Superconducting properties ofA−15 V75−xMxGa25compounds:M=Ti,Cr,Mn,Fe,Co,andNi

Superconducting transition temperatures $T_c$ and their pressure derivatives $\frac{d{T}_c}{\mathrm{dp}}$ have been determined for ${\mathrm{V}}_3$Ga based ternary compounds containing $3d$-element substitutions for V. The superconducting transition temperatures decrease while the normalized pressure derivatives, $\left(\frac{1}{T_c}\right)\left(\frac{d{T}_c}{\mathrm{dp}}\right)$ increase systematically for all 3rd element additions. These results suggest that the form of the $T_c$ equation is not affected by rapid variations in the electronic density of states $N\left(E\right)\mathrm{}$, as suggested by some models for superconductors of the $A-15\mathrm{}$-structure type. Analysis of the data in terms of the McMillan $T_c$ equation and the more recent $T_c$ equation of Birnboim show that the electron-phonon coupling parameter $\lambda$ scales uniquely with the electron-per-atom ratio $\mathcal{z}$ and is approximately proportional to the electronic density of states at the Fermi level, $N\left(E_F\right)$. The volume derivative of $\lambda$, as determined from $\frac{d{T}_c}{\mathrm{dp}}$ data, remains approximately constant as the electron-per-atom ratio varies near that of ${\mathrm{V}}_3$Ga ($\mathcal{z}=4.5\mathrm{}$). The only significant difference in the two model $T_c$ equations, i.e., McMillan or Birnboim, is in the absolute values of the parameters $\lambda$ and $\frac{d\ln \lambda }{d\ln V}$, not in systematic variations.