Search for a spin-nematic phase in the quasi-one-dimensional frustrated magnet LiCuVO${}_4$

Abstract

We have performed NMR experiments on the quasi one-dimensional frustrated spin-1/2 system LiCuVO${}_4$ in magnetic fields $H$ applied along the c-axis up to field values near the saturation field $H_{\mathrm{sat}}$. For the field range $H_{c2}<H<H_{c3}$ ($\hat{I}{¼}_0{H}_{c2}\approx 7.5$T and $\hat{I}{¼}_0{H}_{c3}=[40.5\pm 0.2]$T) the ${}^{51}$V NMR spectra at $T$ = 380mK exhibit a characteristic double-horn pattern, as expected for a spin-modulated phase in which the magnetic moments of Cu${}^{2+}$ ions are aligned parallel to the applied field $H$ and their magnitudes change sinusoidally along the magnetic chains. For higher fields, the ${}^{51}$V NMR spectral shape changes from the double-horn pattern into a single Lorentzian line. For this Lorentzian line, the internal field at the ${}^{51}$V nuclei stays constant for $\hat{I}{¼}_{0}H>41.4$T, indicating that the majority of magnetic moments in LiCuVO${}_4$ are already saturated in this field range. This result is inconsistent with the previously observed linear field dependence of the magnetization $M(H)$ for $H_{c3}<H<H_{\mathrm{sat}}$ with $\hat{I}{¼}_0{H}_{\mathrm{sat}}=45$T [L. E. Svistov {\it et al}., JETP Letters {\bf 93}, 21 (2011)]. We argue that the discrepancy is due to non-magnetic defects in the samples. The results of the spin-lattice relaxation rate of ${}^7$Li 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 $\hat{I}{¼}_{0}H\approx 41$T. Our results suggest that the theoretically predicted spin-nematic phase, if it exists in LiCuVO${}_4$, can be established only within the narrow field range $40.5<\hat{I}{¼}_{0}H<41.4$T .

8 pages, 9 figures