Phase Separation And Related Properties In La₂CuO₄ System

In this doctoral dissertation,the effects of several dopants on phase separation in La₂CuO₄ system were investigated under the regime of light doping level.First of all,a series of La₂CuO_4+δ samples with light increased oxygen contents were prepared by carefully controlling the preparing process.It was found that there exists a threshold of excess oxygen content for phase separation.Below the threshold,no phase separation was observed,the antiferromagnetic transition peak becomes broader and the Neel temperature becomes lower with the increase of excess oxygen.While phase separation occurs as soon as the excess oxygen content reaches the threshold.In this case,both superconducting phase with Tc～35K and antiferromagnetic phase coexist,in addition,the antiferromagnetic transition temperature goes up again within a certain region.Secondly, the Cu-doping was investigated to understand the effect of antiferromagnetic exchange interaction on phase separation and superconductivity of the system.Magnetization and transport measurements show that the Cu dopant has a role to promote the phase separation quite clearly by enhancing the antiferromagnetic interaction due to the spin of 1/2 for Cu²⁺.Meanwhile,the carder density is also increased slightly for the possible substitution of Cu for La.Furthermore,based on HRTEM and electron diffraction observations,both stripe-like structure in ab plane and incommensurable modulation related to c direction were observed in the Cu-doped La₂CuO_4+δ samples.These modulations,which might be a kind of spin and charge order state,are thought to be related with phase separation.In order to understand both the enhanced antiferromagnetic interaction and the increment of holes are necessary for promoting phase separation in the La₂CuO_4+δ system,the investigation of light Sr-doping was also performed.It was shown that Sr-dopants significantly destroy the antiferromagnetic order,this leads to the phase separation difficult to occur though the Sr-doping can effectively cause the increase of holes in the La₂CuO₄ system.The experimental results also show that the competition between the antiferromagnetic interaction and the long-range Coulomb interaction should decide whether phase separation occurs or not.The most significant feature of phase separation in high-Tc superconductor is that the superconductivity can appear in the case that the average hole density is far lower than that at which the superconducting phase transition occurs.Moreover,the Tc obtained in phase separation state can approach the value corresponding to the optimal hole-doping level.