Specific heat study in U1-xThxBe13: Enormous Delta C and strong coupling at x=x(c1) and x(c2); Correlation between gamma and unusual superconductivity

Specific heat measurements of samples of U1-xThxBe13 utilizing extremely fi ne gradations of x near x(c1) and x(c2), where two superconducting phases a re known to occur for x(c1)<x<x(c2), were undertaken to study the genesis o f 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 Delta C at the superconducting transiti on 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 ear lier pressure work. Correlated with the discovery of a sharp increase in De lta C at T-c at both x(c1) and x(c2), the present work observes also a shar p increase in the normal state specific heat gamma at both x(c1) and x(c2), as well as peaks in Delta C/gamma T-c at both critical concentrations. The implications for understanding the superconductivity in U1-xThxBe13 are di scussed, including the possibility that the second transition for x(c1)<x<x (c2) is caused by the anomalous behavior in gamma proportional to N(0)(1+la mbda). [S0163-1829(98)03746-1].