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Piazza BD, Mourigal M, Christensen NB, Nilsen GJ, Tregenna-Piggott P, Perring TG, Enderle M, McMorrow DF, Ivanov DA, Rønnow HM. Fractional excitations in the square lattice quantum antiferromagnet. NATURE PHYSICS 2015; 11:62-68. [PMID: 25729400 PMCID: PMC4340518 DOI: 10.1038/nphys3172] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/10/2014] [Accepted: 10/29/2014] [Indexed: 05/30/2023]
Abstract
Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.
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Affiliation(s)
- B Dalla Piazza
- Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland
| | - M Mourigal
- Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland ; Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France ; Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
| | - N B Christensen
- Department of Physics, Technical University of Denmark (DTU), DK-2800 Kgs. Lyngby, Denmark ; Laboratory for Neutron Scattering, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
| | - G J Nilsen
- Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland ; Department of Chemistry, University of Edinburgh, Edinburgh, EH9 3JJ, United Kingdom
| | - P Tregenna-Piggott
- Laboratory for Neutron Scattering, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
| | - T G Perring
- ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, United Kingdom
| | - M Enderle
- Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France
| | - D F McMorrow
- London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1H 0AH, United Kingdom
| | - D A Ivanov
- Institute for Theoretical Physics, ETH Zürich, CH-8093 Zürich, Switzerland ; Institute for Theoretical Physics, University of Zürich, CH-8057 Zürich, Switzerland
| | - H M Rønnow
- Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland ; RIKEN Centre for Emergent Matter Science (CEMS), Wako 351-0198, Japan
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