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CARLO CALANDRA BUONAURA

Professore emerito
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2018 - Phosphorus Adsorption on Fe(110): An ab Initio Comparative Study of Iron Passivation by Different Adsorbates [Articolo su rivista]
Fatti, G.; Restuccia, P.; Calandra, C.; Righi, M. C.
abstract

From corrosion protection to embrittlement and lubrication, the presence of phosphorus at iron surfaces is critical for a range of processes and applications. However, phosphorus adsorption on iron has never been studied experimentally or theoretically. Here P chemisorption on the most stable surface of iron is analyzed by means of density functional theory. The most stable adsorption geometry and coverage are identified, and the changes induced by P adsorption on the structural and electronic properties of the metal are discussed. A systematic comparison among P, S, N, and O uncovers a peculiar behavior of the P and S species: attractive adatom-adatom interactions at high coverage lead to the formation of two-dimensional overlayers that reduce the metal reactivity and work function. This effect is important in iron embrittlement and for the functionality of P- and S-containing additives included in lubricant oils.

2009 - Role of surface states in the Casimir force between semiconducting films [Relazione in Atti di Convegno]
M., Govoni; CALANDRA BUONAURA, Carlo; A., Benassi
abstract

We present results of first principle calculations of the Casimir force between Si films of nanometric size, which show that it depends dignificantly upon the configuration of the surface atoms and give evidence of the importance of surface states.

2009 - Size quantization effects in thin film Casimirinteraction [Articolo su rivista]
A., Benassi; CALANDRA BUONAURA, Carlo
abstract

We investigate the role of size quantization in the vacuum force between metallic films of nanometric thickness. The force is calculated by the Lifshitz formula with the film dielectric tensor derived from the one-electron energies and wavefunctions under the assumption of a constant potential inside the film and a uniform distribution of the positive ion charge. The results show that quantization effects tend to reduce the force with respect to the continuum plasma model. The reduction is more significant at low electron densities and for film size of the order of few nanometers and persists for separation distances up to 10−50 nm. Comparison with previous work indicates that the softening of the boundary potential is important in determining the amount of the reduction. The calculations are extended to treat Drude intraband absorption. It is shown that the inclusion of relaxation time enhances the size quantization effects in the force calculations.

2008 - Effects of phase transitions in devices actuated by the electromagnetic vacuum force [Articolo su rivista]
Benassi, A; CALANDRA BUONAURA, Carlo
abstract

We study the influence of the electromagnetic vacuum force on the behaviour of a model device based on materials, like germanium tellurides, that undergo fast and reversible metal-insulator transitions on passing from the crystalline to the amorphous phase. The calculations are performed at finite temperature and fully accounting for the behaviour of the material dielectric functions. The results show that the transition can be exploited to extend the distance and energy ranges under which the device can be operated without undergoing stiction phenomena. We discuss the approximation involved in adopting the Casimir expression in simulating nano- and micro- devices at finite temperature.

2008 - Role of vacuum fluctuation forces in thin metal film stability [Articolo su rivista]
Benassi, A; CALANDRA BUONAURA, Carlo
abstract

Thin metal films are subject to the pressure caused by the zero-point oscillations of the electromagnetic field, which depends upon the film optical properties and the substrate reflectivity. It has been suggested that the force may be relevant in determining the stability of deposited pseudomorphic films with respect to buckling or island formation. We present a detailed analysis of its behaviour as a function of the optical parameters and we illustrate the conditions under which it may play some role.

2008 - Vacuum fluctuation forces between ultra-thin films [Articolo su rivista]
Benassi, A; CALANDRA BUONAURA, Carlo
abstract

We have investigated the role of the quantum size effects in the evaluation of the force caused by electromagnetic vacuum fluctuations between ultra-thin films, using the dielectric tensor derived from the particle in a box model. Comparison with the results obtained by adopting a continuum dielectric model shows that, for film thicknesses of 1–10 nm, the electron confinement causes changes in the force intensity with respect to the isotropic plasma model which range from 40% to few percent depending upon the film electron density and the film separation. The calculated force shows quantum size oscillations, which can be significant for film separation distances of several nanometers. The role of electron confinement in reducing the large distance Casimir force is discussed.

2007 - A study of the electromagnetic-fluctuation-induced forces on thin metallic films [Articolo su rivista]
Benassi, A; CALANDRA BUONAURA, Carlo
abstract

Using the plasma model for the metal dielectric function we have calculated the electromagnetic fluctuation induced force of a free standing metallic film in vacuum as a function of the film size and plasma frequency. If the film is deposited onto a substrate or interacts with a plate, both the sign and the value of the force are modified. Il is shown that the force can change sign from attractice to repulsive upon changing the substrate plasma frequency.

2007 - Theoretical simulation of core level photoemission in transition metal oxides [Abstract in Atti di Convegno]
Manghi, Franca; Rozzi, Ca; CALANDRA BUONAURA, Carlo
abstract

core level

2005 - Theoretical simulation of core-level photoemission in transition-metal oxides [Articolo su rivista]
Rozzi, Ca; Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract

We discuss the 2p and 3s line shape in NiO and MnO photoemission, and we show that the main features of all the spectra for both systems are reproduced within an unique theoretical scheme that takes into account the dynamics of the created photohole, together with the detailed band structure of the compounds. The solid state bonding and the antiferromagnetic order are found to deeply influence the characteristics of core level spectra; in particular, the large difference in the spin unbalance of MnO and NiO explains the different response of the two compounds to the creation of a core hole.

1997 - Localized and itinerant character of electron states in the photoemission from CuGeO3 [Articolo su rivista]
Villafiorita, A; Manghi, Franca; Parmigiani, F; CALANDRA BUONAURA, Carlo
abstract

It is shown that electron correlation among Cu d-electrons explains the photoemission data of CuGeO3 and is responsible for the insulating behaviour and of the observed complex satellite structures. The analysis and interpretation of the experiments require the use of a multi-band Hamiltonian which explicitly includes both on site electron correlation and a detailed description of the single-particle band states.

1996 - Electronic structure of the 1x1 YBa2Cu3O7/PrBa2Cu3O7 superlattice: a local spin density approximation with on-site Coulomb interaction [Articolo su rivista]
M., Biagini; Ossicini, Stefano; CALANDRA BUONAURA, Carlo
abstract

Electronic structure calculations based on the local-spin-density approximation (LSDA) fail to reproduce the antiferromagnetic ground state of PrBa2Cu3O7 (PBCO). We have performed linear muffin-tin orbital–atomic sphere approximation calculations, based on the local-spin-density approximation with on-site Coulomb correlation applied to Cu(1) and Cu(2) 3d states. We have found that inclusion of the on-site Coulomb interaction modifies qualitatively the electronic structure of PBCO with respect to the LSDA results, and gives Cu spin moments in good agreement with the experimental values. The Cu(2) upper Hubbard band lies about 1 eV above the Fermi energy, indicating a CuII oxidation state. On the other hand, the Cu(1) upper Hubbard band is located across the Fermi level, which implies an intermediate oxidation state for the Cu(1) ion, between CuI and CuII. The metallic character of the CuO chains is preserved, in agreement with optical reflectivity [K. Takenaka et al., Phys....

