Cu NQR Study of the Spin Dynamics in High-T_c Superconductor La_2−xSr_xCuO₄

Abstract

The nuclear spin-lattice relaxation time T₁ of ⁶³Cu in La_2−xSr_xCuO₄ was measured in the concentration range of x=0.075∼0.15 near the magnetic phase boundary around x=0.05. At low Sr concentration and low temperature, (T₁T)⁻¹ was found to increase markedly with a development of strong antiferromagnetic (AF) spin correlation among Cu spins. In normal state, (T₁T)⁻¹ exhibits a temperature dependence of C⁄(T+θ) associated with the Curie-Weiss law of the staggered susceptibility χ_Q(T) at the zone boundary, Q=(π⁄a, π⁄a), above 60 K and approaches a nearly constant value in a narrow T-region just above T_c. T_c decreases with decreasing θ, becoming zero at x=0.05 when θ is extrapolated to zero. The rapid decrease of 1⁄T₁ below T_c has commonly been observed irrespectively of further development of the AF correlation with decreasing Sr content.