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 exponentially, 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, scenarios such as anisotropic s-wave or multicomponent order parameter are called 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 intrinsic electronic properties other than flux pinning.