First-principles Study Of Exotic Excitonic Phenomena In Quasi-two-dimensional Semiconductors

Due to the exotic quantum properties and underlying high efficiency ratio,two-dimensional?2D?semiconductors are proposed as ideal materials to realize next gener-ation quantum devices in energy storing and transforming.One of the most prominent characteristics of 2D semiconductors is the significantly enhanced excitonic effect,which usually denotes their absorption spectra and photoelectric properties.Based on first-principles density functional method and many-body perturbation theories,we revealed a scaling universality of 2D excitons,and additionally proposed a principle for realizing excitonic insulators in 2D semiconductors and ferromagnetic insulators.Optically active,i.e.,bright excitons usually attract wide attention in both fundamen-tal experiments and device applications.By performing systematical calculations on 232D semiconductors,we found that for bright excitons in 2D semiconductors,there exists a robust linear scaling law between the quasiparticle band gap?E_g?and the exciton binding energy?E_b?,namely,E_b?E_g/4,regardless of their geometric and electronic properties.By deriving a simple expression for the 2D polarizability with respect to E_gand adopting the 2D screened hydrogen model for E_b,the linear relation can be deduced analytically.This work provides a deeper understanding on the influence of quantum confinement ef-fect on exciton behaviors,and offers a powerful guidance for modulation of exciton based devices.In contrast,optically inactive,i.e.,dark excitons usually receive relatively low atten-tion for their inert interaction with light.On the other hand,being lack of interactions also promises other remarkable phenomena such as a considerably longer lifetime.We pro-posed that by combining direct-gap 2D semiconductor with dark excitons,one can realize an intrinsic excitonic insulator?an excitonic Bose–Einstein condensate?by decoupling the exciton binding energy from the quasiparticle band gap.Here the optical inactivity is guaranteed by the same parity of band-edge states.2D Ga As and single-layer Ti S₃which has been experimentally exfoliated are given as concrete supports to this principle.The excitonic effect in spin-polarized 2D insulators is also worth our concern.Cal-culations reveal an unusual electronic state,dubbed as half excitonic insulator,in ferro-magnetic monolayer 1T-M X₂?M=Co,Ni and X=Cl,Br?.Its one spin channel has an excitonic insulator ground state,while the other is characterized by a conventional band insulator gap.This disparity arises from a competition between the band gap and exciton binding energy,which exhibits a spin-dependence due to different orbital occupations.Such a state can be identified by optical absorption measurements and angle-resolved photoemission spectroscopy?ARPES?.Our theory includes magnetism into excitonic insulator,suggesting that strongly-correlated materials could be fertile candidates for ex-citonic insulators,and providing a new respective to understand the nature of many body effect.