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

Abstract

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.

4 pages, 7 figures, submitted to J. Phys. Soc. Jpn, (modified version)

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• Universal pair-breaking in transition metal-substituted iron-pnictide superconductors
• Fermi-Suface Evolution by Transition-metal Substitution in the Iron-based Superconductor LaFeAsO
• Effect of realistic finite-size impurities on Tc in Fe-based superconductors based on the five-orbital tight-binding model
• Critical Slowing Down of Quadrupole and Hexadecapole Orderings in Iron Pnictide Superconductor
• Progress in nonmagnetic impurity doping studies on Fe-based superconductors
• Superconducting Symmetry Studied from Impurity Effects in Single-Crystal Fe1-yMySe0.3Te0.7 (M = Co, Ni, Zn)
• Evidence of unconventional superconductivity in the Ni-doped NbB2 system