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

TL;DR

This study uses first-principles calculations to determine the atomic and electronic structures of cleaved $A$Fe$_2$As$_2$(001) surfaces, revealing surface terminations, reconstructions, and electronic states relevant to experimental observations.

Contribution

It provides a detailed theoretical analysis of surface structures and electronic properties of $A$Fe$_2$As$_2$ (A=Ba, Sr, Ca) surfaces, explaining experimental STM patterns and surface states.

Findings

Most favorable surface is A-terminated with specific reconstructions.

Surface A atoms can diffuse, affecting surface ordering.

Surface states are present near the Fermi energy.

Abstract

By the first-principles electronic structure calculations, we find that energetically 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 that the cleaving process may destroy the $A$ atoms ordering. At the very low temperatures this may result in an As-terminated surface with the $A$ atoms in randomly assembling. The As-terminated BaFe$_2$As$_2$ surface in orthorhombic phase is $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ buckled, giving rise to a switchable $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ STM pattern upon varying the applied bias. No any reconstruction is found for the other As-terminated surfaces. There are surface states crossing or nearby the Fermi energy in the As-terminated and $(1\times 2)$ $A$-terminated surfaces. A unified physical picture is thus established to help understand the cleaved $A$Fe$_2$As$_2$(001) surfaces.