High-energy damping by particle-hole excitations in the spin-wave spectrum of iron-based superconductors

Using a degenerate double-exchange model, we investigate the spin excitation spectra of iron pnictides. The model consists of local spin moments on each Fe site, as well as itinerant electrons from the degenerate $d_{xz}$ and $d_{yz}$ orbitals. The local moments interact with each other through antiferromagnetic $J_1\text{−}{J}_2$ Heisenberg interactions, and they couple to the itinerant electrons through a ferromagnetic Hund coupling. We employ the fermionic spinon representation for the local moments and perform a generalized random-phase approximation calculation on both spinons and itinerant electrons. We find that in the $(\pi ,0)$ magnetically ordered state, the spin-wave excitation at $(\pi ,\pi )$ is pushed to a higher energy due to the presence of itinerant electrons, which is consistent with a previous study using the Holstein-Primakoff transformation. In the paramagnetic state, the particle-hole continuum keeps the collective spin excitation near $(\pi ,\pi )$ at a higher energy even without any $C_4$ symmetry breaking. The implications for recent high-temperature neutron scattering measurements will be discussed.

Figure 1

The temperature dependence of the mean-field order parameters $M_{\mathrm{loc}}$ and $M_{\mathrm{itn}}$.

Figure 2

The temperature dependence of the orbital polarization.

Figure 3

The total spin susceptibility $\text{Im}{\overline{\chi }}_{\mathrm{tot}}^{+-}$ of the degenerate double-exchange model along the path $(0,0)\text{−}(\pi ,0)\text{−}(\pi ,\pi )\text{−}(0,0)$ in the (a) ordered and (b) paramagnetic state. The tight-binding parameters are $t_1=-0.5,t_2=0.65$, and $t_3=t_4=-0.425$, with an itinerant band filling of $n=2.1$. The superexchange couplings are $J_1=0.16$ and $J_2=0.6J_1$. The Hund coupling is $J_H=4$.

Figure 4

The itinerant components of the bare susceptibility $\text{Im}{\chi }_0^{+-}$ in the (a) ordered and (b) paramagnetic state. The particle-hole continuum dampens the spin-wave excitations.

Figure 5

The spin susceptibility along the wave vector $(\pi ,\pi )$ at various temperatures. As temperature increases, excitations around $(\pi ,\pi )$ soften and become more damped.