Evidence for strong electron-phonon coupling in the thermal conductivity of YBa2Cu3O7

We have measured the ab-plane thermal conductivity on single-crystal YBa2Cu3O7- (0.08) in the temperature range 10T 300 K. Our main focus is an analysis of the slope change in (T), which occurs just below the superconducting transition. The reduced-temperature (t=T/Tc) derivative of the normalized thermal conductivity, d(s/n)/dtt=1, as determined from the data, is rather small, 1.1. From measurements of the electrical conductivity on the same specimens and application of the Wiedemann-Franz Law, we determine the relative contributions to the heat conduction from the carriers and the lattice, and discuss the normal-state phonon-scattering mechanisms. Employing these results we calculate the slope of the lattice thermal conductivity at Tc and infer that the slope of the carrier conductivity must be very large, 6. This result implies strong coupling for some of the carriers.