Theory of triplon dynamics in the quantum magnet BiCu$_2$PO$_6$

TL;DR

This paper develops a theoretical model of triplon excitations in the quantum magnet BiCu$_2$PO$_6$, explaining experimental neutron scattering data through a minimal spin Hamiltonian with anisotropic interactions.

Contribution

It introduces a minimal spin Hamiltonian incorporating Dzyaloshinskii-Moriya interactions to explain triplon behavior in BiCu$_2$PO$_6$ based on neutron data.

Findings

Splitting of low energy triplon modes explained by anisotropic interactions.

Magnetic field dependence of triplon dispersions accounted for.

Decay of triplons into two-triplon continuum modeled successfully.

Abstract

We provide a theory of triplon dynamics in the valence bond solid ground state of the coupled spin-ladders modelled for BiCu$_2$PO$_6$. Utilizing the recent high quality neutron scattering data [Nature Physics (2015), DOI: 10.1038/NPHYS3566] as guides and a theory of interacting triplons via the bond operator formulation, we determine a minimal spin Hamiltonian for this system. It is shown that the splitting of the low energy triplon modes and the peculiar magnetic field dependence of the triplon dispersions can be explained by including substantial Dzyaloshinskii-Moriya and symmetric anisotropic spin interactions. Taking into account the interactions between triplons and the decay of the triplons to the two-triplon continuum via anisotropic spin interactions, we provide a theoretical picture that can be used to understand the main features of the recent neutron scattering experimental data.