CARLO MARIA BERTONI - personale UniMoRe

Nuova ricerca

CARLO MARIA BERTONI

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

Pubblicazioni

2016 - First-Principle Molecular Dynamics of Sliding Diamond Surfaces: Tribochemical Reactions with Water and Load Effects [Articolo su rivista]
Righi, Maria Clelia; Zilibotti, Giovanna; Corni, Stefano; Ferrario, Mauro; Bertoni, Carlo Maria
abstract

Ab initio molecular dynamics offers an unexpected tool to understand many aspects of complex and macroscopic phenomena, like friction, lubrication, and surface passivation through chemical reactions induced by load and confinement, as found in recent works (Zilibotti et al., in Phys. Rev. Lett. 111:146101, 2013; De Barros Bouchet et al., J Phys Chem C 116:6966, 2012). Here we review the results of first-principle molecular dynamics simulations of diamond interfaces interacting with water molecules, at different concentrations. We found that the molecular confinement induced by the applied load promotes water dissociation. The consequent surface passivation prevents the formation of carbon bonds across the interface, reducing adhesion and friction. The possibility to extend the use of an atomistic approach to understand the kinetics of tribochemical reactions and their effects on friction will also be discussed.

2015 - Quantum Description of the Matter-Radiation Interaction [Capitolo/Saggio]
Bertoni, Carlo Maria
abstract

Quantum physics describes the radiation field, matter systems (like atoms, molecules and solids) and their interactions. The processes of interaction consist of absorption, emission and diffusion of photons, the basis of many experimental techniques to study materials and their electronic properties. The states of the electron systems can be studied in a single-particle picture or in a many-electron scheme; the two approaches will be compared in the study of the elementary processes of interaction with light at different levels of approximation for the initial, final and intermediate states of the matter system.

2015 - Topological invariants in interacting quantum spin Hall: a cluster perturbation theory approach [Articolo su rivista]
Grandi, F.; Manghi, Franca; Corradini, Olindo; Bertoni, Carlo Maria; Bonini, Alfredo
abstract

Using cluster perturbation theory we calculate Greenʼs functions, quasi-particle energies and topological invariants for interacting electrons on a 2Dhoneycomb lattice, with intrinsic spin–orbit coupling and on-site e–e interaction. This allows us to define the parameter range (HubbardUversus spin– orbit coupling) where the 2D system behaves as a trivial insulator or quantum spin Hall insulator. This behavior is confirmed by the existence of gapless quasi-particle states in honeycomb ribbons.Wehave discussed the importance of the cluster symmetry and the effects of the lack of full translation symmetry typical of CPT and of most quantum cluster approaches. Comments on the limits of applicability of the method are also provided.

2015 - Topological properties of the bond-modulated honeycomb lattice [Articolo su rivista]
Grandi, F.; Manghi, Franca; Corradini, Olindo; Bertoni, Carlo Maria
abstract

We study the combined effects of lattice deformation, e−e interaction, and spin-orbit coupling in a two-dimensional (2D) honeycomb lattice. We adopt different kinds of hopping modulation—generalized dimerization and a Kekulé distortion—and calculate topological invariants for the noninteracting system and for the interacting system. We identify the parameter range (Hubbard U, hopping modulation, spin-orbit coupling) where the 2D system behaves as a trivial insulator or quantum spin Hall insulator.

2011 - Ab initio calculation of adhesion and potential corrugation of diamond (001) interfaces [Articolo su rivista]
Zilibotti, Giovanna; Ferrario, Mauro; Bertoni, Carlo Maria; Righi, Maria Clelia
abstract

The system consisting of two diamond (001) surfaces in contact was studied by means of plane-wave/pseudopotential density functional calculations. Different hydrogen coverages, ranging from fully hydrogenation to bare surfaces, were considered. The adhesion energy was calculated as a function of both the separation and the lateral displacement of the two surfaces. The effects of dangling carbon bonds on the adhesion and potential corrugation are quantitatively discussed.

