Transport and optical studies of single crystals of the 80-K Bi–Sr–Ca–Cu–O superconductor

Abstract

One of the most challenging problems in solid-state physics is to elucidate the mechanism of the high-transition-temperature (high-T_c) superconductivity in cuprates. Two classes of cuprates, (La, Sr)₂CuO₄ (LSCO) and YBa₂Cu₃O_7−y, (YBCO)^1,2, are now well established as high-T_c superconductors. Several characteristic features common to LSCO and YBCO have provided the basis for theoretical models of high-T_c superconductivity³. Various properties, in particular the transport properties, exhibit large anisotropy owing to the layered structure. The dominant charge carriers are holes, which are supplied in the Cu–O plane by the substitution of La with Sr in La₂CuO₄ or by adding oxygen to YBa₂Cu₃O₆. Recently, Maeda et al.⁴ announced the discovery of new superconducting oxides in the system Bi–Sr–Ca–Cu–O, with T_c above 100 K. Their materials exhibit the superconducting transition in two steps, at ∼80 K and 105 K, indicating the presence of two superconducting phases. Although the detailed crystal structure is not known yet, several groups have reported that at least the 80-K superconductor has the formula Bi₂(Sr, Ca)₃Cu₂O_y^5,6. Transmission electron microscopy reveals that the material possesses a layered structure similar to those of LSCO and YBCO, with Cu–O layers isolated between Bi–O layers⁷. Here we report the synthesis of single crystals of Bi–Sr–Ca–Cu–O with T_c = 80 K; and measurements of their basic properties. The similarity of the measured properties to those of LSCO and YBCO indicates the importance in the new materials, as in their predecessors, of two-dimensional Cu–O layers.