Full ferromagnetic saturation of a two-dimensional quantum antiferromagnet

Cs2CuCl4 is a 2D frustrated quantum magnet that has recently been shown to display a very unusual quantum spin liquid state. The excitations are not spin-1 magnons as observed in other un-frustrated 2D quantum magnets but instead are spinons carrying a fractional spin of 1/2, as predicted by a resonating-valence-bond picture. Here we use high magnetic fields to induce a transition from this fractional quantum spin-liquid phase to the fully-polarized phase where spins are ferromagnetically aligned along the field. In this field-induced phase all quantum fluctuations are quenched by the external field and the system is expected to behave like a classical magnet with spin-1 magnon excitations. Measurements are made in fields up to 12 T and temperatures below 0.2 K. Ferromagnetic saturation is observed at the critical field Bc=8.44(1) T∥a. Above Bc the measured excitations lineshapes show well-defined, almost resolution-limited peaks as expected for spin-1 magnons and are gapped throughout the zone. The gap to the lowest energy excitation decreases linearly upon decreasing field and closes at Bc, below which the system orders into a cone phase. This transition provides an opportunity to study how ordered phases arise from the condensation of excitations.