Search for a spin-nematic phase in the quasi-one-dimensional frustrated magnet LiCuVO₄

Abstract

We have performed nuclear magnetic resonance (NMR) experiments on the quasi-one-dimensional frustrated spin-1/2 system LiCuVO4 in magnetic fields H applied along the c axis up to field values near the saturation field Hsat. For the field range Hc2<H<Hc3 (μ0Hc2≈7.5 T and μ0Hc3=[40.5±0.2]T), the V51 NMR spectra at T=380 mK exhibit a characteristic double-horn pattern, as expected for a spin-modulated phase in which the magnetic moments of Cu2+ ions are aligned parallel to the applied field H and their magnitudes change sinusoidally along the magnetic chains. For higher fields, the V51 NMR spectral shape changes from the double-horn pattern into a single Lorentzian line. For this Lorentzian line, the internal field at the V51 nuclei stays constant for μ0H>41.4 T, indicating that the majority of magnetic moments in LiCuVO4 are already saturated in this field range. This result is inconsistent with the previously observed linear field dependence of the magnetization M(H) for Hc3<H<Hsat with μ0Hsat=45 T [L. E. Svistov et al., JETP Lett. 93, 21 (2011)]. We argue that the discrepancy is due to nonmagnetic defects in the samples. The results of the spin-lattice relaxation rate of Li7 nuclei indicate an energy gap which grows with field twice as fast as the Zeeman energy of a single spin, therefore suggesting that the two-magnon bound state is the lowest-energy excitation. The energy gap tends to close at μ0H≈41 T. Our results suggest that the theoretically predicted spin-nematic phase, if it exists in LiCuVO4, can be established only within the narrow field range 40.5<μ0H<41.4 T.