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Fractional excitations in the square-lattice quantum antiferromagnet

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Piazza B. Dalla, Mourigal M., Christensen N. B., Nilsen G. J., Tregenna-Piggott P., Perring T. G., Enderle M., McMorrow D. F., Ivanov D. A., Ronnow H. M.,
Project Quantum Magnetism - Spinons, Skyrmions and Dipoles
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Original article (peer-reviewed)

Volume (Issue) 11(1)
Page(s) 62 - 68
Title of proceedings NATURE PHYSICS
DOI 10.1038/nphys3172

Open Access

Type of Open Access Repository (Green Open Access)


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 spin-1/2 particles is far from complete. The quantum square-lattice Heisenberg antiferromagnet, 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 wavevector (pi,0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound Cu(DCOO)(2)center dot 4D(2)O, a known realization of the quantum square-lattice Heisenberg antiferromagnet 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 wavevector, 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.