Bound spinons in an antiferromagnetic S=1/2 chain with a staggered field

TL;DR

This study uses neutron scattering to observe how magnetic excitations in an antiferromagnetic S=1/2 chain evolve under magnetic fields, revealing bound states consistent with the quantum sine-Gordon model.

Contribution

It provides experimental evidence for bound spinon states and their field dependence in a S=1/2 chain, confirming theoretical predictions of the sine-Gordon model.

Findings

Observation of dispersive bound states emerging from a two-spinon continuum.

Agreement of excitation energy ratios with sine-Gordon breather and soliton solutions.

Data supports predicted polarizations and scattering cross sections for solitons and breathers.

Abstract

Inelastic neutron scattering was used to measure the magnetic field dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl_2 2(dimethylsulfoxide) (CDC) in the presence of uniform and staggered fields. Dispersive bound states emerge from a zero-field two-spinon continuum with different finite energy minima at wave numbers q=pi and q_i approx pi (1-2<S_z>). The ratios of the field dependent excitation energies are in excellent agreement with predictions for breather and soliton solutions to the quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a staggered field. The data are also consistent with the predicted soliton and n=1,2 breather polarizations and scattering cross sections.