Phase diagram and spin dynamics in volborthite with a distorted kagome lattice.

We report a 51 V-NMR study on a high-quality powder sample of volborthite Cu 3 V 2 O 7 (OH) 2 · 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 1K. The low temperature phase shows anomalies such as a Lorentzian line shape, a 1/T 1 ∞ 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.