Specific heat study in U1−xThxBe13: Enormous ΔC and strong coupling at x=xc1 and xc2; Correlation between γ and unusual superconductivity

Abstract

Specific heat measurements of samples of U1−xThxBe13 utilizing extremely fine gradations of x near xc1 and xc2, where two superconducting phases are known to occur for xc1<x<xc2, were undertaken to study the genesis of the second transition. Surprisingly, this precise delineation of x in the critical regions of the phase diagram has revealed a strong peaking of the discontinuity in the specific heat ΔC at the superconducting transition that varies rapidly with x as the two transition concentration regime is approached from either side in the phase diagram. This was not seen in earlier pressure work. Correlated with the discovery of a sharp increase in ΔC at Tc at both xc1 and xc2, the present work observes also a sharp increase in the normal state specific heat γ at both xc1 and xc2, as well as peaks in ΔC/γTc at both critical concentrations. The implications for understanding the superconductivity in U1−xThxBe13 are discussed, including the possibility that the second transition for xc1<x<xc2 is caused by the anomalous behavior in γ∝N(0)(1+λ).

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