High-resolution, hard x-ray photoemission investigation of ${\text{BaFe}}_2{\text{As}}_2$: Moderate influence of the surface and evidence for a low degree of $\text{Fe}3d–\text{As}4p$ hybridization of electronic states near the Fermi energy

Photoemission data taken with hard x-ray radiation on cleaved single crystals of the barium parent compound of the $M{\text{Fe}}_2{\text{As}}_2$ pnictide high-temperature superconductor family are presented. Making use of the increased bulk sensitivity upon hard x-ray excitation, and comparing the results to data taken at conventional vacuum ultraviolet photoemission excitation energies, it is shown that the ${\text{BaFe}}_2{\text{As}}_2$ cleavage surface provides an electrostatic environment that is slightly different to the bulk, most likely in the form of a modified Madelung potential. However, as the data argue against a different surface doping level, and the surface-related features in the spectra are by no means as dominating as seen in systems such as ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_x$, we can conclude that the itinerant, near-$E_F$ electronic states are almost unaffected by the existence of the cleavage surface. Furthermore, exploiting the strong changes in photoionization cross section between the Fe and As states across the wide photon energy range employed, it is shown that the degree of energetic overlap between the iron $3d$ and arsenic $4p$ valence bands is particularly small at the Fermi level, which can only mean a very low degree of hybridization between the $\text{Fe}3d$ and $\text{As}4p$ states near and at $E_F$. Consequently, this means that the itinerancy of the charge carriers in this group of materials involves mainly the $\text{Fe}3d\text{-Fe}3d$ overlap integrals with at best a minor role for the $\text{Fe}3d\text{-As}4p$ hopping parameters and that the states which support superconductivity upon doping are essentially of $\text{Fe}3d$ character.

Figure 1

(Color online) Hard x-ray photoemission data from Ba122. All data taken at room temperature. (a) Overview spectrum taken with $h\nu =3000\text{eV}$. The inset shows a scanning tunneling microscopy topograph $\left(150\times 150{\text{Å}}^2\right)$ from room-temperature cleaved Ba122, displaying a clear surface reconstruction that lacks long-range order [taken from Ref. 14]. (b) Zoom of the near valence-band region displaying the $\text{Ba}5p$ (green shaded) and $\text{As}4s$ core levels (red shaded) and an approximate inelastic background in blue. Representative core-level spectra $\left(h\nu =2010\text{eV}\right)$ for all three elements are shown in (c). Displayed are spectra taken in a normal emission geometry (black) and $35°$ off normal (red), the latter decreasing the bulk sensitivity, as the mean-free path length of the electron is determined by the photon energy and the escape depth thus by the emission angle (illustrated schematically in the inset of the rightmost panel). The $\text{Fe}2p_{3/2}$ peak is fitted with a single Doniac-Sunjic line shape, displayed in green. Note that the normal and off normal recorded spectra for As 2p and Fe 2p fall exactly on top of each other, but that the off normal recorded spectrum for Ba 3d has additional spectral weight on the high binding energy side. The inset to the $\text{Fe}2p_{3/2}$ core-level spectrum shows a broadened and background corrected version of this spectrum including the $\text{Fe}2p_{1/2}$ line (blue) together with $\text{Fe}2p$ core lines from metallic Fe(111) taken from Ref. 15 in green. Note the excellent resemblance. For the $\text{Ba}3d_{3/2}$ line shown in the rightmost panel of (c), the difference between normal and off-normal emissions is highlighted in green. Note that the normal and off normal recorded spectra for As 2p and Fe 2p fall exactly on top of each other, but that the off normal recorded spectrum for Ba 3d has additional spectral weight on the high binding energy side.

Figure 2

(Color online) Comparison of x-ray and VUV photoemission valence-band data (a) Valence-band spectra taken with $h\nu =2010\text{eV}$, $T=\text{room}$ temperature and $h\nu =125\text{eV}$, $T=20\text{K}$ (Ref. 19). The inset shows a broadened version of the $h\nu =125\text{eV}$ spectrum, representing the $\text{Fe}3d$ pDOS of the valence band. This curve has been subtracted from the one in the left panel to obtain the $\text{As}4p$ pDOS (shown as the red curve in the inset). (b) Shallow core levels of Ba122 recorded with $h\nu =125\text{eV}$, $T=20\text{K}$. The inset shows a zoom of the $\text{Ba}5p$ and As core levels, with their Lorentzian peak fits (green shaded and red shaded areas, respectively).

Figure 3

(Color online) Surface versus bulk electronic states of Ba122 from comparison of x-ray and VUV photoemission data; (a) $\text{Ba}4d$ (left) and $\text{As}3d$ (right) core levels recorded with $h\nu =140\text{eV}$ (black). The surface $\left(S\right)$, and bulk $\left(B\right)$, contributions have been determined by a fit with four Gaussian-broadened Lorentzians and are shown with blue and green shaded areas, respectively. (b) For comparison, the same core levels recorded with $h\nu =3000\text{eV}$ (red) are shown together with the data from panel (a) (thin black) and Gaussian-broadened versions of the latter (blue) to compensate for the difference in experimental resolution. (c) $\text{Ba}4d$ core levels of cleaved ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_x$ $\left(x=6.75\right)$ taken with different experimental geometries and photon energies in the hard x-ray regime. Data shown courtesy of Maiti and co-workers. Note the large spectral weight of the surface contribution here and the large energy difference between bulk $\left(b\right)$ and surface $\left(s\right)$ states, indicated in the figure with green and blue vertical lines, respectively.