Topological Hopf and chain link semimetal states and their application to Co2MnGa (Theory and Materials Prediction)

TL;DR

This paper introduces a new class of topological semimetals protected by mirror symmetries, featuring complex linked band crossings like Hopf links, and predicts Co2MnGa as a candidate material through first-principles calculations.

Contribution

It proposes a novel mechanism for topological semimetals involving mirror-protected linked band crossings and predicts Co2MnGa as a material realization.

Findings

Identification of Hopf link and chain-like band crossings in Co2MnGa

Protection of topological states by mirror symmetry

Prediction of unconventional surface states

Abstract

Topological semimetals can be classified by the connectivity and dimensionality of the band cross- ing in momentum space. The band crossings of a Dirac, Weyl, or an unconventional fermion semimet- al are 0D points, whereas the band crossings of a nodal-line semimetal are 1D closed loops. Here we propose that the presence of perpendicular crystalline mirror planes can protect 3D band crossings characterized by nontrivial links such as a Hopf link or a coupled-chain, giving rise to a variety of new types of topological semimetals. We show that the nontrivial winding number protects topolog- ical surface states distinct from those in previously known topological semimetals with a vanishing spin-orbit interaction. We also show that these nontrivial links can be engineered by tuning the mirror eigenvalues associated with the perpendicular mirror planes. Using first-principles band structure calculations, we predict the ferromagnetic full Heusler compound Co2MnGa as a candidate. Both Hopf link and chain-like bulk band crossings and unconventional topological surface states are identified.