Fig. 3.

Field-induced transition revealed by the thermal conductivity. (A) Temperature dependence of the in-plane thermal conductivity divided by temperature, $\kappa /T$. Arrow marked $T_{\text{c}}$ indicates the onset temperature of the superconductivity determined by zero resistivity (Fig. 1A) and zero thermoelectric power (SI Text, section 2 and Fig. S3C). (Inset) $\kappa /T$ at low temperatures. (B) Magnetic-field dependence of $\kappa /T$ at low temperatures ($\mathit{H}\Vert c$). No hysteresis with respect to the field-sweep direction is observed. $\kappa /T$ shows a plateau-like behavior in a wide field range. At $H^{\mathrm{\ast }}$, $\kappa /T$ shows a cusp-like peak, suggestive of a nearly temperature-independent transition (dashed line). At 1.5 K, the cusp disappears and a weak structure (within the accuracy of the measurement) is observed at lower field. (C) Magnetic-field dependence of $\kappa /T$ in the zero-temperature limit, ${\kappa }_0/T$, obtained by linear extrapolation of $\kappa /T$ versus T at low temperatures.