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Original publication:

Title: Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain
Author(s): Martin Mourigal, Mechthild Enderle, Axel Klöpperpieper, Jean-Sébastien Caux, Anne Stunault, Henrik M. Rønnow
As Contributors: Jean-Sébastien Caux
Journal ref.: Nat Phys 9, 435-441
Date: 2013-06-16


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 created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71% of the spectral weight and 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 spin-1/2 Heisenberg antiferromagnetic chain compound, 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 experimental line shape resembles a rescaled two-spinon one with similar boundaries allows us to develop a simple picture for understanding multi-spinon excitations.

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