Neutron Scattering Study of the Fe Oxypnictide Superconductors NdFeAsO$_{1-x}$F$_x$ and LaFeAsO$_{0.87}$F$_{0.13}$*

We report the neutron scattering studies of NdFeAsO$_{1-x}$F$_x$ (x=0, 0.2)[1] and LaFeAsO$_{0.87}$F$_{0.13}$[2]. In NdFeAsO, there is a tetragonal to orthorhombic structural transition at Ts$\approx$150 K, where an anomaly in resistivity also occurs. A long range magnetic order with the wave-vector (1/2, 1/2, 0)$_T$ forms below T$_N$=1.96 K. This long range order is dominated by the rare earth Nd ions, however, both the Nd and smaller Fe moments contribute to the antiferromagnetic structure. Neither the magnetic ordering nor the structural distortion occurs in the superconducting samples NdFeAsO$_{0.80}$F$_{0.20}$ and LaFeAsO$_{0.87}$F$_{0.13}$ at temperatures down to 1.6 K. In LaFeAsO$_{0.87}$F$_{0.13}$, no magnetic-resonance peak was observed in the superconducting state at 1.6 K. Two phonon peaks at 12 and 17 meV were observed, consistent with theoretical calculation. Reference: [1] Y. Qiu et al., arXiv:0806.2195 accpeted by PRL(2008) [2] Y. Qiu et al., Phys. Rev. B 78, 052508(2008)