Low-Temperature Anomaly in Thermal Conductivity of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}({\mathrm{Cu}}_{1-\mathit{x}}{\mathrm{Ni}}_{\mathit{x}}{)}_2{O}_8$: Second Superconducting Phase?

We report the discovery of a low-temperature anomaly in thermal conductivity of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}({\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x{)}_2{\mathrm{O}}_8$ high-temperature superconductor. This anomaly manifests itself as a sharp reduction of the thermal conductivity of the samples at a temperature $T_c^{*}\approx 200\mathrm{mK}$. The temperature dependence of the thermal conductivity changes dramatically from $T^{\alpha }$ with $\alpha$ between 1.6 and 1.75 above $T_c^{*}$ to $T$-linear behavior below it. Application of a small magnetic field suppresses the low-temperature anomaly. We propose that such behavior is consistent with a phase transition into a second bulk low-temperature superconducting state at $T_c^{*}$. The presence of a second superconducting state would constitute direct evidence for unconventional superconductivity in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$. Within the suggested scenario, time-reversal symmetry is likely to be violated in the low-temperature phase.