ESR study of the frustrated S=12 chain magnet LiCuVO4 in spiral and spin-modulated phases

We report on the electron spin resonance (ESR) study of ${\mathrm{LiCuVO}}₄$ in spiral and spin-modulated phases ($0<{\ensuremath{\mu}}₀H<10$ T). The measurements have been made on high quality samples that allowed getting rid of absorption lines arising from defects. In the field range of spiral phase ($0<{\ensuremath{\mu}}₀H<7.5$ T) all observed branches of the spectrum can be ascribed to uniform oscillations of planar spin structure. The ESR spectrum observed in spin-modulated phase (${\ensuremath{\mu}}₀H>7.5$ T) has two branches. One of them can be explained by uniform oscillations of spin structure, whereas the second branch, surprisingly demonstrates only a weak dependence on field orientation, which indicates the exchange nature of this mode. Allegedly this mode can be associated with soliton excitations.