Thermal conductivity of an untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystal and a new interpretation of the superconducting state thermal transport

We have measured the anisotropic thermal conductivity of an untwinned single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in the most conducting a-b plane from 14 to 200 K. Unlike previous analyses, ours attributes the observed rapid rise in thermal conductivity in the superconducting state to the electronic contribution of the Cu-O plane. We propose that strong suppression of the quasiparticle scattering rate with decreasing temperature is responsible for the large enhancement of the superconducting-state thermal conductivity.