NMR study of magnetic excitation in LiVX2 (X = O, S)

Abstract

Magnetic excitation in the two-dimensional triangular lattice LiVO2 and its itinerant analogue LiVS2 are investigated by 51V- and 7Li-NMR experiments. The nuclear spin-lattice relaxation rate 1/T1 on 51V and 7Li nuclei shows thermally activated behavior with different size of spin gap, Δ/kB ∼ 3400 K for LiVO2 and ∼ 1900 K for LiVS2, below the transition temperature Tc from magnetic to non-magnetic ground state. In 51(1/T1(T))/51(1/T1(Tc)) vs. T/Tc plot, the normalized relaxation rate for both materials seems to fall on the same curve. This scaling behavior indicates that the ground states of LiVO2 and LiVS2 are the same, i.e., V3+ (3d2 S = 1) spins form spin singlet with the formation of triangular vanadium clusters, although the replacement of oxygen by sulfur may broaden the band width and largely suppresses Tc from ∼ 500 K in LiVO2 to ∼ 310 K in LiVS2. However, the magnetic properties in the high temperature phase are different between LiVO2 and LiVS2. The value of 7(1/T1T) in LiVS2 becomes smaller with decreasing temperature, which suggests the development of pseudogap above Tc, in qualitatively consistent with the results from the magnetic susceptibility and the small change in entropy at Tc. This is a contrast to the case in LiVO2 where 51(1/T1T) is proportional to 1/T and it is explained by a localized spin model well above Tc.