Evidence for strong electron-phonon coupling in the thermal conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have measured the ab-plane thermal conductivity (κ) on single-crystal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (δ≊0.08) in the temperature range 10≤T≤300 K. Our main focus is an analysis of the slope change in κ(T), which occurs just below the superconducting transition. The reduced-temperature (t=T/${\mathit{T}}_{\mathit{c}}$) derivative of the normalized thermal conductivity, d(${\mathrm{\kappa }}^{\mathit{s}}$/${\mathrm{\kappa }}^{\mathit{n}}$)/dt${\mathrm{\Vert }}_{\mathit{t}=1\mathrm{}}$, 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 ${\mathit{T}}_{\mathit{c}}$ and infer that the slope of the carrier conductivity must be very large, ≤6. This result implies strong coupling for some of the carriers.