Surface structures of ternary iron arsenidesAFe2As2(A=Ba, Sr, or Ca)

Abstract

By the first-principles electronic structure calculations, we find that energetically the most favorable cleaved $A{\text{Fe}}_{2}{\text{As}}_{2}(001)$ surface is $A$-terminated with either $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ or $1\ifmmode\times\else\texttimes\fi{}2$ order. For ${\text{BaFe}}_{2}{\text{As}}_{2}$, the surface Ba atoms are predominantly in a $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ order. A low-temperature cleaving may generate a metastable As-terminated surface. The As-terminated surface in the antiferromagnetic orthorhombic ${\text{BaFe}}_{2}{\text{As}}_{2}$ also takes a $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ order, resulting from the buckling of the surface As atoms and giving rise to a switchable $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ scanning tunneling microscopy (STM) pattern upon varying the applied bias. In a $1\ifmmode\times\else\texttimes\fi{}2$-ordered $A$-terminated surface, the high-density spots in the STM image do not directly correspond to the positions of surface atoms. Our results are consistent with the STM observations and resolve the discrepancy between different experimental groups. The study helps establish a physical picture to understand the cleaved $A{\text{Fe}}_{2}{\text{As}}_{2}$ surfaces.