Interacting triplons in frustrated spin ladders: Binding and decay in BiCu$_2$PO$_6$

TL;DR

This paper investigates the complex spin dynamics in BiCu$_2$PO$_6$, revealing that triplon interactions and bound states significantly influence its magnetic excitations, with implications for understanding frustrated quantum magnets.

Contribution

It demonstrates the existence of a two-triplon bound state and clarifies the limitations of hybridization models in explaining dispersion downturns.

Findings

Identification of a two-triplon bound state with singlet character

Hybridization explains high-energy downturns but not low-energy ones

Triplon-triplon interactions are crucial for spin dynamics understanding

Abstract

Establishing a comprehensive model of the rich spin dynamics in BiCu$_2$PO$_6$ has been a challenge over the last decade. Inelastic neutron scattering experiments revealed that its elementary triplons are non-degenerate showing the existence of significant anisotropic spin couplings. Evidence for triplon decay into two triplons has been found, but two prominent downturns in the dispersions eluded an explanation. Level repulsion due to hybridization of single triplons with the continuum of two-triplon scattering states has been proposed as explanation. We show that this concept may explain the weak downturn at higher energies, but fails for the most pronounced downturn at lower energy. In turn, we provide evidence that this downturn is the signature of a two-triplon bound state of essentially singlet character pointing to triplon-triplon interaction as the second crucial ingredient of the spin dynamics in this exemplary system.