Impact of disorder outside the CuO_2 planes of electron-doped superconductors: Different effect on localization and T_c between Gd and Sm doping

摘要

The thermopower S(T), Hall coefficient R_H, and resistivity ρ(T) are studied for Nd_(1.85-x)M_xCe_(0.15)CuO_(4±δ) (A/=Gd and Sm) single crystals. Disorder is introduced into the cation sites outside the CuO_2 planes in the two systems and its degree is controlled by changing M content. Such doping nominally does not change the doped carrier density, which is confirmed by R_H S(T) is analyzed in terms of a semiempirical model above 120 K for both doping, which assumes the coexistence of a narrow electron band and a broad one. In this model, both the bandwidth for the density of states and the bandwidth of the effective conductivity broaden with increasing x for both doping, while the tendency for localization is increased for Gd doping but nearly unchanged for Sm doping, which could be understood by stronger electronic disorder for Gd doping than that for Sm doping. This may be resulted from the difference in ionic radius (Gd~(3+)