Contribution of $\text{Fe}3d$ states to the Fermi level of ${\text{CaFe}}_2{\text{As}}_2$

We present density functional theory (DFT) calculations and soft x-ray spectra (soft x-ray spectroscopy and x-ray photoelectron spectra) measurements of single-crystal ${\text{CaFe}}_2{\text{As}}_2$. The experimental valence-band spectra are consistent with our DFT calculations. Both theory and experiment show that the $\text{Fe}3d$ states dominate the Fermi level and hybridize with $\text{Ca}3d$ states. The simple shape of x-ray photoelectron $\text{Fe}2p$ core level spectrum (without any satellite structure typical for correlated systems) suggests that the $\text{Fe}3d$ electrons are weakly or at most moderately correlated.

Figure 1

XPS survey spectrum of ${\text{CaFe}}_2{\text{As}}_2$.

Figure 2

Summary of spectra for ${\text{CaFe}}_2{\text{As}}_2$. Left panels show the $\text{Fe}{L}_{2,3}$ spectra, right panels the $\text{Ca}{L}_{2,3}$ spectra. Excitation energies for $\text{Fe}{L}_{2,3}$ XES are indicated by arrows in the TFY spectra in the top left panel. Note the presence of second-order $\text{Fe}{L}_{2,3}$ appearing in the $\text{Ca}{L}_{2,3}$ XES; the $\text{Fe}{L}_{2,3}$ nonresonant XES (curve a in the left panel) at half the normal energy scale has been replotted here in gray.

Figure 3

$\text{Fe}{L}_{2,3}$ XES spectra and comparison of the $\text{I}\left(L_2\right)/\text{I}\left(L_3\right)$ ratios (inset) for metallic Fe, LaOFeAs, ${\text{CaFe}}_2{\text{As}}_2$, and correlated FeO. The $\text{I}\left(L_2\right)/\text{I}\left(L_3\right)$ was calculated from the ratios of the integral under the $L_2$ and $L_3$ peaks, respectively.

Figure 4

Calculated DOS for ${\text{CaFe}}_2{\text{As}}_2$ and ${\text{BaFe}}_2{\text{As}}_2$. The dotted lines in the $\text{Fe}3s$, $3p$ DOS plot refer to the $3s$ states increased by a factor of 10. The $y$ axis in the total DOS plot is in units of [states/eV/unit cell].

Figure 5

Comparison of calculated DOS and measured spectra. (a) XPS VB spectra and total DOS (light gray, proportional to [states/eV/unit cell]), the $\text{As}4s$, $4p$ DOS (dark gray, proportional to [states/eV/atom]). (b) $\text{Fe}{L}_3$ peak of $L_{2,3}$ spectra, $\text{Fe}3d$ DOS, best-fit curve, and the four pseudo-Voigt components contributing to the best-fit curve. (c) $\text{Ca}{L}_3$ peak of $L_{2,3}$ spectra, $\text{Ca}3d$ DOS, best-fit curve, and the three pseudo-Voigt components contributing to the best-fit curve. The insets on (b) and (c) show the normalized fit parameter $F^{″}$ (note that the “worst” fit has a normalized $F^{″}$ of 1.0, for 1 component in Fe this is off the scale of the plot) for fits with different numbers of pseudo-Voigt components. The estimated Fermi level is indicated in each plot.