Phase Diagram and Spin Dynamics in Volborthite with a Distorted Kagome Lattice

We report a V-51-NMR study on a high-quality powder sample of volborthite Cu3V2O7(OH)(2) center dot 2H(2)O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagome lattice formed by isosceles triangles. In the magnetic fields below 4.5 T, a sharp peak in the nuclear spin-lattice relaxation rate 1/T-1 accompanied with line broadening revealed a magnetic transition near 1 K. The low temperature phase shows anomalies such as a Lorentzian line shape, a 1/T-1 alpha T behavior indicating dense low-energy excitations, and a large spin-echo decay rate 1/T-2 pointing to unusually slow fluctuations. Another magnetic phase appears above 4.5 T with less anomalous spectral shape and dynamics.