Mass Acquisition of Dirac Fermions in Bi4I4 by Spontaneous Symmetry Breaking

TL;DR

This study demonstrates the emergence of massive Dirac fermions in Bi4I4 surface states due to spontaneous symmetry breaking, without breaking time-reversal symmetry, revealing new pathways for topological material engineering.

Contribution

It provides experimental evidence of massive Dirac fermions arising from spontaneous symmetry breaking in Bi4I4, combining microscopy, spectroscopy, and first-principles calculations.

Findings

Massive Dirac fermions observed in Bi4I4 surface states.

Spontaneous symmetry breaking induces nondegenerate edge states.

Potential for engineering Dirac fermions in device applications.

Abstract

Massive Dirac fermions, which are essential for realizing novel topological phenomena, are expected to be generated from massless Dirac fermions by breaking the related symmetry, such as time-reversal symmetry (TRS) in topological insulators or crystal symmetry in topological crystalline insulators. Here, we report scanning tunneling microscopy and angle-resolved photoemission spectroscopy studies of {\alpha}-Bi4I4, which reveals the realization of massive Dirac fermions in the (100) surface states without breaking the TRS. Combined with first-principle calculations, our experimental results indicate that the spontaneous symmetry breaking engenders two nondegenerate edges states at the opposite sides of monolayer Bi4I4 after the structural phase transition, imparting mass to the Dirac fermions after taking the interlayer coupling into account. Our results not only demonstrate the formation of the massive Dirac fermions by spontaneous symmetry breaking, but also imply the potential for the engineering of Dirac fermions for device applications.