Origin of the modulation structure of Pb-substituted Bi2212 superconductors

抄録

  To elucidate the effect of Pb substitution on the modulation structure of the Bi2212 phase, we prepared Pb-free Bi₂Sr_1.8(Ca_1-xY_x)Cu₂O_y and Pb-substituted Bi_1.6Pb_0.4Sr_1.8(Ca_1-xY_x)Cu₂O_y. For both the Pb-free and Pb-substituted Bi-2212 phases, the modulation wave number σ increased with increasing oxygen content y. This tendency is explained by the excess oxygen that is inserted into the BiO layer and leads to the formation of the modulation structure. For the Pb-free Bi2212 phase, σ increased with increasing O-Cu-O length l. This tendency is explained by the lattice mismatch that exists between the O-Bi-O and O-Cu-O layers. In contrast, for the Pb-substituted Bi2212 phase, σ changed only slightly with increasing l, revealing that the relaxation mechanism for lattice mismatch in the Pb-substituted Bi2212 phase is different from that of the Pb-free Bi2212 phase. A possible mechanism for relaxation of lattice mismatch is the change in Pb valence. As the Pb valence increases from 2+ to 4+, the ionic radius of Pb decreases, suggesting that l decreases and the mismatch between the O-Cu-O and Bi(Pb)O lengths decreases.