Spin Dynamics of the Spin-1/2 Kagome Lattice Antiferromagnet   ZnCu_3(OH)_6Cl_2
  We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagome lattice antiferromagnet Zn Cu_3 (OH)_6 Cl_2. The susceptibility indicates a Curie-Weiss temperature of ~ -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law with exponent less than or equal to 1. These results suggest that an unusual spin-liquid state with essentially gapless excitations is realized in this kagome lattice system. 
J.S. Helton, K. Matan, M.P. Shores, E.A. Nytko, B.M. Bartlett, Y.   Yoshida, Y. Takano, A. Suslov, Y. Qiu, J.-H. Chung, D.G....