Nuclear spin-spin coupling in La_2-xSr_xCuO₄ studied by stimulated echo decay

We have performed copper nuclear quadrupole resonance experiments in high-temperature superconductors YBa₂Cu₄O₈, YBa₂Cu₃O₇, and La_2-xSr_xCuO₄ (x=0.12 and x=0.15), using the stimulated echo technique which utilizes the rf-pulse sequence π/2-(τ)-π/2-(T-τ)-π/2. The τ and T dependences of the stimulated echo intensity is analyzed by a model that includes the spin-lattice relaxation process (T₁ process) and the fluctuating local field due to nuclear spin-spin coupling. The model gives quantitative account of the experimental results in YBa₂Cu₄O₈ and YBa₂Cu₃O₇, using the known values of 1/T₁ and 1/T_2G, the Gaussian decay rate of the spin-echo intensity. The same model applied to La_2-xSr_xCuO₄ enables us to extract the value of T_2G. Our results indicate that T₁T/T_2G^z shows significant temperature dependence for z=2 but is approximately independent of temperature for z=1. This implies that the dynamic exponent is close to one in La_2-xSr_xCuO₄.