Optical properties of monolayer bismuthene in electric fields.

Optical excitations of monolayer bismuthene present very rich and unique absorption spectra. The optical energy gap corresponding to the threshold frequency is not equal to an indirect energy gap, and it becomes zero under the critical electric field. The frequency, number, intensity, and form of the absorption structures are dramatically changed when an external electric field is applied. The prominent peaks and the observable shoulders, respectively, arise from the constant-energy loop and the band-edge states of parabolic dispersions. These directly reflect the unusual electronic properties, being very different from those in monolayer graphene. The novel optical properties of bismuthine that are easily manipulated by electric fields may find a lot of various applications in optoelectronics, either combined with or complementary to those graphene-based systems.

Prediction of Quantum Anomalous Hall Effect in MBi and MSb (M:Ti, Zr, and Hf) Honeycombs.

The abounding possibilities of discovering novel materials has driven enhanced research effort in the field of materials physics. Only recently, the quantum anomalous hall effect (QAHE) was realized in magnetic topological insulators (TIs) albeit existing at extremely low temperatures. Here, we predict that MPn (M =Ti, Zr, and Hf; Pn =Sb and Bi) honeycombs are capable of possessing QAH insulating phases based on first-principles electronic structure calculations. We found that HfBi, HfSb, TiBi, and TiSb honeycomb systems possess QAHE with the largest band gap of 15 meV under the effect of tensile strain. In low-buckled HfBi honeycomb, we demonstrated the change of Chern number with increasing lattice constant. The band crossings occurred at low symmetry points. We also found that by varying the buckling distance we can induce a phase transition such that the band crossing between two Hf d-orbitals occurs along high-symmetry point K2. Moreover, edge states are demonstrated in buckled HfBi zigzag nanoribbons. This study contributes additional novel materials to the current pool of predicted QAH insulators which have promising applications in spintronics.