Nuclear Relaxation Measurements in the Superconducting and Normal States of SomeV3 XCompounds

Abstract

The temperature variation of the spin-lattice relaxation time $T_1$ and the spin-spin relaxation time $T_2$ has been measured in five compounds possessing the $\beta -\mathrm{W}$ crystal structure and the chemical composition ${\mathrm{V}}_{3}X$ ($X=\mathrm{S}\mathrm{i},\mathrm{G}\mathrm{a},\mathrm{G}\mathrm{e},\mathrm{P}\mathrm{t},\mathrm{I}\mathrm{r}$). The temperature region under investigation was between 1.3 and 500°K. With the exception of ${\mathrm{V}}_3$Ir, it is found that $\frac{1}{T_{1}T}$ has a temperature dependence for temperatures between 20 and 500°K which indicates that the conduction electrons have an unusually large density of states at the Fermi surface. The effect is greatest for those compounds possessing a high superconducting transition temperature $T_c$ (e.g., ${\mathrm{V}}_3$Si and ${\mathrm{V}}_3$Ga). An additional increase in $\frac{1}{T_{1}T}$ is observed at the temperature of the recently discovered crystal-structure transformation in ${\mathrm{V}}_3$Si; similar increases have also been observed in ${\mathrm{V}}_3$Ge and ${\mathrm{V}}_3$Pt, which indicates that the change in crystal structure has a substantial effect on the electron bands. Measurements of $T_1$ in the superconducting state show the presence of the energy gap. At temperatures below $\frac{1}{4} T_c$, a contribution to the spin-lattice relaxation rate in addition to that expected of a simple energy gap in ${\mathrm{V}}_3$Si is observed. A marked increase in $T_2$ is observed in the superconducting state. Also, a dependence of $T_2$ on the magnitude of the pulsed rf field strength, the magnetic field, and the rate at which the external magnetic field is turned on ($\frac{d{H}_0}{\mathrm{dt}}$) is observed in the superconducting state. An interpretation of these changes in $T_2$ based on microscopic field inhomogeneities caused by a fluxoid structure is presented. Measurements of the ac magnetic susceptibility of ${\mathrm{V}}_3$Si show a dependence on $\frac{d{H}_0}{\mathrm{dt}}$ similar to that of $T_2$.