Low-Temperature Phase Transition in Bi_2Sr_2Ca(Ni_xCu_{1-x})_20_8: Evidence for Unconventional Superconductivity

We report the discovery of a low-temperature phase transition in Bi_2Sr_2Ca(Ni_xCu_{1-x})_20_8 high temperature superconductor. This transition manifests itself as a sharp reduction of the thermal conductivity of the samples at a temperature of T_c* \approx 200 mK. The temperature dependence of the thermal conductivity changes dramatically from T^\alpha with \alpha between 1.6 and 1.75 above the transition to T-linear behavior below the transition. Application of a small magnetic field suppresses the low-temperature phase. We interpret this behavior as a phase transition into a second bulk low-temperature superconducting state in which the time-reversal symmetry is likely to be broken. This discovery constitutes direct evidence for unconventional superconductivity in high temperature superconductors.