Evidence of Two-Spinon Bound States in the Magnetic Spectrum of Ba${}_3$CoSb${}_2$O${}_9$

Abstract

Recent inelastic neutron scattering (INS) experiments of the triangular antiferromagnet Ba${}_3$CoSb${}_2$O${}_9$ revealed strong deviations from semiclassical theories. We demonstrate that key features of the INS data are well reproduced by a parton Schwinger boson theory beyond the saddle-point approximation. The measured magnon dispersion is well reproduced by the dispersion of two-spinon bound states (poles of the emergent gauge fields propagator), while the low-energy continuum scattering is reproduced by a quasifree two-spinon continuum, suggesting that a free-spinon gas is a good initial framework to study magnetically ordered states near a quantum melting point.

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