Anisotropic spin fluctuations in the quasi one-dimensional frustrated magnet LiCuVO_4

TL;DR

This study uses NMR experiments to investigate anisotropic spin fluctuations and different magnetic orders in the quasi one-dimensional frustrated magnet LiCuVO_4, revealing field-dependent spin dynamics and supporting a bound magnon pair theory.

Contribution

It provides detailed experimental evidence of field-dependent helical and SDW orders and their associated spin fluctuations in LiCuVO_4, supporting the bound magnon pair model.

Findings

Helical order at 4 T with transverse spin fluctuations peak.

SDW order at 10 T with divergent longitudinal fluctuations.

Energy gap in transverse spin excitation spectrum due to bound magnon pairs.

Abstract

We report results of NMR experiments on a single crystal of the quasi one-dimensional frustrated magnet LiCuVO_4. The NMR spectra of ^7Li and ^{51}V nuclei indicate a helical spin order in a magnetic field of 4 T with the helical spin plane perpendicular to the field and a spin-density-wave (SDW) order at 10 T with modulation in the magnitude of the moments aligned along the field, in agreement with earlier reports. The nuclear spin-lattice relaxation rate 1/T_1 at ^{51}V nuclei, which is selectively coupled to the transverse spin fluctuations perpendicular to the field, shows a pronounced peak near the helical ordering temperature in the field of 4 T applied along the a-axis. In the field of 10 T, however, such a peak is absent. Instead 1/T_1 at ^7Li nuclei probing longitudinal spin fluctuations shows divergent behavior towards the SDW ordering temperature. These results are qualitatively consistent with the theoretical description that the SDW correlation is due to bound magnon pairs, which produce an energy gap in the transverse spin excitation spectrum.