Excitations in a quantum spin liquid with random bonds
  We present results of inelastic neutron scattering study on two bond disordered quasi twodimensional quantum magnets (C$_4$H$_12$N$_2$)Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ with x=0.035 and 0.075. We observe the increase of spin gap, reduction of magnon bandwidth and a decrease of magnon lifetimes compared to x=0 sample. Additional magnon damping is observed at higher energies away from zone center which is found to follow the density of single particle states. 
D. Huvonen, S. Zhao, G. Ehlers, M. M{\aa}nsson, S. N. Gvasaliya, A.   Zheludev
cond-mat.str-el