Investigation of the band structure of germanium chalcogenides by means of photoelectron spectroscopy

The valence band photoelectron spectra of amorphous GeS, GeSe and GeTe have been measured. The ultraviolet and X-ray excited spectra show three bands associated with bonding p-states, anti-bonding and bonding s-states, respectively. In the ultraviolet photoemission spectra the p-band shows a pronounced fine structure. Features of the valence band density of states estimated on the basis of a tightbinding model are in reasonable agreement with experiment. Some problems connected with the extension of the ionicity concept of Phillips and van Vechten to the IV–VI compounds are discussed.     

Valence-level structure of phosphides of 3d-metals on the basis of XRS and ESCA data

For the first time, comprehensive X-ray spectroscopic and X-ray photoelectron data have been obtained on the energy spectra of valence electrons in phosphides of the transition metal series: TiP, CrP, MnP, FeP, NiP.Analysis of the experimental data on the electronic structure of phosphides of 3d-metals in the TiPNiP series indicates that the mechanism of interaction of the M 3d-states occurring near the top of the valence band with the s,p-states of phosphorus in the following sub-bands resides in the latter being induced near the d-sub-band as a result of the M 3d—P 3s,p interaction with the density maximum of the nearest P 3p-states being oppositely shifted as the number of d-electrons of the metal increases. Analysis of X-ray spectroscopic and X-ray photoelectron data on phosphides of transition metals along with those of metals of groups I(Cu, Ag) and II(Zn, Cd), and comparison of these data with the results of similar studies involving sulphides and silicides of the same metals indicate that the occupancy and the associated position of the d-shell of the metal within the valence band are the determining factors as far as the mutual arrangement of energy sub-bands and the symmetry of respective states are concerned.     

Theoretical study of X-ray photoelectron spectra of BN and SiC crystals

The tight-binding method has been used to calculate the density of states and X-ray photoelectron spectra of valence electrons in BN and SiC cubic crystals. It is shown that s- and p-states of atoms of various components contribute differently to the spectrum.     

UV-photoemission measurements on erbium and samarium

Photoemission measurements have been made on samarium and erbium in the photon energy range 4 to 21 eV. The photoelectron energy distributions are dominated by electron emission from valence band states whereas emission from 4f-states is unimportant. The width and energy of the occupied and unoccupied 5d-bands has been determined as well as the energy relative to the Fermi level of the bottom of the valence band. A model for the unscattered yield is presented allowing a determination of the hot electron scattering length for some rare-earths using available optical and photoemission data.     

A local mixed valence model

We consider a purely electronic model for local valence fluctuations consisting of a spinless localized 4f-level interacting with spinless extended 5d-states. We show that (a) the impurity model is related to the anisotropic Kondo problem, (b) a single 4f-level behaves like a resonant level with temperature-dependent width and (c) these results, when extended to a concentrated system of local incoherent mixed valence levels, are qualitatively in agreement with the main experimental features for mixed valence compounds.     

Energy Band Structure and Optical Properties of LiNaSO<Subscript>4</Subscript> Crystals

The energy band structure of a LiNaSO₄ has been calculated within the framework of density functional theory for space group P31c. It has been established that bandgap width E_g in the GGA approximation is 5.49 eV. The valence band top is generally formed of oxygen p-electrons, while the conduction band bottom is composed of lithium and sodium s-states. The effect of the cationic substitution Na → K → Rb → NH₄ on the electron structure of LiBSO₄ group crystals is considered. Based on the calculated dielectric functions, the spectral dependences of the refractive indices and extinction coefficient of a crystal were calculated and compared with experimental data.     

Electronic structure of compounds at platinum - silicon (111) interface

The electronic structure of Platinum silicides produced by thin film reaction is studied using ultraviolet photoemission and Auger spectroscopy. Spectra have been taken during the various stages of Si-Pt intermixing, in order to monitor the changes in the valence band, which take place during the reaction. The experimental data are compared with semi-empirical LCAO calculations. The importance of the coupling between Silicon p and Platinum d-states in determining the basic features of the chemical bond is discussed.     

