Line Nodes in the Superconducting Gap Function of Noncentrosymmetric ${\mathrm{CePt}}_3\mathrm{Si}$

The superconducting gap structure of recently discovered heavy fermion ${\mathrm{CePt}}_3\mathrm{Si}$ without spatial inversion symmetry was investigated by thermal transport measurements down to 40 mK. In zero field a residual $T$-linear term was clearly resolved as $T\to 0$, with a magnitude in good agreement with the value expected for a residual normal fluid with a nodal gap structure, together with a $T^2$ dependence at high temperatures. With an applied magnetic field, the thermal conductivity grows rapidly, in dramatic contrast to fully gapped superconductors, and exhibits one-parameter scaling with $T/\sqrt{H}$. These results place an important constraint on the order parameter symmetry; that is, ${\mathrm{CePt}}_3\mathrm{Si}$ is most likely to have line nodes.

Figure 1

(a) Main panel: $a$-axis thermal conductivity of ${\mathrm{CePt}}_3\mathrm{Si}$ for the $\mathit{H}\parallel b$ axis, plotted as $\kappa /T$ vs $T$ (●, 0 T; ○, 0.2 T; ▲, 0.5 T; △, 1 T; ■, 1.5 T; □, 2 T; ▼, 3 T; ▽, 4 T). The solid line is a linear fit in zero field: $\kappa /T=A+BT$. The dashed line is the contribution of phonons ${\kappa }_{\mathrm{ph}}$. The dash-dotted line is the thermal conductivity in $s$-wave superconductors. Inset: $T$ dependence of $a$-axis resistivity for the $\mathit{H}\parallel b$ axis. (b) The same data at low temperatures plotted as $\kappa /T$ vs $T^2$. The solid line is a linear fit in the normal state: $\kappa /T=a+b{T}^2$. The dashed line represents ${\kappa }_{\mathrm{ph}}$. Inset: $T$ dependence of $\kappa /T$ in zero field.

Figure 2

(a) $H$ dependence of the $T$-linear coefficient of $\kappa /T=A+BT$. (b) $H$ dependence of $\kappa$, normalized by the normal state value ${\kappa }_n$, as a function of $H/H_{c2}$ at $T=100\mathrm{mK}$ and at $T\to 0$. We used $H_{c2}=3.6\mathrm{T}$. The dashed line represents $\kappa /{\kappa }_n=0.15+0.54H^{0.49}$. The solid line is the result of fitting obtained by using Eq. (3). For comparison, data for the typical $s$-wave superconductor Nb at 2 K is shown (◆). For details, see the text.

Figure 3

Scaling plot $F\equiv {\kappa }_e(T,H)/{\kappa }_e(T,0)$ vs $x=(T/T_c)/\sqrt{H/H_{c2}}$. Note that the data collapse into the same curve.