Bare Density of States at the Fermi Level inV3Ga1−xSnx: A Nuclear-Magnetic-Resonance Study

Abstract

The superconducting transition temperature of the pseudobinary $A15$ compounds ${\mathrm{V}}_3{\mathrm{Ga}}_{1-x}{\mathrm{Sn}}_x$ ($0\le x\le 1$) falls sharply from a peak of about 14 °K at $x=0\mathrm{}$ to about 4 °K at $x=1\mathrm{}$. In order to understand the dependence of $T_c$ on the electronic structure, we have measured the spin-lattice relaxation rate, isotropic and axial Knight shift, and electric field gradient of ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ in the normal and superconducting state at frequencies between 8 and 48.5 MHz. The composition dependence of the relaxation rate and the electric field gradient are calculated by a tight-binding model and the theory of Watson, Gossard, and Yafet, respectively. The results are in quantitative agreement with the $d$-subband densities of states based on an interpolation of the augmented-plane-wave band-structure calculations of Mattheiss. Symmetry considerations suggest that the difference in the electron-phonon coupling parameter for ${\mathrm{V}}_3$Ga and ${\mathrm{V}}_3$Sn could be due to an anomalously large phonon renormalization in ${\mathrm{V}}_3$Ga caused by a peak in the $\pi$-subband density at the Fermi level.