Absence of superconductivity down to 80 mK in graphite intercalated BaC6

Following recent theoretical and experimental investigations of superconductivity in the graphite intercalated compounds CaC6 and SrC6, we report on a very low temperature magnetisation study on BaC6 down to 80 mK. The data show no trace of superconductivity even at fields as low as 0.7 Oe. Using a McMillan parametrisation of the BCS parameters, we conclude that the Coulomb pseudopotential is expected to be as large as 0.19 in both BaC6 and SrC6, i.e. 40% larger than in CaC6. As an alternative scenario, we argue that extrinsic effects such as intercalant disorder may depress superconductivity in both BaC6 and SrC6, as in the case of CaC6. (C) 2007 Elsevier Ltd. All rights reserved.