Scanning tunneling spectroscopy of a superlattice superconductor: Bi₂Sr₂CaCu₂O₈

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 Bi₂Sr₂CaCu₂O₈, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied to a-b BiO cleavage planes of T_c≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local dI/dV spectra (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 dI/dV spectra 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 dI/dV spectra 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 CuO₂ planes. 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 CuO₂ and two BiO layers, which act as single composite electrodes for the Josephson junctions.