From weakly interacting spinons to tightly bound triplons in the frustrated quantum spin-Peierls chain

TL;DR

This study investigates the transformation from fractionalized spinons to bound triplons in a frustrated quantum spin-Peierls chain, revealing how frustration and dimerization influence quasiparticle confinement and spectral properties.

Contribution

It provides the first detailed experimental and theoretical analysis of the transition from spinons to triplons in CuGeO3, highlighting the role of frustration and dimerization.

Findings

Identification of a transition from deconfined spinons to bound triplons.

Observation of a structured two-triplon continuum with a van Hove singularity.

Demonstration that triplon-like excitations persist even with weak dimerization.

Abstract

Fractionalized quasiparticles and their confinement into emergent bound states lie at the heart of modern quantum magnetism. While the evolution into magnonic bound states has been well characterized, experimental insight into the analogous transition to triplons remains limited. Here, using high-resolution neutron spectroscopy and state-of-the-art spin dynamics simulations, we uncover the transformation from weakly interacting spinons to tightly bound triplons in the spin-Peierls compound CuGeO3. Quantitative comparisons between the measured spectra and tensor network simulations reveal substantial next-nearest-neighbor frustration and weak external dimerization, placing the system deep within the spontaneously dimerized regime and near the exactly solvable Majumdar-Ghosh point. We further show an energy- and temperature-dependent evolution between two contrasting quasiparticle regimes: deconfined spinons with markedly suppressed interactions by frustration, and coherent triplonic bound states with no observable spinon degrees of freedom. Remarkably, triplon character 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 challenge the conventional view that robust triplons require strong external dimerization and demonstrate how the interplay between frustration and dimerization can reshape fractionalization and confinement.