  We synthesized polycrystalline MgB2 from a stoichiometric mixture of Mg and
the 11 B isotope under different conditions. All the samples showed bulk
superconductivity with Tc = 38~39 K. The samples containing the least amount of
unreacted Mg showed the highest Tc and the sharpest transition width
(Delta_Tc). A residual resistivity ratio (RRR) of ~ 5.8, and a
magnetoresistance (MR), at 40 K, of 12% were obtained for these samples.
Moreover, there was no upturn of resistivity in a low temperature region at 10
Tesla. The samples containing appreciable amounts of unreacted Mg showed quite
different behaviors; the values of Delta_Tc, 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.
