Ab-initio overestimation of the topological region in Eu-based compounds

TL;DR

This study uses hybrid functional methods to accurately compute band gaps in Eu-based compounds, revealing their topological nature and magnetic properties, and addressing previous underestimations of energy gaps in ab-initio calculations.

Contribution

It provides the first comprehensive computational analysis of EuCd2X2 and related compounds, confirming their topological and magnetic characteristics with improved accuracy.

Findings

EuSn2As2, BaIn2As2, EuCd2Bi2, EuCd2SbBi are topological insulators.

EuCd2P2, EuCd2As2, EuCd2Sb2 are trivial semiconductors with gaps 1.38-1.48 eV, 0.72-0.79 eV, 0.46-0.49 eV.

Magnetic field causes a strong red shift in trivial compounds due to exchange coupling.

Abstract

An underestimation of the fundamental band gap values by the density functional theory within the local density approximation and associated approaches is a well-known challenge of ab-initio electronic structure computations. Motivated by recent optical experiments [D. Santos-Cottin et al., arXiv:2301.08014], we have revisited first-principle results obtained earlier for EuCd2As2 and extended the computational studies to the whole class of systems EuCd2X2 (X = P, As, Sb, Bi), to EuIn2X2 (X = P, As, Sb), and to nonmagnetic AEIn2As2 (AE= Ca, Sr, Ba) employing a hybrid functional method. We find that our approach provides the magnitude of the energy gap for EuCd2As2 in agreement with the experimental value. Actually, our results indicate that EuSn2As2, BaIn2As2, EuCd2Bi2 and EuCd2SbBi are robust topological insulators, while all other compounds are topologically trivial semiconductors. The trivial band gaps of EuCd2P2, EuCd2As2 and EuCd2Sb2 are in the range of 1.38-1.48 eV, 0.72-0.79 eV and 0.46-0.49 eV, respectively. The topologically trivial Eu-based systems are antiferromagnetic semiconductors with a strong red shift of the energy gap in a magnetic field caused by the exchange coupling of the band states to spins localized on the 4f-shell of Eu ions. Additionally, the EuIn2X2 (X = P, As) compounds show altermagnetic exchange-induced band spin-splitting, particularly noticeable in the case of states derived from 5d-Eu orbitals.