DIRECT STM MAPPING OF THE SUPERCONDUCTING ENERGY-GAP IN SINGLE-CRYSTAL BI2SR2CACU2O8+X

Scanning tunneling spectroscopy applied to BiO cleavage planes of 90 K T(c) BSCCO 2212 at 4.2 K simultaneously provides topography and local dI/dV spectra (superconducting DOS). The spectra, which are similar t o recent photoemission spectra, confirm a large gap parameter DELTA(x, y) associated with an apparently gapless DOS on the uppermost layer. Transverse spatial variations of DELTA on a 100 angstrom scale are att ributed to variations in BiO metallicity, presumably originating in ox ygen stoichiometry variations in an unannealed crystal. We identify tw o characteristic dI/dV spectral shapes with regions of metallic and no nmetallic BiO layers, and can relate these by the McMillan model of th e superconducting proximity effect. A sharply peaked spectral shape, s imilar to that observed in photoemission, is predicted for the metalli c BiO layer induced superconducting by its proximity to the underlying CuO2 planes. The short mean free path and short coherence length impl y that both tunneling and photoemission spectra are heavily weighted t oward contributions from the BiO layer in fully metallic 2212. The pre sent results and analysis thus suggest that the superconducting proxim ity effect influences the lineshapes seen by both techniques.