Orbital symmetry of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ superconductors probed with x-ray absorption spectroscopy

The orbital symmetries of electron-doped iron-arsenide superconductors $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ have been measured with x-ray absorption spectroscopy. The data reveal signatures of $\text{Fe}d$ electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the $\text{Fe}{d}_{\text{xz}+\text{yz}}$ states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied $s$ and $d$ states are found to be in remarkably good agreement with the predictions of standard density-functional theory.

Figure 1

(Color online) $\text{Fe}{L}_{2,3}$ edge and $\text{O}K$ edge XAS spectra illustrating the effects of surface contamination. (a) $\text{Fe}{L}_{2,3}$ edge XAS spectra measured in TEY at 200 K for a pristine-oxide surface and a freshly cleaved surface in UHV $\left(5\times {10}^{-8}\text{Torr}\right)$ of the ${\text{BaFe}}_2{\text{As}}_2$ parent compound. The incident beam is at grazing incidence $\left(\theta =25°\right)$ to the surface with the electric field polarized in the H and V directions, as denoted by the double headed arrows in the inset. The $x$ and $y$ axes denote the direction of the Fe-Fe bonds. The presence of an additional Fe-oxide peak visible in the pristine sample at 710 eV is indicative of surface contamination. (b) XAS $\text{O}K$ edge spectra taken on a ${\text{BaFe}}_{1.8}{\text{Co}}_{0.2}{\text{As}}_2$ sample freshly cleaved in UHV $\left(2\times {10}^{-9}\text{Torr}\right)$ at room temperature, but still showing occurrence of contamination. The presence of the $\text{O}K$ edge signal indicates an immediate interaction of the freshly cleaved surface in UHV with molecules containing O in the vacuum system (see text), a token of the high reactivity of the surfaces of these compounds, even when freshly exposed in UHV at pressures in the low ${10}^{-9}\text{Torr}$ range.

Figure 2

(Color online) $\text{Fe}{L}_3$ edge XAS spectra measured in TEY for ${\text{BaFe}}_2{\text{As}}_2$ surfaces freshly cleaved in UHV $\left(3\times {10}^{-10}\text{Torr}\right)$ and comparison with theory. The insets show the schematic layout of the experimental geometries. The incident beam is at normal $\left(\theta =90°\right)$ and grazing incidence $\left(\theta =25°\right)$ to the surface with H and V polarizations, as denoted by the double headed arrows. The $x$ and $y$ axes denote the direction of the Fe-Fe bonds. The thick black line denotes the XAS spectrum calculated using orientation averaged matrix elements and DOS calculated with DFT without magnetism. The curve denoted as “V-H” is the dichroic signal obtained by subtracting the H polarization spectra from the V polarization spectra at grazing incidence. The error bar of the amplitude of the dichroic signal, roughly equal to twice the noise level in the XAS spectra, is indicated on the right of the V-H curve. Bottom: orbital projections of the $\text{Fe}s$ and $\text{Fe}d$ p-DOS calculated with DFT. The $\text{As}{p}_{\text{x}+\text{y}}$ states are also indicated to illustrate the hybridization with the $\text{Fe}s$ and $\text{Fe}{d}_{\text{xz}+\text{yz}}$ states.

Figure 3

(Color online) $\text{As}{L}_3$ edge XAS spectra measured in TEY for ${\text{BaFe}}_2{\text{As}}_2$ surfaces freshly cleaved in UHV $\left(5\times {10}^{-8}\text{Torr}\right)$ and comparison with theory. The insets denote the experimental geometries, as in Fig. 2. The thick black line denotes the XAS spectrum calculated using orientation averaged matrix elements and DOS calculated with DFT without magnetism. The curve denoted as V-H is the dichroic signal. Bottom: orbital projections of the $\text{As}s$ and $\text{As}d$ p-DOS calculated with DFT. The error bar of the amplitude of the dichroic signal, roughly equal to twice the noise level in the XAS spectra, is indicated on the right of the V-H curve.