Topological surface states and Andreev bound states in superconducting iron pnictides

TL;DR

This paper investigates the topological surface states and Andreev bound states in superconducting iron pnictides, revealing their robustness and coexistence depending on the superconducting gap structure.

Contribution

It provides a detailed analysis of surface and Andreev bound states in iron pnictides with unconventional superconductivity using exact diagonalization.

Findings

Topological states are robust in the superconducting phase.

Andreev bound states coexist with topological states for small gaps.

Surface states partly remain within the superconducting gap.

Abstract

The nontrivial topology of the electronic structure of iron pnictides can lead to the appearance of surface states. We study such states in various strip geometries with a focus on the superconducting phase. In the presence of unconventional superconducting pairing with $s_\pm$-wave gap structure, the topological states are quite robust and partly remain in the superconducting gap. Furthermore, Andreev bound states appear, which coexist with the topological states for small superconducting gaps and merge with them for larger gap values. The bulk and surface dispersions are obtained from exact diagonalization for two-orbital and five-orbital models in strip geometries.