Interband Plasmon Research Articles

Electron Energy Loss Spectroscopy of YBa2Cu3O7–x

AbstractElectronic excitations of Y–Ba–Cu–O polycrystalline material in superconducting form have been investigated by electron energy loss spectroscopy in reflection. The data obtained confirm recent band structure models for this material. Low electron energy losses correspond to energy levels close to the Fermi energy with an upper and lower Hubbard subband and an anionic O 2p band in the middle. Loss peaks due to interband transitions, plasmon excitations, and core electron excitations have been observed, the origin of which is interpreted by a schematic energy level diagram.

Interband transitions, plasmons, and dispersion in hexagonal boron nitride.

We have measured inelastic-electron-scattering spectra of several hexagonal-boron-nitride samples with momentum transfer both in and out of the a-b plane and obtained the dielectric and optical constants from 0 to 60 eV. The low-q energy-loss spectrum with momentum in the plane is dominated by the \ensuremath{\pi}-electron plasmon at 8.5 eV and the total (\ensuremath{\sigma}+\ensuremath{\pi}) plasmon at 26.4 eV. The \ensuremath{\pi} plasmon arises from two strong interband transitions at 6.1 and 6.95 eV, and a continuum threshold at 7.6 eV. The plasmons are well described as collective oscillations of bound electrons. We have inferred a band gap of 5.9 eV by observing the intrinsic absorption threshold in a series of samples of varying purity. The dispersion in the plasmons and the second interband transition is quadratic for 0<q<1.0 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, while the first interband transition disperses upward in energy up to 0.6 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, above which its energy remains almost constant. The dispersion of the \ensuremath{\pi} plasmon is equal to that of the second interband transition, and its width remains constant up to a critical momentum, indicating that its width is dominated by decay into single-particle transitions. The energy-loss function with q along c shows three collective oscillations at 7.7, 11.7, and 23 eV. The interband spectrum is similar to that with q in the plane, except that an additional transition appears at 9.9 eV and the oscillator strength is shifted to higher energies. The similarity in the spectra for q in and out of the plane indicates nearly degenerate occupied \ensuremath{\sigma} and \ensuremath{\pi} states near ${E}_{F}$, which is inconsistent with existing band-structure calculations.

The dielectric description of inelastic electron scattering

Abstract It is shown that the dielectric permittivity ϵ is a useful concept for the description of interband transitions and plasmon excitation in electron energy loss spectroscopy (EELS). After a brief introduction to what is necessary for understanding EELS in the framework of Maxwell theory (which is best suited to reveal the close relationship between such different experimental techniques as optical spectroscopy and EELS), examples are given from recent work in the field. Two problems from plasmon spectroscopy, viz. dispersion of the plasmon in Al and plural scattering, demonstrate that the theory of collective oscillations in the electron gas is far from being solved. Recently, powerful techniques have been devised for quantitative interpretation of interband transition in EELS. Examples show that EELS is advantageous over optical spectroscopy in many aspects. The dielectric theory of layered two-dimensional systems is of current interest in the theoretical and experimental investigation of modulation-doped superlattices and intercalated structures. There is reason to hope that, by properly choosing the parameters of the superlattice, new decay channels can be opened for excited electrons in a formerly inaccessible energy-momentum range, thus giving rise to new and unexpected phenomena. At present, surface plasmons are of interest mainly in the study of the two-dimensional electron gas (2DEG). Besides a number of unique electronic features, collective surface modes have gained practical importance in the design of far-infrared emitters (FIREs), solar cells and flat TV screens. Two examples are discussed: the FIRE, a simple device which in the long run might replace infrared light-emitting diodes, and the use of surface modes in high resolution electron spectroscopy.

Electronic excitations of the YBa2Cu3O7-x superconductor: A study by transmission electron-energy-loss spectroscopy with an electron microprobe.

Interband and core-level transitions and plasmon resonance in the high-temperature oxide superconductor $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ were studied by transmission electron-energy-loss spectroscopy using an electron microprobe \ensuremath{\sim}5-10 \AA{} in size. Both single-crystal and polycrystalline ceramic samples have been examined. No significant change in electronic structure was observed at the grain boundaries. A 13.5-eV interband-transition peak was found to be sensitive to damages created by ion-beam bombardment.

Theory of the overlayer plasmon on the Si(001)2 x 1-K surface.

The dispersion of the overlayer plasmon on Si(001)2\ifmmode\times\else\texttimes\fi{}1-K surface is investigated theoretically based on structure models in which alkali-metal adatoms form one-dimensional chains on the ridge of Si dimers. First, a simplified rod model is introduced to clarify qualitative features of the overlayer plasmon in such systems. Then a first-principles band-structure calculation is performed to elucidate the detailed electronic structure of Si(001)2\ifmmode\times\else\texttimes\fi{}1-K surface. Based on the surface bands obtained, the dispersion of the overlayer plasmon on this surface is calculated nonempirically by the use of the random-phase approximation. The interband plasmon mode between K 4s--like and K 4${p}_{x}$--like bands is found to reproduce the observed plasmon dispersion very well.

