Axionic Instability of Periodic Weyl-Semimetal Superstructures

TL;DR

This paper predicts that Coulomb interactions in Weyl-semimetal superstructures can induce a spontaneous chiral symmetry breaking, leading to a dynamical axion insulator state that stabilizes spiral magnetic order without charge density waves.

Contribution

It introduces a novel mechanism where nesting and Coulomb interactions cause chiral symmetry breaking and axionic states in Weyl semimetals, with potential material realization in magnetically doped Bi2Se3.

Findings

Coulomb interactions induce chiral symmetry breaking in Weyl semimetals.

Formation of a dynamical axion insulator state without charge-density-wave order.

Stabilization of spiral magnetic order through axionic condensation.

Abstract

Weyl-semimetal superstructures with a spiraling position of a pair of Weyl nodes of opposite chirality can host a chiral-symmetry preserving Fermi-arc metal state, where the chirality is carried by cylindrical Fermi surfaces, electron- and hole-like depending on the chirality. The Fermi surfaces nest at vanishing momentum separation (zero nesting vector) at the electron-hole-compensation energy because the nesting is topologically protected by vanishing spatial overlap of any pair of equal-momentum opposite-chirality states. In this work we show that the nesting and Coulomb interaction drive a spontaneous chiral symmetry breaking in such a Fermi arc metal, which leads to a dynamical axion insulator state but without breaking translational symmetry (no charge-density-wave order) as in a conventional Weyl semimetal. As for material realization, we discuss magnetically doped Bi$_2$Se$_3$, for which the Weyl-node positions depend on the order of the magnetic dopands. In this case, the axionic condensation can itself stabilize a spiral order of the magnetization, and hence the spiraling node positions, even if the magnetic interaction is intrinsically ferromagnetic.