The Study On The Elementary Excitations Of Topological Nodal-line Semimetal ZrSiS

Elementary excitations,including single-particle excitations and collective excitations,are important concepts in condensed matter physics and play a decisive role in the physical properties of materials.In recent years,condensed matter physics has made great progresses in the research of topological properties of materials.Topological materials,such as topological insulators and topological semimetals,have not only greatly expanded our understanding of fundamentals of solid-state physics,but also brought abundant application prospects.The research on the elementary excitations of topological materials is the key to understand their properties.Taking a prototype nodalline semimetal Zr Si S as an example,this dissertation focuses on the exploration of the elementary excitations of topological materials.On one hand,the interaction between the unique surface states and surface phonons of Zr Si S is explored.On the other hand,the unique plasmon properties related to its topological bands are studied.This work will be helpful for the potential applications of topological nodal-line semimetals in low-dimensional electronics and plasmonics.In the first part,using our self-developed high-resolution electron energy loss spectrometer,we obtained for the first time the phonon dispersion along the highsymmetry line covering the whole first Brillouin zone.An optical mode with anomalous softening was observed.With the help of an electron-phonon interaction model,the softening is attributed to the Kohn anomaly induced by the interaction of surface phonons with the peculiar floating surface states of Zr Si S.On this basis,a detailed physical picture for the surface electron-phonon interactions are depicted.Under this picture,the average electron-phonon coupling constant is extracted to be ?? 0.15 by fitting the imaginary part of the phonon self-energy.This study will help to understand the low-dimensional transport properties of Zr Si S.In the second part,we observed for the first time three plasmons of Zr Si S in the energy range 0.1 to 1.5 e V.At the center of the Brillouin zone,the energies of these plasmons are 0.20?0.50?0.90 e V,respectively,in the range of near/mid-infrared frequency.All the three plasmons are shown to be isotropic,which is different from the theoretical prediction on the ideal nodal-ring semimetals.Temperature-dependent experiments show good thermal stability for these three plasmons.Combining the analysis of the surface electronic states of Zr Si S based on first-principle calculations and loss function calculations under random phase approximation,the electronic origins of these plasmons are revealed.All the plasmons are found to be related to the topological bands of Zr Si S,providing direct evidences of the plasmon origins from the topological nodal lines.Besides,we also tried to build a magnetic helium atom scattering facility with the prospect to detect surface magnetic order and surface magnons,together with the capability of detecting surface corrugations and surface phonons.This part involves the design and build of the apparatus with its electronic control system.The mechanical parts involve the setup for generation,excitation and detection of monochromic helium beam,while the electronic parts involve the design and implementation of the electronic boards and control modules,and the development of the software.This work provides the technical basis for the realization of magnetic helium atom scattering spectrometer.