1996 - Structural and electronic properties of Sb islands on GaAs (110) [Articolo su rivista]
Magri, Rita; Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract

We present the results of theoretical calculations of the electronic structure of the Sb/GaAs(110) interface in the submonolayer coverage regime performed in the full ab initio self-consistent pseudopotential scheme. Different structural models for the edges of the extended Sb islands have been considered and their equilibrium geometry has been determined by total energy minimization. The single particle band structure shows interface states arising mainly from the incomplete bonding of the Sb adatoms at the terrace edge to the substrate, which fall within the optical gap. Distinct features are associated to different island terminations. The interface turns out to be metallic in all the considered cases with a partially occupied peak at the Fermi level. We have also studied the effect of including explicitly the on-site Hubbard electron-electron correlation in the calculation of the quasiparticle spectrum, obtaining the observed semiconducting interface when the Coulomb interaction parameter U is larger than 3 eV. The interface states within the optical band gap can be present also at higher coverages when some disorder exists, evidentiating a general mechanism for the Fermi level pinning at this interface.

1995 - ELECTRONIC-STRUCTURE OF PRBA2CU3O7 - A LOCAL-SPIN-DENSITY APPROXIMATION WITH ON-SITE COULOMB INTERACTION [Articolo su rivista]
Biagini, M; CALANDRA BUONAURA, Carlo; Ossicini, Stefano
abstract

Electronic structure calculations based on the local-spin-density approximation (LSDA) fail to reproduce the antiferromagnetic ground state of PrBa2Cu3O7 (PBCO). We have performed linear muffin-tin orbital-atomic sphere approximation calculations, based on the local-spin-density approximation with on-site Coulomb correlation applied to Cu(1) and Cu(2) 3d states. We have found that inclusion of the on-site Coulomb interaction modifies qualitatively the electronic structure of PBCO with respect to the LSDA results, and gives Cu spin moments in good agreement with the experimental values. The Cu(2) upper Hubbard band lies about 1 eV above the Fermi energy, indicating a Cu-II oxidation state. On the other hand, the Cu(1) upper Hubbard band is located across the Fermi level, which implies an intermediate oxidation state for the Cu(1) ion, between Cu-I and Cu-II. The metallic character of the CuO chains is preserved, in agreement with optical reflectivity [K. Takenaka et al., Phys. Rev. B 46, 5833 (1992)] and positron annihilation experiments [L. Hoffmann et al., Phys. Rev. Lett. 71, 4047 (1993)]. These results support the view of an extrinsic origin of the insulating character of PrBa2Cu3O7.

1995 - Hole filling and interlayer coupling in YBa2 Cu3 07/PrBa2 Cu307 superlattices [Articolo su rivista]
M., Biagini; CALANDRA BUONAURA, Carlo; Ossicini, Stefano
abstract

Charge transfer effects in YBa2Cu3O7/PrBa2Cu3O7 superlattices have been proposed by many authors as the origin of the experimentally observed strong depression of the critical temperature. We performed self-consistent LMTO-ASA calculations and found that no remarkable change in the electronic structure of the superconducting CuO2 planes occurs in the studied structures, when the PBCO layer thickness is varied. The observed depression of the critical temperature does not seem to be originated intrinsically by a severe modification of the electronic structure or by the hole-filling mechanism.

1994 - Electron electron correlation for Sb on GaAs(110) [Relazione in Atti di Convegno]
Magri, Rita; Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract

We have studied the effect of including the on-site Hubbard electron-electron correlation in the calculation of the quasiparticle spectrum, obtaining the observed semiconducting interface when the Coulombinteraction parameter U is larger than 3 eV. The interface states within the optical band gap can bepresent also at higher coverages when some disorder exists, evidentiating a general mechanism forthe Fermi level pinning at this interface.

1994 - Physics of Solid Surfaces: Electronic and Vibrational Properties [Monografia/Trattato scientifico]
CALANDRA BUONAURA, Carlo; Chiarotti, G.; Gradmann, U.; Jacobi, K.; Manghi, Franca; Maradudin, A. A.; Tong, S. Y.; Wallis, R. F.
abstract

n the last two decades surface physics has experienced an explosive expansion caused by the development and/or substantial improvement of surface sensitive techniques and UHV appa-ratus. It has grown into a mature field of research, with data of the highest accuracy and repro-ducibility. Surface physics is of great importance for technological applications like control of the work function of semiconductors,interface physics,field effect devices, molecular beam epitaxy, chemisorption and catalysis, corrosion, surface hardening etc.. The present volume is restricted to the so-called "clean" surfaces, i.e. to surfaces atomically clean and well characterized,leaving the more complex field of the contaminated surfaces and overlayers to a later occasion.

1994 - Quasi-particle band structure of NiO: The Mott-Hubbard picture regained [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo; Ossicini, Stefano
abstract

We demonstrate that the Hubbard correlation among Ni 3d electrons is able to reproduce the insulating character of NiO, the correct value of the gap, the orbital character of the valence band edge, and the presence of satellite structures. We have determined the quasiparticle spectra starting from the complex single particle band structure of NiO and including the on-site Hubbard repulsion according to a recently developed three-body scattering theory which allows us to treat highly correlated and highly hybridized systems. The calculated quasiparticle band structure is in excellent agreement with photoemission data.

1994 - SURFACE TERMINATION OF YBA2CU3O7-X SYSTEMS [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

Owing to the complexity of their structure, YBa2Cu3O7-x systems can have several different terminations of the crystal. Even considering basal plane surfaces only, six possible terminations are found for an ideal truncation of the crystal, having different electronic properties. The characterization of the real surfaces produced by fracturing sintered materials, by cleaving a mono-crystal or by growing a thin film is a very difficult task, due to the occurrence of processes such as contaminant segregation, oxygen loss from the external planes and surface reconstruction. Here we review the experimental work on this subject and critically compare the outcomes of different spectroscopical experiments. The data are discussed in the light of the available theoretical results for the surface band structure of the ideal basal plane surfaces. The role of the surface in determining a non-superconducting surface phase is illustrated. Possible morphological and structural models for the observed surfaces are presented.

1994 - Three-Body Scattering Theory of Correlated Hole and Electron States [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We describe a theoretical scheme for the determination of quasiparticle spectra starting from the energy band structure of a metal and treating the on-site Hubbard repulsion beyond the mean-field theory. The method relies upon an expansion of the many-body states in terms of uncorrelated states with a different number of electron-hole pairs. By truncating the expansion to include configurations with one or two pairs one can achieve approximate expressions of the electron and hole self-energies. The interparticle scattering matrices appearing in such expressions are conveniently calculated using the Faddeev approach. Calculations performed for a constant density of states allow us to illustrate the role of three-body correlations in the metal-insulator transitions.

