Abstract
We have prepared extremely high-purity samples of and and conducted a study of the specific heat in zero field on the as-prepared and annealed samples. In addition we have measured the low-temperature specific heat of the 3 at. % Th sample in 0.25, 0.5, 0.75, 1.0, 1.25, 2, and 3 T magnetic fields in order to accurately determine for both transitions. The high-purity unannealed 3.8 at. % Th samples show larger and sharper transitions than previously reported; annealing further improves the transitions such that ΔC() for the 3 at. % Th sample is more than twice as large as previously reported, with being approximately the same. A γ value of 2300 mJ/mol for is derived from entropy considerations and the 0-field and 3-T specific-heat data. The separation between the two transitions is significantly improved with annealing—ΔC(), which is essentially only a shoulder on ΔC() for unannealed 3.8 at. % Th samples, becomes a clearly distinct transition upon annealing. The results of the field measurements are that the (T) data for the two transitions are, within our (rather good) precision, parallel; this includes the data as H→0, i.e., () for the two transitions is the same, ≈-45±3 T/K. This result is consistent with the value reported for pure suppresses. These field results contradict the two previous specific-heat-in-field studies (on unannealed lower-purity samples with significantly more scatter in the data) of the two transitions in . The equality of (), (), and for pure demonstrated here is difficult to explain on the basis of current theories that consider the lower transition as a magnetic superconducting state.
- Received 18 March 1991
DOI:https://doi.org/10.1103/PhysRevB.44.6921
©1991 American Physical Society