Ghost in the machine: Theory of inelastic neutron scattering in a field-induced spin-nematic state

TL;DR

This paper develops a theoretical framework to identify spin-nematic order via inelastic neutron scattering in frustrated ferromagnets, providing specific predictions for experimental detection in candidate materials like BaCdVO(PO_4)_2.

Contribution

It introduces a soluble spin-1 model and a continuum field theory to predict neutron scattering signatures of spin-nematic states in two-sublattice systems.

Findings

Predicted a ghostly Goldstone mode in neutron scattering data.

Provided quantitative predictions for BaCdVO(PO_4)_2.

Showed feasibility of experimental detection of spin-nematic order.

Abstract

The spin-nematic state has proved elusive, due to the difficulty of observing the order parameter in experiment. In this article we develop a theory of spin excitations in a field-induced spin-nematic state, and use it to show how a spin-nematic order can be indentified using inelastic neutron scattering. We concentrate on 2-dimensional frustrated ferromagnets, for which a two-sublattice, bond-centered spin-nematic state is predicted to exist over a wide range of parameters. First, to clarify the nature of spin-excitations, we introduce a soluble spin-1 model, and use this to derive a continuum field theory, applicable to any two-sublattice spin-nematic state. We then parameterise this field theory, using diagrammatic calculations for a realistic microscopic model of a spin-1/2 frustrated ferromagnet, and show how it can be used to make predictions for inelastic neutron scattering. As an example, we show quantitative predictions for inelastic scattering of neutrons from BaCdVO(PO_4)_2, a promising candidate to realise a spin-nematic state at an achievable h\sim 4T. We show that in this material it is realistic to expect a ghostly Goldstone mode, signalling spin-nematic order, to be visible in experiment.