Multi-frequency ESR Study of S = 1/2 Frustration Systems ZnxCu4-x(OH)6Cl2

Multi-frequency ESR measurements of S=1/2 phyrochlore system ZnxCu4-x(OH)(6)Cl-2 (x = 0 similar to 0.5) have been performed using the pulse magnetic field up to 16 T and in the frequency region from 30 GHz to 360 GHz. Temperature dependence ESR measurement of the distorted pyrochlore system (x = 0) from 1.76 K to 265 K shows the linewidth broadening below 20 K, and the coexistence of AFMR modes and the peculiar mode is suggested below 6 K. Moreover, the x dependence measurements at 1.8 K suggest the existence of phase boundaries around x=0.3 and 0.5. Possible model to interpret the result will be presented.