Influence of Rb, Cs, and Ba on the superconductivity of magnesium diboride

Magnesium diboride has been thermally treated in the presence of Rb, Cs, and Ba. The magnetic susceptibility shows the possible onsets of superconductivity in the resulting samples at 52 K (Rb), 58 K (Cs), and 45 K (Ba). Room-temperature(11)B NMR indicates the cubic symmetry of the electric field gradient at the boron site for the samples reacted with Rb and Cs, in contrast to the axial symmetry in the initial MgB2 and in the sample treated with Ba.