Temperature-dependent surface states and transitions of Si(111)-7x7
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Abstract : Ultraviolet photoemission and electron-energy-loss measurements of Si(111) - 7x7 between T = 15 and 300 K reveal significant temperature-dependent changes in the occupied surface states and their transitions which can be associated with electron-phonon coupling at the surface. Several new surface states and transitions are determined at low temperatures, including a highly localized (about 2-meV-wide), half-occupied state that resides within a 100-meV-wide surface-state band gap and determines the Fermi-level position.Keywords:
Surface States
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Electron spectroscopy
Vibronic spectroscopy
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Inverse photoemission spectroscopy
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Valence-band photoemission at a photon energy of 32 eV has been carried out on Ge(100) from below room temperature to 1173 K. The $c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{\rightarrow}2\ifmmode\times\else\texttimes\fi{}1$ phase transition is accompanied by a shifting of a back-bond-derived surface state. The high-temperature $2\ifmmode\times\else\texttimes\fi{}\stackrel{\ensuremath{\rightarrow}}{1}1\ifmmode\times\else\texttimes\fi{}1$ transition is apparent in the discontinuity in the measured emission intensity of both a bulk and a surface electronic state. A further discontinuity occurs in both of these features and of the Fermi level intensity at higher temperature, approximately 1075 K, indicating the presence of a further reversible phase transition whose nature is discussed.
Inverse photoemission spectroscopy
Intensity
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Brillouin zone
Inverse photoemission spectroscopy
Surface States
Photon energy
Dangling bond
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Angle-resolved photoemission spectra from valence bands of a clean W(011) single crystal, obtained using synchrotron radiation in the range 1100-1250 eV and emission normal to the surface, were observed to show strong dependences on both photon energy and temperature. The spectral variations with excitation energy at ambient temperature agree very well with the predictions of a simple bulk direct-transition model assuming free-electron final-state dispersion and constant matrix elements. The strong temperature dependence observed indicates the importance of phonon-assisted nondirect transitions in Brillouin-zone averaging.
Brillouin zone
Photon energy
Atmospheric temperature range
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We have investigated coupling constants in elementary electron-phonon scattering processes on a graphite surface by the combined use of high-resolution electron-energy-loss spectroscopy (HREELS) and very low-energy electron diffraction (VLEED). HREELS is used to measure the modulations of electron transition probabilities from incoming electrons in vacuum to outgoing electrons in vacuum where the transition includes one-phonon scattering processes inside a solid. Determining the electronic band structures of graphite with VLEED, we defined electronic states of the solid surface that electrons entered before and after scattering off phonons. Thus, we observed that the measured electron transition probabilities significantly depended on whether the electrons were in a bulk Bloch state or an evanescent state before scattering off the phonons. This result clearly indicates that the measured electron transition probabilities reflect the strength of the coupling constants in the solid.
Electron spectroscopy
Electron scattering
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uv photoemission and electron-energy-loss measurements of Si(111)-7 \ifmmode\times\else\texttimes\fi{} 7 between $T=15 \mathrm{and} 300$ K reveal significant temperature-dependent changes in the occupied surface states and their transitions which can be associated with electron-phonon coupling at the surface. Several new surface states and transitions are determined at low temperatures, including a highly localized (\ensuremath{\sim}2-meV-wide), half-occupied state that resides within a 100-meV-wide surface-state band gap and determines the Fermi-level position.
Surface States
Inverse photoemission spectroscopy
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C 60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali-doped C 60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions.
Quasi Fermi level
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We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on graphite single crystal (kish graphite). We have successfully determined the electronic band structure in the close vicinity of the Fermi level and found an extremely small hole-like Fermi surface centered at the K(H) point. We also found a weakly dispersive band near the Fermi level around the K(H) point, which is not predicted by the bulk band calculation. The origin of this anomalous feature is discussed in relation to the electronic states characteristic of the stepped surfaces.
Inverse photoemission spectroscopy
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Complex structure in photoemission spectra of Be(0001) surface states near the Fermi energy is observed and explained as the effect of strong electron-phonon coupling. The weak momentum dependence of the electron-phonon contribution to the electron self energy \ensuremath{\Sigma} is exploited to determine \ensuremath{\Sigma} by direct inversion of the spectra.
Inverse photoemission spectroscopy
Self-energy
Sigma
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