a-b plane anisotropy of the superconducting gap in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$

We report a-b plane electronic Raman scattering (ERS) measurements of superconducting ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ single crystals with ${\mathit{T}}_{\mathit{c}}$’s ranging from 90 K down to 65 K on both sides of the doping curve. At low temperatures, a peak associated with the opening of a superconducting energy gap Δ(k) forms in the electronic Raman continuum. Polarization selection rules yield ERS sensitivity to k dependence and a clear anisotropy is observed. As the doping level is varied, anomalies in the observed continuum peak positions indicate that scattering in the ${\mathit{B}}_{1\mathit{g}}$ symmetry is not a direct probe of Δ(k).