High-energy spin excitations in ${\text{BaFe}}_2{\text{As}}_2$ observed by inelastic neutron scattering

We report neutron-scattering measurements of cooperative spin excitations in antiferromagnetically ordered ${\text{BaFe}}_2{\text{As}}_2$, the parent phase of an iron pnictide superconductor. The data extend up to $\sim 100\text{meV}$ and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17 eV. The large characteristic spin-fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.

Figure 1

(Color online) Crystal and magnetic structure of ${\text{BaFe}}_2{\text{As}}_2$. On the left is the crystal structure with the conventional unit cell for the low-temperature orthorhombic $Fmmm$ structure. The three-dimensional AFM ordering of Fe spins is indicated. On the right is a single layer of Fe spins showing the in-plane AFM order and the nearest- and next-nearest-neighbor exchange interactions.

Figure 2

(Color online) Neutron scattering spectra of ${\text{BaFe}}_2{\text{As}}_2$. The data were recorded at a temperature of 7 K with incident neutron energies of (a) 200 meV (logarithmic scale) and (b) 50 meV. The pillars of scattering centered near $Q=1.2{\text{Å}}^{-1}$ and $Q=2.6{\text{Å}}^{-1}$ are caused by highly dispersive collective excitations associated with the antiferromagnetically ordered Fe spins (see Fig. 1). (b) is a higher resolution image showing the magnetic signal emerging from $Q=1.2{\text{Å}}^{-1}$.

Figure 3

(Color online) The left and right panels show a series of constant-energy cuts through the magnon signals at $Q=1.2{\text{Å}}^{-1}$ and $Q=2.6{\text{Å}}^{-1}$, respectively. The data were averaged over the energy ranges indicated. Successive cuts are displaced vertically for clarity. The symbols represent different neutron incident energies: 25 meV (red circles), 50 meV (blue squares), 200 meV (green triangles), and 400 meV (gray inverted triangles). In the left panel the lines are fits to Gaussian peaks on a sloping background. In the right panel the lines are constant-energy cuts (averaged over the same energy ranges as the data) through the powder-averaged spin-wave spectrum calculated with parameters $S{J}_2=2S{J}_{1a}=2S{J}_{1b}=35\text{meV}$, $J_c=0$, $S{K}_{ab}=S{K}_c=0.042\text{meV}$, and $S_{\text{eff}}=0.28$ (see Fig. 4). The inset shows the integrated intensities of the magnon scattering measured at $Q=1.2{\text{Å}}^{-1}$ as a function of energy.

Figure 4

(Color online) Simulation of the powder-averaged spin-wave spectrum of ${\text{BaFe}}_2{\text{As}}_2$. The simulation covers the same $Q$ and energy range as the data in Fig. 2a. The parameters used for the simulation are given in the caption to Fig. 3.