Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor C$_6$Yb

We report measurements of the in-plane electrical resistivity $\rho$ and thermal conductivity $\kappa$ of the intercalated graphite superconductor C$_6$Yb to temperatures as low as $T_c$/100. When a magnetic field is applied along the c-axis, the part of $\kappa$ associated with fermionic quasiparticles increases exponentially for $H_{c1}