Studies Of Interplay Between Heavy Electron State And Several Electron Spin- Related States

Strongly correlated electron system is a research hotspot in the field of condensed matter physics and challenges researchers for several decades.The Coulomb interactions among electrons in solid are non-negligible and thus make the traditional band theory invalid when applied in the study of strongly correlated electron systems.Traditional band theory just consists of the interaction between electrons and periodic lattice and regards that there is no interaction between electrons.When the Coulomb interactions among electrons are strong enough,there will be much novel phenomena,such as high-temperature superconductivity,Mott transition,colossal magnetoresistance effect,fractional quantum hall effect,heavy electron state and so on.In this dissertation,we mainly use angle-resolved photoemission spectroscopy（ARPES）and scanning tunneling microscopy/spectroscopy（STM/STS）to study the electronic structures of seveal kinds of heavy electron materials（CeIn₃,Fe₃GeTe₂ and CeSb₂）and emphasize the interaction between heavy electron state and several spin-related states,including:1)the localized/itinerant character of the f electrons in an antiferromagnetic f-electron heavy electron compound and its relationship with antiferromagntism.2)the first observation of the heavy electron state in a 3d-electron ferromagnet and the study of the relationship between ferromagnetic state and heavy electron state.3)the observation of the Dirac-like band structure in a traditional f-electron heavy electron compound.We obtain the three-dimensional topology of the Fermi surface and electronic structure around the Fermi level of an antiferromagnetic heavy electron compound CeIn₃by ARPES,STM/STS and the first-principles calculations.The localization of the f electrons is dominated at low temperature and only a small portion of f electrons hybridize with conduction electrons.The formation of the heavy electron state can well explain the phenomena observed by specific heat,resistivity and magnetic susceptibility measurements.With the decrease of temperature,we find the hybridization strength between f electrons and conduction electrons does not disappear but is suppressed in the antiferromagnetic state.Compared with the heavy electron supercondutors CeCoIn₅ and CeIrIn₅,the c-f hybridization strength of CeIn₃ is weaker.The observed phenomena in this dissertation are quite important to understand the relationship between heavy electron state,magnetism and even superconductivity.We mainly use ARPES,STM/STS and the first-principles calculations to study the topology of the Fermi surface and electronic structures of the 3d-electron itinerant ferromagnet Fe₃GeTe₂.We find that the electronic structures of Fe₃GeTe₂ are significantly renormalized when the compound goes into the ferromagnetic state,including the enhanced density of states around the Fermi level,enlarged Fermi surface volume,decreased Fermi velocity around the K point in the brillouin zone and enhanced effective electron mass.Additionally,when temperature keeps going down,the density of states around the Fermi level is further enhanced.The STS reveals a Fano resonant lineshape.Furthermore,the resistivity,magnetic susceptibility and specific heat all indicate the emergence of heavy electron state in this compound.Together with the first-principles calculations,we propose that in this compound,the heavy electron states emerge in the ferromagnetic state and the ferromagnetism makes important roles for the formation of heavy electron states.This is the first observation of the heavy electron states in a 3d-electron ferromagnet and needs furtuer theoretical researches on this field.We use ARPES,electrical transport and magnetic susceptibility measurements to illustrate the complicated magnetic structures of the heavy electron compound CeSb₂.The topology of the Fermi surface and electronic structure of CeSb₂ are obtained.We find that except for the traditional parabolic-like band structures,there is a photon energy independent and Dirac-like band structure in valence band,implying its quasi-two dimensional character.The electrical transport measurement reveals the heavy electron behavior in this compound at low temperature.Additionally,when the applied magnetic field is parallel to the direction of electrical current,the value of the magnetic resistance is negative,implying the possible Weyl fermion state in this compound.However,this phenomenon needs furtuer inverstigations.To summarize,we systematically studied the electronic structures of the three kinds of heavy electron compounds.On one hand,we have a better understanding of how heavy electron state evolves in f-and d-electron systems as a function of temperature and its relationship with magnetism.On the other hand,we futher realize the coexistence of heavy electron state and the possible Weyl fermion state in 4f-electron systems.Those results not only chanllenge the researches,but also make them exciting in the field of heavy electron compounds.