Electroreflectance observation of localized and itinerant electron states in NiO

We have observed electroreflectance spectra in NiO at 6 eV which we believe represent transitions from the oxygen 2_p derived valence band to the nickel 4s derived conduction band. We also observe the spectrum seen earlier by McNatt near 4 eV but interpret it differently in terms of transitions from the localized 3d⁸ state to the 4s band. These interpretations are consistent with the recent model of Adler and Feinleib.     

Conduction electron distributions in europium and gadolinium by soft x-ray spectroscopy

A study of the 5d6s conduction band in Eu and Gd by soft X-ray spectroscopy is presented. Our observation that the density of occupied d states is lower for Eu than for Gd is supported by band structure calculations. For Gd, the convolution integrals between the 3p_3/2 inner level Lorentzian distribution and the A.P.W. densities of states are compared with our experimental M_m spectra. The position of the occupied 4f states with respect to the Fermi level is determined, compared with X-ray photoemission data and discussed for both metals.     

Resonant photoemission in DyNi2Mn x rare-earth intermetallides

The electronic structure of the DyNi₂Mn _x rare-earth (RE) intermetallides whose cubic structure is similar to the structure of RT₂ compounds is studied. Resonant photoemission and X-ray absorption methods are used in the vicinity of the 2p- and 3p-excitation thresholds of transition elements and the 3p-, 3d-, and 4d-thresholds of RE metals to find the Ni, Mn 3d-, and R 4f-partial densities of the states in the valent band. The use of resonant photoemission allows us to establish features of the interaction between the unfinished 4f-shells of ions of RE metals with ions of the transition 3d-elements in RNi₂Mn _x compounds. The contributions from atoms of various elements to the structure of the valent band are separated, and the basic regularities of band formation during the introduction of manganese atoms are found.     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

Investigation of electronic states of infinite-layer SrFeO2 epitaxial thin films by X-ray photoemission and absorption spectroscopies

We investigated the electronic states of a single-crystal SrFeO₂ epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2p–3d resonant photoemission measurements revealed that the Fe 3d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3d-derived conduction band hybridized with O 2p states; these can be assigned to Fe 3d_xy, 3d_xz + 3d_yz, and 3d_x²_–y². In addition, the indirect bandgap value of the SrFeO₂ film was determined to be 1.3 eV by transmission and absorption spectroscopies.     

Observation of occupied 5d states in f.c.c. Yb metal

We propose experimental evidences for the sd character of the occupied band states of f.c.c. Yb metal. Photoionization cross-section σ¹ in the range 24–65 eV, covering the 5p core electron energies have been measured for both Yb (4ƒ¹⁴6s²5d⁰) and Lu (4ƒ¹⁴6s²5d¹). Both the hv dependence of σ¹ for the Yb valence band and the presence of photoemission resonances above the 5p edges of Lu and Yb indicate the presence of occupied 5d states at the top of the valence band of solid Yb. The Yb 4ƒ σ¹ is presented in order to distinguish the 5d band resonances from the 5p-5d giant dipole autoionization decays. L_{2, 3} X-ray absorption white lines for Yb provide a measure of the 5d bandwidth.     

Valence bands of layer structure transition metal chalcogenides

The valence band structure of representative MX₂ layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation. Chalcogen s and p and metal d band are identified and their width and position obtained. The results are compared with u.v. and He II photoemission and with recent band structure calculations.     

Die Änderung der Elektronenstruktur des Nickels durch Wasserstoffbeladung

TheK-absorption spectrum of nickel charged with hydrogen has been investigated using a Bragg spectrograph with photographic registration. The fine structure of the absorption edge of the nickel hydride as compared with that of nickel shows that the electrons of the dissolved hydrogen fill up the empty 3d- and 4s-states of the conduction band of the metal.     

Localized electronic excitations in nickel oxide

Valence-electron ionization and optical excitation energies of NiO have been calculated by the SCF?Xα cluster method and transition-state procedure. Ni 3d-like spin orbitals are localized above the O 2p band, leading to well defined peaks in the photoemission spectra. Ligand-to-metal charge-transfer excitations are responsible for strong optical absorption between 4 and 7eV.     

Surface photoemission in the 4d band from polycrystalline silver surfaces

Surface induced local d-band states in the upper 4d band between ～ 4 and ～ 5.2 eV below E_Fermi have been identified for polycrystalline silver films in photoemission experiments using synchroton radiation. A thin over-coat (10 å) by an Al film leads to a depression of these surface induced local states whereas a change from s- to p-polarized excitation leads to an enhancement. Deposition of additional silver (～ 3 Å) at 120 K induces additional emission 4.2 eV below E_F with a FWHM of only ～ 0.4 eV.     

Electronic structure of copper halides CuI and CuCl: A comparative X-Ray photoelectron and absorption spectroscopy study

The energy distributions of the occupied and unoccupied electronic states for copper halides CuCl and CuI have been investigated using X-ray photoemission and absorption spectroscopy with a highenergy resolution on the equipment of the Russian-German beamline for outlet and monochromatization of synchrotron radiation from the electron storage ring BESSY II. A quasi-molecular analysis of the obtained experimental spectra has revealed that there is a fundamental similarity of the energy structures of the valence band and the conduction band of copper halides CuX (X = Cl, I) due to the identical atomic structure of the studied compounds. The differences in the positions of individual energy subbands in the valence band and the conduction band of CuX and in their intensities in the spectra are associated with different degrees of hybridization of the Cu 3d, 4s and X(n + 1)s, np valence states, as well as with different sizes of structural units (CuCl₄ and CuI₄ quasi-molecules) of the studied crystals.     

