Superconductivity in Dense ${\mathrm{MgB}}_2$ Wires

${\mathrm{MgB}}_2$ becomes superconducting just below 40 K. Whereas porous polycrystalline samples of ${\mathrm{MgB}}_2$ can be synthesized from boron powders, in this Letter we demonstrate that dense wires of ${\mathrm{MgB}}_2$ can be prepared by exposing boron filaments to Mg vapor. The resulting wires have a diameter of $160\mu \mathrm{m}$, are better than $80\%$ dense, and manifest the full $\chi =-1/4\mathrm{}\pi$ shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that ${\mathrm{MgB}}_2$ is a highly conducting metal in the normal state with $\rho \left(40\mathrm{K}\right)=0.38\mathrm{}\mu \Omega \mathrm{cm}$. By using this value, an electronic mean-free path, $l\approx 600Å$ can be estimated, indicating that ${\mathrm{MgB}}_2$ wires are well within the clean limit. $T_c$, $H_{c2\mathrm{}}\left(\mathrm{T}\right)$, and $J_c$ data indicate that ${\mathrm{MgB}}_2$ manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.