Temperature-Dependent Pair Breaking inLaCeandLaGd

Using multiple-pair-breaking theory, we have analyzed our $H_{c2\mathrm{}}$ data for pure La and for $\mathrm{La}\mathrm{Ce}$ and $\mathrm{La}\mathrm{Gd}$ alloys. For the $\mathrm{La}\mathrm{Gd}$ samples, we find a deviation from simple-pair-breaking theory, which is caused by the presence of the strong exchange field from the spin-$\frac{7}{2}$ impurities. This deviation implies significant Pauli paramagnetic pair breaking for this type of superconductor where the total internal field is large, even though the (externally applied) critical field is small. The $\mathrm{La}\mathrm{Ce}$ data provide a direct measurement of the spin-flip scattering rate as a function of temperature. The strong temperature dependence observed implied that $\mathrm{La}\mathrm{Ce}$ is a Kondo system with a characteristic temperature much less than 1 K. The $T_c$-vs-concentration study shows that impurity-impurity interactions play a large role.