7Liand51VNMR study of the heavy-fermion compoundLiV2O4

Abstract

${}^{7}\mathrm{Li}$ and ${}^{51}\mathrm{V}$ NMR Knight shift, linewidth, spin-lattice, and spin-spin relaxation rate data are reported as a function of temperature $T$ (1.5--800 K and 74--575 K for Li and V nuclei, respectively) in the heavy fermion compound ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$. The ${}^{7}\mathrm{Li}$ Knight shift $K$ exhibits a broad maximum at about 25 K and is nearly constant below 4.2 K, as is the linewidth. The ${}^{51}\mathrm{V}$ Knight shift is negative at 575 K and it decreases strongly as the temperature is lowered. Above \ensuremath{\sim}80 K, both ${}^{7}\mathrm{Li}$ and ${}^{51}\mathrm{V}$ Knight shifts are proportional to the susceptibility and from the slope the hyperfine coupling constant can be obtained for both nuclei. The ${}^{7}\mathrm{Li}$ spin-lattice relaxation rate increases as the temperature is lowered below room temperature reaching a maximum at about 50 K. Below 5 K the relaxation rate decreases linearly with $T$ as for normal metals but with a very high value of the Korringa product ${(T}_{1}{T)}^{\ensuremath{-}1}$. Even so, the ${}^{7}\mathrm{Li}$ Korringa ratio ${K}^{2}{T}_{1}{T/S}_{\mathrm{Li}}$ below $\ensuremath{\sim}$ 10 K is on the order of unity as in conventional metals, thus indicating heavy Fermi liquid behavior. From an analysis of the ${}^{7}\mathrm{Li}$ and ${}^{51}\mathrm{V}$ relaxation rate data we infer an approximate square root temperature dependence of the V local moment spin-relaxation rate at high temperatures $(T\ensuremath{\gtrsim}50$ K). A simplified analysis of the data yields a value of 17 meV for the magnitude of the exchange coupling constant between the local moments and the conduction electrons and 2.3 meV for that between neighboring local moments. Quantitative discrepancies in the ${}^{7}\mathrm{Li}$ and ${}^{51}\mathrm{V}$ relaxation rates at low $T$ from what is expected for a uniform heavy Fermi liquid indicate that effects such as Kondo effect screening of the V local moment and/or dynamical vanadium ${t}_{2g}$ orbital occupation correlation/fluctuation effects may be present in our system.