Angular position of nodes in the superconducting gap of YBCO

The thermal conductivity of a YB_a2Cu₃O_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.