Electronic structure studies of ${\text{BaFe}}_2{\text{As}}_2$ by angle-resolved photoemission spectroscopy

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of ${\text{BaFe}}_2{\text{As}}_2$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\Gamma$ point and an electron pocket at each of the $X$ points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

Figure 1

(Color online) Brillouin zone of ${\text{BaFe}}_2{\text{As}}_2$ in the tetragonal phase. The presented ARPES data are focused around the $\Gamma$ and around the $X$ point.

Figure 2

(Color online) measuring geometry for the ARPES data taken near the $\Gamma$ point. The entrance slit (ES) of the analyzer, the crystal (C), the vertical (v) and the horizontal (h) linear polarizations of the incoming photons (ph), and the trajectory of the outgoing photoelectrons (e) are shown. The direction of the photons and the photoelectrons define the scattering plane. This scattering plane is almost horizontal for photoelectrons traveling to the center of the entrance slit $\left(k_y=0\right)$.

Figure 3

Band structure calculation for tetragonal paramagnetic ${\text{BaFe}}_2{\text{As}}_2$ for the energy region close to the Fermi level including an assignment of the dominant $\text{Fe}3d$ orbital character of the bands in the coordinate frame $x=a$ and $y=b$. (a) The $\Gamma \text{−}M$ direction. (b) The $\Gamma \text{−}X$ direction.

Figure 4

(Color online) calculated constant energy contours for tetragonal paramagnetic ${\text{BaFe}}_2{\text{As}}_2$ around the $\Gamma$ point for $k_z=0$. $k_x$ is along the $\Gamma \text{−}M$ direction.

Figure 5

(Color online) calculated constant energy contours for tetragonal paramagnetic ${\text{BaFe}}_2{\text{As}}_2$ around the $X$ point for $k_z=0$. $k_{x^{\prime }}$ is along the $\Gamma \text{−}X$ direction.

Figure 6

(Color online) ARPES data around the center of the Brillouin zone ($\Gamma$ point) of ${\text{BaFe}}_2{\text{As}}_2$ (BFA65K crystal) measured with a photon energy $h\nu$. The top line shows the two-dimensional Brillouin zone together with the scanning range (dashed line). It also gives the polarization of the photons (in this case vertical and parallel to $k_y$) and the recording temperature (here 20 K). (a) Momentum distribution maps for different binding energies derived by an integration over an energy range of 10 meV. (b) Spectral intensities as a function of $k_y$ and binding energy for $k_x=0$. (c) Momentum distribution curves for $E-E_F=50$ to $-700\text{meV}$. (d) Energy distribution curves for wave vectors parallel to $k_y$ ranging from $-1$ to $1{\text{Å}}^{-1}$.

Figure 7

(Color online) analogous data as in Fig. 6 but now recorded with a horizontal photon polarization (parallel to $k_x$) and at a temperature $T=300\text{K}$.

Figure 8

(Color online) ARPES momentum distribution curves for $E-E_F=30–150\text{meV}$ near the $\Gamma$ point for $k_x=0$ along the $k_y$ direction for a BFA100K and a BFA65K crystal using horizontally polarized photons with energies $h\nu =50\text{eV}$ and $h\nu =75\text{eV}$, respectively. The solid lines are based on a fit to the data. A geometry was used as depicted in Fig. 7. (a) BFA100K crystal at $T=20\text{K}$. (b) BFA100K crystal at $T=300\text{K}$. (c) BFA65K crystal at $T=20\text{K}$. (d) BFA65K crystal at $T=300\text{K}$.

Figure 9

(Color online) similar data as in Fig. 7 measured with a photon energy $h\nu =75\text{eV}$ but now around the $X$ point and at a temperature was $T=20\text{K}$. Horizontal photon polarization (parallel to $k_{x^{\prime }}$) has been used. The horizontal $k_{x^{\prime }}$ direction is aligned parallel to the $\Gamma \text{−}X$ direction.

Figure 10

(Color online) similar data as in Fig. 9 around the $X$ point but now with the recording temperature $T=300\text{K}$. Again a horizontal photon polarization has been used.

Figure 11

(Color online) similar data as in Fig. 9 taken at a temperature of 20 K but with vertical photon polarization (parallel to $k_{y^{\prime }}$).

Figure 12

(Color online) ARPES data from a BFA100K crystal recorded around the center of the Brillouin zone ($\Gamma$ point) measured with horizontal photon polarization and variable photon energies ranging from $h\nu =50$ to 175 eV. Data are recorded in normal emission at a temperature of $T=20\text{K}$. $k_x$ is parallel to the $\Gamma \text{−}M$ direction.