On the Residual Resistivity Ratio in $\hbox{MgB}_{2}$ Superconductors

It is found that there exist two different but related parameters that control the value of the residual resistivity ratio (RRR) in MgB
₂
, namely, the stoichiometry of the starting powder and the synthesis parameters. The origin of the high RRR in the polycrystalline MgB
₂
samples has been attributed to the enhanced crystallinity of the samples due to higher Mg concentration in the starting composition. Scanning-electron-microscope images show the improved crystalline nature of the tape samples synthesized with the excess Mg in the starting powder mixture.