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内容記述 |
Heavy fermions, one of the strongly correlated electrons systems (SCES), have attractedmuch attention due to the wide ground states from magnetism to non-magnetism through aquantum critical point (QCP). This phase variation shows the similar tendency with theothers of SCES, i･e･ the High-Tc cuprate superconductors and organic conductors, despite ofa lack evidence of connection among them. Therefore, the explicit understanding of thephase variation in heavy fermion will provide the clues to resolve the problems of SCES. Inheavy fermion system, the ground state varies as a function of the cf-hybridization strength, responsible for Kondo effects, between the local 4f electrons and the itinerant conductionelectrons in the Doniach phase diagram. In spite of the numerous research for the heavyfermions, the role of the cf-hybridization through the QCP remains poorly understood in theview of the electronic structure, because of the difficulty of a systematic experiments andthe direct observation of the cf-hybridization band.<br /><br /> In his thesis, the two heavy fermion systems, CeNi<small>1-x</small>Co<small>x</small>Ge<small>2</small> and CeCoGe<small>1.75</small>Si<small>1.25</small>, aremainly studied by several kinds of photoemission (PE) spectroscopies. First, in order to investigate Ce 4f characters across the QCP, he has performed the resonant, hard X-ray and high-resolution PE on the isostructural heavy fermion CeNi<small>1-x</small>Co<small>x</small>Ge<small>2</small> system, where the ground state changes from an antiferromagnetic (0 ≦ x ≦ 0.2) to a non-magneticregime (0.4 ≦ x ≦ 1) through the QCP(x = 0.3). In the resonant PE, the bulk properties of Ce4f electrons are obtained from the quantitative analysts of the non-crossing approximation(NCA) with considering the surface sensitivity of Ce 4d-4f and 3d-4f resonant PE spectra, respectively. The obtained bulk properties reveals the detail electronic structure, e.g. crystalline electric field (CEF) effects, and firstly shows that the Ce 4f electronic structurecontinuously develops across the QCP with increasing cf-hybridization intensity. In the hard X-ray PE, Ce 3d core-level was measured at photon energy, hv = 7941.5 eV, where the obtained spectra almost reflect the bulk properties due to the large escape depth, λ－200 Å Analysis of Ce 3d<small>5/2</small> peaks also shows the continuity of Ce 4f electronic structureacross the QCP in agreement with the results of the resonant PE. Multiplet structures areclearly observed in Ce 3d<small>3/2</small> peaks. And it is first found in the X-ray PE experiment thatthese multiplet structures are strongly related to CEF effects. Moreover, the quasi-Particle peak due to cf-hybridization and the detail electronic structuredue to CEF effects is directly observed in the high-resolution PE, showing the consistencywith the results of both resonant and hard X-ray PE. <br /><br />Secondly, the angle resolved photoemission (ARPES) study on a heavy fermionCeCoGe<small>1.75</small>Si<small>1.25</small> has been performed to explicitly understand cf-hybridization band in amomentum space. Even though the single impurity Anderson model (SIAM) well explainsthe spectra of the angle-integrated photoemission, Ce 4f electrons are actually affected bythe periodic potential in the solids. It surely makes the cf-hybridization bands. ARPES datashow that CeCoGe<small>1.75</small>Si<small>1.25</small>system is the quasi two-dimensional system: there is smalldispersion along the c-axis. Here, he presents the first observation of both the renormalizedCe 4f band and conduction band due to cf-hybridization by uslng Ce 4d-4f resonant ARPES. The quasi-particle peak, the so-called tail of the Kondo resonance, is strongly enhancedacross the intersected place of Ce 4f band and Co 3d conduction band in contrast to theSIAM expectation. This may be a reason why SIAM model well explains theangle-integrated PE. Furthermore, the cf-hybridization band is scaled by using the simpleperiodic Anderson model (PAM). Surprisingly, the cf-hybridization strength exhibits theanisotropy in the momentum space, which can not be expected in SIAM and simple PAMwhere the cf-hybridization strength is isotropic. This reveals that in heavy fermion system, cf-hybridization takes place in a lattice and the anisotropy of Kondo coupling should beconsidered. |
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