Spin dynamics of S = 1/2 kagome lattice antiferromagnets observed by high-field ESR

Due to the existence of strong spin frustration in a system, the spin dynamics of S= 1/2 kagome lattice antiferromagnet at low temperature has attracted much interest. High-field ESR has been measured on its model substances, Cu 3 V 2 O 7 (OH) 2 · 2H 2 O (volborthite) and BaCu 3 V 2 O 3 (OH) 2 (vesignieite), down to 1.8 K using pulsed magnetic fields up to 16 T. The measurements are performed for 160 and 315 GHz using polycrystalline samples. Although both samples showed the g-shift and the change of linewidth at low temperature, volborthite showed a small gap excitation of the order of 40 GHz (1.9 K) while vesignieite showed a paramagnetic behavior down to 1.9 K. Observed difference will be discussed in connection with the crystal structure, and the possible spin liquid state in vesignieite will be discussed.