Single-crystalline N${\mathrm{d}}_{1.84}$C${\mathrm{e}}_{0.16}$Cu${\mathrm{O}}_4$ with a superconducting transition temperature of 19.3 K has been studied above and below $T_c$ using polarized Raman scattering. Electronic, crystal-field, and vibrational excitations have been detected. In the superconducting state, the formation of an almost isotropic gap $2\Delta$ is observed which varies between 4.1 and 4.9 in units of ${\mathrm{kT}}_c$. This indicates that N${\mathrm{d}}_{1.84}$C${\mathrm{e}}_{0.16}$Cu${\mathrm{O}}_4$ is different from the other cuprate systems.

• Received 27 January 1995

DOI:https://doi.org/10.1103/PhysRevLett.74.4911