Existence, Character, and Origin of Surface-Related Bands in the High Temperature Iron Pnictide Superconductor ${\mathrm{BaFe}}_{2-x}{\mathrm{Co}}_x{\mathrm{As}}_2$

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped ${\mathrm{BaFe}}_{2-x}{\mathrm{Co}}_x{\mathrm{As}}_2$ ($x=0.1,0.17$). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating $1/2$ Ba layer accompanied by distortions of the underlying $\mathrm{As}\mathrm{\text{−}}{\mathrm{Fe}}_2\mathrm{\text{−}}\mathrm{As}$ block. As a result, surface-related Fe $3d$ states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the ${\mathrm{BaFe}}_{2-x}{\mathrm{Co}}_x{\mathrm{As}}_2$ system for accurate determination of self-energies using ARPES.

Figure 1

(a) Typical LEED images for $E_0=114$ and 292 eV for ${\mathrm{BaFe}}_{2-x}{\mathrm{Co}}_x{\mathrm{As}}_2$ with $x=0.17$. (b) $E_0=200\mathrm{eV}$ for $x=0.1$. In each image, representative (1,1) spots are highlighted with red circles. All images taken at 17 K. (c) Real-space and $q$-space structures of the $2\times 1$ and $\sqrt{2}\times \sqrt{2}$ structures (green, Ba; purple, As; orange, Fe).

Figure 2

Half integer IV curves for both terminations. Left-hand side: $2\times 1$ termination; right-hand side: $\sqrt{2}\times \sqrt{2}$ termination. The Pendry $R$ factor is indicated for each spot.

Figure 3

Top view (a) and side view of half a slab (b) used in the DFT calculations (green, Ba; purple, As; orange, Fe). The blue shaded area in (b) indicates distorted atomic positions, as obtained from the LEED experiments (green, Ba; purple, As; orange, Fe. Coloring identical to Fig. 1).

Figure 4

(a) Fermi surface of ${\mathrm{BaFe}}_{1.83}{\mathrm{Co}}_{0.17}{\mathrm{As}}_2$ measured with 74 eV, $p$-polarized photons. Indicated in yellow is the 1st Brillouin zone and the $\Gamma$ and $M$ points. The dashed yellow line indicates the cuts shown in Fig. 5. (b) Undoped bulk FS from DFT. (c) Unfolded FS for the $2\times 1$ termination, from DFT using the relaxed atomic positions and virtual crystal approximation doping of 0.17. Dark (red), bulk; light (green), surface; brown, mixture of bulk and surface states. (d) Same as (c), but for the $\sqrt{2}\times \sqrt{2}$ structure.

Figure 5

(a) $I(k,E)$ image taken along the cut indicated in Fig. 4a. On the side we show an energy distribution curve taken along the line indicated by the arrow. The dashed curve indicates the surface state (SS). (b) Same as (a) but taken after cycling in temperature. (c),(d) Calculated band structure for the same momentum window. Note that the energy window is twice as large for the DFT data.