Scanning tunneling spectroscopy of a superlattice superconductor: Bi 2Sr 2CaCu 2O 8

Abstract

It is clear that many of the cuprate superconductors are truly superlattices, composed of sheets whose individual superconducting critical temperatures may approach bulk values. Such a cuprate is Bi 2Sr 2CaCu 2O 8, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied to a- bBiO cleavage planes of T c ≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local d I/d Vspectra (superconducting density of states: DOS). The spectra, which are similar to those obtained from angle-resolved photoemission spectroscopy, confirm a large gap parameter Δ( x, y) on the uppermost layer. The d I/d Vspectra do not unequivocally select order parameter symmetry, but are probably consistent with d-wave or anisotropic s-wave states. Spatial variations of Δ on a 100 Å scale are attributed to variation in BiO metallicity, originating in oxygen stoichiometry variations. A model is presented to explain the different d I/d Vspectra which are seen, and associated with different local oxygen concentrations. This model, based on the superconducting proximity effect, assumes that in some regions the BiO uppermost layer is insulating and in other regions it is metallic, in the latter case induced superconductivity by proximity to the CuO 2planes. Our STM measurements appear to sample only the uppermost half cell of the crystal, and contain no obvious superlattice features. Recent measurements have confirmed Josephson radiation from voltage biased c-axis pillars of BSCCO. From the point of view of the present work, the superconducting systems which weakly couple along the c-direction to create Josephson junctions are probably half-cell slabs of height 15.4 Å, each containing two CuO 2and two BiO layers, which act as single composite electrodes for the Josephson junctions.