Multiple Superconducting Phases in New Heavy Fermion Superconductor ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$

The superconducting gap structure of recently discovered heavy fermion superconductor ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ was investigated by using thermal transport measurements in magnetic field rotated relative to the crystal axes. We demonstrate that a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state, giving a clear indication of the presence of two distinct superconducting phases with twofold and fourfold symmetries. We infer that the gap functions in both phases have a point node singularity, in contrast to the familiar line node singularity observed in almost all unconventional superconductors.

Figure 1

Main panel: $T$ dependence of ${\kappa }_{zz}/T$ in zero field and above $H_{c2}$ ($\mathit{H}\parallel [100]$). Upper inset: $T$ dependence of ${\kappa }_{zz}(H=0)/{\kappa }_{zz}(H=2.5\mathrm{T})$ near $T_c$. Lower left inset: $H$ dependence of ${\kappa }_{zz}(\mathit{H}\parallel [100])$ at 0.4 K. Lower right inset: schematic figure of the $\mathit{H}=H(\sin \theta \cos \varphi ,\sin \theta \sin \varphi ,\cos \theta )$ rotation.

Figure 2

(a),(b) Angular variation of ${\kappa }_{zz}$ in $\mathit{H}$ rotating within the $ab$ plane ($\theta =90°$ in the lower right inset of Fig. 1) at 0.52 K above and below $H_{c2}(\simeq 2.0\mathrm{T})$.

Figure 3

(a) The amplitude of twofold (open circles) and fourfold (filled circles) symmetries plotted as a function of $H/H_{c2}$ at $T=0.52\mathrm{K}$. For details, see the text. $H^{*}$ indicates the phase boundary (see Fig. 4). (b) Angular variation of ${\kappa }_{zz}(\mathit{H},\varphi )$ at $\theta =90°$, $45°$, and $30°$ measured by rotating $\mathit{H}$ ($|{\mu }_B\mathit{H}|=1.2\mathrm{T}$) conically around the $c$ axis as a function of $\varphi$.

Figure 4

The phase diagram of the superconducting gap symmetry determined by the present experiments. The filled circles represent the magnetic field $H^{*}$ at which the transition from fourfold to twofold symmetry takes place. The open circles represent $H_{c2}$. The area of the gap function with fourfold symmetry is shown by pink ($A$ phase) and the area of the gap function with twofold symmetry is shown by blue ($B$ phase).