Traditional & Exotic Semiconductors in the Two-Dimensional Limit*

Recent years have witnessed an amazing expansion in the family of 2D materials, which has led to many exotic physical properties distinctly different from their bulk counterparts. An example is the traditional II-VI compound semiconductors in their monolayer limit [1], for which the experiments have been catching up rapidly. Besides the monolayer honeycomb (MLHC) hBN-like structure, a double layer honeycomb (DLHC) structure was also predicted to be stable over a large portion of ordinary semiconductors [2]. It is becoming clear that when the thickness of a solid is below certain critical value, irrespective of its commonly-observed bulk form, a universal van der Waals stacking may happen, leading to unexpected physical properties. Taking GaAs, a well-known covalent semiconductor, as an example, non-trivial topological properties emerge as a result of the stacking of DLHCs [2], as well as the formation of an excitonic insulator (EI) where the exciton binding energy magically becomes larger than its band gap [3]. In strongly-correlated 2D systems such as MX₂ where M = Ni, Co and X = Cl, Br, on the other hand, half excitonic insulator can also form where one spin channel is an ordinary semiconductor while the other spin channel is an EI, leading to Bose-Einstein condensation [4].
[1] H. Zheng, et al., Phys. Rev. B 92, 115307 (2015).
[2] M. C. Lucking, et al., Phys. Rev. Lett. 120, 086101 (2018).
[3] Z. Jiang, Y. Li, S. B. Zhang, and W. Duan, Phys. Rev. B 98, 081408(R) (2018).
[4] Z. Jiang, Y. Li, W. Duan, and S. B. Zhang, Phys. Rev. Lett. 122, 236402 (2019).