Pseudogap behavior in Pr0.5Sr0.5MnO3: A photoemission study

The valence band electronic structure of Pr_0.5Sr_0.5MnO₃ has been investigated across its paramagnetic metallic (PMM)–ferromagnetic metallic (FMM)–antiferromagnetic insulator (AFMI) transition. Using surface sensitive high resolution photoemission we have conclusively demonstrated the presence of a pseudogap of magnitude 80 meV in the near Fermi level electronic spectrum in the PMM and FMM phases and finite intensity at the Fermi level in the charge ordering (CO)-AFMI phase. The pseudogap behavior is explained in terms of the strong electron–phonon interaction and the formation of Jahn Teller (JT) polarons, indicating the charge localizations. The finite intensity at the Fermi level in the insulating phase showed a lack of charge ordering in the surface of the Pr_0.5Sr_0.5MnO₃ samples.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

YNi4Cu: XPS measurements and electronic structure calculation

The valence band and the core levels of the YNi₄Cu compound are studied using the X-ray photoemission spectroscopy. The valence band is compared with the theoretical calculation by the spin-polarized Tight Binding Linear Muffin Tin Orbital method. The dominance of the Ni 3d and Cu 3d states down to 5 eV below the Fermi level is found. The modification of the bands’ widths and positions can be well explained by the d–d repulsion model. The Ni 2p and Y 3d peaks resemble the results for pure metals.     

Ultraviolet photoemission from polymeric sulfur nitride polycrystalline films and single crystals

The (SN)_x valence band structure, for polycrystalline films as well as for single crystal samples, has been studied using He I and He II resonance radiation. In angle-resolved photoemission energy distributions from single crystals, structure in the spectra is selectively enhanced offering a possibility of assigning the photoemission as originating from particular regions of the Brillouin Zone. The observed onset of photoemission 0.2 eV below the Fermi edge is discussed.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Conduction and valence band density of states of SnS2: Theory and experiment

The band structure of SnS₂ has been investigated over a wide energy range by pseudopotential band structure calculations and synchrotron radiation photoemission spectroscopy techniques. A good correspondence has been found between energy positions of the theoretical density of states features and structure in the constant initial state (CIS) and energy distribution curves (EDC's) for the conduction and the valence bands respectively. In the energy region between — 8 eV and 15 eV from the top of the valence band we observe four valence band and six conduction band peaks.     

Unified model of pseudogap features of conductivity in HTSCs

The pseudogap phenomenon in underdoped and optimally oxygen-doped high-temperature superconductors (HTSCs) of the Y₁Ba₂Cu₃O_x system is explained from a unified point of view within the model of negative U centers. It is shown that the pseudogap features of conductivity are not related directly to the superconductivity but arise due to the existence of statistical interaction of negative U centers with valence-band holes. Specifically due to this interaction, the hole density in the valence band does not remain constant. It differently changes with temperature for different mutual positions of the Fermi level and the valence band top. These differences lead to different temperature dependences of conductivity for underdoped and optimally doped samples.     

Magnetic and electronic properties of the antiferromagnetic YbFe4Al8 compound

The magnetic, electrical and electronic properties of the tetragonal ternary YbFe₄Al₈ compound have been investigated. This compound was supposed to be an antiferromagnetic superconductor due to the negative magnetization signal appearing at a low field of the field cooling mode, however, based on the measurements of the temperature dependence of magnetization and resistivity we do not confirm the presence of superconductivity in this material and we ascribe the negative magnetization to the complicated non-collinear magnetic structure. A switch to the antiferromagnetic order at about 150 K has been visible both on the M(T) and ρ(T) curves. The valence state of the Yb ions has been studied by X-ray photoemission spectroscopy. The valence band spectrum at the Fermi level exhibits the domination of the hybridized Yb(4f) and Fe(3d) states.     

Experimental evidence for the p-d hybridization in the Cd-Ca quasicrystal: origin of the pseudogap

Ca 2p-3d resonant photoemission spectroscopy of a Cd6Ca crystalline approximant unambiguously demonstrates that the low-lying unoccupied 3d levels of calcium are lowered below the Fermi energy by the formation of the approximant, as suggested from electronic structure calculations [Phys. Rev. Lett. 87, 206408 (2001)]]. Moreover, the Ca 3d partial density of states (DOS) obtained near the Fermi energy is in reasonable agreement with theoretical Ca 3d DOS. These results verify the unconventional picture that the origin of the pseudogap in the Cd-based quasicrystals is due to hybridization of the Ca 3d band with the Cd 5p band.     

Photoemission from Cerium systems

The analysis of the valence band photoemission spectra of Ce compounds is inspected. It is found that the data and the models agree in that they give a 4f hole state at a binding energy of 2 eV and a 4f band near (within 100 meV) the Fermi energy. The implications of photoemission data for the electronic structures of heavy fermion systems is discussed.     

