Direct STM mapping of the superconducting energy gap in single crystal BI₂SR₂CACU₂O_8+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 to recent photoemission spectra, confirm a large gap parameter Δ(x, y) associated with an apparently gapless DOS on the uppermost layer. Transverse spatial variations of Δ on a 100 Å scale are attributed to variations in BiO metallicity, presumably originating in oxygen stoichiometry variations in an unannealed crystal. We identify two characteristic dI/dV spectral shapes with regions of metallic and nonmetallic BiO layers, and can relate these by the McMillan model of the superconducting proximity effect. A sharply peaked spectral shape, similar to that observed in photoemission, is predicted for the metallic BiO layer induced superconducting by its proximity to the underlying CuO₂ planes. The short mean free path and short coherence length imply that both tunneling and photoemission spectra are heavily weighted toward contributions from the BiO layer in fully metallic 2212. The present results and analysis thus suggest that the superconducting proximity effect influences the lineshapes seen by both techniques.