Electronic structure of the ${\text{BaFe}}_2{\text{As}}_2$ family of iron-pnictide superconductors

We use high-resolution angle-resolved photoemission to study the electronic structure of the ${\text{BaFe}}_2{\text{As}}_2$ pnictides. We observe two electron bands and two hole bands near the X point, $\left(\pi ,\pi \right)$ of the Brillouin zone, in the paramagnetic state for electron-doped $\text{Ba}{\left({\text{Co}}_{0.06}{\text{Fe}}_{0.94}\right)}_2{\text{As}}_2$, undoped ${\text{BaFe}}_2{\text{As}}_2$, and hole-doped ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$. Among these bands, only the electron bands cross the Fermi level, forming two electron pockets around X while the hole bands approach but never reach the Fermi level. We show that the band structure of the ${\text{BaFe}}_2{\text{As}}_2$ family matches reasonably well with the prediction of local-density approximation calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.

Figure 1

(Color online) Measurements on ${\text{BaFe}}_2{\text{As}}_2$ taken at 140 K $\left(h\nu =40\text{eV}\right)$. (a) EDCs along the high-symmetry line $\Gamma \text{-X}$ marked in (b). (b) Symmetrized FS map. The red square bounded by –1 to 1 in $k_x$ and $k_y$ marks the 1st BZ. Top left quadrant shows in green dots the outline of the electron pockets obtained from $k_F$ points. Green and magenta lines mark the LDA calculated electron and hole pockets at X and $\Gamma$, respectively, after a nonuniform shift of $E_F$ (down by 0.14 eV at X and up by 0.01 eV at $\Gamma$). (c) LDA bands near X along the $\Gamma \text{-X-}\Gamma$ direction shown in dashed lines, resulting from hybridization of two hole band $\left(h_1,h_2\right)$ and two electron bands $\left(e_1,e_2\right)$.

Figure 2

(Color online) Spectra of ${\text{BaFe}}_2{\text{As}}_2$ taken at 140 K $\left(h\nu =40\text{eV}\right)$ along cuts near X, with positions labeled in Fig. 1b. The light polarization is perpendicular to the $\Gamma \text{-X}$ direction in all cuts. (a) Spectra divided by the Fermi-Dirac function to reveal band dispersions above $E_F$. (b) LDA bands $\left(k_z=\pi \right)$ (Ref. 22) overlaid on corresponding cuts in (a) after a shift of $E_F$ down by 0.14 eV and a bandwidth renormalization of a factor of 1.5. (c) Second derivative plots of (a), overlaid with LDA bands. (d) MDCs of (a) for the corresponding cuts, marking band dispersions.

Figure 3

(Color online) Spectra of ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ $\left(T_c=37\text{K}\right)$ taken at 45 K $\left(h\nu =27\text{eV}\right)$. (a) FS maps around $\Gamma$ and X, overlaid with LDA FS $\left(k_z=0\right)$ (Ref. 22) calculated for ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ after a shift of $E_F$ down by 0.09 eV at X and no shift at $\Gamma$. (b) Spectra along parallel cuts marked in (a). (c) LDA bands $\left(k_z=0\right)$ for corresponding cuts, (see text).

Figure 4

(Color online) Spectra of $\text{Ba}{\left({\text{Co}}_{0.06}{\text{Fe}}_{0.94}\right)}_2{\text{As}}_2$ $\left(T_c=25\text{K}\right)$ taken at 35 K $\left(h\nu =25\text{eV}\right)$. (a) Measured FS, overlaid with LDA FS $\left(k_z=0\right)$ calculated for ${\text{BaFe}}_2{\text{As}}_2$ after a shift of $E_F$ down by 0.13 eV at X and up by 0.03 eV at $\Gamma$. (b) Spectra along cuts marked in (a). (c) LDA bands $\left(k_z=0\right)$ for the corresponding cuts, (see text).