  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-xF0.11+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 2 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 (=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 LaFe1-yCoyAsO0.89F0.11 and LaFeAsO0.89-xF0.11+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 Tc 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-reversing 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 the
sign-reversing is excluded.
