Unconventional Suppression of Antiferromagnetic Spin Fluctuation in the High-T_c Cu Oxide La_2-xSr_xCuO₄

Abstract

The doping dependence of low-frequency spin dynamics is discussed from a viewpoint of the self-consistently renormalized spin-fluctuation theory for the experimental planar Cu nuclear spin-lattice relaxation rate 1/T₁ in the doped high-T_c Cu oxides, La_2-xSr_xCuO₄ (x=0.06, 0.15 and 0.20). We report that the crossover temperature T^* of T₁T deviating from the high-T Curie-Weiss law increases with reduced carrier density. We found that the doping dependence of T^* is not consistent with the current spin-fluctuation theories in which the actual metal-insulator transition is regarded as the disorder-to-order transition. The present suppression of the staggered spin fluctuation is an anomaly near half-filling, inclusive of the pseudo spin-gap.