Persistent dynamics in the S=1/2 quasi-one-dimensional chain compound   Rb4Cu(MoO4)3 probed with muon-spin relaxation
  We report the results of muon-spin relaxation measurements on the low-dimensional antiferromagnet Rb4Cu(MoO4)3. No long-range magnetic order is observed down to 50 mK implying a ratio T_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 x 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. 
T. Lancaster, P.J. Baker, F.L. Pratt, S.J. Blundell, W. Hayes and D.   Prabhakaran
cond-mat.str-el