Angle-Resolved Photoemission Soectroscopy Study On Heavy-Fermion Superconductor CeCu₂Si₂ And Related 4f-electron Systems

In heavy-fermion compounds and some other related 4f-electron systems,the entanglement among electrons' different degrees of freedom serves as a catalyst for plenty of interesting physical phenomena,and these materials have aroused much attention in the research of condensed matter physics.The main topic of this thesis is to study several important 4f-electron compounds by angle-resolved photoemission spectroscopy(ARPES).And combining ARPES with first-principles calculations and other measurement methods,we analyze and explain the inner physical mechanism within the terminology of electronic structure.CeCu2Si2 is a typical heavy-fermion superconductor,and its unique superconductivity mechanism is still an unresolved research topic which draws much interest.The superconductivity of CeCu2Si2 origins from its 4f electrons which hybridize with conduction electrons and raise quasiparticles with large effective mass,such heavy quasiparticles are impossible to be explained with the electron-phonon coupling mechanism in BCS theory.In early years people widely agreed that the superconductivity paring symmetry of CeCu2Si2 is d-wave,and its pairing glue probably comes from the magnetic fluctuation near the quantum critical point.However,recent works have proposed new paring symmetries like s± and d+d,thus,the exact superconductivity mechanism of CeCu2Si2 is still under debate.For solving this problem,it is crucial to get the precise Fermi surface information.However,since CeCu2Si2 is rather difficult to be cleaved,the direct momentum-resolved electronic structure(especially the heavy quasiparticle dispersions)is still lacking.In this main work of the thesis,we have conquered several technical difficulties and systematically studied the electronic structure of CeCu2Si2.We have determined the detailed Fermi surface constructions and the multiband nature of CeCu2Si2,and given the direct evidence that there is strong hybridization between conduction and 4f electrons.This hybridization is momentum-dependent,which raises quasiparticles with different effective mass.We have also revealed the existence of heavy electron Fermi surface with almost full 4f weight near the M point in momentum space,which is important for its heavy-fermion superconductivity.We have also revealed some relatively lighter quasiparticles near the Fermi energy,which is possibly related to the multi-band superconductivity in CeCu2Si2,and their 4f weight isogenesis could provide a platform for intra or inter band scattering or even superconductivity pairing.Our measurement gives a direct depict for the electronic structure of CeCu2Si2,which would provide important experiment basis for further theoretical and calculation works.YbPtBi is the "heaviest" one among the heavy-fermion compound family.We have systematically investigated its electronic structure and found that the low carrier density and multiband semimetal nature dominate the transport properties of YbPtBi,and such low carrier density is also responsible for its ultra-low Kondo temperature and fragile antiferromagnetic order.We have also observed a possible crystal field splitting of Kondo resonance,which shall be further verified.Another interesting phenomenon is the Rashba state observed in Bi-terminated sample,which could be effectively tuned by electron doping.Rare-earth mono-pnictides exhibit extreme magnetoresistance at low temperature,and we pick Pr/Sm mono-antimonides/bismuthides to investigate the origin of extreme magnetoresistance and the possible influence from 4f electrons.Combining photoemission with transport measurements and calculations,we have found such large magnetoresistance can be well explained with electron-hole compensation mechanism.This conclusion could be a reference for other compounds which exhibit similar magnetoresistance behavior.And we have also confirmed the localization property of 4f electrons in the materials.