2009 - Atomic and electronic structure of the nonpolar GaN (1 1̄ 00) surface [Articolo su rivista]
Bertelli, Matteo; Löptien, P.; Wenderoth, M.; Rizzi, Angela; Ulbrich, R. G.; Righi, Maria Clelia; Ferretti, Andrea; Martin Samos, L.; Bertoni, Carlo Maria; Catellani, A.
abstract

We present a cross-section scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and ab initio density-functional theory simulations study of the cleaved nonpolar (1[overline 1]00) surface (m-plane) of n-type HVPE GaN free-standing quasisubstrates. Atomically resolved empty and filled states STM topographies show that no reconstruction occurs upon cleavage, as predicted by theory. STS measurements on clean and atomically flat cleaved surfaces (defect concentration sigmad<=2×10**12 cm−2) show that the Fermi energy is not pinned and the tunneling current flows through Ga-like electronic states lying outside the fundamental band gap. On surface areas with defect concentration sigmad>=3×10**13 cm−2, the Fermi energy is pinned inside the band gap in defect-derived surface states and tunneling through filled (empty) N-like (Ga-like) states takes place.

2007 - Atomic and electronic structure of the cleaved 6H-SiC (11 2̄ 0) surface [Articolo su rivista]
Bertelli, Matteo; Homoth, J.; Wenderoth, M.; Rizzi, Angela; Ulbrich, R. G.; Righi, Maria Clelia; Bertoni, Carlo Maria; Catellani, A.
abstract

We present a combined cross-section scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) and ab initio simulations study of the nonpolar (11 2̄ 0) cleaved surface of 6H-SiC. The experimental results show an unreconstructed surface in agreement with theory. Upon truncation, two surface bands appear inside the semiconductor band gap: one empty band localized on the Si atoms and one filled band on the C atoms. According to the STS experimental results on n -doped samples, the Fermi energy is pinned at the surface inside the band gap. By comparison of STM filled and empty states topographies we propose that on the fresh cleaved surface the Fermi level lies at the bottom of the Si-like band. The calculated STM images reproduce the experimental contrast of the 6H stacking sequence and its bias dependence very well. © 2007 The American Physical Society.

2007 - Polarization properties of (1-100) and (11-20) SiC surfaces from first principles [Articolo su rivista]
Brandino, Gp; Cicero, G; Bonferroni, Benedetta; Ferretti, Andrea; Calzolari, Arrigo; Bertoni, Carlo Maria; Catellani, A.
abstract

We report on first-principles density functional calculations of nonpolar low-index surfaces of hexagonal silicon carbide. We provide an accurate analysis of the macroscopic bulk spontaneous polarization as a function of the hexagonality of the compound, and we describe in detail the electronic and structural properties of the relaxed surfaces. We revise the methodology to achieve a detailed description of the surface polarization effects. Our results on low-index surfaces reveal a strong in-plane polar contribution, opposing the spontaneous polarization field present in hexagonal polytypes. This in-plane surface polarization component has not been considered before, although it is of significant impact in adsorption experiments, affecting functionalization and growth processes, as well as the electronic properties of confined, low-dimensional systems.

2006 - Angular and polarization dependence of x-ray resonant elastic scattering in transition metals [Articolo su rivista]
Marri, Ivan; Bertoni, Carlo Maria; Ferriani, P; Joly, Y.
abstract

We present a comparison of the x-ray elastic scattering at the 2p threshold of Ni calculated both in a single-ion picture with a full description of the multiplet manifolds of the electron configurations and in a one-electron model using a multiple scattering approach for the excited intermediate state. We study the case of perpendicular geometry of the circularly polarized incoming light and analyze the variation of the intensity of the emitted light along a conical scan. We discuss the results obtained in the two approaches for the total and dichroic signals, that confirm the presence of a significant anisotropy as previously predicted in the inelastic case.

2006 - Optical dichroism: E1-M1 integral relations [Articolo su rivista]
Marri, Ivan; P., Carra; Bertoni, Carlo Maria
abstract

The present paper discusses optical dichroism in noncentrosymmetric systems. The cases of circular and linear polarizations are considered. Integrated spectra are interpreted using effective two-electron operators, which are derived within a localized (atomic) model. As a consequence, our theory is not suitable for quantitative predictions; nevertheless, it identifies microscopic origins of natural, nonreciprocal and Jones' dichroisms.

