Angular Position of Nodes in the Superconducting Gap of YBCO

Abstract

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\ifmmode^\circ\else\textdegree\fi{} 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.