obeys Mott-Hubbard theory. The other is correlated with the substitution of Pb^2+/4+ for Bi³⁺, which does not obey Mott-Hubbard theory. The results strongly suggest that Pb substitution for Bi does not necessarily dope a hole but decreases the hole concentration.