1993 - CHARGE AND ENERGY EXCHANGE BETWEEN A METAL AND A 2-LEVEL PARTICLE IN A METASTABLE STATE [Articolo su rivista]
Zanardi, P; CALANDRA BUONAURA, Carlo
abstract

The interaction with a metal of a two-level particle having two electrons in the triplet configuration has been studied using a time-dependent Newns-Anderson Hamiltonian under the assumption that the intra-atomic Coulomb repulsion terms are large compared to the matrix elements of the metal-particle interaction potential. Using perturbation theory explicit expressions of the transitions amplitudes for the processing occurring to second order in the interaction potential have been derived, thus allowing for the calculation of the spectral function up to fourth order. Results for a model system are presented that emphasize the importance of correlation terms in the various processes.

1992 - BASAL-PLANE SURFACES OF YBA2CU3O7 - SINGLE-PARTICLE RESULTS AND VALENCE-BAND SPECTRA [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Minerva, Tommaso
abstract

YBa2Cu3O7 has a complex structure, which allows for six different basal-plane surfaces. For each of them we have calculated the local density of states in the outermost crystal layers using a tight-binding method, and found that surface states with dangling-bond character are present. The surfaces obtained by cutting the bond between in-plane copper ions and axial oxygen atoms appear to be electrostatically stable, the charge distribution in the outermost planes closely resembling the bulk one. Their surface band structures show dangling-bond states with significant dispersion, which are proposed to be responsible for the surface feature observed by angle-resolved photoemission. By using a model Hamiltonian that includes correlation effects, we have calculated the hole self-energy and determined the valence-band satellite distribution for different crystal terminations and compositions. The results show that satellites are significantly affected by the surface conditions. It is concluded that the role of the surface is crucial in modeling the photoemission spectra.

1991 - SATELLITES FROM HYBRIDIZED CU-O STATES IN YBA2CU3O7 [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Minerva, Tommaso
abstract

The satellite structure of YBa2Cu3O7 compound is calculated starting from an accurate description of the bulk band structure and evaluating the self-energy effects arising from the intra-atomic Coulomb repulsion on the copper and oxygen sites. A continuum of satellites of d8 character originating from the highly hybridized Cu-O bands is found between 8 and 13 eV binding energy. Comparison with the experimental data allows one to explain the behavior of the Cu satellites in resonant photoemission and provides indications about the intrinsic character of the satellite at 9 eV.

1990 - Hole density and Tc behaviour in YBa2Cu3O7 - x [Articolo su rivista]
Minerva, T.; Calandra, C.
abstract

The behaviour of the hole number in YBCO systems as a function of the oxygen deficiency x is studied theoretically by calculating the density of electron states for several different superstructures formed by periodic arrangements of the oxygen vacancies. The characteristic features of the electronic properties for chain copper sites with different coordination are pointed out and their relevance in determining the number of carriers for oxygen planes is illustrated. It is shown that the curve of the hole number as a function of x shows plateaus and jumps similar to those found in the Tc versus x curves if the de-oxygenation process takes place by forming fragmented chains. Structures formed by filled and empty chains only give a smooth hole behaviour as a function of the composition. The results of the numerical calculations can be understood by a simple model giving the Fermi energy shift in terms of the concentrations of Cu(1) sites with different coordination. © 1990.

1990 - OXYGEN-INDUCED SURFACE-STATES IN YBA2CU3O7 [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Minerva, Tommaso
abstract

We present the results of theoretical calculations of the surface electronic structure of YBa2Cu3C7 assuming different crystal terminations. Through a detailed analysis of the distribution of the valence charge near the surface we show that hole distribution may be significantly modified at the surface, due to the band narrowing and to the presence of surface states. The role of these modifications in determining the shape of the one-hole spectra is discussed.

1990 - THEORETICAL INVESTIGATION OF THE FERMI LEVEL PINNING AT THE SB-GAAS(110) INTERFACE [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract

We present the results of theoretical calculation of the electronic structure of the Sb-GaAs(110) interface in the submonolayer coverages perormed in the semiempirical TB scheme. Iterface states arising from the incomplete bonding of Sb atoms to the substarte apepar in the band gap; such states can be prsent also at higher coverages when some disorder exists, evidentiating a general mechanism for fermi lev pinning at the interface

1990 - Theoretical interpretation of valence band photoemission spectra in YBa2Cu3O7 [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Minerva, Tommaso
abstract

Angle resolved one-hole spectra have been calculated starting from a realistic tight-binding Hamiltonian and including correlation and surface effects. The hole self-energy has been evaluated using the t-matrix approximation for the hole-hole scattering. To account for surface effects the local density of states of the outermost atomic layers has been determined assuming an ideal termination of the crystal and a non polar basal plane surface. The theoretical curves allow one to assign the main features of the spectra at high symmetry points of the Brillouin zone, except for the peak observed at 8.5 eV binding energy. It is suggested that the discrepency may arise from the neglecting of the k-dependence in the self-energy or from the extrinsic nature of the peak.

1990 - VACANCY BEHAVIOR AND HOLE NUMBERS IN YBA2CU3O7-X [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Minerva, Tommaso
abstract

We have performed a systematic theoretical study of the effects of de-oxygenation on the plane hole numbers in YBa2Cu3O7 − x compounds, by calculating the total and partial densities of electronic states for several different orthorhombic structures corresponding to various forms of vacancy orderings. We show that as x increases the Fermi energy Ef moves upwards and the density of states at Ef decreases. The movement of the Fermi energy depends upon the type of structure under consideration and for structures having fragmented chains Ef remains constant over a large interval of compositions. This behaviour leads to plateaus in the hole number versus x curve which can be related to the behaviour of the critical temperature.

1989 - Correlated one-hole spectra of YBa2Cu3O7 in the bulk and at the surface [Articolo su rivista]
Calandra, C.; Minerva, T.
abstract

The one-hole valence spectrum of YBa2Cu3O7 is calculated using an accurate tight-binding hamiltonian and a Hubbard model to treat correlation effects. Intra-atomic Coulomb integrals for holes sitting on Cu or O sites are considered and the one-hole self-energy is computed in the low density approximation. The calculated spectrum is compared with photoemission and X-ray emission experiments. This allows to point out a number of problems that arise in assigning the observed structures to a specific atom and to understand some aspects of the data. To evaluate the role of the surface in determining the shape of the photoemission energy distribution curves, the one-hole spectrum is calculated for the Ba-O4 surface. Oxygen induced surface states are shown to be important in determining the intensity of the observed structures. © 1989 Springer-Verlag.

