We studied the effect of disorder on the superconducting properties of polycrystalline MgB 2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2Δ 0 / k B T c = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses T c and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of d H c 2 /d T at T c can be obtained without sacrificing more than a few degrees in T c . The upper critical field of the sample after irradiation exceeds 28 T at T → 0.
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Wang et al. (2003) studied this question.
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