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

TL;DR

This paper shows that in the triangular antiferromagnet Ba$_3$CoSb$_2$O$_9$, the magnetic spectrum features two-spinon bound states and a continuum, explained by a parton Schwinger boson theory beyond simple approximations.

Contribution

It introduces a theoretical approach using parton Schwinger boson theory to accurately reproduce experimental neutron scattering data in a frustrated magnet.

Findings

Reproduces magnon dispersion with two-spinon bound states

Matches low-energy continuum scattering with a two-spinon continuum

Supports free-spinon gas as a useful model near quantum melting points

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.