Synthesis of superconducting Bi₂Sr₂CaCu₂O_8+δ single crystals using alumina crucibles with different purities

Abstract

A self-flux method is the simplest technique for synthesizing Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) superconducting single crystals. However, the crystals are inevitably contaminated because of the Bi-2212 stoichiometric melt reacting with the crucible material. In this paper, we investigate the nonsuperconducting subproduct that forms during heating in the self-flux method for synthesizing Bi-2212. This subproduct was identified as BiSr₂CaAl₃O₉ by X-ray diffraction. Bi-2212 crystal growth was performed using A1₂O₃ crucibles with three different purities (nominally 97, 99.7, and 99.9%). For both the 97 and 99.7% purity crucibles, the subproduct was observed in all five samples out of five, whereas for the 99.9% purity crucible, it was observed in only two samples out of five. Furthermore, the 99.9% purity crucible gave a much lower subproduct volume than the 97% purity crucible. The average superconducting critical temperature (T_c,zero) varied depending on the crucible purity; it was 89.6, 90.8, and 91.8 K for the 97, 99.7, and 99.9% purity crucibles, respectively. Finally, we fabricated Bi-2212 stacked devices with intrinsic Josephson junctions (Bi-2212 stack) using as-grown crystals from the 97 and 99.9% purity crucibles. The Bi-2212 stacks exhibited a highly hysteretic current–voltage characteristic even at liquid N₂ temperature and they had identical quality parameters for Josephson junctions (i.e., I_r/I_c and voltage jump V_j). We conclude that the subproduct formation has little effect on the electrical characteristics of a Josephson junction device at 77 K. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.