Mass Acquisition of Dirac Fermions in Bi4 I4 by Spontaneous Symmetry Breaking

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 in topological insulators or crystal symmetry in topological crystalline insulators. Here, we report scanning tunneling microscopy and angle-resolved photoemission spectroscopy studies of α-Bi4I4, which reveals the realization of massive Dirac fermions in the (100) surface states without breaking the time-reversal symmetry. Combined with first-principles calculations, our experimental results indicate that the spontaneous symmetry breaking engenders two nondegenerate edge 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.