<i>Ab initio</i> overestimation of the topological region in Eu-based compounds

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-principles results obtained earlier for ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ and extended the computational studies to the whole class of systems ${\mathrm{EuCd}}_{2}{\phantom{\rule{4pt}{0ex}}X}_{2}$ ($X=\mathrm{P}$, As, Sb, Bi), to ${\mathrm{EuIn}}_{2}{\phantom{\rule{4pt}{0ex}}X}_{2}$ ($X=\mathrm{P}$, As, Sb), and to nonmagnetic ${\mathrm{AEIn}}_{2}{\mathrm{As}}_{2}$ ($AE=\mathrm{Ca}$, Sr, Ba) employing a hybrid functional method. We find that our approach provides the magnitude of the energy gap for ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ in agreement with the experimental value. Actually, our results indicate that ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}, {\mathrm{BaIn}}_{2}{\mathrm{As}}_{2}, {\mathrm{EuCd}}_{2}{\mathrm{Bi}}_{2}$ and ${\mathrm{EuCd}}_{2}\mathrm{SbBi}$ are robust topological insulators, while all other compounds are topologically trivial semiconductors. The trivial band gaps of ${\mathrm{EuCd}}_{2}{\mathrm{P}}_{2}, {\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$, and ${\mathrm{EuCd}}_{2}{\mathrm{Sb}}_{2}$ 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 ${\mathrm{EuIn}}_{2}{\mathrm{X}}_{2}$ ($X=\mathrm{P}$, As) compounds show altermagnetic exchange-induced band spin-splitting, particularly noticeable in the case of states derived from $5d$-Eu orbitals.

Topics & Concepts