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The experimental realization of quantum spin liquids is a longsought goal in physics, as they represent new states of matter. Quantum spin liquids cannot be described by the broken symmetries associated with conventional ground states. In fact, the interacting magnetic moments in these systems do not order, but are highly entangled with one another over long ranges1. Spin liquids have a prominent role in theories describing high-transition-temperature superconductors2,3, and the topological properties of these states may have applications in quantum information4. A key feature of spin liquids is that they support exotic spin excitations carrying fractional quantum numbers. However, detailed measurements of these 'fractionalized excitations' have been lacking. Here we report neutron scattering measurements on single-crystal samples of the spin-1/2 kagome-lattice antiferromagnetZnCu3(OD)6Cl2 (also called herbertsmithite), which provide striking evidence for this characteristic feature of spin liquids. At low temperatures, we find that the spin excitations form a continuum, in contrast to the conventional spin waves expected in ordered antiferromagnets. The observation of such a continuum is noteworthy because, so far, this signature of fractional spin excitations has been observed only in one-dimensional systems. The results also serve as a hallmark of the quantum spinliquid state in herbertsmithite.
In a spin liquid, the atomic magnetic moments are strongly correlated but do not order or freeze even in the limit as the temperature, T, goes to zero. Although many types of quantum spin-liquid states exist in theory, a feature that is expected to be common to all is the presence of deconfined spinons as an elementary excitation from the ground state1. Spinons are spin-half (S51/2) quantum excitations into which conventional spin-wave excitations with S51 fractionalize. In one dimension, this phenomenon is well established for the S51/2 Heisenberg antiferromagnetic chain, where spinons may be thought of as magnetic domain boundaries that disrupt Néel order and are free to propagate away fromeach other. In the one-dimensional compound KCuF3, a continuum of spinon excitations has been well characterized using neutron scattering5. In two dimensions, the nature of the spinon excitations is less clear. First, the existence of two-dimensional magnets with a quantumspin-liquid ground state is still amatter of great debate. Second, the various spin-liquid states which are proposed in theory give rise to a variety of spinon excitation spectra, which...