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