FIG. 8: The spin excitation spectra of two pnictide materials are fit to the linear spinwave spectrum expected at a quantum critical point (QCP) that seperates a cSDW from ferromagnetic (N´ eel) hidden order. [See Eq. (12) and Fig. 2.] The wavenumber in the direction perpendicular to the planes of iron atoms corresponds to antiferromagnetic order in both sets of data. (See refs. 14 and $^{15}$.) 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 18  0  50  100  150  200  250  300 (0,0) (1,0) (2,0) (2,1) (2,2) (1,1) (0,0) SPIN-WAVE ENERGY (meV) MOMENTUM ( /2a) J$_{1}$ ||( $^{)}$ = 0, J$_{1}$  (||$^{)}$ = 115.8 meV, J$_{2}$|$^{|}$ = 43.7 meV = J$_{2}$ , J$_{0c}$ = - 57.0 meV CaFe$_{2}$ As$_{2}$ IN cSDW (ZHAO ET AL.) QCP: cSDW TO HIDDEN ORDER  0  50  100  150  200  250  300 (0,0) (1,0) (2,0) (2,1) (2,2) (1,1) (0,0) SPIN-WAVE ENERGY (meV) MOMENTUM ( /2a) J$_{1}$ ||( $^{)}$ = 0, J$_{1}$  (||$^{)}$ = 147.2 meV, J$_{2}$|$^{|}$ = 47.3 meV = J$_{2}$ , J$_{0c}$ = - 105.4 meV SUPERCONDUCTING BaFe$_{2-x}$ Co$_{x}$ As$_{2}$ (HAYDEN ET AL.) QUANTUM CRITICAL POINT: cSDW TO HIDDEN ORDER