Tunneling measurements are reported for break junctions on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{O}_{8-\delta }$ single crystals with various oxygen concentrations. Superconducting energy gaps $\Delta$ are observed in the underdoped samples which are considerably larger $\left(\sim 30\%\right)$ than found in optimal doped crystals. The trend of decreasing $\Delta$ and $2\Delta {/kT}_c$ with increasing hole doping is continued into the overdoped region. Thus the superconducting gap and strong-coupling ratio change monotonically and dramatically over a narrow doping region where $T_c$ exhibits a maximum.

• Received 20 August 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.157