1989 - One hole spectra at YBa2Cu3O7 surfaces [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Minerva, Tommaso
abstract

We present a theoretical study of the modification of the one-hole spectra induced by the surface in YBa2Cu3O7. Basal plane surfaces terminated either with Ba or CuO planes are considered. Correlation effects are included by using a Hubbard model hamiltonian and by calculating the self-energy in the low density approximation. The results indicate that both the main bands and the satellites are sensitive to the choice of the surface.

1989 - SURFACE AND MIXED-VALENCE EFFECTS IN XPS SPECTRA FROM YBCO [Articolo su rivista]
Parmigiani, F; Samoggia, G; CALANDRA BUONAURA, Carlo
abstract

We report X-ray photoemission data from YBa2Cu3O7 showing that the measured composition and the electronic structure depend significantly upon the photo-electron escape depth. For large take-off angles the valence band spectrum near the Fermi edge is well structured and shows a number of features, arising from Cu---O hybrid states. Their intensities are drastically reduced in the energy distribution curves measured at grazing angles. The results are interpreted assuming that the surface is mainly composed by Ba---O planes. This interpretation is shown to be consistent with electronic structure calculations for the bulk and the Ba---O surface.

1989 - SURFACE STOICHIOMETRY AND VALENCE ELECTRONIC-STRUCTURE OF YBA2CU3O7-X [Articolo su rivista]
F., Parmigiani; G., Samoggia; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We report x-ray photoemission data from YBa2Cu3O7−x showing that the measured composition and the electronic structure changes significantly upon the photoelectron escape depth. For large take-off angles the valence band spectrum near the Fermi edge is well structured and shows a number of features arising from Cu-O hybrid states. Their intensities are drastically reduced in the energy distribution curves measured at grazing angles. These results are interpreted assuming that the surface is mainly composed by Ba-O planes. This interpretation is shown to be consistent with electronic structure calculations for the bulk and for a YBa2Cu3O7−x surface terminated with a Ba plane and produced by cutting the Cu2 O4 bond normal to the c axis. The local density of states calculated on this basis and taking into account electron correlation effects is shown to be consistent with many significant behaviors experimentally observed in the YBa2Cu3O7−x valence band. Journal of Applied Physics is copyrighted by The American Institute of Physics.

1989 - Search for stable configuration of (GaAs)1(InAs)1 (111) superlattice [Articolo su rivista]
Magri, Rita; CALANDRA BUONAURA, Carlo
abstract

The epitaxial formation energy of a (GaAs)1 (InAs)1 (III) superlattice grown on an InPsubstrate is calculated by using a self-consistent pseudopotential approach within the densityfunctional theory. Atomic configurations are considered which have two distinct bonddistances and correspond to different relaxations along the [111] direction. The minimumenergy is found assigning to the superlattice the same bond distances obtained by minimizingthe total energy of the two binary systems under the epitaxial constraint. For all thegeometries we find that the superlattice is unstable with respect to disproportionation intothe constituents•

1989 - Structural stability and valence charge density in (GaAs)1(InAs)1 (111) superlattice [Articolo su rivista]
Magri, Rita; CALANDRA BUONAURA, Carlo
abstract

We present a theoretical study of the structural stability and the ground-state electronic distribution of a (GaAs)1/(InAs)1 superlattice grown along the [111] direction. The calculations were carried out self-consistently using norm-conserving pseudopotentials in the local-density approxima- tion. We find that the unrelaxed superlattice with a single anion-cation bond length is unstable with respect to disproportionation into the binary systems. By allowing for relaxation through plane displacements along the [111] directions under the constraints of having only two different bond lengths, the energy may be significantly lowered, but the superlattice remains unstable. The effects of deformation, chemical exchange, and relaxation on the valence charge distribution are investigated and the peculiar characteristics of the chemical bond in the superlattice are pointed out.

1989 - Surface effects in the electronic properties of YBa2Cu3O7 [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Goldoni, Guido; Manghi, Franca; Magri, Rita
abstract

The electronic structure of basal plane surfaces of the high-T, superconducting material YBa,Cu,O, has been determined using a tint-binding Hamilto~~n derived from a first-p~nciple calculation of the bulk phase. The surface local density of states has been obtained for four different surface configurations, which differ for the yttrium and oxygen content. The results show that the presence and state of the surface have a strong influence on the distribution of single particle states. The nature of the surface induced modifications is detailed and the relevance of the results to the understanding of experimental data is shortly discussed.

1988 - X-ray Absorption Spectroscopy in YBa2Cu3O7-x superconductors with variable oxygen con¬tent [Articolo su rivista]
M., Sacchi; Corni, Federico; G. M., Antonini; CALANDRA BUONAURA, Carlo; F. C., Matacotta; R., Frahm
abstract

We report the results of XANES and EXAFS measurements performed on YBa2Cu3O7–x compounds withx ranging from 0 to 1. Data for Cu and YK-edges are presented and definite trends in the behaviour of the main near edge structures are singled out, which allow to address the question of the Cu and Y oxidation states and to elucidate the role of the chemical environment in determining the shape of the absorption lines. The inter-atomic distances as obtained by EXAFS generally agree with the results of X-ray diffraction data with the exception of the Y–O distances. Possible sources of this discrepancy are indicated.

1987 - Atomic Effects in the Auger and Absorption Spectra Involving the M2,3 Levels in Cr and Cr-Compounds [Articolo su rivista]
DEL PENNINO, Umberto; CALANDRA BUONAURA, Carlo; L., Grandi; C., Mariani; Valeri, Sergio
abstract

We determined that in the Cr-M2,3VV spectra autoionization increases with oxygen exposure. These effects are interpreted due to multiplet term splitting.

1987 - Atomic Local Coordinations and Multivalent States in YBa2Cu3O9-δ Superconductors [Articolo su rivista]
G. M., Antonini; CALANDRA BUONAURA, Carlo; Corni, Federico; F. C., Matacotta; M., Sacchi
abstract

EXAFS and XANES spectra at the edges of Cu and Y have been measured in the YBa2Cu3O9-δ(δ 2) superconductor. The oxygen coordination around the two metals is found to generally agree with the model suggested by diffraction experiments, especially as the occurrence of two distinct distances between Cu and O along the perovskite structure c-axis is concerned. Conversely, some evidence is found for a partial occupation of the oxygen sites in the yttrium plane, a result excluded by diffraction experiments. This is better supported by EXAFS spectra at the Y-edge. The XANES spectra of the superconductor are remarkably different from those of Cu oxides suggesting the presence of both divalent and trivalent Cu, in agreement with EXAFS and diffraction results.

1987 - Bonding and surface electronic structure of an Sb overlayer on GaP(110) [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo; Molinari, Elisa
abstract

We calculate the electronicstructure of an Sboverlayer on GaP(110) using a self-consistent pseudopotential approach. A detailed analysis of the energy position and the orbital nature of the overlayer induced states shows that the main contribution to the bonding between the adatoms and the substrate comes from states around 4 eV below the valence band maximum. A model description of the outcomes of the full calculation is presented, which differs from previous interpretations.

