The thermal conductivity of a YBCO 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 temperatures below T_c and at a fixed field of 30 kOe. A four-fold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45 degrees in k-space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a four-fold symmetry such as sample shape or anisotropic pinning. 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.
@article{arxiv.cond-mat/9611063,
title = {Angular position of nodes in the superconducting gap of YBCO},
author = {H. Aubin and K. Behnia and M. Ribault and R. Gagnon and L. Taillefer},
journal= {arXiv preprint arXiv:cond-mat/9611063},
year = {2009}
}