Interplay of thermal and quantum spin fluctuations in the kagome lattice compound herbertsmithite

We present a Raman spectroscopic investigation of the herbertsmithite ZnCu3(OH)(6)Cl-2, a realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. Focusing on its magnetic excitation spectrum we find two components, a high-temperature quasielastic signal and a low temperature, broad maximum. The latter has a linear low-energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a gapless spin liquid ground state.