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

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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 two electrons to the host conduction bands and behave as nonmagnetic impurities. The superconducting transition temperature T_{\text{c}} of LaFe1-yNiyAsO0.89F0.11 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 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 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.
2011-08-15