  Neutron spectroscopy and diffuse neutron scattering on herbertsmithite [ZnCu3
(OH)6Cl2], a near-ideal realisation of the s = 1/2 kagome antiferromagnet,
reveal the hallmark property of a quantum spin liquid; instantaneous
short-ranged antiferromagnetic correlations in the absence of a time-averaged
ordered moment. These dynamic antiferromagnetic correlations are only very
weakly dependent of neutron-energy transfer and temperature, and persist up to
25 meV and 120 K. At low energy transfers a shift of the magnetic scattering to
low Q is observed with increasing temperature, providing evidence of gapless
spinons. It is argued that these observations provide important evidence in
favour of RVB theories of (doped) Mott insulators.
