Angle-Rresolved Photoemission Spectroscopy Studies On Topological Insulators

As the application of topology theory in condensed matter physics,topological quantum materials achieved great success in past decades,from topological insulators to topological semimetals,exotic topological character has greatly inspired enthusiasm of physicists.With the progressive elaboration of topological band theory,researchers have a deeper understanding of macroscopic quantum states.From symmetry indicators to topological invariant,theoretical physicists have established a complete classifying framework of topological quantum states.The experimental demonstration of topological states has also made significant progress.Re-examine previous research,we find that there are still some problems that remain unresolved,some important topological states are waiting to be demonstrated.The authors re-sorted the framework of topological insulators,then focused on the topological crystalline insulators and weak topological insulators,meanwhile took careful research on relevant topological materials by using angle-resolved photoelectron spectroscopy.The main research results are listed below:（1）Experimental discovery of new classes of topological crystalline insulators in Sr Pb.Rotation symmetry protected topological crystalline insulators has gapless Dirac cone on the surface preserving the rotation symmetry,which are connected by one-dimensional（1d）helical states on the hinges parallel to the rotation axis.Through theoretical calculations,it was found that Sr Pb has a nontrivial C_2y rotational topological invariant!!!"=1,and its（010）surface has two rotational symmetry-protected Dirac surface states.We carefully measured the electronic structure of Sr Pb using angle-resolved photoelectron spectroscopy,we observed two oblique Dirac cones in the band gap successfully,which confirmed the existence of rotation symmetry-protected topological crystalline insulators in Sr Pb.This discovery enriches the classification of topological phases and provides an important experimental platform to study the topological crystalline insulators.（2）Experimental discovery of weak topological insulators in Ta As₂.Weak topological insulators have spin currents in specific direction,this exotic character has stimulated further research.However weak topological insulators which has been demonstrated are very rare.Through theoretical calculations,Z₂ topological invariant of Ta As₂ are computed to be（0;111）,and there are even number of Dirac surface states on its（001）surface and（100）surface.Subsequently,using angle-resolved photoelectron spectroscopy,we observed corresponding nontrivial topological surface states on the（100）surface of Ta As₂,which providing important experimental evidence for weak topological insulators in Ta As₂.The author believe in the near future,with the ongoing exploration of topological band theory and continuous efforts of physicists,more novel topological quantum states will be discovered and studied,which providing new ideas for the application of topological materials.