Temperature-invariant photoelectron spectra in cerium heavy-fermion compounds: Inconsistencies with the Kondo model.

4f levels in Ce heavy-fermion compounds are examined using resonant photoemission. We find the following inconsistencies with the predictions of the Kondo model: (a) All temperature dependence can be accounted for simply by phonon broadening and the Fermi function; (b) the spectral weights of the features near EF do not scale with TK; and (c) the line shape of the feature previously identified as the Kondo resonance is Lorentzian and about an order of magnitude broader than predictions. Instrument resolution is not a limiting factor. © 1992 The American Physical Society

Temperature-invariant photoelectron spectra in cerium heavy-fermion compounds: Inconsistencies with the Kondo model.

4f levels in Ce heavy-fermion compounds are examined using resonant photoemission. We find the following inconsistencies with the predictions of the Kondo model: (a) All temperature dependence can be accounted for simply by phonon broadening and the Fermi function; (b) the spectral weights of the features near EF do not scale with TK; and (c) the line shape of the feature previously identified as the Kondo resonance is Lorentzian and about an order of magnitude broader than predictions. Instrument resolution is not a limiting factor. © 1992 The American Physical Society

PHOTOEMISSION IN YBCU2SI2 - PROBLEMS WITH THE KONDO IMPURITY MODEL

We report valence band photoemission results for YbCu2Si2, The 4f13(J = 7 2) final state peak, centered 60 meV below the Fermi level ε{lunate}F, lacks the temperature dependence and is broader than predicted for a Kondo resonance. Together with recent photoemission results for cerium compounds, these results raise serious doubts about the Kondo impurity explanation of heavy fermion photoemission. © 1992

Photoemission spectra of CeAl3, CeBe13, CeSi2, and CeCu2Si2: Weights and widths of the 4f emission features.

We present valence-band photoemission spectra for CeAl3, CeBe13, CeSi2, CeCu2Si2, and related conventional rare-earth counterpart compounds, taken at photon energies corresponding to the giant 4d resonance with resolution ∼150 meV. We take into account the 5d emission, which comprises 30% of the valence-band emission at the 4d resonance. We compare the resulting 4f emission to the predictions of the Anderson impurity model calculated in a low-order 1/N expansion, and including spin-orbit, crystal-field, and finite Coulomb correlation effects. The calculation gives order-of-magnitude fits to the data, but underestimates the spectral weight near the Fermi level by a factor of 2 and overestimates the width of the main f emission at 2 eV by a factor of 4. We argue that Ce photoemission remains an open problem and discuss several experimental and theoretical issues which need to be resolved to make further progress. © 1993 The American Physical Society

Photoemission spectra of CeAl3, CeBe13, CeSi2, and CeCu2Si2: Weights and widths of the 4f emission features.

We present valence-band photoemission spectra for CeAl3, CeBe13, CeSi2, CeCu2Si2, and related conventional rare-earth counterpart compounds, taken at photon energies corresponding to the giant 4d resonance with resolution ∼150 meV. We take into account the 5d emission, which comprises 30% of the valence-band emission at the 4d resonance. We compare the resulting 4f emission to the predictions of the Anderson impurity model calculated in a low-order 1/N expansion, and including spin-orbit, crystal-field, and finite Coulomb correlation effects. The calculation gives order-of-magnitude fits to the data, but underestimates the spectral weight near the Fermi level by a factor of 2 and overestimates the width of the main f emission at 2 eV by a factor of 4. We argue that Ce photoemission remains an open problem and discuss several experimental and theoretical issues which need to be resolved to make further progress. © 1993 The American Physical Society

Inconsistencies with the single-impurity Anderson model in photoelectron spectra of cerium heavy fermion compounds

A series of cerium heavy fermion compounds have been studied in order to check for the systematics with TK and the temperature dependence of the Kondo resonance predicted by the Kondo model. Neither the systematics nor the temperature dependence is found, the latter primarily determined from a detailed study in CeSi2. The qualitative shapes of the features at EF remain nearly constant irrespective of TK, while all the temperature dependence can be explained as resulting from phonon broadening of core-like states as well as Fermi function broadening. In addition, if the d electron contribution to the spectra is subtracted, one obtains a symmetric, lorentzian line shape for the "main" 4f peak. © 1992

Inconsistencies with the single-impurity Anderson model in photoelectron spectra of cerium heavy fermion compounds

A series of cerium heavy fermion compounds have been studied in order to check for the systematics with TK and the temperature dependence of the Kondo resonance predicted by the Kondo model. Neither the systematics nor the temperature dependence is found, the latter primarily determined from a detailed study in CeSi2. The qualitative shapes of the features at EF remain nearly constant irrespective of TK, while all the temperature dependence can be explained as resulting from phonon broadening of core-like states as well as Fermi function broadening. In addition, if the d electron contribution to the spectra is subtracted, one obtains a symmetric, lorentzian line shape for the "main" 4f peak. © 1992

Photoemission and the Kondo model. Serious discrepancies between theory and experiment in heavy fermion systems

Resonant photoelectron spectroscopy has been used to examine the electronic structure of Ce- and Yb-based heavy fermions. The experimental results are compared with Gunnarsson-Schönhammer calculations which include crystal field and finite Uff effects. Comparison of the model and experimental results reveals a large disparity between predictions and actual results. In particular, the Kondo model predictions of width, energy position and spectral weight for features near the Fermi level are not realized. Additionally, there is disparity between calculation and photoemission data in the Fermi-level crossing positions for spectral features very near the Fermi level. These discrepancies are present for both Ce and Yb heavy fermion systems. We see no evidence of spectral features in the photoemission data which can be uniquely identified with the Kondo resonance while our calculations show the resolution is adequate to clearly distinguish such features. © 1993

Crystal fields, linewidths and temperature dependence in the photoelectron spectra of heavy fermion Ce and Yb compounds

We report high-resolution photoemission results for the 4f levels in YbCu2Si2 and CeSi2. The data are compared to the predictions of the Anderson impurity model for valence band photoemission. These predicted spectra are obtained both in an approximate fashion by broadening crystal field (CF) states whose parameters are determined from existing neutron scattering data, as well as from a Gunnarson-Schonhammer calculation. Inclusion of crystal fields does not affect our two fundamental conclusions: that there is a natural linewidth in the problem (≈130meV) which is much greater than predicted by the Anderson model, and that the expected temperature dependence of the Kondo resonance is absent. Due to the large natural linewidth, experimental resolution (60-95meV) is not a limiting factor. © 1992