Effects of Zn and Ni substitution on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_(Zn,Ni)_y_O_4_ with x=0.15-0.20 studied by the muon spin relaxation and magnetic susceptibility

摘要

We have investigated effects of Zn and Ni on the Cu-spin dynamics andsuperconductivity from the zero-field muon-spin-relaxation (ZF-muSR) andmagnetic-susceptibility, chi, measurements forLa_2-x_Sr_x_Cu_1-y_(Zn,Ni)_y_O_4_ with x=0.15-0.20, changing y up to 0.10 infine step. In the optimally doped x=0.15, it has been concluded that theformation of a magnetic order requires a larger amount of Ni than that of Zn,which is similar to our previous results of x=0.13. From the estimation ofvolume fractions of superconducting (SC) and magnetic regions, it has beenfound for x=0.15 that the SC region is in rough correspondence to the regionwhere Cu spins fluctuate fast beyond the muSR frequency window for both Zn- andNi-substituted samples. According to the stripe model, it follows that, evenfor x=0.15, the dynamical stripe correlations of spins and holes are pinned andlocalized around Zn and Ni, leading to the formation of the static stripe orderand the suppression of superconductivity. These may indicate an importance ofthe dynamical stripe in the appearance of the high-T_c_ superconductivity inthe hole-doped cuprates. In the overdoped regime of x=0.18 and 0.20, on theother hand, the SC region seems to be in rough correspondence to the regionwhere Cu spins fluctuate fast beyond the muSR frequency window, though itappears that the Cu-spin dynamics and superconductivity are affected by thephase separation into SC and normal-state regions.