Electronic structure and collective excitations of alkali metal chain on Si(100)2×1 surface

Abstract An alkali-adatom chain on a Si(100)2×1 surface shows peculiar properties originating from the unique overlayer structure. The insulator-metal change observed at θ ~ 0.6 is correlated with the onset of chain formation around this coverage. Surface bands are calculated by the LCAO-Xα-slab method. Three surface bands with the character of antibonding K 4s-dimer σ, bonding K 4p x -dimer π ∗ , bonding K 4p z -dimer π are found in the gap region. General properties of the overlayer plasmons of such adatom chains are clarified by a simple rod model and by non-empirical calculations based on the surface bands. Overlayer plasmons observed by AREELS are assigned to the s → p x ( x ⊥ chain axis, z | surface) interband plasmon mode.

Theory of Surface Plasmons of the Alkali-Adatom-Chain Model on Si(100)2×1 Surfaces

With the use of the random-phase approximation, we obtained the dispersion relation of the inter- and intra-surface-band plasmon of the parallel-rod model, which simulates the alkali-metal adatom chains on Si(100)2\ifmmode\times\else\texttimes\fi{}1 surfaces. There are two interband plasmon modes: One corresponds to the polarization vertical to the surface and the other to the polarization vertical to both the rod axis and the surface normal. The results explain fairly well the plasmon modes of the Si(100)(2\ifmmode\times\else\texttimes\fi{}1) K surface observed recently.

Valence and core excitation spectra in K, Rb, and Cs alkali-metal stage-1 intercalated graphite

Excitation spectra of the alkali-metal stage-1 intercalated graphites K${\mathrm{C}}_{8}$, Rb${\mathrm{C}}_{8}$, and Cs${\mathrm{C}}_{8}$ have been measured by high-resolution (0.1 eV) electron-energy\char22{}loss spectroscopy. The $\ensuremath{\pi}$ intraband and interband plasmon excitations in the region below 10 eV are the same in all three compounds. However, in the valence region between 15 and 30 eV, small peaks are observed in Rb${\mathrm{C}}_{8}$ and Cs${\mathrm{C}}_{8}$ which are weaker than those previously observed in K${\mathrm{C}}_{8}$. In K${\mathrm{C}}_{8}$, these structures were identified as both excitations to the backfolded graphite bands created by the introduction of the intercalant and also as characteristic metal-atom core excitations. Within this interpretation, the weakening of the structure due to the backfolded bands with intercalant atomic number is consistent with weakening the coupling far above the Fermi level between the graphite and intercalant atoms. The carbon $1s$ core shell excitation at \ensuremath{\sim}285 eV is also reported. We find that both the spectral shape and threshold energy are unaffected by the choice of alkali-metal intercalant (K, Rb, Cs), although the resulting spectrum differs from that of pristine graphite. Hence the graphite $p$ states just above the Fermi level which are probed by the C $1s$ excitation are identically perturbed by the K, Rb, and Cs intercalations, suggesting that the degree of hybridization and charge transfer in these materials is the same.

Low energy electron loss studies of cuInSe2 surfaces

AbstractThe low energy loss spectra (LELS) of clean and oxygen contaminated (111) p‐type CuInSe 2 single crystals are reported. Losses due to interband transitions and plasmon excitations are observed. By varying the primary electron energy (100–500 eV) both surface and bulk features of the conduction bands are probed. As oxygen is adsorbed on the surface of the CuInSe 2 (at room temperature), the In and Se atoms seem to associate with themselves and with oxygen, whereas the Cu atoms do not seem to play a direct role in the oxidation process. LELS Spectra are presented as a function of primary beam energy and oxygen absorption.

The electronic structure of ferric chloride intercalated graphite

Abstract The electronic excitation spectrum in energy and momentum space of stage 1 and stage 2 FeCl 3 intercalated graphite has been measured from 0.2 to 287 eV by high resolution (0.1 eV) electron energy loss spectroscopy. The dispersion of intra- and interband plasmons was measured and the dielectric function ϵ( q, E ) computed by a Kramers-Kronig analysis of the energy loss data. Critical points in the dielectric function and the interband plasmon dispersion indicate that the carbon π bands are only weakly perturbed. A tight binding model of the carbon π bands is used to compute ϵ( q, E ) and the intraband plasmon dispersion thus providing an accurate measure of the amount of charge transfer. In addition, the spectrum of excitations of carbon 1S core states to empty carbon 2P states directly probes the new states which appear after intercalation and provides a direct measurement of Fermi level lowering in intercalated graphite. Charge transfer per intercalant layer is constant in the various compounds and with respect to pure graphite the Fermi level has been lowered by 0.9 and 0.7 eV in stage 1 and stage 2 FeCl 3 -graphite, respectively.

