We report ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation data for ${\mathrm{Ba}}_2$${\mathrm{YC}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in the superconducting state obtained by nuclear quadrupole resonance in zero applied magnetic field. The temperature dependences of the relaxation rates reveal striking differences in the excitation spectra for quasiparticles on the chain-forming Cu1 and planar Cu2 lattice sites, corresponding to substantially different pairing energies for electrons on the planes and chains. Values of the gap ratio 2Δ/${\mathrm{kT}}_{\mathrm{c}}$ 8.3 and 2.4 are obtained for the Cu1 and Cu2 sites, respectively.

• Received 14 August 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.1860