Intrinsic features of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ tunneling spectra: Scaling and symmetry of the energy gap

Tunneling measurements have been made on a series of high-quality ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ (BSCCO) single crystals with transition temperatures (${\mathit{T}}_{\mathit{c}}$) ranging from 79 to 92 K. From these systematic studies it has been possible to delineate the intrinsic features of the low-energy electronic spectrum in the BSCCO materials. These results demonstrate that the magnitude of 2Δ at 4.2 K is nearly constant in the different ${\mathit{T}}_{\mathit{c}}$ samples, and thus, that 2Δ/${\mathit{kT}}_{\mathit{c}}$ increases as ${\mathit{T}}_{\mathit{c}}$ decreases. In addition, intrinsic quasiparticle excitations are detected within the gap irrespective of the sample ${\mathit{T}}_{\mathit{c}}$’s. Analyses of these electronic spectra using a variety of pairing models suggest that gap functions having a large anisotropy can provide a good fit to the experimental data.