Study On Elemental Excitations And Exotic Phase Transitions In Typical Semimetallic Materials

The study of elementary excitation is a way to describe the interactions between particles in a system.It treats particles with many-body interactions as excitations with special properties.Hence,the interaction between particles can be understood by study-ing the properties of elementary excitations.The carrier density in semimetals is be-tween that in metals and insulators.Hence,properties in semimetals are neither like in metals,where the carriers can almost completely shield the Coulomb interaction and the electron-phonon interaction,nor like in insulators,where there are so few carriers that they have little effect on the system.Therefore,in semimetals,the Coulomb interaction and the electron-phonon interaction play important roles,and many exotic phase transi-tions induced by interactions between particles can be observed.Since the elementary excitation in the condensed matter system is determined by interactions between par-ticles,the interaction mechanism related to exotic phase transitions can be revealed by analyzing the properties of elementary excitations in semimetallic materials.In this thesis,using high-resolution electron energy loss spectroscopy with reflec-tive scattering geometry and the first-principles calculation,the properties of elementary excitations and particle interactions are explored in two typical semimetallic systems with exotic phase transitions.The first is 1T-Ti Se₂,a candidate for the excitonic in-sulator with a charge-density-wave transition.In 1T-Ti Se₂,a previous work reported a plasmon softening at large momentum and claimed that it was the signature of an excitonic insulator.Here,we perform high-precision experimental measurements com-bined with theoretical calculations and find the Landau damping behavior of plasmons at small momentum,which does not support the previous claim that plasmon softening is the signature of the excitonic insulator.With the detailed analysis of phonons,we demonstrate that the experimental evidence regarded as plasmon softening in previous work is a misidentification of phonon dispersions.Our results indicate that the search for the experimental fingerprint of excitonic insulators remains an important challenge.At the same time,we find that the plasmon exhibits the same temperature dependence as the charge-density-wave gap,and a phenomenological model is used to reveal that the temperature dependence of the plasmon is due to changes in the strength of the electron screening.The second is Zr Te₅,a Dirac semimetal with a possible topological transition.In Zr Te₅,we measured the elemental excitations in the low energy range（<30 me V）,and find strong phonon-plasmon interference,revealing the possibility of the existence of Dirac polarons.By comparison of the temperature dependence between the plas-mon and the carrier density,we prove that Zr Te₅is a three-dimensional massive Dirac electron system and the band gap is gradually opened with increasing temperature.Our results rule out the mechanism of the transition between strong and weak topological in-sulators at the resistance anomaly temperature,and we attribute the resistance anomaly to the inherent property of extrinsic three-dimensional Dirac systems.In addition,we developed a program for quantitatively fitting the electron energy loss spectra at the low energy range.In conclusion,the plasmon and phonon behaviors can reveal the characteristics of Coulomb interaction and electron-phonon interaction,and demonstrate the relationship between themselves and the microscopic mechanism of physical properties in semimetal-lic materials.Hence,in this thesis through the measurement and analysis of plasmons,phonons,and other elementary excitations in semimetallic materials,the mechanism of exotic phase transitions is clarified from the perspective of Coulomb interaction and electron-phonon coupling.