Spin dynamics in the high-field phases of volborthite
  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. 
M. Yoshida, K. Nawa, H. Ishikawa, M. Takigawa, M. Jeong, S. Kramer, M.   Horvatic, C. Berthier, K. Matsui, T. Goto, S. Kimur...
cond-mat.str-el