Topologically trivial semiconducting behavior and polaronic effects in antiferromagnetic EuZn$_2$As$_2$ and EuCd$_2$Sb$_2$

TL;DR

This study combines experimental and theoretical methods to show that EuZn$_2$As$_2$ and EuCd$_2$Sb$_2$ are topologically trivial semiconductors with defect-induced polaronic effects explaining their magnetoresistance.

Contribution

It provides the first combined experimental and computational evidence that these Eu-based compounds lack topological surface states and highlights defect-driven polaron formation.

Findings

No topological surface or edge states detected.

Intrinsic defects trap charge carriers.

Defects may lead to magnetic polaron formation.

Abstract

The Eu-based EuA$_2$X$_2$ (A = Zn, Cd, In, Sn; X = P, As, Sb) family of compounds has recently attracted significant attention as a promising platform for exploring magnetic topological materials, with several members either predicted or reported to exhibit nontrivial topological properties. We investigate the previously reported topological semimetals, EuZn$_2$As$_2$ and EuCd$_2$Sb$_2$, using scanning tunneling microscopy and spectroscopy, complemented by various first-principles computational approaches. Through examination of the cleaved surfaces, step-edges, and defect states, we determine the trivial semiconducting behavior in both material systems, with no evidence of topological surface or edge states. These experimental results are consistent with our theoretical analysis revealing the absence of topological band inversion in either system. Our experimental observations also reveal numerous intrinsic defects that trap charge carriers. These defects may facilitate the formation of magnetic polarons, providing a natural explanation for the colossal negative magnetoresistance observed in many of the EuA$_2$X$_2$ material systems.