Atomic and electronic structures of ternary iron arsenides $A$Fe$_2$As$_2$(001) surfaces ($A$=Ba, Sr, or Ca)

摘要

By the first-principles electronic structure calculations, we find thatenergetically the most favorable cleaved $A$Fe$_2$As$_2$(001) surface ($A$=Ba,Sr, or Ca) is $A$-terminated with a $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ or$(1\times 2)$ order. The $(1\times 2)$ ordered structure yields a $(1\times 2)$dimerized STM image, in agreement with the experimental observation. The $A$atoms are found to diffuse on the surface with a small energy barrier so thatthe cleaving process may destroy the $A$ atoms ordering. At the very lowtemperatures this may result in an As-terminated surface with the $A$ atoms inrandomly assembling. The As-terminated BaFe$_2$As$_2$ surface in orthorhombicphase is $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ buckled, giving rise to aswitchable $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ STM pattern upon varying theapplied bias. No any reconstruction is found for the other As-terminatedsurfaces. There are surface states crossing or nearby the Fermi energy in theAs-terminated and $(1\times 2)$ $A$-terminated surfaces. A unified physicalpicture is thus established to help understand the cleaved $A$Fe$_2$As$_2$(001)surfaces.