1986 - Importance of Coulomb correlation in Silicide spectra [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Bisi, Olmes; DEL PENNINO, Umberto; Valeri, Sergio; Xi, Yiming
abstract

We present an experimental and theoretical study of the valence band spectra of Ni and Cr silicides. Evidence is provided that the mare features of both photoemission and Auger lineshapes cannot be understood if correlation effects are not included into the theoretical treatment

1985 - Coulomb correlation in Chromium compounds [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Bisi, Olmes
abstract

It is shown that the main features of chromium silicide photoemission spectra, unexplained by single-particle theory, can be understood quantitatively by the inclusion of Coulomb correlation effects. Our results provide the first direct evidence of the importance of correlation in metallic Cr compounds.

1985 - Electronic properties of Silicon - transition metal interface compounds [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Bisi, Olmes; Ottaviani, Giampiero
abstract

The article reviews recent experimental and theoretical work on the electronic properties of transition metal-silicides and of silicon-silicide interfaces. After a short description of the contactreaction between silicon and transition metal films, we give a detailed presentation of the results concerning the electronic structure of bulk silicides of near-noble, noble and refractory materials.We discuss both single- and two-particle spectra and illustrate, the importance of electronic correlation in determining the spectral lineshapes. We then survey the results for several interfaces, stressing the relation between the geometry and composition of the interface and its electronic properties and specifying the current status of understanding of the experimental spectra. The last part of the article is devoted to a discussion of Schottky barriers in these interfaces and to a reviewof the mechanisms responsible for barrier formation.

1985 - Theoretical study of the electronic structure of the GaP-Si(110) interface [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Bertoni, Carlo Maria
abstract

Self-consistent pseudopotential calculations have been performed to determine the electronicstructure of a Si overlayer on GaP(110) surface. The results show that both filled and empty Si-induced states appear in the gap. The consequences for the Fermi level pinning are discussed and recent experimental results are explained.

1984 - Correlation effects in valence band spectra of Nickel Silicides [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo; DEL PENNINO, Umberto; P., Sassaroli; Valeri, Sergio
abstract

We discuss the role of the electronic correlation in determining the line shapes of valence-band photoemission and Auger spectra of Ni silicides. Theoretical one-particle spectra, with correlation effects included by a perturbation approach to a degenerated Hubbard Hamiltonian, are compared with experimental energy-distribution curves. It is shown that for these compounds the electronic correlation is of crucial importance for describing valence-band photoemission. We also present the experimental line shapes of Ni L3VV transitions and compare them with theoretical two-hole spectral densities. It is concluded that a band-theoretical approach can account for some important features of the experimental Auger spectra, but multiplet effects should be included for a more detailed description.

1984 - ELECTRONIC-PROPERTIES OF THE Cs-GaAs(110) INTERFACE AT MONOLAYER COVERAGE [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo; Bertoni, Carlo Maria; Molinari, Elisa
abstract

The electronic properties of the interface obtained by a monolayer deposition of Cs onto GaAs(110) are studied theoretically by modelling the overlayer with a thin jellium slab of appropriate electron density. The calculations are carried out self-consistently with a pseudopotential approach for two different substrate geometries, corresponding to the ideal and the relaxed configuration of the surface atoms. In both cases a band of Cs-induced states is found in the gap which hybridizes with the dangling and back bond states of the GaAs surface. The change in the work function upon Cs deposition is calculated and found for both the substrate geometries in good agreement with the experimental data. Comparison with electron energy loss and photoemission experiments indicates that the model with the surface in the relaxed configuration is the most appropriate one.

1983 - ELECTRON-STATES OF AN Sb-ORDERED OVERLAYER ON GaAs(110) [Articolo su rivista]
Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca; Molinari, Elisa
abstract

The electronic properties of an Sb overlayer deposited onto a GaAs(110) surface have been calculated using a self-consistent-pseudopotential approach and assuming the ordered-overlayer geometry proposed in recent low-energy electron diffraction studies. The results show that Sb adatoms are bound by strong covalent bonds to the substrate and that various overlayer or chemisorption-induced states appear throughout the valence band. Comparison with photoemission data allows us to assign a major Sb-induced structure appearing in the energy distribution curves.

1983 - Effects of chemical environment in the lineshape of Silicon LVV Auger spectra of Nickel Silicides [Articolo su rivista]
DEL PENNINO, Umberto; P., Sassaroli; Valeri, Sergio; Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

The Si L2,3VV Auger lineshape in nickel silicides of different compositions has been studied both experimentally and theoretically with the purpose of understanding the behaviour of silicon states in compounds where the Si atom has different local chemical environments. The experimental spectra provide evidence of significant modifications in the lineshapes, which are associated with changes in the distribution of the valence electrons. These modifications can be explained by theoretical calculations based on a single-particle description of the bulk electronic structure. Some discrepancies between theory and experiments can be attributed to approximations in the treatment of the final state of the Auger electron and to the neglect of surface effects in the electronic structure calculation.

1983 - Electronic structure of Cr Silicides and Si-Cr interface reactions [Articolo su rivista]
A., Franciosi; J. H., Weaver; D. G., O'Neill; F. A., Schmidt; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

We present synchrotron radiation photoemission studies of bulk CrSi2 and silicide phases grown on Si by thermal processing of the Si-Cr interface. Experiment shows that Si-Cr interface formation at room temperature results in reacted phases that differ from both bulk CrSi2 and in situ—grown Si-rich CrSi2. Extended—Hückel-theory linear combination of atomic orbitals calculations of the density of states of Cr3Si, CrSi, and CrSi2 show that Si—Cr bond formation involves Si p and Cr d states with minimal charge transfer.

1983 - NONLOCAL EXCHANGE AND CORRELATION AND SEMICONDUCTOR BAND-STRUCTURE [Articolo su rivista]
Manghi, Franca; G., Riegler; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; G. B., Bachelet
abstract

We present the results of self-consistent band-structure calculations for bulk GaAs, in which the density-functional theory is applied with a nonlocal expression for the exchange and correlation energy, Exc[n(r⃗)]. Using the same ionic potentials and basis set, other local expressions for exchange and correlation are also considered for comparison. It will be shown that the inclusion of nonlocality in the exchange and correlation potential improves the description of the valence bands, but does not solve the "gap problem" experienced by the simpler local expressions (i.e., too small a gap).See AlsoErratum: F. Manghi, G. Riegler, C. M. Bertoni, C. Calandra, and G. B. Bachelet, Erratum: Nonlocal exchange and correlation and semiconductor band structure, Phys. Rev. B 29, 5966 (1984).For revisions.

