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 (nominal value $x_{nom}$ = 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 $\kappa_0/T$ 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.