Muon Spin Relaxation Measurements on a Spin-Ladder Material Na_2Co_2(C_2O_4)_3(H_2O)_2(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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Positive muon spin-relaxation (μ^+SR) measurements on a spin-ladder material Na_2Co_2(C_2O_4)_3(H_2O)_2 are reported. We find that the relaxation rate, λ, under a longitudinal field of 3950 Oe increases with decreasing temperature, T, and is a maximum at T_<peak>≃18K. Below T_<peak>, λ decreases with decreasing T. This finding implies the observation of a spin gap formation by μ^+SR technique. The approach of the exponent β in the relaxation function to 0.5 with decreasing T suggests a freezing of dilute moments. Below about 5K, we observe a sinusoidal oscillation in the time spectrum in zero applied field, which indicates an onset of long-range magnetic order. All these observations give microscopic evidence that the spin-ladder material is close to a quantum critical point.
社団法人日本物理学会の論文
2007-02-15