Two-dimensional Dirac semiconductor and its material realization

We propose a new concept of a two-dimensional (2D) Dirac semiconductor which is characterized by the emergence of four-fold degenerate band crossings near the band edge, and provide a generic approach to realize this novel semiconductor in the community of material science. Based on the first-principle calculations and symmetry analysis, we discover recently synthesized triple-layer (TL) Bi2OS2 is such a 2D Dirac semiconductor that features a Dirac cone at the X/Y point, protected by a centrosymmetric nonsymmorphic space group. Due to the sandwich-like structure, each Dirac fermion in TL-Bi2OS2 can be regarded as a combination of two Weyl fermions with opposite chiralities, degenerate in momentum-energy space but separated in real space. Such a 2D Dirac semiconductor carries hidden layer-dependent helical spin textures. Moreover, novel topological phase transitions are flexibly achieved in TL-Bi2OS2: (i) a vertical electric field can drive it into the Weyl semiconductor with switchable spin polarization direction; (ii) an extensive strain is able to generate ferroelectric polarization and actuate it into the Weyl nodal ring around the X point and into another type of four-fold degenerate point at the Y point. Our work extends the Dirac fermion into semiconductor systems and provides a promising avenue to integrate spintronics and optoelectronics in topological materials.