Interplay between magnetic, metal/insulator and topological phases in Hg Mn Te alloys: prediction of a ferromagnetic Weyl semimetal at x = 0.25.

We report a theoretical investigation of magnetic, electronic, and topological properties of Hg Mn Te alloys. We consider periodic structures with Mn concentrations as x  =  0, 0.25, 0.5, 0.75, and 1. Our hybrid DFT/Hartree-Fock calculations for the bandgaps of antiferromagnetic (ground-state) phases are in good agreement with experiments. The calculations also show that the modification of the magnetic ordering from anti- to ferromagnetic leads to a significant bandgap reduction, resulting in a metal/insulator transition at higher Mn concentrations. We show that a ferromagnetic Weyl semimetal phase is achieved for x  =  0.25, where a single pair of Weyl nodes mirrored by the [Formula: see text] point in the momentum space is observed. The non-trivial topological property of the ferromagnetic HgMnTe is confirmed by the calculation of the chirality of each Weyl node, which are connected by a surface Fermi arc of a semi infinite HgMnTe.