The Study Of Collective Excitations In Topological Materials Using High Resolution Electron Energy Loss Spectroscopy

Recent years,research of topological materials?TMs?is a new and developing field in condensed matter physics?CMP?.TMs are those materials that have topologically nontrivial electronic bands.Based on different types of band structures,TMs can be classified into topological insulators?TIs?,topological semimetals?TSMs?and more.TMs show unique properties in the single particle picture due to the existence of topological protected states,which has generated great breakthroughs in both theoretical and experimental studies.Elemental excitation is an important concept used to describe the low-energy excited states in CMP,which plays a key role in understanding emergent phenomena.Therefore,if collective excitations?such as plasmons?in TMs show exotic properties and if they are under topological protection as in the single particle picture are extremely valuable to study.In this dissertation,we have systematically studied the behaviors of collective excitations and their interactions in two typical TMs-a 3D TI Bi₂Se₃ and a Type-II Weyl Semimetal?WSM?MoTe₂,using high-resolution Electron Energy Loss Spectroscopy with the capacity of two-dimensional energy/momentum mapping.In 3D TI Bi₂Se₃,we observed an anomalous acoustic plasmon mode which is originated from the topologically nontrivial surface state.Different from the plasmons originating from the bulk conduction band,this mode exhibits an almost linear dispersion up to the second Brillouin zone center with weak damping due to the topological protection.What's more,the energy of the mode is much lower than previous theoretical predictions,suggesting a strong interaction with phonons.In Type-II WSMs MoTe₂,the analyses of the collective excitations are very difficult due to the complex Fermi surfaces with contributions from both topologically trivial and nontrivial states.We find a trivial plasmon mode in its topologically trivial phase?1T'phase?splits into two nontrivial modes after MoTe₂ enters the topologically nontrivial phase?Td phase?.Combining with first-principle calculations,we show that plasmon modes in the Td phase are attributed to the interplay of Weyl fermions and normal nonrelative fermions.This result demonstrates that plasmons in complex systems are not only simply from contributions of intraband transitions,but also from the contributions of interband couplings.Furthermore,we have also observed an anomalous acoustic mode with a dispersion like regular acoustic phonons,but surprisingly its period is twice of the measured acoustic phonon branch.The origin of this unusual mode is still unknown and the relevant work is to be continued.In conclusion,in our research of collective excitations in TMs,we observe several exotic properties of TMs and reveal their origins related to the topological protections.These works broaden the knowledge in understanding collective excitations in TMs,and potentially pave the way to follow-up studies in relevant fields.