Waves in the unseen : theory of spin excitations in a quantum spin nematic

TL;DR

This paper develops a theoretical framework for detecting spin-nematic states in quantum magnets through their characteristic excitations, which are invisible to traditional magnetic probes but observable via inelastic neutron scattering.

Contribution

It introduces an SU(3) non-linear sigma model to describe long-wavelength excitations in spin-nematic states, providing explicit experimental predictions.

Findings

Predicts wave-like excitations detectable by neutron scattering

Identifies symmetry-breaking signatures in spin-nematic states

Provides a theoretical basis for experimental detection of invisible magnetic order

Abstract

The idea that a quantum magnet could act like a liquid crystal, breaking spin-rotation symmetry without breaking time-reversal symmetry, holds an abiding fascination. However, the very fact that spin nematic states do not break time-reversal symmetry renders them "invisible" to the most common probes of magnetism - they do not exhibit magnetic Bragg peaks, a static splitting of lines in NMR spectra, or oscillations in muSR. Nonetheless, as a consequence of breaking spin-rotation symmetry, spin-nematic states do possess a characteristic spectrum of dispersing excitations which could be observed in experiment. With this in mind, we develop a symmetry-based description of long-wavelength excitations in a spin-nematic state, based on an SU(3) generalisation of the quantum non-linear sigma model. We use this field theory to make explicit predictions for inelastic neutron scattering, and argue that the wave-like excitations it predicts could be used to identify the symmetries broken by the otherwise unseen spin-nematic order.