Fractional excitations in the square-lattice quantum antiferromagnet

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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“…This hybridization is strongly enhanced by a strong attraction between pairs of magnons leading to significant continua. Our interpretation is supported by the noticeable agreement with recent experimental results [16].…”

“…Since the seminal paper by Anderson [13] there are wide-spread activities to seek for fractionalization of magnons into spinons [14][15][16]. By contrast, here we present strong evidence that spinons do not appear as the elementary excitations at any wavevector.…”

“…Here we perform a detailed comparison between the theoretical results and the experimental data from inelastic scattering of polarized neutrons [15,16]. First, we consider the applicability of the simple Heisenberg Hamiltonian (1).…”

Spin wave excitations in the tetragonal double perovskiteSr2CuWO6

“…The low energy dynamics of QSLHAF are well described using linear spin wave theory with quantum corrections. However, inelastic neutron scattering measurements on a range of Cu 2+ QSLHAF have revealed a glaring anomaly at high energy in the vicinity of q = (π,0), where the intensity of the otherwise sharp excitations is completely wiped out [5,[40][41][42][43]. Identifying the origin of this effect is complicated by the presence of additional exchange terms such as electronic ring exchange [5,40] and further neighbor exchange [41], as is also present in Sr 2 CuWO 6 .…”

“…By analogy with one-dimensional (1D) systems, these are referred to as spinons. In 1D spinons have been identified in a number of materials, but observing 2D analogs has proved more challenging until recently [43]. It would therefore clearly be very interesting to measure the excitations in single crystal Sr 2 CuWO 6 .…”