Superconductivity Modulated by Binary Doping in Nd<sub>1-x</sub>Ba<sub>x</sub>FeAsO<sub>1-2x</sub>F<sub>2x</sub>

Binary doping effect is studied for the Fe-based superconductors of Nd_1-xBa_xFeAsO_1-2xF_2x(x = 0.02, 0.05, 0.1, 0.15, 0.2). The X-ray diffractions show that the c-axis lattice constant decreases monotonously with the doping content, in contrast to the little change in the a-axis. Temperature dependences of electric resistivity and magnetic susceptibility reveal that the superconductivity for the studied system emerges at x = 0.1, and enhances together with H_c2(0) as the doping content x increases further. In case of x = 0.2, the superconducting critical temperature reaches as high as 50 K, which is the first demonstration of superconductivity with a high fluorine-doping induced by both electron and hole doping in this family. Negative Hall coefficient (R_H) indicates that electron-type carriers are dominated in the present samples. The complicated temperature dependence of R_H, is believed to arise from a multiband effect together with a complicated scattering, especially at the tem-perature near the T_C