Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor C6Yb

We report measurements of the in-plane electrical resistivity rho and thermal conductivity kappa of the intercalated graphite superconductor C(6)Yb down 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)/2. This activated behavior is compelling evidence for an s-wave order parameter, and is a strong argument against the possible existence of multigap superconductivity.