A New High-Temperature Superconductor: Bi ₂ Sr _3-x Ca _x Cu ₂ O _8+y

TLDR

The structure contains copper‑oxygen sheets as in La₂CuO₄ and YBa₂Cu₃O₇, but lacks the copper‑oxygen chains present in YBa₂Cu₃O₇. Single‑crystal X‑ray diffraction revealed a pseudo‑tetragonal A‑centered orthorhombic structure (a = 5.399 Å, b = 5.414 Å, c = 30.904 Å) composed of alternating double Cu‑O and Bi‑O sheets with Ca²⁺ and Sr²⁺ cations interleaved between them. The material exhibits a superconducting transition with onset near 120 K, flux exclusion beginning at ~116 K and zero resistivity at ~91 K, and displays an incommensurate superstructure along the a axis (≈5× the subcell) observed by electron microscopy and X‑ray diffraction, though twinning can make it appear along both a and b axes.

Abstract

A new superconductor that displays onset behavior near 120 K has been identified as Bi 2 Sr 3- x Ca x Cu 2 O 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 La 2 CuO 4 and YBa 2 Cu 3 O 7 , but the copper-oxygen chains present in YBa 2 Cu 3 O 7 do not occur in Bi 2 Sr 3- x Ca x Cu 2 O 8+ y . The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca 2+ and Sr 2+ cations between the adjacent Cu-O sheets; Sr 2+ 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 Bi 2 Sr 3- x Ca x Cu 2 O 8+ y show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.