Cu NQR Study of the Spin Dynamics in High-<i>T</i>_cSuperconductor La_2-<i>x</i>Sr_<i>x</i>CuO₄

Abstract

The nuclear spin-lattice relaxation time T 1 of 63 Cu in La 2- x Sr x CuO 4 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, ( T 1 T ) -1 was found to increase markedly with a development of strong antiferromagnetic ( AF ) spin correlation among Cu spins. In normal state, ( T 1 T ) -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/ T 1 below T c has commonly been observed irrespectively of further development of the AF correlation with decreasing Sr content.