Anisotropy of the thermal conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$

The thermal conductivity, both in-plane (${\kappa }_{\mathrm{ab}}$) and out-of-plane (${\kappa }_c$), has been measured in ‘‘90-K’’ crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$ from 10 to 330 K. ${\kappa }_{\mathrm{ab}}$ displays an 18 to 40% increase below the transition temperature $T_c$ (as in ceramic data). ${\kappa }_c$, however, is four to five times smaller and shows no anomaly at $T_c$. Comparing these results to ${\kappa }_{\mathrm{ab}}$ of insulating crystals, we find evidence for very strong but highly anisotropic electron-phonon coupling in the normal state. Our results are consistent with a phonon origin for the peak in ${\kappa }_{\mathrm{ab}}$. .AE