The thermal conductivity of a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ detwinned single crystal has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu-O planes. Measurements were carried out at several different temperatures below $T_c$ for a field of 30 kOe. A fourfold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45° in $k$ space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a fourfold symmetry and our results impose an upper limit of 10% on the weight of the $s$-wave component of the essentially $d$-wave superconducting order parameter of YBCO.

• Received 8 November 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.2624