Nucleation of Superconductivity in Thin Type-I Foils

The nucleation of the superconductive phase in macroscopic type-I foils in a slow, continuously decreasing perpendicular magnetic field is observed to first occur at the field $H_{c3}$, and reinitiate at $H_{c2}$. The observed variation of $H_{c2}$ with temperature further yields Ginzburg-Landau parameters for the materials approximately a factor 2 below tabulations, consistent with previous determinations in small grains.