We report measurements of the in-plane electrical resistivity ρ and the thermal conductivity κ of the intercalated graphite superconductor C6Yb to temperatures as low as Tc/100. When a field is applied along the c-axis, the residual electronic linear term κ0/T evolves in an exponential manner for Hc1<H<Hc2. 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.
@article{arxiv.cond-mat/0603664,
title = {Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor C$_6$Yb},
author = {Mike Sutherland and Nicolas Doiron-Leyraud and Louis Taillefer and Thomas Weller and Mark Ellerby and S. S. Saxena},
journal= {arXiv preprint arXiv:cond-mat/0603664},
year = {2009}
}