Electronic structure of CsAu

X-ray photoemission spectra of CsAu prepared in vacuum are in good agreement with the ionic character expected for this material. The Cs 5p doublet lies well below flat Au 5d bands. The Au 6s valence band is located somewhat closer to the 5d states than recent band structure calculations indicate. The charge transfer to gold is in the range 0.6 to 0.8 electrons.     

Observation of p- and d-like surface states on CuCl(100) by angle-resolved photoemission

In the course of a systematic ultraviolet photoemission study of the electronic band structure of CuCl, we have identified two occupied surface states on CuCl(100), situated at 0.25 and 3.0 eV below valence band maximum in normal emission spectra. They essentially show pure p- and d-like orbital symmetry, respectively. We interpret them as a chlorine p_x,y-like occupied antibonding resonance and a copper Γ₁₂-derived state split off from the bulk orbitals by the surface potential. We also present critical point energies along Γ-X and Γ-L.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

Resonant photoemission at the 3p-core threshold from quasiatomic Ni in nickel-phthalocyanine

In nickel-phthalocyanine a resonant enhancement at the 3p?Ni photo-absorption maximum ($\text{hv ? 68.5 eV}$) of a peak approximately 6 eV below the maximum of the Ni 3d derived states is observed in photoemission experiments with variable excitation energy (synchrotron radiation). Our observation of this resonance, explained by configuration interaction of Ni 3p→3d and 3d-continuum transitions, gives direct evidence for the atomic nature of the 6 eV resonance structure in photoemission from bulk Ni.     

Development of electronic structure of Ni thin films on Cu(0 0 1) surfaces

We have traced the development of the Ni electronic structure with thickness variation on flat and nano-structured Cu(0 0 1) surfaces by means of photoemission spectroscopy. The binding energy of the Ni 2p_3/2 main peak and satellite peak is found to have shifted monotonically in the direction opposite to each other, with the increase of Ni coverage. The reduced binding energy of the thin film’s main peak is strongly correlated to the Cu 4s/Ni 3d interfacial hybridization effect (s/d IHE) and the narrowing of the d band with the reduction of dimensions, while the increased satellite binding energy results from the combination of interface hybridization and expansion of an extended 4s-like state towards the vacuum. The center of the Ni d_xy band is predicted to shift closer to the Fermi level with increasing film thickness. Enhanced satellite intensity in thin films is observed, correlating to the narrowing of the d band with decreased film thickness. Through comparison of Ni films grown on flat versus nano-structured Cu(0 0 1) surfaces, the mixing of Cu and Ni atoms is found to be enhanced at the terrace edge region and consequently a larger s/d IHE is predicted for Ni on the nano-structured surface.     

On the interpretation of valence band photoemission spectra of NiO

The valence band photoemission spectrum and the L₃VV Auger spectrum of NiO are compared. The satellite found in the valence band of NiO and other Ni compounds is interpreted as an unscreened 3d⁷ final state, whereas the main d-emission is a 3d⁷ final state screened by a d electron in an exitonic state.     

Angle-resolved photoemission spectroscopy study of the (101) surface of the Kondo insulator CeNiSn

The electronic structure of CeNiSn, which is considered a possible topological Kondo insulator, has been investigated by employing synchrotron radiation excited angle-resolved photoemission spectroscopy (ARPES). We have found that the easy cleavage plane in CeNiSn is (101), for which we have investigated the Fermi surface (FS) and band structures. The measured FS and ARPES for the (101) plane are described well by the calculated FS and band structures, obtained from the DFT calculations. The measured ARPES bands and photon energy map show that the metallic states crossing the Fermi level have the 3D nature, casting a negative suspicion for the existence of the topological surface states of the 2D character in CeNiSn. The Ce 4f Kondo resonance peak is observed in Ce 4d → 4f resonant photoemission spectroscopy, suggesting the importance of the Ce 4f electrons in determining the temperature-dependent topological electronic structure of CeNiSn.     

Photoemission of LaI2 and CeI2

The electronic structure of the layered compounds LaI₂ and CeI₂ was investigated by photoemission and electron energy loss spectroscopy. From the experimental results we are able to confirm the metallic nature of these compounds, and by using photon energy dependent measurements of the valence band we can identify the orbital character of the conduction band as essentially 5d¹-like. A detailed analysis of the Ce 3d and 4f spectra yields a remarkably small 4f-5d hybridization strength, almost completely decoupling the f-electron from the conduction band, which makes CeI₂ a somewhat unusual system compared to other metallic Ce compounds. Band structure calculations by Jepsen and Andersen [1] confirm these experimental results.     

Separation of the partial s- and p-densities of valence states of ZnO from UPS-measurements

UPS-spectra of the cleaved (0001) Zn and (000$\text{1}$) O surfaces of ZnO are taken at hv = 16.8, 21.2, 26.9, and 40.8 eV. Two maxima in the spectra at constant final energy are ascribed to high densities of conduction band states. Using the hv-dependence of the valence band emission, the partial s- and p-densities of states are separated. They yield similar excitation probabilities for Zn-4s-, 3d-, and O-2p-electrons.