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

We report a ${}^{51}\mathrm{V}$-NMR study on a high-quality powder sample of volborthite ${\mathrm{Cu}}_3{\mathrm{V}}_2{\mathrm{O}}_7(\mathrm{OH}{)}_2·2{\mathrm{H}}_2\mathrm{O}$, 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 $1/T_1\propto T$ 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.