Study of Ni-Doping Effect of Specific Heat and Transport Properties

Specific heats and transport quantities of the LaFe1� yNiyAsO0:89F0:11 system have been measured, and the results are discussed together with those reported previously by our group mainly for LaFe1� yCoyAsO0:89F0:11 and LaFeAsO0:89� xF0:11þx systems. The y dependence of the electronic specific heat coefficientcan 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 Tc of LaFe1� yNiyAsO0:89F0:11 becomes zero as the carrier density p (¼ 2y þ 0:11) doped to LaFeAsO reaches its critical value, pc � 0:2. This pc value of � 0:2 is commonly observed for LaFe1� yCoyAsO0:89F0:11 and LaFeAsO0:89� xF0:11þx systems, in which the relations p ¼ x þ 0:11 and p ¼ y þ 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 Tc value. We have not observed appreciable effects of pair breaking, which originates from the nonmagnetic impurity scattering of conduction electrons and strongly suppresses Tc values of systems with sign reversal of the order parameter over the Fermi surface(s). On the basis of the results, the so-called ssymmetry of the order parameter with sign reversal is excluded.