1982 - Chemical bonding at the Si-metal interface: Si-Ni and Si-Cr [Articolo su rivista]
A., Franciosi; J. H., Weaver; D. G., O'Neill; Y., Chabal; J. E., Rowe; J. E., Poate; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

Chemical bonding at the interface of a near‐noble‐metal (Ni) and a transition metal (Cr) with Si is examined through synchrotron radiation photoelectron spectroscopy studies of in situ formed interfaces, of cleaved bulk silicides, and of disordered surfaces prepared by sputter etching of the silicides. Interpretation of these experimental results is guided by parallel linear combination of atomic orbitals (LCAO) (extended Huckel approximation) calculations of stoichiometric Ni and Cr silicides.

1982 - Electronic properties of metal rich Au-Si compounds and interfaces [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo; L., Braicovich; I., Abbati; Rossi, Giorgio; I., Lindau; W. E., Spicer
abstract

A theoretical analysis of the electron density of states in the delta -phase Au3Si compound is given and the main aspects of the hybridisation between Si p and gold d orbitals are discussed. Differences in the electronic behaviour of Au-Si compounds with respect to those produced by nearly noble metals are pointed out. The theoretical results are compared with new photoemission data, which have been taken with synchrotron radiation using photon energy at the Cooper minimum, in order to distinguish the orbital contribution to the main structures. The general features of the experimental results agree with the theoretical model. A description of the main aspects of the chemical bond at the interface between Si and noble metals is provided.

1982 - SELF-CONSISTENT PSEUDOPOTENTIAL CALCULATION OF THE ELECTRONIC-PROPERTIES OF THE INP (110) SURFACE [Articolo su rivista]
Manghi, Franca; Molinari, Elisa; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo
abstract

Self-consistent pseudopotential calculations for InP(110) surface have been carried out using the repeated slab method and assuming relaxed surgace geometry. Various surface states have been found and their character and symmetry properties have been investigated by analyzing the corresponding pseudocharge density maps. Comparison between self-consistent and non-self-consisting calculations shows significant differences.The results are in agreement with the experimental finding obtained by different techniques.

1982 - THEORETICAL INVESTIGATION OF HYDROGEN CHEMISORPTION ON GA-CONTAINING III-V-COMPOUNDS [Articolo su rivista]
Manghi, Franca; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Molinari, Elisa
abstract

A fully self-consistent pseudopotential calculation of the electronic properties of atomic hydrogen chemisobed on GaAs and GaP (110) surface is reported. Different chemisorption geometries ans substrate coverages are considered. The results are compared with the experimental information to select a structural model.

1982 - Valence band and core lines investigation on the Ge-Au system by photoelectron spectroscopy with synchrotron radiation [Articolo su rivista]
P., Perfetti; A. D., Katnani; TE XIU, Zao; G., Margaritondo; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

Core level and valence band photoemission experiments coupled to theoretical calculations reveal the formation of a stoichiometric Ge–Au phase during gold deposition onto Ge(111) at room temperature. This result is significantly different with respect to that found for the Si–Au system.

1982 - W (111): angle-resolved photoemission from the clean and H2 - covered surface [Articolo su rivista]
F., Cerrina; J. R., Anderson; G. J., Lapeyre; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

We report the results of new photoemission experiments from a clean and hydrogen-covered W(111) surface. Symmetry-related polarization effects are used to discriminate the symmetry of the initial states. Through a careful analysis of angle-resolved energy distribution curves and angle-resolved constant final-state spectra we are able to locate several surface bands in the clean surface emission and the hydrogen-induced states in the spectra from the covered surface. The experimental data are compared with the outcomes of semiempirical linear combination of atomic orbitals calculations both for the clean surface and for the adsorbed one, assuming a chemisorption geometry with H atoms bridging metal atoms of different layers. The model used shows a good agreement with the experiment and indicates the possible mechanisms of hydrogen bonding on this surface.

1981 - Copper on Ni(111): the electron states from submonolayer to several monolayer coverages [Articolo su rivista]
Abbati, ; L., Braicovich; A., Fasana; Bertoni, Carlo Maria; Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract

Experimental and theoretical results are presented for Cu on an Ni(111) surface. The monolayer case is discussed in detail in connection with theoretical results of a tight-binding calculation. The theory accounts for the Cu 3d-peak position and the modification of the substrate density of states. Also, the measured Cu 3d line shape can be understood on the basis of the theory.

1981 - Theoretical study of the electronic structure of GaP(110) [Articolo su rivista]
Manghi, Franca; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Molinari, Elisa
abstract

A self-consistent pseudopotential approach has been used to calculate the electronic structure of GaP(110) surface in both ideal and relaxed configurations. Calculations have been performed using the repeated slab method and a local form of the bare ionic pseudopotential. An efficient self-consistent procedure, which allows us to obtain quick convergence and eliminates some difficulties found in previous applications of the method, has been used. Particular care has been devoted to have complete consistency between bulk and slab calculations. Our results for the ideal surface show various surface states, whose distribution and nature are similar to those found in tight-binding calculations. For the geometry of the relaxed surface we assumed a rotation-relaxation model determined by a recent low-energy electron diffraction study. With this geometry our results show that a nonvanishing density of empty surface states, to a large extent due to backbonds, remains in the gap. The orbital composition of these states, as well as of all the other surface features, is detailed, together with the mirror-plane symmetries relevant in the interpretation of angle-resolved photoemission data. Our results are in agreement with the experimental data provided by various different measurements.

1981 - Transition metal Silicides: aspects of the chemical bond and trends in the electronic structure [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

This paper presents a theoretical investigation of the electronic properties of bulk silicides of near-noble metals (NiSi2, NiSi, Ni2,Si, PdSi, Pd2Si, PtSi and Pt2Si). The theoretical approach is provided by the iterative extended Huckel method. The densities of states of various compounds, differing in silicon content and in the crystal structure, are presented and the main features of the chemical bond are discussed. A detailed comparison with the available experimental data and with other theoretical work is presented. The relevance of the present results for the interpretation of the silicide/silicon interfaces is also discussed.

1981 - Two dimensional band structure of chemisorbed chlorine on GaAs(110) [Articolo su rivista]
G., Margaritondo; J. E., Rowe; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

The two-dimensional electronic energy bands of an ordered Cl overlayer on GaAs (110) were mapped by angle-resolved photoemission spectroscopy with synchrotron radiation. The results were compared to bands calculated with an empirical tight-binding approach. This comparison shows that the Cl adatoms are bound to As atoms and that the substrate atoms are relaxed from their positions in a way qualitatively similar to that of clean-surface GaAs (110).

1980 - An experimental and theoretical study of the electronic structure of the W (111) surface [Capitolo/Saggio]
F., Cerrina; G. P., Williams; J., Anderson; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

We report results of synchrotron angular resolved photoemission experiments from W(111) together with a theoretical analysis of the data. For normal emission several different surface sensitive features are found depending on the s or p polarization of the incident radiation. The symmetry assignements derived from this polarization dependance are compared with theoretical results based on a tight binding calculation of the electronic structure of a 16 layer thick slab. The same analysis is extended to off-normal data collected in the mirror plane.

