Phononic Weyl Nodal Straight Lines in High-Temperature Superconductor MgB$_2$

TL;DR

This study predicts topological phononic Weyl nodal lines in MgB₂, revealing unique straight-line features in its phonon spectrum with potential for experimental exploration of topological phonons.

Contribution

First-principles calculations uncover phononic topological Weyl nodal lines in MgB₂, a high-temperature superconductor, highlighting their unique straight-line configuration and topological properties.

Findings

Weyl nodal lines located along high-symmetry boundary of BZ

Surface states form drumhead-like non-trivial states

Breaking mirror symmetry lifts degeneracy to form Weyl points

Abstract

Based on first-principles calculations, we predict that the superconducting MgB$_2$ with a AlB$_2$-type centrosymmetric lattice host the so-called phononic topological Weyl nodal lines (PTWNLs) on its bulk phonon spectrum. These PTWNLs can be viewed as countless Weyl points (WPs) closely aligned along the straight lines in the $-$H-K-H direction within the three-dimensional Brillouin zone (BZ). Their topological non-trivial natures are confirmed by the calculated Berry curvature distributions on the planes perpendicular to these lines. These lines are highly unique, because they exactly locate at the high-symmetry boundary of the BZ protected by the mirror symmetry and, simultaneously, straightly transverse the whole BZ, in different from known classifications including nodal rings, nodal chains or nets, and nodal loops. On the (10$\bar{1}$0) crystal surface, the PTWNLs-induced drumhead-like non-trivial surface states appear within the rectangular area confined by the projected lines of the PTWNLs with opposite chirality. Moreover, when the mirror symmetry is broken, the double-degenerate PTWNLs are further lifted to form a pair WPs with opposite chirality. Our results pave the ways for future experimental study on topological phonons on MgB$_2$ and highlights similar results in a series of isostructural AlB$_2$-type metallic diborides.