Inductive Measurements of Heavy Fermion Superconductors
Abstract
Experimental results are presented on the temperature dependence of the penetration depth, lambda (T), in UPt_3, and on the upper critical field, B_{rm c2 }(T), of UBe_{13}. The objective of this work was to obtain a better understanding of the unusual superconductivity in these materials. The inductive response of eight UPt_3 samples obtained from seven different materials fabrication groups was measured from 50 to 700 mK at frequencies varying between 32 Hz and 33 MHz. The low frequency ( <=4.7 kHz) data suggest a linear temperature dependence of lambda(T/T _{rm c} <= 0.5) for the samples possessing a double feature near the superconducting transition temperature, T_{rm c}. We have verified that this double feature present in lambda(T) for some of the samples corresponds to the double jump observed in the specific heat. On the other hand, lambda (T/T_{rm c} <= 0.5) was found to have a quadratic temperature dependence for unannealed specimens which exhibit only a single transition at T_{rm c}. The linear temperature dependence in lambda(T) is consistent with the presence of line nodes in the basal plane, while the quadratic dependence found for other samples indicates that impurity scattering dominates in these specimens. The double transition in lambda(T) has been studied in magnetic fields up to 1.6 T for one of the specimens, and the resulting phase diagram is consistent with the one constructed from thermodynamical measurements. The high frequency ( >=3 MHz) data suggest lambda (T/T_{rm c} <= 0.5) ~ {rm T}^eta, where 2<=eta<=4. The possible origins of the frequency dependence of lambda(T) are discussed, as well as the effect of the surface quality on eta. Using sensitive mutual induction techniques, we have systematically measured B_{rm c2}(T) for three UBe_{13 } single crystals in fields up to 8 T oriented along the (100) and (110) directions, with particular emphasis on the region near the zero field critical temperature. In that low field regime and within our experimental uncertainties of 0.5 mK and 0.5 mT, no anisotropy in B_ {rm c2}(T) was observable for any of the samples. For one of these specimens, an anomaly in chi^'(B,T) was observed below the superconducting transition. The complete B_{rm c2}(T) phase diagrams are presented and compared to the results of other workers. The temperature dependence of the penetration depth down to 50 mK and in zero field is measured on two of the crystals.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT.......332S
- Keywords:
-
- URANIUM;
- PLATINUM;
- BERYLLIUM;
- Physics: Condensed Matter; Engineering: Materials Science