Dynamical Piezomagnetic Effect in Time-Reversal Invariant Weyl Semimetals with Axionic Charge-Density Waves

TL;DR

This paper predicts a novel dynamical piezomagnetic effect in time-reversal invariant Weyl semimetals with charge-density waves, where strain induces bulk orbital magnetization without relying on fluctuations, revealing a new 3D strain-related topological phenomenon.

Contribution

It introduces the dynamical piezomagnetic effect (DPME) in Weyl semimetals with CDWs, a bulk-constant strain effect originating from a valley axion field and characterized by a 3D Chern-Simons form, distinct from previous 2D Berry curvature effects.

Findings

DPME occurs in static, homogeneous bulk CDWs without relying on phason fluctuations.

A discontinuous change in DPME signals a topological quantum phase transition at the surface.

DPME acts as a bulk signature of boundary topological phase transitions in Weyl-CDW systems.

Abstract

Charge-density waves (CDWs) in Weyl semimetals (WSMs) have been shown to induce an exotic axionic insulating phase in which the sliding mode (phason) of the CDW acts as a dynamical axion field, giving rise to a large positive magneto-conductance [Wang et al., Phys. Rev. B 87, 161107(R) (2013); Roy et al., Phys. Rev. B 92, 125141 (2015); J. Gooth et al., Nature 575, 315 (2019)]. In this work, we predict that dynamical strain can induce a bulk orbital magnetization in time-reversal- (TR-) invariant WSMs that are gapped by a CDW. We term this effect the "dynamical piezomagnetic effect" (DPME). Unlike in [J. Gooth et al., Nature 575, 315 (2019)], the DPME introduced in this work occurs in a bulk-constant (i.e., static and spatially homogeneous in the bulk) CDW, and does not rely on fluctuations, such as a phason. By studying the low-energy effective theory and a minimal tight-binding (TB) model, we find that the DPME originates from an effective valley axion field that couples the electromagnetic gauge field with a strain-induced pseudo-gauge field. In particular, whereas the piezoelectric effects studied in previous works are characterized by 2D Berry curvature, the DPME represents the first example of a fundamentally 3D strain effect originating from the Chern-Simons 3-form. We further find that the DPME has a discontinuous change at a critical value of the phase of the CDW order parameter. We demonstrate that, when there is a jump in the DPME, the surface of the system undergoes a topological quantum phase transition (TQPT), while the bulk remains gapped. Hence, the DPME provides a bulk signature of the boundary TQPT in a TR-invariant Weyl-CDW.