Angular Position of Nodes in the Superconducting Gap of YBCO

The thermal conductivity of a YBa{sub 2}Cu{sub 3}O{sub 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{sub c} for a field of 30kOe. A fourfold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45{degree} 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. {copyright} {ital 1997} {ital The American Physical Society}