Field-Induced Freezing of a Quantum Spin Liquid on the Kagome Lattice

We report O-17 NMR measurements in the S = 1/2 (Cu2+) kagome antiferromagnet Herbertsmithite ZnCu3(OH)(6)Cl-2 down to 45 mK in magnetic fields ranging from 2 to 12 T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-Kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments less than or similar to 0.1 mu(B) and gapped excitations. The H - T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field mu H-0(c) = 1.55(25) T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagome Heisenberg antiferromagnet model.