The effect of randomness on the quantum spin system Tl_1-xK _xCuCl₃ with x = 0:44 studied by the zero-field muon-spin-relaxation (ZF-μSR) method

Zero-field muon-spin-relaxation (ZF-μSR) measurements were carried out down to 80 mK on the randomness bond system Tl_1-xK _xCuCl₃ with x = 0.44. Time spectra are well fitted by the stretched exponential function exp(-λt)^β. The muon spin relaxation rate λ increases rapidly with decreasing temperature, and β tends to 0.5 at 80 mK. The divergent increase of λ suggests the critical slowing down of the frequency of the Cu-3d spin fluctuations toward a spin frozen state below 80 mK, and the root-exponential-like behavior of the time spectrum indicates that the origin of the relaxation is possibly the spatially-fixed fluctuating dilute moments.