Superconductivity in Dense MgB₂ Wires

MgB₂ becomes superconducting just below 40 K. Whereas porous polycrystalline samples of MgB₂ can be synthesized from boron powders, in this Letter we demonstrate that dense wires of MgB₂ can be prepared by exposing boron filaments to Mg vapor. The resulting wires have a diameter of 160 μm, are better than 80% dense, and manifest the full χ  =  -1/4π shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that MgB₂ is a highly conducting metal in the normal state with ρ(40 K)  =  0.38 μΩ cm. By using this value, an electronic mean-free path, l≈600 Å can be estimated, indicating that MgB₂ wires are well within the clean limit. T_c, H_c2(T), and J_c data indicate that MgB₂ manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.