Magnetic catalysis and axionic charge-density-wave in Weyl semimetals

TL;DR

This paper investigates how magnetic fields induce charge-density-wave order in Weyl and Dirac semimetals, leading to axionic phases with unique topological and spectral properties, and explores the competition with spin-density-wave order.

Contribution

It demonstrates the emergence of axionic charge-density-wave phases in Weyl and Dirac semimetals under magnetic fields and analyzes their spectral gaps, susceptibilities, and topological features.

Findings

Spectral gap scales with weak interactions and magnetic field.

Charge-density-wave order is favored over spin-density-wave in weak coupling.

Topological phenomena like axion strings and surface Hall conductivity are discussed.

Abstract

Three-dimensional Weyl and Dirac semimetals can support a chiral-symmetry-breaking, fully gapped, charge-density-wave order even for sufficiently weak repulsive electron-electron interactions, when placed in strong magnetic fields. In the former systems, due to the natural momentum space separation of Weyl nodes the ordered phase lacks the translational symmetry and represents an axionic phase of matter, while that in a Dirac semimetal (neglecting the Zeeman coupling) is only a trivial insulator. We present the scaling of this spectral gap for a wide range of subcritical (weak) interactions as well as that of the diamagnetic susceptibility with the magnetic field. A similar mechanism for charge-density-wave ordering at weak coupling is shown to be operative in double and triple-Weyl semimetals, where the dispersion is linear (quadratic and cubic, respectively) for the z (planar) component(s) of the momentum. We here also address the competition between the charge-density-wave and a spin-density-wave orders, both of which breaks the chiral symmetry and leads to gapped spectrum, and show that at least in the weak coupling regime the former is energetically favored. The anomalous surface Hall conductivity, role of topological defects such as axion strings, existence of one-dimensional gapless dispersive modes along the core of such defects, and anomaly cancellation through the Callan-Harvey mechanism are discussed.