Fractionalized excitations in the spin-liquid state of a kagome-lattice antiferromagnet

• Published in: Nature (London) 2012-12, Vol.492 (7429), p.406-410
• Main Authors: Han, Tian-Heng, Helton, Joel S., Chu, Shaoyan, Nocera, Daniel G., Rodriguez-Rivera, Jose A., Broholm, Collin, Lee, Young S.
• Format: Article
• Language: English
• Subjects: 639/766/119/1001; 639/766/119/997; 639/766/119/999; Antiferromagnetism; Atoms & subatomic particles; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Crystal structure; Electric properties; Electromagnetic waves; Electron spin; Exact sciences and technology; Fourier transforms; Halides; Humanities and Social Sciences; letter; Low temperature; Magnetic properties; Magnetic properties and materials; Magnetically ordered materials: other intrinsic properties; Measurement; multidisciplinary; Neutrons; Physics; Scattering; Science; Spin waves; Studies of specific magnetic materials
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11659

Neutron scattering measurements on single-crystal samples of the mineral herbertsmithite, which is a spin-1/2 kagome-lattice antiferromagnet, provide evidence of fractionalized spin excitations at low temperatures, indicating that the ground state of herbertsmithite may be a quantum spin liquid. Creating a quantum spin liquid Quantum spin liquids are exotic states of matter with atomic magnetic moments that are highly correlated but resist ordering even when cooled to absolute zero. They display remarkable collective behaviour, of potential relevance for understanding high T c superconductivity, and host exotic excitations with fractional quantum numbers. On the downside, conclusive evidence for their existence is still missing. Tian-Heng Han et al . now report an exciting result from neutron scattering measurements on large single crystals of 'herbertsmithite', a two-dimensional frustrated antiferromagnet. Specifically, they observe the emergence of fractional spin excitations at low temperature, which is a hallmark signature of quantum spin liquids. Fractional spin excitations have so far only been seen in one-dimensional systems. The experimental realization of quantum spin liquids is a long-sought 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 ranges 1 . Spin liquids have a prominent role in theories describing high-transition-temperature superconductors 2 , 3 , and the topological properties of these states may have applications in quantum information 4 . 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 antiferromagnet ZnCu 3 (OD) 6 Cl 2 (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. T