  We have synthesized high-quality single crystals of volborthite, a seemingly
distorted kagome antiferromagnet, and carried out high-field magnetization
measurements up to 74 T and 51V NMR measurements up to 30 T. An extremely wide
1/3 magnetization plateau appears above 28 T and continues over 74 T at 1.4 K,
which has not been observed in previous study using polycrystalline samples.
NMR spectra reveal an incommensurate order (most likely a spin-density wave
order) below 22 T and a simple spin structure in the plateau phase. Moreover, a
novel intermediate phase is found between 23 and 26 T, where the magnetization
varies linearly with magnetic field and the NMR spectra indicate an
inhomogeneous distribution of the internal magnetic field. This sequence of
phases in volborthite bear a striking similarity to those of frustrated spin
chains with a ferromagnetic nearest-neighbor coupling J1 competing with an
antiferromagnetic next-nearest-neighbor coupling J2.
