Origin for the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-Tc cuprates

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We report measurements of the anisotropy of the spin echo decay for the inner layer Cu site of the triple layer cuprate, Hg₀.8Re₀.2Ba₂Ca₂Cu₃O₈ (Tc=126 K) in the pseudogap T regime below Tₚg 170 K and the corresponding analysis for their interpretation. As the field alignment is varied, the shape of the decay curve changes from Gaussian (H₀ ∥ c) to single exponential (H₀ ⊥ c). The latter characterizes the decay caused by the fluctuations of adjacent Cu nuclear spins caused by their interactions with electron spins. The angular dependence of the second moment (T₂ₘ⁻² ≡ <Δω²>) deduced from the decay curves indicates that T₂ₘ⁻² for H₀ ∥ c, which is identical to T₂G⁻² (T₂G is the Gaussian component), is substantially enhanced, as seen in the pseudogap regime of the bilayer systems. Comparison of T₂ₘ⁻² between H₀ ∥ c and H₀ ⊥ c indicates that this enhancement is caused by electron spin correlations between the inner and the outer CuO₂ layers. These results provide the answer to the long-standing controversy regarding the opposite T dependences of (T₁T)⁻¹ and T₂G⁻² in the pseudogap regime of bi- and trilayer systems.