Abstract
Nanocrystalline superconducting MgB2 bulks were prepared by high pressure sintering of ball milled MgB2 powders. The disorder produced in these nanocrystalline materials shows a significant improvement in the upper critical field Hc2(0) of 38 T without severe suppression the superconducting transition temperature Tc. The band gap Δσ(0) decreases from 6.3 ± 0.1 meV to 5.4 ± 0.1 meV, while Δπ(0) keeps nearly fixed with the decrease in grain size to 20 nm. The ratios of the mean free path to the BCS coherence lengths li/ξ0,i(i = σ, π) turn out to be far less than 1, confirming that the two bands of the nanocrystalline bulks are both in the dirty limit. Strong intraband scatterings are responsible for the enhancement of Hc2(0).
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