In-plane symmetry of the superconducting gap in YBCO probed by the angular dependence of thermal magnetoresistance

We present a study of the thermal conductivity of a detwinned YBa₂Cu₃O_6.9 single crystal as a function of the angle between the crystal axes and a rotating magnetic field. Measurements were done at several different temperatures below T_c and at a fixed field of 30 kOe. When the field rotates in a plane parallel to the Cu-O planes, thermal conductivity shows a four-fold angular variation with maxima at 45 degrees off the principal symmetry axes. This is caracteristic of an anisotropic superconducting gap associated with a d_x2-y2 order parameter. An upper limit on the weight of the possible s-wave component can be deduced from the angular positions of the extrema.