The unoccupied electronic structure characterization of hydrothermally grown ThO2 single crystals

Single crystals of thorium dioxide ThO₂, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L₃, M₃, M₄, and M₅ X‐ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n‐type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

Photoemission from Co on Pt(111) using synchrotron radiation in the range 40 eV ω 150 eV

Photoelectron spectra for CO adsorbed on the (111)-face of Pt have been measured using synchrotron radiation of energy 40 eV ω 150 eV. A dramatic increase of the molecular orbital (MO) intensity relative to the intensity of the Pt 5d valence band (VB) is observed for ω > 100 eV, to a ratio at that is a factor of three higher than at . The energy variation of the Pt 5d VB photoemission peak intensity has been derived independently in the 40–200 eV range from measurements on clean Pt. The 5d peak intensity is found to decrease steeply (by more than an order of magnitude) between 100 and 150 eV. The observed increase of the MO peak intensity relative to that of the 5d VB is attributed to this cross-section effect. The Pt VB peak nearest the Fermi energy which is mainly t_2g in character, is found to decrease in intensity on adsorption of CO. In the present case synchrotron radiation in the ω > 100 eV range appears to be especially valuable for studies of adsorbates.     

Electronic structure of HgGa2S4

The electronic structure and chemical bonding in HgGa₂S₄ crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa₂S₄ is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS₂ and SrGa₂S₄ compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa₂S₄ crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.     

Mass-selected Ag<Subscript>3</Subscript> clusters soft-landed onto MgO/Mo(100): femtosecond photoemission and first-principles simulations

The electronic structure of supported mass-selected Ag₃ clusters is analyzed by joint femtosecond photoemission spectroscopy and ab initio theoretical investigations. A wide band gap insulating magnesia ultra-thin film on Mo(100) has been chosen as substrate in order to minimize the electronic interaction between metal clusters and support. After magnesia ultra-thin film preparation no photoemission from the molybdenum substrate is observed anymore, instead very weak two photon photoemission is detected possibly originating from surface or subsurface oxide defect states. Soft-landing deposition of 2 of atomic monolayer equivalents of Ag₃ clusters results in the disappearance also of the MgO two photon photoemission signal, while a strong single photon photoemission signal is detected from states located directly below the Fermi level. The theoretical study of structural, electronic and optical properties of Ag₃ at two model sites of MgO (100), the stoichiometric MgO(100) and an F_S-center defect, based on the DFT method and the embedded cluster model provides insight into the interactions between the cluster and the support which are responsible for the characteristic spectroscopic features.     

Resonant valence band satellite in palladium

Normal incidence photoemission has been used to investigate the existence of a valence band satellite as in the case of Ni. It is observed at 8.5 eV below the Fermi level and has an intensity which is only measurable when a photon energy is reached which corresponds to the threshold for excitation of an electron from the 4p levels to the Fermi level.     

Electronic structure of Cr1−δS (δ=0, 0.17) with NiAs-type crystal structure

Valence-band and conduction-band the electronic structure of the CrS (δ=0) and Cr₅S₆ (δ=0.17) has been investigated by means of photoemission and inverse-photoemission spectroscopies. The bandwidth of the valence bands of Cr₅S₆ (8.5 eV) is wider than that of CrS (8.1 eV), though the Cr 3d partial density of states evaluated from the Cr 3p-3d resonant photoemission spectroscopy is almost unchanged between the two compounds concerning shapes as well as binding energies. The Cr 3d (t_2g) exchange splitting energies of CrS and Cr₅S₆ are determined to be 3.9 and 3.3 eV, respectively.     

Photoemission study of the adsorption of O2, CO and H2 on GaAs(110)

Ultraviolet photoemission spectroscopy with hv < 12 eV has been used to study O₂, CO, and H₂ adsorption on the cleaved GaAs(110) face. It was found that O₂ exposures above 10⁵ L(1LM = 10^?6 Torr sec) were required to produce changes in the energy distribution curves. At O₂ exposures of 10⁶ L on p-type and 10⁸ L on n-type an oxide peak is observed in the EDC's located 4 eV below the valence band maximum. On p-type GaAs, O₂ exposures cause the Fermi level at the surface to move up to a point 0.5 eV above the valence band maximum, while on n-type GaAs O₂ exposures do not remove the Fermi level pinning caused by empty surface states on the clean GaAs. CO was found to stick to GaAs, but to desorb over a period of hours, and not to change the surface Fermi level position. H₂ did not affect the EDC's, but atomic H lowered the electron affinity and raised the surface position of the Fermi level on p-type GaAs. A correlation is found in which gases which stick to the GaAs cause an upward movement of the Fermi level at the surface on p-type GaAs, while gases which stick only temporarily do not change the surface position of the Fermi level.     

The electronic structure of the [CuO5]8− unit: A photoemission study of Y2BaCuO5

A photoemission study of the green phase Y₂BaCuO₅, including core level spectra of the elements, CuL ₃VV Auger, and XPS and UPS valence band spectra has been performed. The crystal structure of Y₂BaCuO₅ contains discrete [CuO₅]^8– units with divalent copper. The surface status of the Y₂BaCuO₅ samples is discussed with respect to the problems encountered in photoemission studies of the YBaCuO superconductors. The photoemission data are analysed within a simple cluster configuration interaction model by considering in addition previously reported optical spectra. Y₂BaCuO₅ is a charge transfer insulator with the charge transfer energy 0.5 eV being much smaller than thed–d Coulomb interaction energyU6eV. Cluster model parameters and valence band shapes of Y₂BaCuO₅ are compared with those of Nd₂CuO₄ the structure of which contains divalent copper within an extended copper-oxygen network. More detailed cluster and impurity models for cuprates described in literature are discussed in view of the present results.