A new high-temperature superconductor: Bi₂Sr_3-x Ca_x Cu₂O_8+y

Abstract

A new superconductor that displays onset behavior near 120 K has been identified as Bi₂Sr_3-xCa_xCu₂O_8+y, with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 Å, b= 5.414 Å, and c = 30.904 Å. The structure contains copper-oxygen sheets as in La₂CuO₄ and YBa₂Cu₃O₇, but the copper-oxygen chains present in YBa₂Cu₃O₇ do not occur in Bi₂Sr_3-xCa_xCu₂O_8+y. The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca²⁺ and Sr²⁺ cations between the adjacent Cu-O sheets; Sr²⁺ cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi₂Sr_3-xCa_xCu₂O_8+y show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.