Heat-capacity analysis of a large number ofA15-type compounds

We analyze the low- and medium-temperature specific heat of 25 samples based on eleven $A15\mathrm{}$ binary compounds, with $T_c$'s ranging from less than 0.015 to 18 K. Experimentally determined "moments" of the phonon spectra ($\overline{\omega }$,${\overline{\omega }}_2$,${\omega }_{log}$) are included in the analysis. Values are tabulated for ${\overline{T}}_c$, ${\overline{\alpha }}^2$, $\eta$, $〈I^2〉$, $N_{\mathrm{bs}}\left(E_F\right)$, $M{\overline{\omega }}_2^2$, $H_c\mathrm{}\left(0\right)\mathrm{}$, and $\frac{2\Delta \mathrm{}\left(0\right)\mathrm{}}{k_B{T}_c}$. We note the following: (i) The Debye temperature is generally a bad estimate of ${\omega }_{log}$. (ii) $\lambda$ is governed mainly by the "electronic parameter" $\eta$; $\lambda =0.175\mathrm{}\eta (\mathrm{eV}/{\AA }^2)\pm 0.2$ for all $A15\mathrm{}$ compounds studied. (iii) $\eta$ is proportional to the density of states at the Fermi level and this density of states agrees well with band-structure calculations of Jarlborg in Nb-based compounds. In V-based compounds, the observed bad correlation may reflect the presence of spin fluctuations. (iv) The values for the reduced gap $\frac{2\Delta \mathrm{}\left(0\right)\mathrm{}}{k_B{T}_c}$ range from 3.4 to 4.9 and they are correlated with $\frac{T_c}{{\omega }_{log}}$.