The thermal conductivity $\kappa$ of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ was measured along the $a$ and $b$ axes of detwinned single crystals. Both ${\kappa }_a$ and ${\kappa }_b$ show the usual peak below $T_c=93.8\mathrm{}\mathrm{K}$, but a clear additional contribution to the $b$-axis conduction sets in well below $T_c$. The difference ${\kappa }_b-{\kappa }_a$ exhibits a peak similar to the peak in the plane conductivity ${\kappa }_a$ but with an onset temperature of 55 K. This result suggests that the CuO chains give rise to a sudden growth in superfluid density at that temperature, as predicted by models of single-electron tunneling between chains and planes.

• Received 2 December 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1976