Fractional excitations in the square-lattice quantum antiferromagnet

TL;DR

This paper demonstrates the existence of fractional quasiparticles in a 2D square-lattice quantum antiferromagnet through experimental neutron spectroscopy and a Gutzwiller projection theoretical framework, revealing a continuum indicative of deconfined spin-1/2 pairs.

Contribution

It provides the first experimental and theoretical evidence for fractional quasiparticles in a simple 2D antiferromagnet without frustration.

Findings

Observation of an isotropic continuum at the anomaly point

Identification of deconfined fractional spin-1/2 quasiparticles

Recovery of conventional spin-wave spectrum away from the anomaly

Abstract

The square-lattice quantum Heisenberg antiferromagnet displays a pronounced anomaly of unknown origin in its magnetic excitation spectrum. The anomaly manifests itself only for short wavelength excitations propagating along the direction connecting nearest neighbors. Using polarized neutron spectroscopy, we have fully characterized the magnetic fluctuations in the model metal-organic compound CFTD, revealing an isotropic continuum at the anomaly indicative of fractional excitations. A theoretical framework based on the Gutzwiller projection method is developed to explain the origin of the continuum at the anomaly. This indicates that the anomaly arises from deconfined fractional spin-1/2 quasiparticle pairs, the 2D analog of 1D spinons. Away from the anomaly the conventional spin-wave spectrum is recovered as pairs of fractional quasiparticles bind to form spin-1 magnons. Our results therefore establish the existence of fractional quasiparticles in the simplest model two dimensional antiferromagnet even in the absence of frustration.