Spontaneous mass generation due to phonons in a two-dimensional Dirac fermion system

TL;DR

This paper investigates how strong electron-phonon interactions in a two-dimensional Dirac fermion system induce a spontaneous gap and transition into a Hall insulating phase with broken symmetries, revealing quantized conductance.

Contribution

It demonstrates the spontaneous generation of mass and quantized Hall conductance due to phonons in Dirac fermion systems, highlighting novel parity-breaking excitations.

Findings

Spontaneous gap formation at strong electron-phonon coupling.

Quantized Hall plateaux with conductance twice as large in binodal models.

Presence of scale-invariant parity-breaking excitations resembling Chern-Simons terms.

Abstract

Fermions with one and two Dirac nodes are coupled to in-plane phonons to study a spontaneous transition into the Hall insulating phase. At sufficiently strong electron-phonon interaction a gap appears in the spectrum of fermions, signaling a transition into a phase with spontaneously broken parity and time-reversal symmetry. The structure of elementary excitations above the gap in the corresponding phase reveals the presence of scale invariant parity breaking terms which resemble Chern-Simons excitations. Evaluating the Kubo formula for both models we find quantized Hall plateaux in each case, with conductance of binodal model exactly twice as large as of the mononodal model.