In-plane thermal conductivity and Lorenz number in YBa2Cu3O7−y

Abstract

We make a tentative proposal to extract the electronic contribution to the in-plane thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{el}}(T)$ of YBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}y}$ in the normal state. We have measured ${\ensuremath{\kappa}}_{\mathrm{ab}}(T)$ for single crystals with various oxygen contents $(6.06<~$ 7$\ensuremath{-}y$ $<~6.93)$. The systematic study of the oxygen-content dependence of ${\ensuremath{\kappa}}_{\mathrm{ab}}$ for the insulating phase enables us to estimate the phononic contribution ${\ensuremath{\kappa}}_{\mathrm{ph}}(T)$ for the metallic phase to some extent. Based on the estimated ${\ensuremath{\kappa}}_{\mathrm{ph}},$ we examined ${\ensuremath{\kappa}}_{\mathrm{el}}$ from the view point of whether the Wiedemann-Franz law holds or not. The estimated ${\ensuremath{\kappa}}_{\mathrm{el}}$ appears to show only a weak $T$ dependence and no significant change at a characteristic temperature ${T}^{*}$ below which the electrical conductivity ${\ensuremath{\sigma}}_{\mathrm{ab}}(T)$ is enhanced possibly due to the opening of a spin gap. The difference between ${\ensuremath{\kappa}}_{\mathrm{el}}$ and ${\ensuremath{\sigma}}_{\mathrm{ab}}$ suggests the violation of the Wiedemann-Franz law below ${T}^{*}$. We discussed the ambiguity of our analyses and other possible interpretations of the present result as well as the picture of the charge transport speculated from the above suggestion.