1980 - MICROSCOPIC ASPECTS OF SI-GE HETEROJUNCTION FORMATION [Articolo su rivista]
Nannarone, Stefano; F., Patella; P., Perfetti; C., Quaresima; A., Savoia; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We present the results of an investigation of the electronic structure of heterojunctions formed by deposition of Germanium on Si (lll) surfaces. Energy Loss Spectra for various Ge coverages have been taken and compared with theoretical calculations. The results indicate that the interface is abrupt and at low coverages Ge is deposited uniformly on the substrate. Above 2 monolayers coverage modifications in the electronic structure appear probably caused by the growth mechanism.

1980 - The electronic properties of the Si (111) - transition metals interfaces [Articolo su rivista]
I., Abbati; L., Braicovich; B., DE MICHELIS; Bisi, Olmes; CALANDRA BUONAURA, Carlo; DEL PENNINO, Umberto; Valeri, Sergio
abstract

We present the results of a joint experimental and theoretical investigation of the electronic properties of Silicon – transition metal interfaces. We carried out ultraviolet photoemission experiments on Si-Pt, Si-Ni and Si-Pd interfaces at different coverages. Our results indicate that these interfaces are reactive. Comparison with semi-empirical LCAO calculations allows to understand the main features of the observed spectra.

1980 - W(111) surface: an experimental and theoretical photoemission study [Articolo su rivista]
F., Cerrina; G. P., Williams; J., Anderson; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

A detailed analysis of W(111) surface has been carried out by means of photoemission spectroscopy, together with a theoretical study of the electronic structure of the surface based on a tight binding Hamiltonian. During such a study the results from angular integrated and angular resolved ultraviolet photoemission spectroscopy have been compared with the computed surface density of states. This allowed the unique assignement of the surface sensitive features to surface states and resonances; the use of the polarized continuum of Synchrotron Radiation excitation source made possible to extend such an analysis to the determination of the state symmetries. The picture which emerges for the W(111) surface shows a remarkable feature: the surface states and resonances bands have a very small dispersion in the 2D BZ, due to the large surface atoms separation. The effect is so large that the K parallel integrated surface DOS is interpretable in terms of single d-orbital derived peaks.

1979 - A THEORETICAL STUDY OF THE ELECTRONIC STRUCTURE OF PD (111) CLEAN SURFACE [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

A detailed investigation of the electronic structure of Pd(111) clean surface is presented in terms of a LCAO band model. Results including surface bands and local densities of states are given and a comparison with recently performed photoemission experiments is presented. For hυ = 21.2 eV we find that the angle resolved distribution curves can be explained in terms of initial state density and k║ conserving transitions. For lower photon energies a mixture of bulk transition calculations and surface density of states seems more appropriate. Comparison with previous theoretical work is also presented.

1979 - CHEMISORPTION GEOMETRY ON CLEAVED III-V SURFACES - CL ON GAAS, GASB, AND INSB [Articolo su rivista]
Margaritondo, G; Rowe, Je; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

Experimental angle-integrated photoemission curves taken on Cl-covered (110) surfaces of GaAs, GaSb, and InSb have been compared to tight-binding calculations of the local density of states. The results clearly demonstrate that the Cl adatoms are bound to the anion substrate atoms rather than to the cation substrate atoms. Some qualitative information is also provided on surface relaxation and on chemisorption bond lengths.

1979 - Chemisorption geometry on cleaved GaAs (110) surfaces [Relazione in Atti di Convegno]
G., Margaritondo; J. e., Rowe; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

An experimental and theoretical study of Cl chemisorption on GaAs(110) shows that adatoms are bound to surface As atoms

1979 - Surface electron states at the (110) surfaces of III-V semiconductors [Relazione in Atti di Convegno]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We present a theoretical investigation of the effect of surface relaxation on the electronic structure of the cleavage faces of III-V compounds. The energy position of the surface states, their localization and orbital composition are discussed.

1978 - A theoretical study of ordered monolayer films of Copper [Relazione in Atti di Convegno]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

The aim of this communication is to present some new results of a theoretical study of the density of states of ultra-thin metallic copper films deposited on metal substrates and to compare them with the experimental information. Our results for Cu on Zn(0001) and for Cu on Ni(111) shows that strong modifications can occur in the electronic structure of the overlayer with respect to the isolated monolayer case.

1978 - Electronic Structure of an Ordered Monolayer of Cu on Zn(0001) [Articolo su rivista]
I., Abbati; L., Braicovich; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We have measured the energy distribution curves of photoelectrons (hν=21.2 eV) from a Zn single-crystal (0001) face covered by an ordered Cu monolayer and from bulk Cu. We have found a narrow (2.3-eV total width) Cu d-state peak 1.20 eV below the position of the peak of bulk Cu 3d structure. We have also calculated the density of states for both a Cu monolayer on Zn and an unsupported Cu monolayer. Theory is in good agreement with experiments. The overlayer density of states is shown to be very similar to that of the unsupported atomic layer.

1978 - Surface bands in relaxed cleavage surface of GaP [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We have studied the dependence of the surface states upon relaxation in GaP (110) surface. Calculations were performed using a tight binding model with an approximate treatment of the self‐consistency. Different relaxation models, involving both rotation and stretching of the bonds were considered. The location and orbital composition of the surface bands their dispersion and the local density of states at the surface are presented for the various models. Unlike the other III–V compounds, we find that the relaxation does not remove the empty surface states from the band gap. Such a conclusion agrees with the experimental information on the pinning of the Fermi level in n‐doped samples of GaP. A comparison with partial‐yield photoemission data and a theoretical estimate of the surface excitonic binding energy are also given.

1978 - Surface bands of Ga containing III-V compounds [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We present calculations of the electronic structure of the relaxed (110) face of all the Ga containing III-V materials. Surface features are identified either by their location inside gaps of the projected bulk band structure or by the localization of the wave function near the surface. The calculation provides the first accurate description of the surface electronic structure of Ga containing III-V compounds and explain the main trends of the experimental data.

1978 - Surface electronic states at the (110) surfaces of III-V compounds [Relazione in Atti di Convegno]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We present a theroetical investigation of the effects of surface relaxation on the electronic structure of the cleavage faces of III-V compounds

1978 - The electronic structure of Au monolayer on (001) W [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

The picture which emerges from our work and from the comparison with the available experimental information is that on the “open” surfaces like the bcc (001) the noble metal atoms are adsorbed in a regular 1 X 1 structure and their electronic structure is some intermediate betweeen the free atom and the bulk. At higher coverages the behaviour changes and structural modifications occur, which change the electronic structure.

