Doping effect of Cu and Ni impurities on the Fe-based superconductor Ba0.6K0.4Fe2As2

Copper and nickel impurities have been doped into the iron pnictide superconductor Ba0.6K0.4Fe2As2. Resistivity measurements reveal that Cu and Ni impurities suppress the superconducting transition temperature T-c with rates of Delta T-c/Cu-1% = -3.5 K and Delta T-c/Ni-1% = -2.9 K, respectively. The temperature dependence of the Hall coefficient RH of these two series of samples shows that both Cu doping and Ni doping can introduce electrons into Ba0.6K0.4Fe2As2. With more doping, the sign of RH gradually changes from positive to negative, and the changing rate of Cu-doped samples is much faster than that of Ni-doped ones. Combining this with the results of first-principles calculations published previously and the nonmonotonic evolution of the Hall coefficient in the low-temperature region, we argue that when more Cu impurities are introduced into Ba0.6K0.4Fe2As2, the removal of Fermi spectral weight in the hole-like Fermi surfaces is much stronger than that in the electron-like Fermi surfaces, which is equivalent to a significant electron doping effect. DC magnetization and the lattice constants analysis reveal that static magnetic moments and notable lattice compression have formed in Cu-doped samples. It seems that superconductivity can be suppressed by the impurities disregarding whether they are magnetic or nonmagnetic in nature. This gives strong support to a pairing gap with a sign reversal, like S+/-. However, the relatively slow suppression rates of Tc show the robustness of superconductivity of Ba0.6K0.4Fe2As2 against impurities, implying that multi-pairing channels may exist in the system. Copyright (C) EPLA, 2013