Weakly interacting spinons and tightly bound triplons in the frustrated quantum spin-Peierls chain

Abstract

Observing fractionalized quasiparticles and their confinement into emergent bound states is a key focus of modern quantum magnetism. The quasi-one-dimensional frustrated antiferromagnet CuGeO3 undergoes a spin-Peierls transition to a low-temperature phase of spin singlet dimers. Here we observe an energy-dependent variation of quasiparticle character—from weakly interacting, deconfined spinons at high energy to tightly bound triplons at lower energy. Comparisons between neutron spectroscopy data and tensor network simulations reveal substantial next-nearest-neighbour frustration and weak explicit dimerization from the three-dimensional structure, placing the system in a regime dominated by spontaneous dimerization. The triplon character of the quasiparticles at lower energies persists into the two-particle regime, forming a structured two-triplon continuum with a spectral feature associated with a van Hove singularity at its lower boundary. These findings, whose understanding requires considering both spontaneous and explicit dimerization, demonstrate how the interplay between frustration and dimerization can reshape fractionalization and confinement.