1745-2473
Fractional excitations in the square-lattice quantum antiferromagnet
Piazza
B. Dalla
Ecole Polytech Fed Lausanne, Lab Quantum Magnetism, CH-1015 Lausanne, Switzerland
Mourigal
M.
Ecole Polytech Fed Lausanne, Lab Quantum Magnetism, CH-1015 Lausanne, Switzerland
Christensen
N. B.
Tech Univ Denmark DTU, Dept Phys, DK-2800 Lyngby, Denmark
Nilsen
G. J.
Ecole Polytech Fed Lausanne, Lab Quantum Magnetism, CH-1015 Lausanne, Switzerland
Tregenna-Piggott
P.
Paul Scherrer Inst, Neutron Scattering Lab, CH-5232 Villigen, Switzerland
Perring
T. G.
Rutherford Appleton Lab, ISIS Facil, Didcot OX11 0QX, Oxon, England
Enderle
M.
Inst Max Von Laue Paul Langevin, F-38042 Grenoble 9, France
Mcmorrow
D. F.
Ivanov
D. A.
ETH, Inst Theoret Phys, CH-8093 Zurich, Switzerland
Rønnow
Henrik M.
2015
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.
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