2006 - Study of arsenic for antimony exchange at the Sb-stabilized GaSb(0 0 1) surface [Articolo su rivista]
Righi, Maria Clelia; Magri, Rita; Bertoni, Carlo Maria
abstract

In this paper we present a first-principle study on the energetics of a single As2 molecule on GaSb(0 0 1) reconstructed surface. In order to shed light into the mechanisms of anion exchange at the Sb-rich GaSb(0 0 1) surface, we studied firstly As2 adsorption and then As for Sb exchange. We identify a surface region where both the processes are energetically favored. The results of this twofold analysis can be combined to derive possible reaction paths for the anion exchange process. © 2005 Elsevier B.V. All rights reserved.

2005 - Acetylene on Si(111) from computer simulations [Articolo su rivista]
C., Sbraccia; Ca, Pignedoli; A., Catellani; R., Di Felice; Pl, Silvestrelli; F., Toigo; F., Ancilotto; Bertoni, Carlo Maria
abstract

The interaction between the acetylene molecule (C_2H_2) and the Si(111)-(7 x 7) surface is studied with the combined use of ab initio and empirical methods. Several possible chemisorption configurations are explored, which may be helpful to rationalize recent experimental results. The kinetics of the chemisorption process can be described by identifying the low-energy paths between pairs of stable configurations. This can be done by means of accurate ab initio calculations only for small-scale models of surface reconstructions. It is shown that large-scale calculations based on the use of semi-empirical potentials, validated by ab initio calculations for small-scale models, can be an efficient tool to study chemisorption reactions of molecules occurring on complex semiconductor surfaces.

2005 - Combined ab initio and kinetic Monte Carlo simulations of C diffusion on the root(3)X root(3) beta-SiC(111) surface [Articolo su rivista]
Righi, Maria Clelia; Pignedoli, Ca; Di Felice, R; Bertoni, Carlo Maria; Catellani, A.
abstract

We investigate the kinetic behavior of a single C adatom on the √3 X √3 β-SiC(111) surface by means of combined ab initio and kinetic Monte Carlo simulations. After identifying the metastable binding locations, we calculate the energy barriers the adatom must overcome when jumping among them. The presence of the √3 X √3 reconstruction creates considerable differences among the diffusion mechanisms that can be thermally activated. This has important implications for the C mobility on the surface, and therefore for SiC growth. The kinetic simulation at realistic temperatures and time scales revealed that C diffusion occurs mostly around the Si adatoms forming the √3 X √3 reconstruction. A reduced adatom mobility, as observed in many studies of surfactant-mediated growth, can favor the formation of a high density of nuclei, and thus promote a layer-by-layer growth. As a further result of the kinetic simulation we obtained the adatom diffusion coefficient, a macroscopic quantity accessible in experiments. ©2005 The American Physical Society.

2005 - First-principles study of Sb-stabilized GaSb(001) surface reconstructions [Articolo su rivista]
Righi, Maria Clelia; Magri, Rita; Bertoni, Carlo Maria
abstract

We report results of ab initio total-energy and electronic- structure calculations for the Sb-stabilized GaSb(001) surface. We consider different reconstruction models proposed in the literature on the basis of experimental observations in typical GaSb growth conditions and present the T=0 surface stability diagram. We found that dimer-based (4 x 3) reconstructions are favored over widely proposed (1 x 3)/c(2 x 6) models that have a similar structure, but do not satisfy the electron counting rule. We discuss also the stability of the beta 2(2 x 4) reconstruction for comparison with the As-based GaAs(001) and InAs(001) surfaces that present this phase in a wide range of surface preparation conditions. We predict the surface band structure of the GaSb(001) stable configurations and identify the nature of the surface bands. All the (4 x 3) reconstructions turned out to be semiconducting with an energy gap slightly smaller than the GaSb bulk value.

2005 - Hydrogen-induced surface metallization of beta-SiC(100)-(3 x 2) revisited by density functional theory calculations [Articolo su rivista]
Di Felice, R; Bertoni, Carlo Maria; Pignedoli, Ca; Catellani, A.
abstract

Recent experiments on the silicon terminated (3x2)-SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of density-functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: in addition to saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, i.e., stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surface.

2005 - Kinetic Monte Carlo simulations of C diffusion on √3 × √3 β-SiC(111) based on ab initio calculations [Articolo su rivista]
Righi, Maria Clelia; Ca, Pignedoli; R., Di Felice; Bertoni, Carlo Maria; A., Catellani
abstract

We performed ab initio calculations to identify and characterize the stationary points of the potential energy surface experienced by a C adatom deposited on the root 3- x root 3 beta-SiC(111) surface. A kinetic Monte Carlo simulation relying on the ab initio calculated parameters allowed us to follow C diffusion at realistic temperature and time scales. We found that the C diffusion occurs mostly around the Si adatoms characteristic of the root 3- x root 3- reconstruction.

