We report the results of muon-spin relaxation measurements on the low-dimensional antiferromagnet Rb${}_4$Cu(MoO${}_4$)${}_3$. No long-range magnetic order is observed down to 50 mK implying a ratio $T_{\mathrm{N}}/J<0.005$ (where $J$ is the principal exchange strength along the spin chains) and an effective ratio of interchain to intrachain exchange of $|J_{\perp }/J|<2\times {10}^{-3}$, making the material an excellent realization of a one-dimensional quantum Heisenberg antiferromagnet. We probe the persistent spin excitations at low temperatures and find that ballistic spin transport dominates the excitations detected below 0.3 K.

• Received 6 March 2012

DOI:https://doi.org/10.1103/PhysRevB.85.184404