content_image/372956.jpg
<img src='content_image/372956.jpg'>

the flat La 4 f bands visible at around 3 eV in Fig. 1 do not result into any evident structure in the Im χ$_{0}$ plot, indicating the small hybridization (thereby small matrix element) of these states with Fe and As states. 

At small values of q and at low frequency, the value of Im χ$_{0}$ grows linearly as a function of the frequency and then drops rapidly to zero (resembling linear response susceptibility for the non-interacting electron gas). From Fig. 7(a), we see that this low frequency feature has two components; the position of the first peak grows with q , saturating at a frequency of ∼ 0.4 eV, while the second one grows up to ∼ 1eV finally merging into the main broad peak. This behaviour is the same along the Γ − M − Γ and Γ − X − Γ lines, except for the presence of an extra low frequency structure ( ω ≤ 0 . 3 eV) visible around the M -point. A detailed view of Im χ$_{0}$ ( q , ω ) and the behaviour of Re χ$_{0}$ ( q , ω ) is given in Fig. 7(b) at X and M , both for the undoped and for the doped systems. Even though doping does not significantly change the general shape of both real and imaginary parts of χ$_{0}$ , in the low frequency region, we notice a different behaviour of Im χ$_{0}$ ( q , ω ) at M (see inset of Fig. 7b). For x = 0 the Im χ$_{0}$ grows linearly as a function of ω , while it only starts to grow at a finite value of ω for x = 0 . 14. The reason for this is closely tied to the nesting function; An highly nested FS gives rise to linear behaviour for x = 0. While in the doped case, deterioration of FS nesting at M , only allows finite energy excitations, leading to a finite starting value for 

