Evidence for weak electronic correlations in iron pnictides

Using x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS), charge dynamics at and near the $\text{Fe}L$ edges is investigated in Fe-pnictide materials and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. The best agreement with experiment is obtained using Hubbard parameters $U\lesssim 2\text{eV}$ and $J\approx 0.8\text{eV}$.

Figure 1

(Color online) (a) X-ray absorption spectra of the 55 K $T_c$ ${\text{SmO}}_{0.85}\text{FeAs}$. TEY and TFY intensities of the $\text{Fe}{L}_{2,3}$ edges are plotted as a function of incident photon energy. The difference between TEY and TFY data is mainly because of the self-absorption effect in TFY. (b) RIXS spectra of ${\text{SmO}}_{0.85}\text{FeAs}$ collected with excitation energy across the $\text{Fe}{L}_{2,3}$ absorption peaks. The number on the left stands for the excitation energy corresponding to the number marked in (a), the value of which is marked with the arrows on the spectra. Inset shows the prominent $\text{Fe}{L}_3$ emission peak collected with excitation energy above the $\text{Fe}{L}_3$ absorption edge (No. 2 to 8), they all overlap nicely with the nonresonant spectrum (No. 8). Note that RIXS spectra were normalized to the $\text{Fe}{L}_3$ emission peak for emphasizing the similar lineshape. The vertical dashed lines are guide lines of major tick marks.

Figure 2

(Color online) (a) $\text{Fe}{L}_{2,3}$ XAS spectra of a ${\text{BaFe}}_2{\text{As}}_2$ single crystal. (b) RIXS spectra of ${\text{BaFe}}_2{\text{As}}_2$ collected with excitation energy labeled and marked in (a). Inset shows that all the $\text{Fe}{L}_3$ emission peaks (scaled to the same intensity) collected with excitation energy above $\text{Fe}{L}_3$ absorption edge (No. 2 to 8) overlap with the nonresonant spectrum (No. 8).

Figure 3

(Color online) (a) $\text{Fe}{L}_{2,3}$ XAS spectra of a ${\text{LaFe}}_2{\text{P}}_2$ single crystal. (b) RIXS spectra of ${\text{BaFe}}_2{\text{P}}_2$ collected with excitation energy labeled and marked in (a). Inset shows that all the $\text{Fe}{L}_3$ emission peaks (scaled to the same intensity) collected with excitation energy above $\text{Fe}{L}_3$ absorption edge (No. 2 to 7) overlap with the nonresonant spectrum (No. 7).

Figure 4

(Color online) (a) $\text{Fe}{L}_{2,3}$ XAS spectra of Fe metal. (b) RIXS spectra of Fe metal collected with excitation energy labeled and marked in (a). Inset shows that, with the elastic peak tracking excitation energy, all the $\text{Fe}{L}_3$ emission peaks collected with excitation energy above $\text{Fe}{L}_3$ absorption edge (No. 2 to 7) overlap with the nonresonant spectrum (No. 7), same as that of iron pnictides.

Figure 5

(Color online) (a) $\text{Fe}{L}_{2,3}$ XAS spectra of $\alpha {\text{-Fe}}_2{\text{O}}_3$ powder. (b) RIXS spectra of $\alpha {\text{-Fe}}_2{\text{O}}_3$ collected with excitation energy labeled and marked in (a). (c) Energy-loss features corresponding to $dd$ excitations marked with gray lines.

Figure 6

(a) Comparison of RIXS data on the noted samples at 708 eV excitation energy. All iron pnictides and iron metal show only fluorescent peaks same as the nonresonant spectrum [insets of Figs. 1, 2, 3, 4], while $\alpha {\text{-Fe}}_2{\text{O}}_3$ displays multiple peaks as the signature of $dd$ excitations. (b) Comparison of XAS data on the same five samples, $\alpha {\text{-Fe}}_2{\text{O}}_3$ again displays very different lineshapes due to the crystal-field splitting.

Figure 7

(Color online) Fe-pnictide $L_3$-edge XAS spectra obtained from small cluster diagonalization for a fixed $J\left(e_g\right)=0.8\text{eV}$ and various $U$. A strong Coulomb repulsion tends to suppress the XAS shoulder peak intensity. The arrows sketch the energy separation of the dominant peak and its shoulder. The inset is a plot for the peak/shoulder energy separation versus the on-site repulsion $U$, from which a naive upper bound of $U\sim 2\text{eV}$ is drawn.

Figure 8

(Color online) Fe-pnictide $L_3$-edge XAS spectra obtained from small cluster diagonalization for a fixed $U=8.6\text{eV}$ and two different values of $J\left(e_g\right)$. The XAS shoulder peak is split into a two-peak structure by a larger $J$, indicated by the black arrows.

Figure 9

(Color online) X-ray absorption spectrum: iron $L$-edge multiplet calculation for ${\text{BaFe}}_2{\text{As}}_2$.

Figure 10

(a) The $\text{Fe}{L}_3$ edge XES signal calculated using FEFF for the four different metallic iron-containing materials considered in the experimental section. (b) The $\text{Fe}{L}_{2,3}$ edge XAS calculated using FEFF for the four different iron-containing materials considered in the experimental section.

Figure 11

The angular-momentum-projected LDOS for each of the metallic materials considered in the experimental section. Only the largest angular momentum contributions from each type of atom are shown; the $p$ DOS is shown for As, O, and P; the $d$ DOS is shown for Ba, La, Sm, and Fe. We note that the Fe DOS shown is that of the absorbing atom with the core hole.

Figure 12

(Color online) The iron $L_{2,3}$ edge XAS and XES, calculated using WIEN2K, for three different iron-containing materials.