Bond randomness induced magnon decoherence in a spin-12ladder compound

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 blue shift, $\ensuremath{\delta}\ensuremath{\Delta}$, and broadening, $\ensuremath{\delta}\ensuremath{\Gamma}$, of the magnon gap excitation compared to the pure compound. At temperatures exceeding the energy scale of the interladder exchange interactions, $\ensuremath{\delta}\ensuremath{\Delta}$ and $\ensuremath{\delta}\ensuremath{\Gamma}$ 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, $\ensuremath{\delta}\ensuremath{\Delta}$ and $\ensuremath{\delta}\ensuremath{\Gamma}$ 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.