Fractional excitations in the square lattice quantum antiferromagnet.
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 fractionalS=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.
Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland.
B Dalla
BD
Piazza
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.
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Mourigal
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.
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Christensen
Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland ; Department of Chemistry, University of Edinburgh, Edinburgh, EH9 3JJ, United Kingdom.
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GJ
Nilsen
Laboratory for Neutron Scattering, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
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P
Tregenna-Piggott
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, United Kingdom.
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Perring
Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France.
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Enderle
London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1H 0AH, United Kingdom.
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McMorrow
Institute for Theoretical Physics, ETH Zürich, CH-8093 Zürich, Switzerland ; Institute for Theoretical Physics, University of Zürich, CH-8057 Zürich, Switzerland.
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DA
Ivanov
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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Rønnow