Fig. (4) Comparison of spin density distribution of LaO_1-xF_xFeAs with different composition x, (a) x=0, (b) x=0.125, (c) x=0.25 and (d) x=0.5 model. The polarization on the As site is induced by hybridizing between As-4p and Fe-3d, which like a bridge connecting with two magnetic Fe ions. With the increase of doped concentration, the polarization of As decreases implying the bridge is gradually broken. The yellow and blue balls are Fe ions with spin-up and spin-down respectively, in which isosurfaces corresponding to 0.030e/ų.