Studies on Superconducting LaFe_1-<i>y</i>Co_<i>y</i>AsO_1-<i>x</i>F_<i>x</i>(<i>x</i>=0.11) –For the Understanding of the Superconducting Symmetry of Fe-Pnictides–

Abstract

NMR longitudinal relaxation rates 1/ T 1 and Knight shifts K have been measured for superconducting samples of LaFe 1- y Co y AsO 1- x F x with y =0.0 and 0.0075 and for a nonsuperconducting metallic sample with y =0.1, where the x values are fixed at 0.11. The temperature ( T ) dependence of 1/ T 1 below the superconducting transition temperature T c has been found to be markedly different from the behavior 1/ T 1 ∝ T 2.5–3.0 reported by many groups in the entire T range measured [(0.1–0.2) T c < T < T c ]. The nonexistence of the coherence peak has also been found. On the basis of the results of the measurements and other kinds of existing data, arguments are presented on the superconducting symmetry of the Fe pnictide systems, where the several points that cannot be easily explained by existing theories, are pointed out. Results of the measurements on the nonsuperconducting metallic samples are also presented.