Angle-resolved Photoemission Spectroscopy (ARPES) Studies On Electron Correlation And Topology In Ce-based Kondo Lattice Systems

The observation of Quantum Hall Effect(QHE)opened the door of the topological matter field in condensed matter physics.Up to now the weak correlated topological systems described by single electron band structure have been systematically well studied,whereas less research of the corresponding topological states in strong correlated systems.In strong correlated electron systems,it is a big challenge to detect the topological states because of the small electron band width;At the same time,theoretical calculations also are very difficlut due to the strong correlation effect.Even so,strong correlated topological states could induce abundant physical phenomena,especially their smaller energy scale provides the broad space for the research of tunning,which is always an important direction in the field of topological matter.An important material system related to correlated topological states is Kondo lattice system,which usually contains f electron.In this thesis,we mainly used angle-resolved photoemission spectroscopy(ARPES)in experiments to detect the topological states and electron correlated effects in Ce-based kondo systems.Our main findings include:the topological surface states in REBi(RE is rare earth element)and it's topological states can be tunned by the 4f electron's occupancy number;the weak valence fluctuation induced fermi surface change and the anisotropy c-f mixing effect in low carrier density Kondo lattice CeBi;the coexistence of charge density wave(CDW)phase,Dirac state and kondo effect in CeSbTe and the interaction among them;the spectra evidence of the competition relationship between Kondo effect and magnetic ordering in low temperature and the detection of predicted topological states in noncentrosymmetric heavy fermion compound CeCoGe3.The details are summarized as follows:1.REBi(RE=Ce,Pr,Sm,Gd)has a simple NaCl-type crystal structure.This system has been predicted by theory to have bulk band inversion induced topological surface states which could be tunned by the f electron' occupancy number.We used ARPES measurements combined with first-principle calculations to systematically studied the materials' band topology and the influence of the f electrons on band structures by tuning the occupancy of the f electron.And we also predicted a possible topological phase transition from topological nontrivial to topological trivial based on our research.2.The change of Ce's valence with temperature in Kondo lattice systems is usually very tiny.Especially in the metallic systems,the carrier density change induced by valence fluctuation is hard to be detected by ARPES experiment.However,the topological nontrivial Kondo semimetal CeBi has the low carrier density property,which makes CeBi to be a possible platform to detect the valence change by ARPES.Actually,we did the temperature dependent ARPES measurements in CeBi,and found an obvious fermi surface expansion induced by the tiny valence change in low temperature.We also compared CeBi with GdBi which has a similar antiferromagnetic(AFM)transition in low temperature but no kondo effect to exclude the possible influence of antiferromagnet in CeBi.By using the resonant ARPES measurements,we concluded that valence fluctuation induced the carrier density change was from the mixing between conduction electrons and f electrons(c-f mixing).3.The research of the interaction between topological states with some other ordered states induced new exotic physics has attracted much attentions recently.By using ARPES combined with first principle calculations together with Low Energy Electron Deflection(LEED),we made a system study on the Kondo Dirac semimetal CeSbTe,and we found there is a very robust CDW phase even above room temperature in this compound.Then we studied the possible interaction between CDW and Dirac point.At the same time,the existence of CDW reduced the carrier density in this material,so the Kondo effect at low temperature was very weak.This result reviewed the competition relationship of the two ground states(heavy fermion state in Kondo screening and CDW state)4.CeCoGe3 is a noncentrosmmetric heavy fermion compound and it was predicted to have topological states which would be renormalized near fermi level in low temperature due to the strong electron correlation effect.We have used ARPES to detect the electronic structures of CeCoGe3 systematically.Even though the band splitting induced by non-centrosymmertic could not be resolved from the measured ARPES spectra,we did observe the photoemission spectra evidence of the competition relationship between Kondo effect and RKKY interaction,this benefits from the relative high AFM transition temperature in CeCoGe3,so it is suitable for ARPES measurements to study this physical problem.In experiments we found the Kondo resonance peak was gragually suppressed when the system went into the magnetic ordered states,which reviewed the competition relationship between these two interactions.This result provides the photoemission evidence to interpret this important competition effect in Kondo lattice systems.