We report on a heat-capacity study of high-quality single-crystal samples of ${\mathrm{LiCuVO}}_4$—a frustrated spin $S=\frac{1}{2}$ chain system—in a magnetic field amounting to 3/4 of the saturation field. A detailed examination of magnetic phase transitions observed in this field range shows that although the low-field helical state clearly has three-dimensional properties, the field-induced spin-modulated phase turns out to be quasi-two-dimensional. The model proposed in this paper allows one to qualitatively understand this crossover, thus eliminating the presently existing contradictions in the interpretations of NMR and neutron-scattering measurements.

• Received 13 January 2015
• Revised 14 April 2015

DOI:https://doi.org/10.1103/PhysRevB.91.174410