Measurement of the energy gap in oxygen-annealed ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ high-${\mathit{T}}_{\mathit{c}}$ superconductors by tunneling spectroscopy

The superconducting energy gap has been determined in homogeneous, oxygen-annealed ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ (BSCCO) single crystals by tunneling spectroscopy. Normal-metal–insulator– superconductor (NIS) point-contact tunneling measurements made on cleaved surfaces of these BSCCO crystals exhibit a well-defined gap with a low conductance, G(V), within the gap region. The conductance and change of G(V) within the gap region differs from the behavior expected for either a gapless or BCS-like superconductor. Analysis of data from both NIS and superconductor-insulator-superconductor junctions provides a consistent scale for superconductivity with 2Δ≊6.8${\mathit{kT}}_{\mathit{c}}$. In addition, since this gap structure is reproducible at different surface positions of these oxygen-annealed crystals we conclude that spatial variations in 2Δ and a high density of states within the gap are not intrinsic to the BSCCO system.