Study of Ni-doping effect of specific heat and transport properties for LaFe_1-yNiyAsO_0.89F_0.11

Specific heats and transport quantities of the LaFe _1-yNiyAsO_0.89F_0.11 system have been measured, and the results are discussed together with those reported previously by our group mainly for LaFe_1-yCoyAsO_0.89F_0.11 and LaFeAsO_0.89-xF_0.11+x systems. The y dependence of the electronic specific heat coefficient γ can basically be understood by using the rigid-band picture, where Ni ions provide two electrons to the host conduction bands and behave as nonmagnetic impurities. The superconducting transition temperature T_c of LaFe_1-yNi_yAsO _0.89F_0.11 becomes zero as the carrier density p (= 2y + 0:11) doped to LaFeAsO reaches its critical value, p_c ∼ 0:2. This p_c value of ∼0:2 is commonly observed for LaFe _1-yCoyAsO_0.89F_0.11 and LaFeAsO _0.89-xF_0.11+x systems, in which the relations p = x + 0:11 and p = y + 0:11 hold, respectively. As we pointed out previously, the critical value corresponds to the disappearance of the hole-Fermi surface. These results indicate that the carrier number solely determines the T_c value. We have not observed appreciable effects of pair breaking, which originates from the nonmagnetic impurity scattering of conduction electrons and strongly suppresses T_c values of systems with sign reversal of the order parameter over the Fermi surface(s). On the basis of the results, the so-called s± symmetry of the order parameter with sign reversal is excluded.