Universal heat conduction in Ce1-xYbxCoIn5: Evidence for robust nodal d -wave superconducting gap

In the heavy-fermion superconductor Ce1-xYbxCoIn5, 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≈0.07 (nominal value xnom=0.2). Here we present systematic thermal conductivity measurements on Ce1-xYbxCoIn5 (x=0.013, 0.084, and 0.163) single crystals. The observed finite residual linear term κ0/T is insensitive to Yb doping, verifying the universal heat conduction of the nodal d-wave superconducting gap in Ce1-xYbxCoIn5. Similar universal heat conduction is also observed in the CeCo(In1-yCdy)5 system. These results reveal a robust nodal d-wave gap in CeCoIn5 upon Yb or Cd doping.