New electric features in Cu-rich La₂CuO_4+δ system

A series of light oxygen doped Cu-rich La₂CuO_4+δ samples, with δ<0.01 and the nominal composition of cations being La:Cu=2:1+x (0≤x≤0.18), have been investigated. Characterization shows that the samples are single phase till x≥0.10. TEM, electron energy loss spectroscopy, magnetization and electric transport measurements show that the excess Cu (the substitution of Cu²⁺ for La³⁺) can enhance the 3D antiferromagnetic (AF) exchange interaction and promote electronic phase separation. In the AF background the hole-rich cluster caused by phase separation becomes superconducting below 34 K, and shows ferromagnetism from the superconducting transition temperature T_c to above the Neel temperature T_N.