Nucleation of Superconductivity in Tantalum in a Decreasing Magnetic Field

Measurements of the surface nucleation field $H_{c3\mathrm{}}$ and the bulk supercooling field $H_{\mathrm{sc}}$, made on pure tantalum, show that these fields are identical only when $\kappa <0.39\mathrm{}$, and that measurable supercooling occurs at all temperatures between $T_c$ and 1.4°K. The measurements verify qualitatively the recent theoretical work of Park. Nearly ideal supercooling of the bulk cylindrical samples is achieved by using small auxiliary-field coils to field coils to eliminate nucleation at the sample ends. A new value for the Ginzburg-Landau parameter of the pure metal ${\kappa }_0=0.340\mathrm{}\pm 0.01$ is derived from the data.