Spin Dynamics of the Spin-1/2Kagome Lattice AntiferromagnetZnCu3(OH)6Cl2

We have performed thermodynamic and neutron scattering measurements on the $S=1/2$ kagomé lattice antiferromagnet ${\mathrm{ZnCu}}_3(\mathrm{OH}{)}_6{\mathrm{Cl}}_2$. The susceptibility indicates a Curie-Weiss temperature of ${\theta }_{\mathrm{CW}}\simeq -300\mathrm{K}$; 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 temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagomé lattice system.