Universal Heat Conduction in Ce1-xYbxCoIn5: Evidence for Robust Nodal d-wave Superconducting Gap

In heavy-fermion superconductor Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ system, Yb doping was reported to cause a possible change from nodal $d$-wave superconductivity to a fully gapped $d$-wave molecular superfluid of composite pairs near $x \approx$0.07(nominalvalue$x_{nom}$ = 0.2). Here we present systematic thermal conductivity measurements on Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ ($x$ = 0.013, 0.084, and 0.163) single crystals. The observed finite residual linear term ${\kappa }_0\mathrm{/}T$ is insensitive to Yb doping, verifying the universal heat conduction of nodal $d$-wave superconducting gap in Ce${}_{1-x}$Yb${}_x$CoIn${}_5$. Similar universal heat conduction is also observed in CeCo(In${}_{1-y}$Cd${}_y$)${}_5$ system. These results reveal robust nodal $d$-wave gap in CeCoIn${}_5$ upon Yb or Cd doping.