Marginal stability of d-wave superconductors

Thermal conductivity of Bi2Sr2Ca(Cu1-xNix)(2)O-8 high-temperature supercond uctor display a low-temperature anomaly. It manifests itself as a sharp red uction of the thermal conductivity of the samples at a temperature T*(c) ap proximate to 200 mK. We interpret this anomaly as an indication of a margin al stability of the d(x2-y2) due to interaction with the spins of magnetic impurities. We suggest that d(x2-y2) + id(xy) state forms below T*(c). (C) 1999 Elsevier Science B.V. All rights reserved.