Fractional excitations in the square-lattice quantum antiferromagnet

The quantum square-lattice Heisenberg antiferromagnet (QSLHAF) 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 used polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound Cu(DCOO)$_2\cdot$4D$_2$O (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 wave functions [1]. The excitation continuum is accounted for by the existence of pairs of fractional S = 1/2 quasiparticles that deconfine over intermediate length-scales. Away from the anomalous wavevector, these fractional excitations are bound and form conventional magnons. Our results reveal the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration. [1] B. Dalla Piazza {\it et al.}, to appear in Nature Physics (December 2014)