Physics – Condensed Matter – Superconductivity
Scientific paper
2009-07-17
J. Phys. Soc. Jpn. 79 (2010) No. 1 014710.
Physics
Condensed Matter
Superconductivity
11 pages, 14 figures, accepted for publication in J. Phys. Soc. Jpn., Fig. 14 added
Scientific paper
Measurements of the electrical resistivities, Hall coefficients, thermoelectric powers, electronic specific heat coefficients have been carried out for samples of LnFe1-yMyAsO1-xFx (Ln=La, Nd; M=Co, Mn; x=0.11) obtained by M atom dopings to the superconducting LnFeAsO1-xFx (Ln1111) system. The NMR longitudinal relaxation rates 1/T1 have also been measured for samples of LaFe1-yCoyAsO1-xFx with various x values. Co atoms doped to the superconducting LnFeAsO1-xFx are nonmagnetic, and the Tc-suppression by the Co atoms has been found to be too weak to understand by the pair breaking effect expected for the S+- superconducting symmetry proposed as the most probable one for the system. It throws a serious doubt whether the symmetry is realized in this system. Instead of the pair breaking, two mechanisms of the Tc-suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet resistance exceeds h/4e2=6.45 kohm, and another is the disappearance or reduction of the hole-Fermi-surfaces around the gamma point in the reciprocal space. The latter mechanism has been observed, when the electron number increases with increasing Co-doping level and the system changes from an anomalous metal to an ordinary one. On the two distinct T dependences of 1/T1 of LaFeAsO1-xFx, 1/T1 T6 reported by our group in the T region from Tc to ~0.4 Tc for samples with the highest Tc values with varying x, and 1/T1 T2.5-3.0 observed by many groups in the almost entire T region studied below Tc, we discuss what the origin of the difference is, and show that, at least, the T2.5-3.0-like dependence of 1/T1 cannot be considered as the experimental evidence for the S+- symmetry of the order parameter.
Kobayashi Yoshiaki
Lee San Chul
Miura Yoko
Sato* Masatoshi
Satomi Erika
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