The electronic structure and chemical bonding in rare earth chalcogenides according to the Xα SW method—I: EuO,EuS and EuSe

Calculations of the electronic structure for (EuO₆)^10? (EuS₆)^10?, and (EuSe₆)^10? clusters, which represent the nearest-neighbour sphere surrounding Eu in EuO, EuS, EuSe, have been made by the Xα SW method. The contour maps for valence MO's have been drawn. The MO energy diagrams are in reasonable agreement with experimentally derived band structures. In spite of the localized nature of 4f-derived states, there are noticeable admixtures of 4f-states to the valence band. The 5d Eu states hydridize with the chalcogenide valence states, and the covalency of the chemical bonding decreases from EuO to EuSe, which can explain the decrease of elastic modulus, microhardness and chemical stability.     

Solution of a spectral problem for electrons in an atom allowing for energy level widths

A method is proposed for describing electron excitations allowing for energy level widths, and it is numerically implemented for a beryllium atom. It is demonstrated that, when the beryllium atom is excited, the 2p-states are split, unequal shift of single-electron energy levels and mixing of the 2s and 2p states at excitation energies of 10 and 14 Ry are observed, and, starting from 6.7 Ry, the atom is stabilized.     

Lifetime measurements in the triplet system of Mg I

In an atomic beam the metastable 3s3p ³ P-states of Mg I were populated by electron impact. A pulsed tunable dye laser was used to excite the metastable atoms to the 3snd³ D-(n=3–7) and to the 3sns³ S ₁-states (n=4and 6). Observing the time dependence of the reemitted resonance light the lifetimes of these states were measured.     

Evidence for an alkali-like conduction band in alkali graphite intercalation compounds

The valence bands of pure graphite and several alkali graphite intercalation compounds (AGIC's) were studied by UPS (hv = 21.2 eV). The most significant observation is an intensity peak at the Fermi energy E_F in the intecalation compounds. This peak is mainly due to alkali-like s-states. The density of states at E_F is enhanced by a factor of 30 compared to pure graphite. The alkali-like conduction bands in the first stage AGIC's are similar to those of pure alkali metals.     

The electron structure and x-ray emission spectra of noble metal aluminides

A calculation of the electron structure of [Al₈Cu₄] and [Al₈Au₆]- clusters was performed by the XαSW SCF-method. The X-ray emission and photoelectron spectra for noble metal aluminides Al₂Cu and Al₂Au are interpreted on the base of these calculations. The question of the possible influence of d-states on the distribution of Al3s-states and the shape of the AlL_2,3-emission spectrum is discussed for Al₂Cu.     

Correlation effects in the photoelectron spectra of nickel

An approximate method is presented to calculate the orbital relaxation in photoelectron spectra for systems with a continuous distribution of electron states. The results for Ni 3d-states explain the discrepancy between experimental data and band structure calculations. The degree of d-state localization is discussed.     

Electronic structure of cubic uranium compounds

Self-consistent cellular multiple scattering techniques and photoemission energy distribution curves obtained at 20<hv<80 eV are used to study the density of states of UN and US. The calculations are based on a model using a finite cluster of atoms in a condensed-matter-like boundary potential. The main results refer to the mixing of thes, p, d, andf-states of uranium into a valence and a conduction band. Thef-states form orbitals with the ligands, within the valence and conduction bands. In the nitride the amount off character in the valence band is only 0.3 electrons and thef electrons are in two resonant levels (of each spin) in the conduction band. Only the first of these levels is occupied for the local, alternate from atom to atom, majority spin. In the sulfide the amount off character in the valence band is 0.59 electrons and the rest of thef-levels are in a resonance state (of majority spin) at the beginning of the conduction band. The conduction band is mainly of localized uranium 6d character. The theoretical results compare favorably with the photoemission data reported here.     

Metalloradical EPR Signals from the YZ·S-State Intermediates in Photosystem II

The redox-active tyrosine residue (Y_Z) plays a crucial role in the mechanism of the water oxidation. Metalloradical electron paramagnetic resonance (EPR) signals reflecting the light-induced Y_Z· in magnetic interaction with the CaMn₄-cluster in the particular S-state, Y_Z·S_X intermediates, have been found in intact photosystem II. These so-called split EPR signals are induced by illumination at cryogenic temperatures and provide means to both study the otherwise transient Y_Z· and to probe the S-states with EPR spectroscopy. The illumination used for signal induction grouped the observed split EPR signals in two categories: (i) Y_Z in the lower S-states was oxidized by P680⁺ formed via charge separation, while (ii) Y_Z in the higher S-states was oxidized by an excited, highly oxidizing Mn species. Applied mechanistic studies of the Y_Z·S_X intermediates in the different S-states are reviewed and compared to investigations in photosystem II at physiological temperature. Addition of methanol induced S-state characteristic changes in the split signals’ formation which reflect changes in the magnetic coupling within the CaMn₄-cluster due to methanol binding. The pH titration of the split EPR signals, on the other hand, could probe the proton-coupled electron transfer properties of the Y_Z oxidation. The apparent pK _as found for decreased split signal induction were interpreted in the fate of the phenol proton.     

