Fluxoid pinning by vanadium carbide precipitates in superconducting vanadium

Vanadium carbide precipitates were formed in pure, annealed vanadium foils by the introduction of carbon in the specimens. Thin, disk-shaped precipitates resulted with mean diameters in the range 100–2600 Å and with number densities from 3 × 10¹⁵ to 4 × 10¹⁷ particles/cm³. The macroscopic, pinning-force density for magnetic fluxoids was measured at temperatures from 2 to 5 K and for magnetic fields from 0 to H_c2(T). Peak pinning-force densities in the range of 3 × 10 to 3 × 10⁶ dyn/cm³ for T=0K were realized in the 30 specimens studied. The pinning force density was found to obey a scaling law for specimens meeting certain requirements with respect to precipitate particle size and number density. These requirements correlate with the temperature-dependent, superconducting coherence length (T). Many specimens obeyed the scaling law at temperatures T < T_c except near T_c, where (T) is large in comparison with the precipitate size.Part of this work was supported by the Applied Research Laboratory of The Pennsylvania State University, under contract with the U.S. Naval Sea Systems Command.