ELS and AES studies of the initial oxidation of Ni(l10) surfaces

Abstract The interaction of oxygen with Ni(110) surfaces was investigated by means of electron loss spectroscopy (ELS) and Auger electron spectroscopy (AES). The ELS of clean Ni revealed the well known losses due to interband transitions and plasmon features. New losses due to oxygen adsorption were measured for exposure >10 L at Δ E = 5.8, 7.4, 13.9 and 23.9eV. The drastic change in the ELS at this stage of Ni oxidation coincides with the splitting of the M 23 loss peak, the changes in the MVV and L 3 VV Auger line shapes and a shift of the O(KLL) peak to lower energy.

Inelastic electron scattering by intra- and interband plasmons in rhenium trioxide, tungsten trioxide, and some tungsten bronzes

Energy-loss spectra were measured by backscattering 500-eV electrons from films of W${\mathrm{O}}_{3}$ grown on single-crystal W(100), and from single crystals of ${\mathrm{Na}}_{x}$ W${\mathrm{O}}_{3}$ ($0.35<x<0.86$) and Re${\mathrm{O}}_{3}$ which were cleaved in ultrahigh vacuum. The spectrum of a crystal with $x=0.61$ was nearly identical to the surface loss function $\mathrm{Im}[\ensuremath{-}{(\ensuremath{\epsilon}+1)}^{\ensuremath{-}1}]$ calculated from $\ensuremath{\epsilon}$ measured by optical reflectivity on a crystal with $x=0.65$. For $x=0.61$, two plasmons are observed, ${\ensuremath{\omega}}_{\ensuremath{-}}=1.90$ eV, and ${\ensuremath{\omega}}_{+}=6.5$ eV. ${\ensuremath{\omega}}_{\ensuremath{-}}$ scales approximately as ${x}^{\frac{1}{2}}$ and is assigned to the conduction-electron resonance. The plasmon lifetime ${\ensuremath{\tau}}_{p}=1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec determined from the observed linewidth agrees well with a value for the conduction-electron relaxation time ${\ensuremath{\tau}}_{e}=2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec we obtained from previously reported reflectivity data, but is shorter than a value ${\ensuremath{\tau}}_{c}=6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec we estimated from previously reported dc conductivity data. The higher-energy plasmon ${\ensuremath{\omega}}_{+}$ is assigned to a screened longitudinal resonance of an interband excitation near 5 eV. ${\ensuremath{\omega}}_{+}$ is essentially independent of $x$ for $0\ensuremath{\le}x<0.9$. By fitting $\ensuremath{\epsilon}$ between 0 and 10 eV with a model dielectric function, we found a value for the mean effective conduction electron mass ${m}^{*}=0.80 {m}_{e}$, the interband contribution to the static dielectric constant ${\ensuremath{\epsilon}}_{s}=4.35$, and values for parameters characterizing the interband resonance. Taking these latter parameters to be independent of $x$, we calculated the bulk and surface loss functions for different $x$ values. These calculations indicate that mode hybridization is unimportant for $x<1$ in the bronzes. The effects of damping or joint density-of-states width, screening, and mode interference on the plasmon spectra are discussed. It is shown that Re${\mathrm{O}}_{3}$ and Ag metal have dielectric and energy-loss functions which are similar to the tungsten bronzes. However, some of the assignments given in the literature as to the identities of the conduction-electron and interband plasmons are interchanged from those given above for the bronzes. We believe that the conduction-plasmon hybridization with interband resonances is less important in those materials than previously realized.

Surface characterisation of indium phosphide

Abstract LEED, characteristic loss spectroscopy, Auger electron spectroscopy, photoemission and light modulated contact potential methods have been employed to study indium phosphide surfaces. Cleaved (110) faces have a surface unit mesh identical to that of the bulk and surface states lead to band bending in the surface region. Reaction with oxygen leads to loss of phosphorus and the formation of an oxide layer near the surface. Features in the characteristic loss spectrum are discussed in terms of interband transitions and plasmon losses and the Auger spectra of oxidized surfaces are thought to contain components due to cross transitions. The surface composition, band bending and photothresholds of etched (100) surfaces of epitaxial InP layers have been established. Surfaces cannot be cleaned by heat treatment alone since decomposition occurs at elevated temperature. The influence of evaporated silver on the sign of the surface photovoltage has also been investigated.