Probing The Electronic Structure Of The Kondo Lattice Systems

Kondo lattice system is a typical strongly correlated electron system.The rich physics and quantum phenomena contained in it make Kondo lattice an important branch of condensed matter physics.However,understanding Kondo lattice systems quantitatively is extremely difficult.The energy scale of Kondo lattice systems is relatively small which makes experimental measurement difficult.Solving Kondo lattice model quantitatively is also a hard problem.In this article,we combine angle-resolved photoemission spectroscopy,molecular beam epitaxy and density functional theory to study the electronic structure of two typical Yb-based and Ce-based Kondo lattice systems:YbPtBi and Ce-intercalated graphene and also the electronic structure of weak ferromagnetic Hund metal MnSi,which bears strong similarities with Kondo lattice systems.We provide a spectroscopic understanding of their physical properties and electron correlation.YbPtBi is a well-known low carrier density heavy fermion system.Recently it was found that there are Weyl fermion excitations in YbPtBi.We first use angle-resolved photoemission spectroscopy to determine that the cleavage plane is either the Yb or Bi atomic layer.Then through comparison of the termination-dependent surface state from theory and experiment and the position of 4f state-derived flat bands,we find for Bi terminated surface,trivalent Yb atoms can explain the experimental results well.While for Yb terminated surface,we discover that the surface and the bulk Yb atoms have different valence.The outmost Yb atoms are divalent while the inner Yb atoms are trivalent.Through first principle calculations of hybridization function of different layers of Yb atoms,we give the reason of the valence transition:the reduced coordination number of nearest neighboring Bi atoms makes the outmost Yb atoms have weakest hybridization strength with conduction electrons.This valence difference has profound influence on the band structure and electron correlation.Our results lay a foundation for the study of surface effects and the induced electronic structure change in Yb-based heavy fermion systems.Ce-based material is another kind of typical Kondo lattice system.In this article we try to introduce flat bands and correlation effects into graphene through Ce intercalation.Through molecular beam epitaxy and angle-resolved photoemission spectroscopy we successfully obtain two distinct phases.The first phase is lightly Ce-doped graphene.In this phase the main contribution of Ce is doping graphene,which apparently shifts the Dirac points downwards.And there is band hybridization between two graphene layers which results in characteristic kinks.The Ce concentration in this phase is very low and there is no periodic lattice formation.So only very weak non-dispersive Kondo resonance forms which can be described by single-impurity Anderson model.The second phase is highly Ce-doped graphene.In this phase Ce and graphene form periodic lattice so we can observe apparent Kondo lattice characteristics:the lower Hubbard band at-2 eV and the 4f quasiparticle band around Fermi level.Interestingly,we observe satellite bands which locate at～100 meV below the 4f flat bands around Fermi level.These satellite peaks can hardly be described by crystal field splitting(the crystal field splitting of most 4f system is smaller than 50 meV)and the most possible explanation is the phonon-assisted Kondo effect.In this process satellite peaks which have energy difference of about the phonon energy will appear and 100 meV energy is indeed in accord with the graphene phonon energy scale.This kind of phononassisted Kondo effect has been studied theoretically before but direct experimental evidences still lack.This system provides a possible platform for observing the phonon-assisted Kondo effect and deserves further investigation in the future.Although Kondo lattice systems mainly contain f electron systems,there are also 3d electron systems which exhibit Kondo-like screening,i.e.local moment and the corresponding screening of moment.In 3d electron systems,due to the larger band width the quasiparticles formed by Kondo screening have much smaller effective mass than in f electron systems.Apart from this,recent research found that in multiorbital 3d electron systems when orbital filling is one electron away from half-filling,the Hund coupling effect between electrons can have profound impact upon correlation effects of the system.The coherence temperature of spin/orbital screening can be greatly suppressed and quasiparticle states varying greatly with temperature arise,which bears similarity with Kondo lattice systems.This kind of system is called Hund metal and has attracted wide interest.The third research in this article is the electronic structure study of the Hund metal candidate MnSi.We successfully grow high-quality MnSi films through molecular beam epitaxy.Using in-situ angle-resolved photoemission spectroscopy,we obtain momentum-resolved electron spectral function.Through extracting the energy distribution curves at different momentum points,we find at some momentum points at low temperature the coherence peak gradually emerges.The coherence peak lies around the Fermi level and has large effective mass and strong temperature dependence,which is in accord with the characteristics of Hund metal,that is the formation of Kondo-like spin/orbital screening.Meanwhile,at some other momentum points we find band splitting which is related with ferromagnetic transition.This reflects the weak ferromagnetism of the system.Well below the ferromagnetic transition temperature,the coherence peak seems to saturate.This reflects the coexistence and competition between Kondo effect and ferromagnetism.This phenomenon bears strong similarity with the coexistence and competition of magnetic order and Kondo screening in magnetic Kondo lattice systems and deserves further study in the future.