Combined experimental, theoretical, and computational investigations have identified the origin of outstanding magnetooptical and magnetotransport properties of EuCd₂As₂ and provided topological and magnetic phase diagrams for various classes of Eu-based compounds.

  Computational search for topological electronic material employing the local density approximation demonstrated that 27% of the compounds are topological [Zhang et al. Nature  566, 475 (2019), Vergniory et al. Nature 566, 480 (2019)]. Motivated by those predictions a number of compounds have been grown, investigated, and claimed to be topological. One of them was EuCd₂As_2, in which both ab initio, photoelectron spectroscopy, and magnetotransport studies appeared univocally pointing to the Weyl semimetal band arrangement. However, recent optical and time-resolved ARPES studies (see Figure), carried-out by the Fribourg-Grenoble collaboration and supported by MagTop’s theoretical input, have demonstrated that this compound has a sizable energy gap of 0.8 eV, and its outstanding magnetooptical and magnetotransport properties result from a strong spd-f exchange interaction [1].

Motivated by those findings, MagTop researchers have revisited ab initio results obtained earlier for EuCd₂As₂, and then unveiled the topological phase diagrams for several Eu-based compounds, such as EuCd₂X₂ (X = P, As, Sb, Bi), EuIn₂X₂ (X = P, As, Sb), and AEIn₂As₂ (AE= Ca, Sr, Ba) [2]. By adding the Coulomb repulsion U for 4f electrons and employing the hybrid functionals for electrons in wide bands derived from spd orbitals, the band gap value of EuCd₂X₂ was reproduced (see Figure) and 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 quantitatively explained. Furthermore, it has been shown within this advanced approach that, for several classes of compounds, the topologically trivial region of the phase diagram is wider than previously estimated. It has also been demonstrated that EuIn₂X₂ (X = P, As) compounds show non-relativistic band spin-splitting in the collinear antiferromagnetic phase, i.e., belong to a novel class of altermagnets.