Impact of disorder outside the CuO₂ planes of electron-doped superconductors: Different effect on localization and T_c between Gd and Sm doping

The thermopower S(T), Hall coefficient R_H, and resistivity ρ(T) are studied for Nd_1.85−xM_xCe_0.15CuO_4±δ (M=Gd and Sm) single crystals. Disorder is introduced into the cation sites outside the CuO₂ 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 (Gd³⁺<Sm³⁺<Nd³⁺). Furthermore, for Gd doping the superconducting T_c is strongly depressed with increasing the doping concentration, while for Sm doping T_c is nearly unchanged with increasing Sm content, which implies that the superconducting T_c may be related to the localization and the band structure of the itinerant carriers.