Specific heat ofA- 15 and bccMo0.4Tc0.6

Abstract

The specific heat of the $A$ - 15 phase ${\mathrm{Mo}}_{0.4}$ ${\mathrm{Tc}}_{0.6}$ and of the allotropic bcc ($A-2$) phase of the same composition prepared by a rapid-quench technique has been measured between 4 and 20 K. The $A$ - 15 allotrope has an electronic-specific-heat coefficient $\gamma$ of 3.3 mJ/(g-atom ${\mathrm{K}}^2$) and a Debye temperature of 271 K. The bcc phase has a $\gamma$ of 6.8 mJ/(g-atom ${\mathrm{K}}^2$) and a softer lattice, with a Debye temperature of 258 K. The energy gaps of both structures are appreciably higher than those of any other known superconductors, with $\frac{2\Delta }{k{T}_c}$ values of 6.5 for the $A$ - 15 and 6.2 for the bcc compared with the previously known maxima of 4.8 for ${\mathrm{Nb}}_3$Sn and 4.6 for Hg. In addition, the specific-heat discontinuity at $T_c$ for the $A$ - 15 ${\mathrm{Mo}}_{0.4}$ ${\mathrm{Tc}}_{0.6}$ is the largest yet observed, with the electronic superconducting specific heat $C_{\mathrm{es}}$ equal to $5.4\gamma T_c$.