Experimental Realization of a 2D Fractional Quantum Spin Liquid

The ground-state ordering and dynamics of the two-dimensional $S=1/2\mathrm{}$ frustrated Heisenberg antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ are explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the resonating valence bond state, arising from pairs of $S=1/2\mathrm{}$ spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.