2004 - Angle resolved resonant inelastic X-ray scattering: intensity and dichroism from 3d transition metals in special geometries [Articolo su rivista]
P., Ferriani; Bertoni, Carlo Maria
abstract

The angular dependence of the resonant inelastic X-ray scattering cross section is studied within the ion-model for 3d metals. A geometrical setup with the incidence direction perpendicular to the magnetic axis and a conical scan of the outgoing radiation is chosen. The total and dichroic intensities have angular asymmetries related to the interference between intermediate states in the process. We show the results of the full calculation of the cross section comparing them with the commonly used fast collision approximation.

2004 - Angle-resolved resonant inelastic x-ray scattering from transition-metal magnetic ions [Articolo su rivista]
Ferriani, P; Bertoni, Carlo Maria; Ferrari, Giulio
abstract

We present a study on the x-ray inelastic scattering at the 2p resonance of transition metals. The theoretical results are discussed in the single-ion model with crystal-field corrections and the full description of multiplet manifolds. The interference effects among the intermediate states are described as a function of the directions and polarizations of the incoming and outcoming beams. We consider the special case where the emission is due to inner-shell recombination. A variety of peculiar effects are discussed, with special reference to the perpendicular geometry of the incoming beam. The simplest cases are analytically examined showing the potential sensitivity of this technique to the electronic structure. Numerical results for the total and dichroic signals for some elements of the 3d series are given.

2004 - Carbon induced restructuring of the Si(111) surface [Articolo su rivista]
Pignedoli, Ca; Catellani, A; Castrucci, P; Sgarlata, A; Scarselli, M; De Crescenzi, M; Bertoni, Carlo Maria
abstract

We present a combined experimental and theoretical investigation of the early carbonization stages of the Si(111)-(7x7) surface upon acetylene exposure. Scanning tunneling microscopy images reveal the formation of a (root3xroot3)R30degrees reconstruction. Ab initio calculations allow us to characterize this reconstruction as a carbon-rich silicon phase due to subsurface incorporation of C atoms.

2004 - Chemisorption sites and reaction pathways for acetylene on Si(111)-(7x7) [Articolo su rivista]
C., Sbraccia; Ca, Pignedoli; A., Catellani; R., Di Felice; Pl, Silvestrelli; F., Toigo; F., Ancilotto; Bertoni, Carlo Maria
abstract

The interaction between the acetylene molecule (C2H2) and the Si(1 1 1)-(7 x 7) surface is studied using both ab initio and empirical methods. Several possible chemisorption configurations are explored, which may be helpful to rationalize recent experimental results. The kinetics of the chemisorption process is also studied by means of accurate ab initio calculations, and low-energy paths leading to stable configurations are identified. It is shown that large-scale calculations based on the use of semi-empirical potentials, in tight cooperation with ab initio calculations, can be an efficient tool to achieve a broad overview of the chemisorption reactions of molecules occurring on complex semiconductor surfaces.

2004 - Sum rules for resonant inelastic x-ray scattering: Explicit form and angular dependence in perpendicular geometry [Articolo su rivista]
Borgatti, F; Ghiringhelli, G; Ferriani, P; Ferrari, G; van der Laan, G; Bertoni, Carlo Maria
abstract

Resonant inelastic x-ray scattering (RIXS) and resonant photoemission spectroscopy (RPES) can be used to selectively measure the ground-state properties of atoms in solid materials. For the two types of experiment we compare the sum rules developed in the past years to extract quantitative information from the measured spectra. We show that if the measurements are not sensitive to the emitted photon polarization state (in RIXS) or to the photoelectron spin orientation (in RPES), the two experiments exhibit the same angular dependence of the spectral intensities but differ in some numerical coefficients in the sum rules. In particular we give explicit expressions for the RIXS sum rules in the so-called perpendicular geometry for all the cases of practical interest. These rules can serve, in combination with the well-known x-ray magnetic circular dichroism sum rules in absorption, to evaluate the quadrupole and octupole moments of the scattering atoms.

