Specific heat study inU1−xThxBe13: EnormousΔCand strong coupling atx=xc1andxc2;Correlation between γ and unusual superconductivity

Abstract

Specific heat measurements of samples of ${\mathrm{U}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{Be}}_{13}$ utilizing extremely fine gradations of x near ${x}_{c1}$ and ${x}_{c2},$ where two superconducting phases are known to occur for ${x}_{c1}<x<{x}_{c2},$ 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 $\ensuremath{\Delta}C$ 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 $\ensuremath{\Delta}C$ at ${T}_{c}$ at both ${x}_{c1}$ and ${x}_{c2},$ the present work observes also a sharp increase in the normal state specific heat \ensuremath{\gamma} at both ${x}_{c1}$ and ${x}_{c2},$ as well as peaks in $\ensuremath{\Delta}C/\ensuremath{\gamma}{T}_{c}$ at both critical concentrations. The implications for understanding the superconductivity in ${\mathrm{U}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{Be}}_{13}$ are discussed, including the possibility that the second transition for ${x}_{c1}<x<{x}_{c2}$ is caused by the anomalous behavior in $\ensuremath{\gamma}\ensuremath{\propto}N(0)(1+\ensuremath{\lambda}).$