Abstract
We synthesized polycrystalline MgB 2 from a stoichiometric mixture of Mg and the 11B isotope under different conditions. All the samples showed bulk superconductivity with T c=38–39 K. The samples containing the least amount of unreacted Mg showed the highest T c and the sharpest transition width ( ΔT c). A residual resistivity ratio (RRR) of ∼5.8, and a magnetoresistance (MR), at 40 K and 10 T, of 12% were obtained for these samples. Moreover, there was no upturn of resistivity in a low temperature region at 10 T. The samples containing appreciable amounts of unreacted Mg showed quite different behaviors; the values of ΔT c, RRR, and MR were much larger. An upturn appeared in resistivity of the samples below about 50 K at 10 T and is thought to be due to the unreacted Mg.
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