Critical-Field Curves for Gapless Superconductors

Superconducting critical-field curves have been measured for pure Th and Th-Gd alloys at magnetic-impurity concentrations up to 60% of the critical concentration required to completely quench superconductivity. These samples all show a Meissner effect and type-I magnetic transitions which are reversible to at least 1% of $H_c$. The Abrikosov-Gor'kov theory has been used to make detailed calculations of the critical-field curves over the entire temperature range, and of lifetime broadening up to $\frac{\Gamma }{\Delta (0, \Gamma )}=1\mathrm{}$. Theory and experiment agree to an accuracy of better than 0.5%.