Order parameter of MgB2: A fully gapped superconductor - art. no. 167003

We have measured the low-temperature specific heat C(T) for polycrystalline MgB2 prepared by high pressure synthesis. C(T) below 10 K vanishes exponen tially, which unambiguously indicates a fully opened superconducting energy gap. However, this gap is found to be too small to account for T, of MgB2. Together with the small specific heat jump DeltaC/gamma T-n(c) = 1.09, sce narios such as anisotropic s-wave or multicomponent order parameter are cal led for. The magnetic field dependence of gamma (H) is neither linear for a fully gapped s-wave superconductor nor H-1/2 for nodal order parameter. It seems that this intriguing behavior of gamma (H) is associated with the in trinsic electronic properties other than flux pinning.