Universal Heat Conduction inYBa2Cu3O6.9

Abstract

The thermal conductivity of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ was measured at low temperatures in untwinned single crystals with concentrations of Zn impurities from 0% to 3% of Cu. A linear term ${\kappa }_0/T=0.19\mathrm{}\mathrm{mW}{\mathrm{K}}^{-2\mathrm{}}{\mathrm{cm}}^{-1\mathrm{}}$ is clearly resolved as $T\to 0$, and found to be virtually independent of Zn concentration. The existence of this residual normal fluid strongly validates the basic theory of transport in unconventional superconductors. Moreover, the observed universal behavior is in quantitative agreement with calculations for a gap function of $d$-wave symmetry.