Probing the Role of Co Substitution in the Electronic Structure of Iron Pnictides

The role of Co substitution in the low-energy electronic structure of $\mathrm{Ca}({\mathrm{Fe}}_{0.944}{\mathrm{Co}}_{0.056}{)}_2{\mathrm{As}}_2$ is investigated by resonant photoemission spectroscopy and density-functional theory. The Co $3d$ state center of mass is observed at 250 meV higher binding energy than that of Fe, indicating that Co possesses one extra valence electron and that Fe and Co are in the same oxidation state. Yet, significant Co character is detected for the Bloch wave functions at the chemical potential, revealing that the Co $3d$ electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and the inadequacy of a rigid-band shift description.

Figure 1

Evolution of the $\mathrm{Ca}({\mathrm{Fe}}_{0.944}{\mathrm{Co}}_{0.056}{)}_2{\mathrm{As}}_2$ RPES upon scanning the photon energy across the [(a) and (b)] Fe and (c) Co $L_3$ absorption edges; the RPES curves, taken in 0.395 eV steps, have been displaced vertically to match the energy scale of the Fe and Co edges shown on the left-hand side of parts (a) and (c). The nonresonant PES signal shown at the bottom of (a) and (c), averaged over 3 spectra at 700 (770) eV for Fe (Co), was subtracted to highlight the RPES behavior. In (b), a false color plot of the spectra from (a), normalized to their total area, is shown; the different characteristics of RPES are emphasized: radiationless Raman Auger scattering, Auger emission, and Fano resonance.

Figure 2

[(a) and (b)] Fano profile observed by cutting the RPES data from Figs. 1a, 1c as a function of photon energy and at fixed binding energy: $E_{\mathrm{B}}=0.41$ and 0.66 eV for Fe and Co, respectively (vertical dashed lines mark the energies of maximum x-ray absorption, 707.1 and 777.3 eV). Solid lines (colored red and blue for Fe and Co, respectively) are a fit to Eq. (1); the corresponding Fano asymmetry parameter $q$ and nonresonant photoemission intensity $I^{\mathrm{PES}}$ are shown in panels (c),(d) and (e),(f), for a fit performed over the binding energy range for which a Fano profile with $q<4$ is observed (the smaller the $q$ value, the larger the asymmetry).

Figure 3

(a) Partial Fe (red) and Co (blue) $3d$ DOS from supercell calculations for pure and 5.6% Co-substituted ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, respectively; the center of gravity for Co is $\sim 0.25\mathrm{eV}$ deeper in binding energy than that for Fe. (b) Difference between Co and Fe DOS (filled area, left axis), and its corresponding integral (blue line, right axis); the latter, when estimated at the Fermi level ($E_{\mathrm{F}}=0$), is a measure of the excess charge associated with Co: $\sim 1$ electron.

Figure 4

(a) Evolution of the Co RPES spectra near $E_{\mathrm{F}}$ upon scanning the photon energy; the curves are displaced vertically to match the energy scale of the Co absorption edge in the left-hand side panel. (b) Integrated intensity of the near-$E_{\mathrm{F}}$ peak vs photon energy, showing a cross-section enhancement starting at the Co $L_3$ edge (dashed line).