Interacting Triplons in Frustrated Spin Ladders: Binding and Decay in BiCu_2PO_6

Establishing a comprehensive model of the rich spin dynamics in BiCu2PO6 has been a challenge over the last decade. Inelastic neutron scattering experiments revealed that its elementary triplons are nondegenerate, 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 an 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 a triplon-triplon interaction as the second crucial ingredient of the spin dynamics in this exemplary system.