Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain

TL;DR

This paper provides experimental evidence that higher-order spinon states in the quantum Heisenberg antiferromagnetic chain account for nearly all spectral weight, confirming theoretical predictions about multi-spinon excitations.

Contribution

The study accurately measures the full spectral weight in a quantum spin chain, establishing the existence and significance of higher-order spinon states through neutron scattering data.

Findings

Higher-order spinon states account for approximately 29% of spectral weight.

The entire spectral weight is confined within the two-spinon continuum within experimental error.

The lineshape of excitations resembles a scaled two-spinon spectrum, supporting a simple physical model.

Abstract

Assemblies of interacting quantum particles often surprise us with properties that are difficult to predict. One of the simplest quantum many-body systems is the spin 1/2 Heisenberg antiferromagnetic chain, a linear array of interacting magnetic moments. Its exact ground state is a macroscopic singlet entangling all spins in the chain. Its elementary excitations, called spinons, are fractional spin 1/2 quasiparticles; they are created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71% of the spectral weight while higher-order spinon states, yet to be experimentally located, are predicted to participate in the remaining. Here, by accurate absolute normalization of our inelastic neutron scattering data on a compound realizing the model, we account for the full spectral weight to within 99(8)%. Our data thus establish and quantify the existence of higher-order spinon states. The observation that within error bars, the entire weight is confined within the boundaries of the two-spinon continuum, and that the lineshape resembles a rescaled two-spinon one, allow us to develop a simple physical picture for understanding multi-spinon excitations.