Bond Randomness Induced Magnon Decoherence in a Spin-1/2 Ladder Compound

TL;DR

This study investigates how bond randomness from Br substitution affects magnon properties in a spin-1/2 ladder antiferromagnet, revealing temperature-dependent effects and limitations of Matthiessen's rule in low-dimensional systems.

Contribution

It provides the first detailed experimental analysis of magnon decoherence due to bond disorder in a spin-1/2 ladder compound, highlighting the breakdown of classical scattering rules.

Findings

Bond defects cause a blueshift and broadening of magnon excitations.

Magnon-defect scattering is temperature independent at high temperatures.

Temperature dependence of magnon properties indicates crossover from 1D to 2D spin correlations.

Abstract

We have used a combination of neutron resonant spin-echo and triple-axis spectroscopies to determine the energy and linewidth of the magnon resonance in IPA-Cu(Cl$_{0.95}$Br$_{0.05}$)$_3$, a model spin-1/2 ladder antiferromagnet where Br substitution induces bond randomness. We find that the bond defects induce a blueshift, $\delta \Delta$, and broadening, $\delta \Gamma$, of the magnon gap excitation compared to the pure compound. At temperatures exceeding the energy scale of the inter-ladder exchange interactions, $\delta \Delta$ and $\delta \Gamma$ are temperature independent within the experimental error, in agreement with Matthiessen's rule according to which magnon-defect scattering yields a temperature independent contribution to the magnon mean free path. Upon cooling, $\delta \Delta$ and $\delta \Gamma$ become temperature dependent and saturate at values lower than those observed at higher temperature, consistent with the crossover from one-dimensional to two-dimensional spin correlations with decreasing temperature previously observed in pure IPA-CuCl$_3$. These results indicate limitations in the applicability of Matthiessen's rule for magnon scattering in low-dimensional magnets.