Evidence for two pairing energies from nuclear spin-lattice relaxation in superconducting Ba sub 2 YCu sub 3 O sub (7-delta).

We report sup (63) Cu nuclear spin-lattice relaxation data for Ba sub 2 YCu sub 3 O sub (7-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 quasi-particles 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 2 DELTA/kT sub c of 8.3 and 2.4 are obtained for the Cu1 and Cu2 sites, respectively.