Raman spectra of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\left({\mathrm{Ca}}_{0.62}{\mathrm{Y}}_{0.38}\right){\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ with $T_c\cong {0.65T}_c^{\max }$ in the underdoped regime of the phase diagram are studied as a function of temperature and polarization. At $B_{2g}$ $\left(\mathrm{xy}\right)$ symmetry a reduction of spectral weight by 10% for frequencies less than $700{\mathrm{cm}}^{-1\mathrm{}}$ or ${15k}_B{T}_c$ is found below approximately 200 K. Below $T_c$, a superconducting gap opens up which closely resembles that observed at higher doping levels. It is compatible with $d_{x^2{-y}^2}$ pairing and its amplitude $2{\Delta }_0$ can be estimated to be ${8k}_B{T}_c$.

• Received 6 February 1997

DOI:https://doi.org/10.1103/PhysRevLett.78.4837