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 Tc≈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.
ASJC Scopus subject areas
- Materials Science(all)
- Condensed Matter Physics
- Electrical and Electronic Engineering