Study of Ni-Doping Effect of Specific Heat and Transport Properties for LaFe1-yNiyAsO0.89F0.11

Specific heats and transport quantities of the LaFe 1-y Ni y AsO 0.89 F 0.11 system have been measured, and the results are discussed together with those reported previously by our group mainly for LaFe 1-y Co y AsO 0.89 F 0.11 and LaFeAsO 0.89-x F 0.11+x systems. The $y$ dependence of the electronic specific heat coefficient $\gamma$ 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 $T_{\text{c}}$ of LaFe 1-y Ni y AsO 0.89 F 0.11 becomes zero as the carrier density $p$ ($=2y+0.11$) doped to LaFeAsO reaches its critical value, $p_{\text{c}}\sim 0.2$. This $p_{\text{c}}$ value of ${\sim}0.2$ is commonly observed for LaFe 1-y Co y AsO 0.89 F 0.11 and LaFeAsO 0.89-x F 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 $T_{\text{c}}$ value. We have not observed appreciable effects of pair breaking, which originates from the nonmagnetic impurity scattering of conduction electrons and strongly suppresses $T_{\text{c}}$ values of systems with sign reversal of the order parameter over the Fermi surface(s). On the basis of the results, the so-called $s_{\pm}$ symmetry of the order parameter with sign reversal is excluded.