First-Principles Studies On Topological Electronic States Driven By Time-Periodic Light

Since the discovery of quantum Hall effect,topological electronic states have attracted tremendous interest due to their unique properties.To date,people already have deep understanding of equilibrium electron topology in both aspects of physical model and material realization.However,because of the absence of research techniques and the complexity of excited states,topological states in nonequilibrium are studied only by simple models.As one of the nonequilibrium setup,time-periodically driven systems exhibit a wealth of topological phases,which makes the prediction of realistic excited systems having topology urgent,and this also indicates the great possibility of discovering new topological phenomena.In this dissertation,by combining first-principles calculations and model analysis,we investigate the topological states in specific crystals under the irradiation of time-periodic laser.To study the behavior of electrons in realistic materials under light,we developed the method of time-dependent ab initio calculation in Wannier basis,then calculated nonequilibrium electronic bands based on Floquet theorem,and simulated time-resolved photoelectron spectrum based on Green function.Using the new method,we carried out the research works in this thesis.Firstly,we discovered that light irradiation can effectively engineer Dirac fermions in graphene.Linearly polarized laser induces type-II Floquet-Dirac fermions,which coexist with type-I Dirac fermions around Fermi level.The two different types of fermions are connected by topological edge states,which provide an ideal channel to realize electron transport between the two types of Dirac states.Besides,simulating time-and angle-resolved photoelectron spectroscopy suggests that the new coexisting state of type-I and type-II fermions is experimentally accessible.Next,we studied the nonequilibrium electronic structures of 3D black phosphorus?BP?under a periodic field of laser.Under the irradiation of circularly polarized light?CPL?,BP exhibits photo-dressed Floquet Dirac semimetal state,which can be continuously tuned by changing the direction,intensity and frequency of incident laser.The topological phase transition from type-I to type-II Floquet-Dirac fermions manifests a new form of type-III phase,which exists in a wide range of intensity and frequency of incident laser.In addition,topological surface states exhibit nonequilibrium electron transport in a direction locked by the helicity of CPL.Furthermore,we found that,the three types of Dirac fermions also exist in light driven 2D BP,which can be used to simulate fermionic black hole?BH?and consequent Hawking radiation.A spatially inhomogeneous system with successively distributed type-II,-III and-I Dirac fermions is illustrated,which acts like an Schwarzschild BH metric to induce electron emission from type-II to type-I region.An effective gravity field corresponding to a striking high temperature T_H?3 K is achieved in the laser-driven 2D BP.Finally,we extended our study from type-III cone to Floquet topological flat band.We show that time-periodic CPL can effectively invert second-nearest-neighbor kinetic hopping in a Kagome lattice and simultaneously enhance spin-orbit coupling in one spin channel,so as to produce an isolated flat Chern band exhibiting high-temperature Floquet FQH effect.In a prototypical Kagome lattice of monolayer Pt₃C₃₆S₁₂H₁₂,the so-formed flat Chern band has a flatness ratio of?/w?40,the largest one ever reached so far.This work shows that,besides laser-created type-III cone with flat band on a particular k path in the above,laser can also create flat bands in the whole k space.In the dissertation,the photo-induced novel topological states will deepen our understanding of band topology,and the prediction of specific light-matter coupled systems lays a foundation for experimental observation of Floquet topological states.