2003 - Ab initio Simulations of Homoepitaxial SiC Growth [Articolo su rivista]
Righi, Maria Clelia; Pignedoli, C. A.; Di Felice, R.; Bertoni, Carlo Maria; Catellani, A.
abstract

An ab initio simulation study of SiC homoepitaxial growth is presented. It is shown that the nonstoichiometric reconstruction plays a relevant role in favoring the attainment of high-quality films. It is observed that the energy gain upon surface stability can induce the reorganization of the deposited material into the crystalline structure, thus revealing that a surface-driven mechanism is able to stabilize defect-free layer deposition on Si-rich surfaces.

2003 - Ab initio investigation of the adsorption of organic molecules at Si(111) and Si(100) surfaces [Articolo su rivista]
R., Di Felice; Ca, Pignedoli; Bertoni, Carlo Maria; A., Catellani; Pl, Silvestrelli; C., Sbraccia; F., Ancilotto; M., Palummo; O., Pulci
abstract

I To investigate the early stages of SiC growth on silicon, we performed an ab initio study of the adsorption of C2H2 and other small organic molecules on different Si surfaces. Our calculations, based both on geometry optimization and on finite-temperature molecular dynamics simulations, show that for all the molecules that we have considered the preferred adsorption sites at low temperature are confined at the surface, with no sub-surface penetration. Adsorption occurs through the formation of Si-C bonds, accompanied by a distortion of the adsorbed molecule to adapt the Si-C distance to the SiC bulk bond length. We discuss similarities and differences upon changing the organic molecule and the crystal face. To complete the study with the computation of directly measurable quantities, we analyze the optical reflectance anisotropy of one simulated structure. (C) 2003 Elsevier Science B.V. All rights reserved.

2003 - Italian family with two independent mutations:3358T/A in BRCA1 and 8756delA in BRCA2 genes. [Articolo su rivista]
Cortesi, L.; Turchetti, D.; Bertoni, Carlo Maria; ZANOCCO MARANI, Tommaso; Silvestri, C.; Vinceti, Marco; Federico, Massimo; Silingardi, Vittorio; Ferrari, Sergio
abstract

Hereditary breast/ovarian cancer is a well-characterized clinical entity, largely attributed to the inheritance of BRCA1 or BRCA2 mutations. Among general population, the mutation's frequency of these genes is very low; therefore, the identification of two independent mutations in the same family is a rare event. This study reports the presence of two mutations, one in the BRCA1 and the second in the BRCA2 gene in an Italian Caucasian kindred. This family is composed of more than 250 individuals, spanning through five generations, among which endogamy was a common phenomenon. Considering the tumor spectrum, this family is characterized by a high incidence of different types of cancer. In our study, we considered only three out of seven family units for BRCA1 and BRCA2 analysis. In one of the family units, we found independent mutations of both BRCA genes. The BRCA1 mutation on exon 11 (3358TA) was identified originally in the index case and subsequently in 18 members of this family, whereas the same mutation was not detected in a related family member with male breast cancer. The male breast cancer patient led to the identification, through mutational analysis, of a new BRCA2 mutation (8756delA). This BRCA2 mutation was also found in the male breast cancer patient's daughter. The discovery of the BRCA2 mutation allowed us to alert the patient's daughter who, otherwise, could be falsely reassured since she had a negative BRCA1 test.

2003 - Radiation-matter interaction: Absoption, Photoemission, Scattering. [Capitolo/Saggio]
Bertoni, Carlo Maria
abstract

Quantum physics describes the free radiation field and the systems of particles (like atom, molecules and solids) as well as their interaction in terms of absorption, emission and diffusion of photons. These processes are at the basis of all experimental techniques for the study of material systems, their ground state properties and excitations using light as a probe. They can be described using perturbation expansions, at the various levels of approximations. Another important point considered in this introduction is the difference between single-particle and and many-electron description of the initial, intermediate, and final states.

