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Nuclear Relaxation Measurements in the Superconducting and Normal States of Some V 3 X Compounds

1967; American Institute of Physics; Volume: 153; Issue: 2 Linguagem: Inglês

10.1103/physrev.153.535

1536-6065

B. G. Silbernagel, M. Weger, W. G. Clark, J. H. Wernick,

Advanced Chemical Physics Studies

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 $\ensuremath{\beta}\ensuremath{-}\mathrm{W}$ crystal structure and the chemical composition ${\mathrm{V}}_{3}X$ ($X=\mathrm{S}\mathrm{i},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{e},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{t},\phantom{\rule{0ex}{0ex}}\mathrm{I}\mathrm{r}$). The temperature region under investigation was between 1.3 and 500\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}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 ${T}_{2}$.