Magnon binding in BaCdVO(PO$_4$)$_2$

TL;DR

This paper provides theoretical evidence that magnons form bound pairs in BaCdVO(PO$_4$)$_2$, supporting the existence of a bond-nematic state just below the saturation magnetic field.

Contribution

It offers a detailed analysis of the magnetic Hamiltonian and demonstrates magnon pairing as a mechanism for bond-nematic order in BaCdVO(PO$_4$)$_2$.

Findings

Magnons condense as bound pairs in the studied material.

Theoretical support for a bond-nematic state near saturation.

Hamiltonian parameters align with experimental neutron scattering data.

Abstract

The bond-nematic state has both long-range ordering of magnetic quadrupoles and the high entanglement typical of spin liquids, and as such can be thought of as a spin liquid crystal. One of the most promising materials in which to find such a state is BaCdVO(PO$_4$)$_2$, in which a magnetically silent phase has been found between a magnetically ordered low-field phase and the high-field polarised phase. Here I study the magnetic Hamiltonian of BaCdVO(PO$_4$)$_2$, and, for the Hamiltonian-parameters determined by fits to inelastic neutron scattering experiments, show that magnons condense out of the polarised state as bound pairs. This is direct theoretical support for the existence of a bond-nematic state just below the saturation field.