NMR studies on iron pnictide superconductors of LaFeAsO _0.89F_0.11 and Ca-Fe-Pt-As

We first show results of (NMR-1/T₁)-T curves of LaFeAsO _1-xF_x samples with various x values. It does not have the coherence peak and absolute values of dlog(1/T₁)/d log(T/T _c) shows the dome shaped x dependence with a maximum at the optimum x value. Considering the T dependent energy broadening of quasi-particles and magnetic susceptibility below T_c, we show that the nonexistence of the coherence peak cannot necessarily be regarded as the evidence for the S _± symmetry of the order parameter. We also present NMR data on the superconducting Ca-Fe-Pt-As system recently found by Kakiya et al. ⁷⁵As-NMR spectra taken for single crystals of the system indicate clear evidence that Pt atoms are doped into the FeAs conducting layers consistently with the results of the structure analyses, indicating that the superconductivity survives even in the existence of strong impurity scattering of the conduction electrons.