The Study Of The Collective Excitations On The Surfaces Of Topological Nodal-line Semimetals ZrSiX(X=S,Se,Te)

The research on the topological properties of materials has become one of the most flourishing frontiers in condensed matter physics(CMP)since the discovery of the quantum Hall effect.Topological materials,such as topological insulators and topological semimetals,exhibit rich novel properties,leading to not only further understanding of fundamentals of CMP and revolution of material classification paradigm,but also promising applications.The exploration of elementary excitations,including collective excitations and single-particle excitations,in topological materials is the key to understand their novel properties.In this dissertation,two kinds of collective excitations,plasmon and phonon,on the surfaces of typical topological nodal-line semimetals ZrSiX((3 = S,Se,Te)are systematically studied using surface-sensitive highresolution electron energy loss spectroscopy(HREELS).We have studied in detail the influence of the topologically nontrivial nodal-line states on the electronic collective excitation,and explored the tunable plasmon and phonon properties in ZrSiX induced by the variable interlayer coupling due to the substitution of (3 atoms.This work will promote the understanding of the physics behind the interesting properties of topological nodal-line semimetals,and provide a research basis for related application fields such as plasmonics and low-dimensional electronics.The main contents of this dissertation are as follows:(1)Through the HREELS measurement of ZrSiX,it is found that several nodalline plasmons,associated with the topological nodal-line electron states,ubiquitously emerge in topological nodal-line semimetals ZrSiX.Combined with the first-principles electronic band and dielectric function calculations,these surface plasmons are assigned to the intra-or inter-band correlations of the surface states related to the nodal-line electrons.In particular,the surface intraband plasmons exhibit temperature effect due to the prominent thermal occupation effect of carriers,which is distinct from nomal metals and Dirac/Weyl semimetals,reflecting the nature of nodal-line plasmons.Furthermore,the variable interlayer coupling in ZrSiX leads to the modulation of the screening of surface states on the bulk states,and a bulk plasmon from the bulk nodal-line states emerges in ZrSi S with the strongest interlayer coupling.These results indicate that ZrSi Se and ZrSi Te are more suitable experimental platforms for studying surface plasmon properties related to topological surface states.(2)Through the HREELS measurement with a lower incident energy of the electron beam,it is found that the surface interband plasmons are replaced by acoustic plasmons.After excluding the influence of the dipole scattering geometric effect in the electron energy loss spectrum at low incident energies,we clarified the dispersion characteristics of the acoustic plasmons,which may originate from the incomplete screening of the surface states from the bulk states.(3)Through the HREELS measurement of surface phonon dispersion spectra,combined with the first-principles phonon calculations and lattice dynamics analysis,it is found that the variable interlayer coupling in ZrSiX significantly affects the dispersion characteristics of the bulk and surface phonons,as well as the broadening of the bulk phonon bands projected on the surfaces.Our investigation on the lattice dynamics of ZrSiX will facilitate the future exploration of electron-phonon interactions in these systems.(4)We also introduced in this dissertation the self-designed and-built magnetic helium atom scattering(MHAS)spectroscopy.Based on the helium atom scattering,the spectroscopy utilizes the long-lived magnetic metastable state of helium atoms,which is capable of characterizing the long-range magnetic ordering and the magnon dispersion of surface/low-dimensional magnetic systems.While introducing the physical principles of MHAS,we showed the pictures of the whole MHAS facility and important components,as well as the preliminary testing results of key singals such as the helium beam and the magnetically excitated states.The completed spectroscopy is expected to play important roles in exploring the fractional magnetic excitation in quantum spin liquid,low-dimensional magnon spintronics,and the surface states of topological magnon systems.