%N 6 %V 98 %A M Sutherland %A N Doiron-Leyraud %A L Taillefer %A T Weller %A M Ellerby %A SS Saxena %T Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor C6Yb %D 2007 %L discovery116271 %J PHYS REV LETT %K THERMAL-CONDUCTIVITY, FIELD, STATE, MODEL %I AMERICAN PHYSICAL SOC %O © 2007 The American Physical Society %X We report measurements of the in-plane electrical resistivity rho and thermal conductivity kappa of the intercalated graphite superconductor C6Yb 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.