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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(Cl0.95Br0.05)(3), a model spin-1/2 ladder antiferromagnet where Br substitution induces bond randomness. We find that the bond defects induce a blue shift, delta Delta, and broadening, delta Gamma, of the magnon gap excitation compared to the pure compound. At temperatures exceeding the energy scale of the interladder 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-CuCl3. These results indicate limitations in the applicability of Matthiessen's rule for magnon scattering in low-dimensional magnets. DOI: 10.1103/PhysRevB.87.020408