Temperature-dependent pair breaking in LaCe and LaGd

Using multiple-pair-breaking theory, we have analyzed our Hc2 data for pure La and for LaCe and LaGd alloys. For the LaGd samples, we find a deviation from simple-pair-breaking theory, which is caused by the presence of the strong exchange field from the spin-72 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 LaCe data provide a direct measurement of the spin-flip scattering rate as a function of temperature. The strong temperature dependence observed implied that LaCe is a Kondo system with a characteristic temperature much less than 1 K. The Tc-vs-concentration study shows that impurity-impurity interactions play a large role.