Field-induced magnetism in the high-Tc superconductor Ca₂₋ₓNaₓCuO₂Cl₂ with flat CuO₂ planes

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The internal magnetic field distribution in a mixed state of a cuprate superconductor, Ca₂₋ₓNaₓCuO₂Cl₂ (T c≃28.5 K, near the optimal doping), was measured by muon spin rotation (μSR) technique up to 60 kOe. The μSR linewidth Λ(B) which exhibits excess broadening at higher fields (B>5 kOe) due to field-induced magnetism (FIM), is described by a relation, Λ(B)∝√B. This suggests that the orbital current and associated quasiparticle excitation plays predominant roles in stabilizing the quasistatic correlation. Moreover, a slowing down of the vortex fluctuation sets in well above T c, as inferred from the trace of FIM observed up to 80 K, and develops continuously without a singularity at T c as the temperature decreases.