Scanning tunneling microscope studies on the atomic structures in Bi 2Sr 2CaCu 2O 8+ δ highly doped with Pb

Abstract

Highly doped high- T c Bi compounds, i.e., Bi 2.2− x Pb x Sr 1.8CaCu 2O 8+ δ ( x=0.7) (denoted by Bi(Pb)-2212) were recently found to have dramatically improved critical current J c [I. Chong, Z. Hiroi, M. Izumi, J. Shimoyama, Y. Nakayama, K. Kishio, T. Terashima, Y. Bando, and M. Takano, Science 276 (1997) 770; J. Shimoyama, Y. Nakayama, K. Kitazawa, K. Kishio, Z. Hiroi, I. Chong, M. Takano, Physica C 281 (1997) 69]. To elucidate atom structures of the Bi(Pb)-2212, ultrahigh vacuum scanning tunneling microscope (UHV-STM) studies have been performed. We observed, for the first time, defected α-phase with fragmented modulation of λ=7 nm. Observed defects are amorphous regions of a few nanometer size embedded in the α-phase and edge dislocations with Burgers vector (1/2) [110]. Both the defects are suggested potentially to be effective pinning centers in Bi(Pb)-2212. Annealing in vacuum at temperatures up to 630°C for 19 h was found sufficient enough to evaporate all the Pb atoms in the surface layers of Bi(Pb)-2212 and consequently to transform Bi(Pb)-2212 with λ=7 nm to pure Bi-2212 with λ=2.7 nm.