content_image/369362.jpg
<img src='content_image/369362.jpg'>

More recent inelastic neutron scattering measurements on the ferro-pnictide supercon- ductor BaFe$_{2}$$_{−}$$_{x}$Co$_{x}$As$_{2}$ have uncovered low-energy spin-wave excitations that disperse up in energy in an anisotropic fashion away from the wavenumbers that correspond to cSDW order; 15 e.g. ( π/a, 0). Paradoxically, no evidence for long-range magnetic order exists in this ferro-pnictide superconductor. Figure 8 displays a fit of the spin-wave spectrum ob- tained by inelastic neutron scattering on BaFe$_{2}$$_{−}$$_{x}$Co$_{x}$As$_{2}$ to the prediction for the linear spin-wave dispersion of a square lattice of spin-1 iron atoms, (Ω$_{+}$Ω$_{−}$) 1 / $^{2}$, at the quantum critical point that seperates the cSDW phase from hidden ferromagnetic (N´ eel) order [see Eq. (12) and Fig. 2]. The spin per orbital is again set to s = 1 / 2, and the following Heisen- berg exchange coupling constants are used: J$_{0}$$_{c}$ = − 105 . 4 meV, J ‖ ( ⊥ ) 1 = 0, J ⊥ ( ‖ ) 1 = 147 . 2 

