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

We report ¹⁷O NMR measurements in the S=1/2 (Cu²⁺) kagome antiferromagnet Herbertsmithite ZnCu₃(OH)₆Cl₂ 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 0.1μB and gapped excitations. The H-T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field μ₀Hc=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.