Chern semi-metal and Quantized Anomalous Hall Effect in HgCr2Se4

Understand

In three dimensional (3D) momentum space of solid crystal, a topological phase boundary separating the Chern insulating layers from normal insulating layers may exist, where the gap must be closed, resulting in a "Chern semi-metal" state with topologically unavoidable band-crossings at fermi level.

• This state, if found to exist, is a condensed-matter realization of chiral fermions (or called Weyl fermions) in (3+1)D, and should exhibit remarkable features, like magnetic monopoles in the bulk and fermi arcs on the surface.
• Here we predict, based on first-principles calculations, that such novel quantum state can be realized in a known ferromagnetic compound HgCr2Se4, with a single pair of Weyl fermions separated in momentum space.
• The characteristic feature of this state in HgCr2Se4 is the presence of quantum Hall effect without external magnetic field in its quantum-well structure.