1977 - A self-consistent calculation of the electronic structure of thin copper films [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We calculated the electronic structure of Cu films one, two and four layers thick using a tight-binding approach. An iterative procedure was used in order to dispose of the self-consistency requirements arising from the changes in the fractional occupancies of the various orbitals. The results show that the electronci structure for films of small thickness is intermediate between that of an isolated atom and that of a perfect crystal. A detailed comparison with previous theoretical work is presented, and the source of the various discrepancies is discussed.

1977 - Dispersion of surface bands in W(100) surface [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo; P., Flaviani; Manghi, Franca
abstract

The dispersion of the d surface bands and resonances along high symmetry directions of the two dimensional Brillouin zone has been calculated theoretically for the (001) face of W. The results show a variety of surface states, not specifically connected with the opening of spin orbit gaps.

1977 - ELECTRONIC-STRUCTURE AND ATOMIC CONFIGURATION AT CLEAVAGE SURFACE OF ZINCBLENDE COMPOUNDS [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Manghi, Franca; Bertoni, Carlo Maria
abstract

The role of self-consistency and of the atomic distortions in determining the electronic structure of the cleavage surfaces of GaAs and ZnSe has been investigated using the layer method and an iterative tight-binding approach, which takes account of the difference between the surface and bulk electronic configurations. Several surface bands and resonances are identified near the gaps and empty lenses of the projected bulk band-structure and their sensitivity to the modifications of the surface potential are discussed. In the case of GaAs the inclusion of the atomic distortions at the surface is essential in order to obtain theoretical results in reasonable agreement with the experimental data. For ZnSe the available experimental information does not permit discrimination between the ideal and distorted configuration of the surface atoms, in spite of the sensitivity of the occupied surface states to the atomic displacements.

1977 - Electron states of ultrathin Pd and Cu films on (001) Ag. [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

We present the results of a theoretical calculation of the electronic structure of a monolayer of Pd and Cu deposited on the (001) surface of Ag. The adsorbate atoms are placed in a central site with the same lateral periodicity as a parallel plane of the substrate. The method of calculation is a parametrized LCAO with hopping integrals determined through a fit to bulk data.Charge transfer effects are taken approximately into account by assuming the intraatomic parameters to vary as the valence orbital ionization potentials. Energy bands and local densitiesof states are given and the nature and location of the adsorbate states are discussed. The results are compared with the experimental data: good agreement is found for the Pd on Ag system.For Cu on Ag we find that our assumptions do not explain the photoemission data and suggest that some clustering of Cu atoms occurs during the deposition.

1977 - Electronic properties of clean (001) surfaces of Ir and Pt [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We present a theoretical investigation of the electronic structure of the ideal (001) surface of Ir and Pt. Surface band structures and local densities of states have been determined using a parametrized linear combinatiion of atomic orbitals for a film of fourteen (001) atomic planes. Various surfaces states and resonances arise in correspondence of the gaps of the projected bulk band structure. Our results explain the experimetnal data, obtained in field ion emission and conventional photoemission experiments, showing a surface sensitive structure in the proximity of the Fermi energy.

1977 - Role of the selfconsistency in the evaluation of the electronic structure of transition metal surfaces [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

The effects of the changes insp-d fractional occupancies and of the charge transfer at the transition metals surfaces are discussed by using a tight-binding description of the electron state. A method is proposed which leads to an iterative procedure for modifying the intra-atomic parameters in such a way as to get consistency with the electronic configuration obtained for the surface and for the first sublayers. The main features of the method are illustrated by presenting detailed results of the calculation of the electronic structure of the (001) surface of paramagnetic iron. The role of self-consistency in this case and for other metals is discussed.

1977 - Surface bands of the (001) surface of molybdenum [Articolo su rivista]
Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

The electronic structure of the ideal (001) surface of Mo is calculated using a parametrized linear combination of atomic orbitals. Surface states and resonances are identified along the high symmetry directions of the two dimensional Brillouin zone. The local density of states near the surface is presented and the main modifications due to the surface states are discussed. Significant surface features are found in the range 0–0.1 Ryd below the Fermi energy in agreement with the experimental findings.

1977 - The electonic structure of the (100) surface of Copper [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; Manghi, Franca
abstract

We present the results of a new calculation of the electronic structure of the (001) surface of Cu, obtained by using the layer method and a tight-binding description of the electron states. The selfconsistency requirements arising from the surface charge transfer are treated by assuming a charge and configuration dependence of the intraatomic matrix elements. It is found that, at variance with what obtained for other transition metals, the self-consistency plays a crucial role in determining the nature and the location of the surface states. A detailed discussion of the main surface features in the layer band structure and in the local densities of states at the topmost layers is given together with a comparison with previous theoretical work. The results are discussed in the light of the available experimental data.

1976 - Electronic structure of nonpolar surfaces of II-VI compounds [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Santoro, Giorgio
abstract

The nature of surface states in cleaved surfaces of II-VI compounds with zincblende structure is studied. The tight-binding method is used to calcualete the electronic structure of a layer od twelvw (110) planes. The results show various surface bands both in the gap and in empty lenses od the projected bulk band structure. The nature of these states is discussed and a comparison with the available experiments is presented. The results support the model of ionic surface states previously proposed for these surfaces.

1976 - sp-d hybridization effects on the electronic structure of the (100) surface in Copper [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; F., Nizzoli; Santoro, Giorgio
abstract

The electronic structure of the (100) surface of Cu has been calculated within a tight-binding description of the electron states, using the layer method. With respect to the calculation with pure d bands, it is found that the inclusion of sp-d hybridization modifies the d surface states and introduces a new state localized in the first two layers. This state is identified as a Schockley-type surface state arising at the lower edge of sp-d gap.

1975 - Perturbative theory and tree-body forces in h.c.p. metals [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; F., Nizzoli
abstract

An analysis is presented of the role of the non pair forces in the lattice dynamics of sp bonded hcp metals: Be, Mg and Li. By expanding the total energy up to third order in the electron-ion pseudopotential, a dynamical matrix is derived which contains terms arising from central pairwise interactions between the ions as well as contributions due to sums of three body forces. The phonon dispersion relations are calculated using a local form of the model potential, which allows for a realistic evaluation of the third order contributions. The results show that the three body forces are very important in Be and are responsible for the peculiarities of the spectrum, as is most clearly seen from the analysis of the frequencies at the K point of the Brillouin zone.

1975 - Perturbative valence charge density of trigonal Se and Te [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo; F., Nizzoli
abstract

The electron valence charge density in Se and Te is calculated using perturbation theory to second order in an average form factor which accounts for the lion-locality of the pseudo-potential. The convergence of the perturbative expansion is tested and discussed, and a comparison is made with previous calculations derived from band structures. The importance of the second order term is related to the covalence of the chemical bond, lattice dynamics, and bonding charge models.