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 T c of MgB 2 . Together with the small specific heat jump ∆C/γ n T c =1.
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