Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors

Abstract

Measurements of the electrical resistivity ρ, Hall coefficient R _H , thermoelectric power S , and the electronic specific heat coefficient γ have been carried out for samples of LnFe _{1- y} M _y AsO _{1- x} F _x (Ln=La, Nd; M=Co, Mn; x =0.11) obtained by doping M atoms into the superconducting LnFeAsO _{1- x} F _x (Ln 1111) system. The NMR longitudinal relaxation rate 1/ T ₁ has also been measured for samples of LaFe _{1- y} Co _y AsO _{1- x} F _x with various x values. Co atoms doped into the superconducting LnFeAsO _{1- x} F _x are nonmagnetic, and the rate of T _c -suppression |d T _c /d x | by Co atoms has been found to be too small to be explained by the pair-breaking effect expected for the S _± superconducting symmetry proposed as the most probable symmetry for the system. This result throws a serious doubt on whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of T _c suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet resistance \(R_{\square}\) exceeds h /4 e ² =6.45 kΩ, and the other is the disappearance (or reduction in the area) of the hole Fermi surfaces around the Γ point in the reciprocal space. The latter mechanism has been observed when the number of electrons increases with increasing Co doping level and the system changes from an “anomalous metal” to an ordinary one. Regarding the two distinct T dependences of the NMR longitudinal relaxation rate 1/ T ₁ of LaFeAsO _{1- x} F _x , (1/ T ₁ ∝ T ⁶ reported by our group in the T region from T _c to ∼0.4 T _c for samples with the highest T _c values with varying x , and 1/ T ₁ ∝ T ^2.5–3.0 observed by many groups in almost the entire T region studied below T _c ), we discuss the origin of such a difference, and show that, at least, the T ^2.5–3.0 -like dependence of 1/ T ₁ cannot be considered as the experimental evidence for the S _± symmetry of Δ.