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 the thermal conductivity $\kappa$ of the intercalated graphite superconductor C$_6$Yb to temperatures as low as $T_c$/100. When a field is applied along the c-axis, the residual electronic linear term $\kappa_0/T$ evolves in an exponential manner for $H_{c1} < H < H_{c2}$. This activated behaviour establishes the order parameter as unambiguously s-wave, and rules out the possibility of multi-gap or unconventional superconductivity in this system. 
Mike Sutherland, Nicolas Doiron-Leyraud, Louis Taillefer, Thomas   Weller, Mark Ellerby, S.S. Saxena