Investigation of the second transition in U1-xThxBe13

We have prepared extremely high-purity samples of U0.97Th0.03Be13 and U0.962Th0.038Be13 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 Hc2' 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(Tc2) for the 3 at. % Th sample is more than twice as large as previously reported, with Tc2 being approximately the same. A γ value of 2300 mJ/mol K2 for U0.97Th0.03Be13 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(Tc1), which is essentially only a shoulder on ΔC(Tc2) for unannealed 3.8 at. % Th samples, becomes a clearly distinct transition upon annealing