2003 - Resonant inelastic X-ray scattering from magnetic systems: Mn in MnFe2O4 [Articolo su rivista]
P., Ferriani; G., Ghiringhelli; G., Ferrari; Bertoni, Carlo Maria; A., Tagliaferri; L., Braicovich; N. B., Brookes
abstract

We present a numerical study of the applicability of the sum rule analysis used, in X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS), to evaluate the atomic spin moment of 3d transition metals. Magnetic circular dichroism (MCD) is present both in XAS and RIXS at the L-2.3 absorption edges, although the two sum rules presented here assume different experimental setups: in XAS-MCD the magnetic field is parallel to the circularly polarised incident photons, in RIXS-MCD the two are orthogonal (perpendicular geometry). For Mn2+ we explore XAS- and RIXS-MCD by performing atomic model calculation in the single ion limit with inclusion of the appropriate crystal and magnetic fields. The comparison of the XAS-MCD spectra with those measured from a MnFe2O4 sample is used to fix the calculation parameters. The applicability of the XAS- and RIXS-MCD sum rules to Mn2+ is numerically checked within the chosen calculation model. (C) 2002 Elsevier Science B.V. All rights reserved.

2003 - Surface effects in GaN growth [Articolo su rivista]
C. A., Pignedoli; R., Di Felice; Bertoni, Carlo Maria
abstract

By means of plane wave pseudopotential periodic supercell density functional theory calculations, we investigated the energetics of several GaN(0 0 0 1) surfaces containing atomic and molecular adsorbates. The surface structures were selected in order to get insight into GaN epitaxial growth, in situations where NH3 molecules are employed as the nitrogen precursor and H-2 molecules as the carrier gas, typical of standard vapor phase growth techniques. Therefore, we considered NH2, complexes and H atoms as molecular and atomic adsorbates, respectively, assuming they derive from molecular dissociation from the gas flows in the growth chambers. We took into account different adsorption stoichiometries, varying the relative proportion of NH2 and H adsorbates, and different degrees of clusterization of the ad-atoms and ad-complexes. For each structure, we obtained the relaxed atomic configuration and the corresponding total energy. Hence, we describe here the relative structural stability in terms of a phase diagram that identifies the optimal growth conditions to attain at intermediate stages the most favorable equilibrium surfaces.

2002 - Magnetic circular dichroism in resonant x-ray emission from impurities: Results at the L-2,(3) edges of Mn in Ni [Articolo su rivista]
Borgatti, F; Ferriani, P; Ghringhelli, G; Tagliaferri, A; De Michelis, B; Bertoni, Carlo Maria; Brookes, Nb; Braicovich, L.
abstract

We present a study of Mn magnetic impurities (about 2%) in Ni with resonant x-ray emission spectroscopy excited at the L-2,L-3 edges of Mn using circularly polarized x rays incident almost parallel/antiparallel to the magnetization. The results are discussed jointly with absorption magnetic circular dichroism and complemented with elastic reflectivity measurements. Moreover, we show that emission spectroscopy is particularly suitable to the impurity problem, since the self-absorption/saturation corrections to the measurements are rather limited. The problem is treated also theoretically with a full multiplet splitting calculation of a single Mn2+ (3d(5)) ion with cubic crystal field. The theory reproduces satisfactorily all experimental trends of the features dispersing with the incident photon energy in the whole L-3 region, whereas it is not adequate just below the L-2 excitation. The joint use of the theory and of the measurements allows us to extract the nondispersive component from the measured spectra. It is shown that this component does not correlate directly with the 3d-Mn local density of states. The general implications of the results are discussed.

2002 - Surface-induced stacking transition at SiC(0001) [Articolo su rivista]
Righi, Maria Clelia; Pignedoli, Carlo Antonio; Borghi, G.; DI FELICE, Rosa; Bertoni, Carlo Maria; Catellani, A.
abstract

We present the ab initio results for the energetics of several SiC surfaces having different underlying bulk polytypes, to investigate the role of surface effects in the mechanisms of stacking inversion in SiC. We considered the Si adatom √3×√3 reconstruction for the cubic SiC(111) and the hexagonal SiC(0001) surfaces, taking into account the different subsurface bulk terminations compatible with the 4H and 6H polytypes, and allowing for two opposite stacking orientations of the topmost surface layer. Our investigation reveals that the energy differences among SiC polytypes are enhanced at the surface with respect to the bulk, and two-dimensional effects favor the formation of cubic SiC. We discuss the relevant role played by the surface energetics in the homoepitaxial growth of SiC.

