  We report single-crystal 51V NMR studies on volborthite Cu3V2O7(OH)2 2H2O,
which is regarded as a quasi-two-dimensional frustrated magnet with competing
ferromagnetic and antiferromagnetic interactions. In the 1/3 magnetization
plateau above 28 T, the nuclear spin-lattice relaxation rate 1/T1 indicates an
excitation gap with a large effective g factor in the range of 4.6-5.9,
pointing to magnon bound states. Below 26 T where the gap has closed, the NMR
spectra indicate small internal fields with a Gaussian-like distribution,
whereas 1/T1 shows a power-law-like temperature dependence in the paramagnetic
state, which resembles a slowing down of spin fluctuations associated with
magnetic order. We discuss the possibility of an exotic spin state caused by
the condensation of magnon bound states below the magnetization plateau.
