Theory of itinerant magnetic excitations in the spin-density-wave phase of iron-based superconductors

We argue that salient experimental features of the magnetic excitations in the spin-density-wave phase of iron-based superconductors can be understood within an itinerant model. We identify a minimal model and use a multiband random-phase approximation treatment of the dynamical spin susceptibility. Weakly damped spin waves are found near the ordering momentum and it is shown how they dissolve into the particle-hole continuum. We show that ellipticity of the electron bands accounts for the anisotropy of the spin waves along different crystallographic directions and the spectral gap at the momentum conjugated to the ordering one. We argue that our theory agrees well with the existing neutron scattering data.

Figure 1

(Color online) (a) Calculated Fermi surfaces for the three-band model. The dashed curves refer to the case of $ϵ=0$, i.e., complete nesting; (b) diagrammatic representations of the density-density type interactions (c) and the Dyson equation for the RPA spin susceptibility in the three-band model.

Figure 2

(Color online) Calculated imaginary part of the transverse (upper panel) and longitudinal (lower panel) component of the physical RPA spin susceptibility as a function of frequency and momentum [plotted along $\left(0,\pi \right)\to \left(\pi ,0\right)$ points of the BZ] for (a)–(c) $ϵ=0$ and (b)–(d) $ϵ=0.5$. The color bars refer to the intensity in units of st./eV. For numerical purposes we set the damping constant $\delta =1/300\text{eV}$.

Figure 3

(Color online) Left panel: calculated $\text{Im}{\chi }_{\text{RPA}}^{+-}$ as a function of frequency and momentum [plotted along $\left(0,2\pi \right)\to \left(0,\pi \right)\to \left(\pi ,\pi \right)$ directions of the BZ] for $ϵ=0.5$. The crosses and circles are the measured points taken from Refs. 11, 8, respectively. Right panel: calculated constant energy cuts of $\text{Im}{\chi }^{+-}\left(\mathbf{q},\Omega \right)$ along $\left(0,0\right)\to \left(\pi ,0\right)\to \left(2\pi ,0\right)$ direction (solid curves) for (a) 25, (b) 48, (c) 64, (d) 100, (e) 115, (f) 135, and (g) 144 (all in meV). The experimental data are taken from Ref. 8. The intensity maxima have been adjusted to the experimental values.