Nuclear Quadrupole Resonance in (La_1−xSr_x)₂CuO₄ System

Abstract

Nuclear quadrupole resonance (NQR) of Cu in (La_1−xSr_x)₂CuO₄ system (0.075≤x≤0.15) has been performed where the system is transformed from the high-T_c superconducting to the normal metallic state. The NQR frequency increases and the spectrum is broadened with increasing x. The nuclear spin-lattice relaxation rate 1⁄T₁ shows a remarkable distribution for more than x=0.125. For the nonsuperconducting compound of x=0.15, each component of 1⁄T₁ is suppressed as compared to that in the superconducting compound of x=0.075 which is strongly enhanced by antiferromagnetic spin correlation of Cu-d hole, and is found to obey the Korringa relation, T₁T=const. from 1.4 to 90 K. This dramatic change of 1⁄T₁ is accompanied by the disappearance of T_c. It is concluded that the superconductivity is realized with the background of the A.F. spin correlation.