On cyclic region solutions of the Hubbard model

Exact many-electron solutions of the extended Hubbard Hamiltonian are found for four electrons in s-states on four atoms. These are discussed in relation to similar finite system solutions and in relation to the problem of characterizing electron correlation effects in infinite lattices.     

Electronic structure and X-ray spectra of transition metal sulfides NiS,CuS, and ZnS

A numerical electronic band structure calculations for sulfides NiS, CuS, and ZnS are carried out. Using the results a detailed analysis of a valence states is performed; obtained partial densities of states are compared with X-ray SL _2,3 and $ SK_{\beta _{1,3} } $ SK_{\beta _{1,3} } -emission spectra. We showed that spectrum lineshape depends on hybridization strength between various Me(3d)-orbitals and 3p-states of sulfur. The hybridization strength and the symmetry of hybrid Me(3d)-orbitals are defined by crystal lattice structure. Finally a well splitted in energy bonding and antibonding states Me(3d)-S(3p) appear while weakly hybridized Me(3d)-states mainly contribute to spectra intensity in the energy between them. A good agreement between the theoretical and the experimental spectra of valence band for considered sulfides is obtained.     

High-lyingM 1-states of spherical nuclei

The cross section of inelastic scattering of slow electrons at large angles with excitation of high-lying 1⁺-states in²⁰⁸Pb are calculated in DWBA. The calculations are carried out both in the random-phase approximation and with the interaction of one-and two-phonon states in the framework of semi-microscopic quasi-particle-phonon model. A group of noncollective 1⁺-states with a large excitation probability in backward (e, é)-scattering is found in the excitation energy region of 17–21 MeV. We discuss also the problem of existence, properties, and possibility of discovery the high-lying collective 1⁺-states (2?ω M 1-resonance) predicted by Speth et al.     

One-electron spin-dependent transport in split-gate structures containing self-organized InAs quantum dots

The paper presents the results obtained in a study of electron transport in split-gate structures prepared from heterostructures with self-organizing InAs quantum dots situated close to a two-dimensional electron gas. Coulomb oscillations of current through InAs quantum dots depending on the voltage on the gate were observed. Coulomb current oscillations persisted up to about 20 K. The Coulomb energy ΔE _C = 12.5 meV corresponding to theoretical estimates for the p-states of quantum dots in our structures was determined.     

Cohesive properties of (Cu,Ni)–(In,Sn) intermetallics: Database,electron-density correlations and interpretation of bonding trends

This paper presents a systematic and comparative study of the composition and volume dependence of the cohesive properties for a large group of Me–X intermetallic phases (IPs) with Me=Cu,Ni and X=In,Sn, which are of interest in relation with the design of lead-free soldering (LFS) alloys. The work relies upon a database with total-energy versus volume information developed by using projected augmented waves (PAW) calculations. In previous papers by the current authors it was shown that these results account satisfactorily for the direct and indirect experimental data available. In the present work, the database is further expanded to investigate the composition dependence of the volume (V₀), and the composition and volume dependence of the bulk modulus (B₀) and cohesive energy (E_coh). On these bases, an analysis is performed of the systematic effects of replacing Cu by Ni in several Me–X phases (Me=Cu,Ni and X=In,Sn) reported as stable and metastable, as well as various hypothetical compounds involved in the thermodynamic modeling of IPs using the Compound-Energy Formalism. Moreover, it is shown that the cohesion-related quantities (B₀/V₀)^½ and (E_coh^½/V₀) can be correlated with a parameter expressing the number of valence electrons per unit volume. These findings are compared in detail with related relations involving the Miedema empirical electron density at the boundary of the Wigner–Seitz cell. In view of the co-variation of the cohesive properties, E_coh is selected as a key property and its composition and structure dependence is examined in terms of a theoretical view of the bonding which involves the hybridization of the d-states of Cu or Ni with the s and p-states of In or Sn, for this class of compounds. In particular, a comparative analysis is performed of the DOS of various representative, iso-structural Me–X compounds. Various effects of relevance to understand the consequences of replacing Cu by Ni in LFS alloys are highlighted and explained microscopically for the first time.