  We have measured the low-temperature specific heat C(T) for polycrystalline
MgB_2 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 Tc of
MgB_2. Together with the small specific heat jump DeltaC/gamma_nTc=1.13,
scenarios like anisotropic s-wave or multi-component 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.

Superconductivity