Ultrafast Optical Spectroscopy Studies On Ce-based F-electron Materials

Quasiparticle is a concept proposed to describe the collective behavior of a large number of particles in a solid.Although quasiparticles are not real particles,they can ex-hibit properties similar to real particles.Typical examples of quasiparticles are phonons,excitons,and holes.Introducing the concept of quasiparticles facilitates the study and un-derstanding of complex physical properties,especially in revealing the special physical properties of strongly correlated electron systems.As a non-equilibrium probing tech-nique,ultrafast spectroscopy has become an important experimental tool for studying the behavior of quasiparticles in materials,such as quasiparticle dynamics,spin dynamics,electron-phonon interactions,and coherent phonons,due to its ultrashort time resolution and tunable excitation energy.In this work,ultrafast spectroscopy techniques were used to study three important cerium-based f electron materials,the ferromagnetic Kondo lat-tice material Ce Rh₆Ge₄,cerium metal films,and weakly hybridized ferromagnetic mate-rial Ce Ag Sb₂.By analyzing the temperature-dependent dynamics of quasiparticles,the orbital hybridization process between localized f electrons and conduction electrons at various temperatures was revealed.The research in this work led to the following main findings:（1）In the study of cerium metal thin films with different electronic structures and crystal structures,two characteristic temperatures,T^*～90 K and T⁺～140 K,were identified in the relaxation process of quasiparticles.The temperature T⁺marks the open-ing of hybridization between conduction electrons and localized f-electrons,accompanied by hybridization fluctuations.T^*represents the development of such hybridization into a coherent state with the opening of a gap in density of states.The quasiparticle relax-ation at high temperatures indicates coexistence of nonthermal electron-electron scatter-ing and inelastic Kondo scattering,while its behavior at low temperatures suggests onset ofαphase in theγ-phase dominated film.In both types of thin films,apart from phonon modes interacting with coherent heavy electron states,anomalies were also observed in the frequencies of phonon modes around T⁺,indicating interactions between fluctuating f-electrons and phonons.（2）In the study of the cerium-based ferromagnetic quantum critical material Ce Rh₆Ge₄,it is discovered that the relaxation process of quasiparticles exhibits two characteristic temperatures,T^*～85 K and T⁺～140 K.Below T^*,the relaxation rate of quasiparticles exhibits fluence-dependent behavior,indicating the presence of coherent heavy fermion state below 85 K.Between T⁺and T^*,the relaxation rate decreases with decreasing temper-ature,suggesting the existence of hybridization fluctuations.Additionally,three optical phonon modes are observed,and below T^*,the frequencies of these three phonons all display anomalous decreases with decreasing temperature,indicating coupling between coherent heavy electron state and phonons.（3）In the study of the weakly coupled ferromagnetic material Ce Ag Sb₂,as the tem-perature decreases from room temperature to the characteristic temperature T⁺～150K,localized f-electrons begin interacting with conduction electrons,involving orbital hy-bridization fluctuations.As the temperature further decreases to the characteristic tem-perature T^*～65 K,the system entered a coherent heavy fermion state,accompanied by the opening of an indirect hybridization energy gap near the Fermi surface in the den-sity of states.The size of this hybridization gap（2Δ）is determined to be approximately 9me V using the Rothwarf-Taylor model.Among the observed two coherent phonon modes with frequencies changing with temperature,only one mode exhibits anomalous behavior with decreasing frequency below the coherent temperature T^*,implying coupling between phonons and coherent heavy electrons within this temperature range.Based on these research findings,it is evident that in cerium f-electron materials,the hybridization process between f-electrons and conduction electrons can be divided into two stages.As the temperature decreases,T⁺marks the onset of hybridization between f-electrons and conduction electrons,yet the system has not formed a stable coherent state at this point,exhibiting hybridization fluctuations.The characteristic temperature T^*sig-nifies the formation of an coherent heavy fermion state with the opening of an indirect narrow energy gap in the density of states.Below temperature T^*,an anomalous soften-ing behavior of phonon frequencies is observed,closely related to collective hybridization in the Kondo lattice.In addition,the significant impact of quasiparticles on the electrical performance of novel semiconductor material is also investigated in this dissertation.