INPLANE 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 diff erent 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 conducti vity 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(x)2(-y)2 order parameter. An u pper limit on the weight of the possible s-wave component can be deduc ed from the angular positions of the extrema.