Energy dispersion of 4f-derived emissions in photoelectron spectra of the heavy-fermion compound YbIr2Si2

Band structures of 4f and 5f materials studied by angle-resolved photoelectron spectroscopy

Recent remarkable progress in angle-resolved photoelectron spectroscopy (ARPES) has enabled the direct observation of the band structures of 4f and 5f materials. In particular, ARPES with various light sources such as lasers (hν ~ 7 eV) or high-energy synchrotron radiations (hν >/~ 400 eV) has shed light on the bulk band structures of strongly correlated materials with energy scales of a few millielectronvolts to several electronvolts. The purpose of this paper is to summarize the behaviors of 4f and 5f band structures of various rare-earth and actinide materials observed by modern ARPES techniques, and understand how they can be described using various theoretical frameworks. For 4f-electron materials, ARPES studies of CeMIn5(M = Rh, Ir, and Co) and YbRh2Si2 with various incident photon energies are summarized. We demonstrate that their 4f electronic structures are essentially described within the framework of the periodic Anderson model, and that the band-structure calculation based on the local density approximation cannot explain their low-energy electronic structures. Meanwhile, electronic structures of 5f materials exhibit wide varieties ranging from itinerant to localized states. For itinerant U5f compounds such as UFeGa5, their electronic structures can be well-described by the band-structure calculation assuming that all U5f electrons are itinerant. In contrast, the band structures of localized U5f compounds such as UPd3 and UO2 are essentially explained by the localized model that treats U5f electrons as localized core states. In regards to heavy fermion U-based compounds such as the hidden-order compound URu2Si2, their electronic structures exhibit complex behaviors. Their overall band structures are generally well-explained by the band-structure calculation, whereas the states in the vicinity of EF show some deviations due to electron correlation effects. Furthermore, the electronic structures of URu2Si2 in the paramagnetic and hidden-order phases are summarized based on various ARPES studies. The present status of the field as well as possible future directions are also discussed.

Structure and unusual magnetic properties of YbMn0.17Si1.88

YbMn0.17Si1.88 was synthesized from the reaction of ytterbium, manganese, and silicon using indium as a flux. The average structure of YbMn0.17Si1.88 was refined in the monoclinic space group P21, with a = 4.0107(8) Å, b = 3.8380(8) Å, c = 14.458(3) Å, β = 97.97(3)°, R1/wR2 = 0.0296/0.0720. The structure can be described as the intergrowth of three AlB2-type layers and one BaAl4-type layer. Magnetic susceptibility measurements suggest that the ytterbium atoms in YbMnxSi2-x exist in a mixed valent or intermediate valent state. YbMn0.17Si1.88 shows weak antiferromagnetic ordering below ∼4.5 K. The magnetic interactions between the Mn and Yb atoms in YbMn0.17Si1.88 are evident from the magnetic susceptibility measurements performed at low field. A negative magnetization is observed on warming and a positive magnetization on cooling. The heat capacity data suggest moderate heavy fermion behavior.

CeFePO: f-d hybridization and quenching of superconductivity

As a homologue to the new, Fe-based type of high-temperature superconductors, the electronic structure of the heavy-fermion compound CeFePO was studied by means of angle-resolved resonant photoemission. It was experimentally found-and later on confirmed by local-density approximation (LDA) as well as dynamical mean-field theory (DMFT) calculations-that the Ce 4f states hybridize to the Fe 3d states of d{3z{2}-r{2}} symmetry near the Fermi level that discloses their participation in the occurring electron-correlation phenomena and provides insight into mechanism of superconductivity in oxopnictides.

Tuning the hybridization at the surface of a heavy-fermion system

Electron-hybridization phenomena in YbRh_{2}Si_{2} were probed by angle-resolved photoemission. It was shown that the Yb 4f-Rh 4d hybridization strength in the surface region of this heavy-fermion material can be varied by deposition of Ag. Site-specific charge transfer from adatoms leads to change of the energy overlap of the interacting states close to the Fermi energy. Our study demonstrates a new way to tune the hybridization between 4f and valence electrons as well as the induced strong correlation effects at the surface of heavy-fermion systems.

Strong mass renormalization at a local momentum space in multiorbital Ca1.8Sr0.2RuO4

We have studied the mass renormalization in Ca2-xSrxRuO4 (x=0.2) using high-resolution angle-resolved photoemission spectroscopy. We observed precise band dispersions near the Fermi level (E_{F}) and the corresponding Fermi surfaces. A characteristic flat band with approximately 4 meV dispersion accompanying sharp quasiparticle (QP) peaks shows up in a limited momentum region around (pi, 0). The QP peak rapidly evolves below the crossover temperature T;{*} approximately 20 K, which agrees well with the mass enhancement behavior indicated by thermal, magnetic, and transport properties. We discuss the origin of the mass renormalization in relation to the local flat band at (pi, 0) possibly derived from the gamma (d_{xy}) band.

Hybridization phenomena in nearly-half-filled f-shell electron systems: photoemission study of EuNi2P2

The mixed-valent compound EuNi2P2 was studied by photoemission. Observed splittings and dispersions of the Eu 4f;{6} final state close to energy crossings of the Eu 4f and Ni 3d states are explained in terms of hybridization by a momentum and energy dependence of the electron hopping matrix element. These data obtained for a system with more than one 4f electron (hole) show that dispersions and hybridization gaps related to Kondo and heavy-fermion behavior can be found in other rare-earth-metal compounds apart from Ce and Yb-based ones.

Direct observation of dispersive Kondo resonance peaks in a heavy-fermion system

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe1.2Si0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.

Photoemission insight into heavy-fermion behavior in YbRh2Si2

As shown by angle-resolved photoemission (PE), hybridization of bulk Yb 4f(2+) states with a shallow-lying valence band of the same symmetry leads in YbRh2Si2 to dispersion of a 4f PE signal in the region of the Kondo resonance with a Fermi-energy crossing close to Gamma[over ]. Additionally, renormalization of the valence state results in the formation of a heavy band that disperses parallel to the 4f originating signal. The symmetry and character of the states are probed by circular dichroism and the photon-energy dependence of the PE cross sections.