Study of a Series ofNb3XCompounds by Nuclear Magnetic Resonance

Abstract

A series of ${\mathrm{Nb}}_{3}X$ ($X=\mathrm{S}\mathrm{n},\phantom{\rule{0ex}{0ex}}\mathrm{A}\mathrm{l},\phantom{\rule{0ex}{0ex}}\mathrm{A}\mathrm{u},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{t},\phantom{\rule{0ex}{0ex}}\mathrm{I}\mathrm{r}$) compounds has been studied by observation of nuclear magnetic resonances between 4.2 and 300 \ifmmode^\circ\else\textdegree\fi{}K. The full quadrupole structure of ${\mathrm{Nb}}^{93}$ NMR was observed by means of pulsed resonance techniques, while $X$-site resonances were also studied with steady-state techniques. Spin components of the magnetic susceptibility and Knight shift were deduced and found to be smaller and less temperature dependent than in the case of the ${\mathrm{V}}_{3}X$ compounds of comparable superconducting transition temperatures. The orbital term of the susceptibility (and the Knight shift) is found to be larger in compounds having higher density of states at the Fermi energy. An attempt is made to account for this correlation in terms of the Labb\'e-Friedel one-dimensional theory of the $d$-band structure. The linear relationship between the electric field gradient and the density of states at the Fermi energy found earlier in the ${\mathrm{V}}_{3}X$ series is not fulfilled in the ${\mathrm{Nb}}_{3}X$ series. An unusual increase in $\frac{1}{({T}_{1}T)}$ slightly above the superconducting transition temperature is observed for ${\mathrm{Al}}^{27}$ nuclei in ${\mathrm{Nb}}_{3}$Al. The possibility that superconducting thermal fluctuations are the source of this effect is considered.