2001 - Dissociative chemisorption of NH3 molecules on GaN(0001) surfaces [Articolo su rivista]
Pignedoli, Ca; Di Felice, R; Bertoni, Carlo Maria
abstract

We performed ab initio calculations to study the structure and the relative stability of hydrogenated GaN(0001) surfaces which may form during the growth of gallium nitride using an NH3 nitrogen source. The surfaces that we have studied contain H ad-atoms and NH2 ad-complexes adsorbed on GaN(0001). We find that adsorption of H, alone or accompanied by other species, stabilizes the surface of GaN and is able to catalyze the formation of an ideal-like geometry. We give an interpretation of this effect in terms of the electron band structure. Our results suggest, on microscopic grounds. that rough surfaces are formed by metal organic chemical vapor deposition under N-rich conditions. Ga-rich and H-rich conditions are suggested for the attainment of high quality films.

2001 - XPS surface analysis of monocrystalline silicon solar cells for manufacturing control [Articolo su rivista]
Braicovich, L.; Borgatti, F.; Tagliaferri, A.; Ghiringhelli, G.; Brookes, N. B.; Ferriani, P.; Bertoni, Carlo Maria
abstract

We discuss the application of resonant inelastic X-ray scattering to the study of magnetic systems in the soft X-ray range. To this end we distinguish two broad areas. In the first the layout of the experiment is such that the absorption magnetic circular dichroism (MCD) is not zero. In the second the magnetisation is perpendicular to the incident helical beam so that the absorption MCD is zero. In the first area we summarise published results on Fe-Co alloys and we present new data on Mn impurities in Ni together with calculations. In the second area we summarise published results on Ni in Ni-ferrite with final 3s shell excitation and we present new results on Co-metal and Co in Co-ferrite measured with a new approach. This is based on the incident energy dependence of the integral of the Raman spectrum in inner shell excitation (integrated resonant Raman scattering). The potentialities and the limitations of the above methods are critically presented.

2000 - BRCA1 mutations and clinico-pathological features in a sample of italian women with early-onset breast cancer [Articolo su rivista]
Turchetti, D.; Cortesi, L.; Federico, Massimo; Bertoni, Carlo Maria; Mangone, L.; Ferrari, Stefano; Silingardi, V.
abstract

Breast cancer in young women is uncommon and often presents with unfavourable biopathological features. Although early age at onset could suggest a genetic susceptibility to cancer, the appropriateness of BRCA1 testing for women with early-onset breast cancer and modest family history (FH) is controversial. 40 Women diagnosed with breast cancer at the age of 35 years or less, unselected for FH, were screened for germ line BRCA1 mutations by automated sequencing of exons 2, 5, 6, 11, 13 and 20. Overall, deleterious mutations were evidenced in 6 (15%) patients. With regard to FH, mutations were detected in 14%, 11% and 29% of women with none, weak and strong FH, respectively. Large tumour size, grade 3, lack of oestrogen receptors and high proliferation rate were significantly more common in mutation carriers (MC). Our data support both the appropriateness of testing young breast cancer patients and the frequency of unfavourable features in BRCA1-related breast cancer. It is hypothesised that BRCA1 mutations partially justify the high rate of aggressive breast cancer in young patients and that combining age and breast cancer phenotype could help to identify probable MC.

2000 - Chapter 4 Electronic structure of adsorbates on surfaces. Adsorption on semiconductors [Capitolo/Saggio]
Bertoni, Carlo Maria; G., Roma; R., DI FELICE
abstract

A review of the data, the calculations and of the methods concerning the ordered chemisorption of atomic and molecular species on surfaces is given.

2000 - First-principle investigation of the formation of Cs dimer chains upon adsorption on InAs(110) [Articolo su rivista]
A., Calzolari; C. A., Pignedoli; R., Di Felice; Bertoni, Carlo Maria; A., Catellani
abstract

We have performed first-principle calculations to investigate the formation of Cs chains on an InAs(110) surface at submonolayer coverage. By studying the energetics of different possible configurations in which Cs atoms adsorb on and between surface ions, we have identified the most stable geometry. The stable geometry is characterized by the presence of asymmetric dimer chains, in agreement with recent scanning tunneling microscopy images. We find an important effect of substrate-mediated Cs-Cs correlation. Adsorption occurs simultaneously at two different surface sites. The binding of Cs atoms to surface atoms essentially involves cations, while there is no direct adsorbate-anion interaction. (C) 2000 Elsevier Science B.V. All rights reserved.

1999 - First-principle study of ß-AlN thin films on ß-SiC(001) [Articolo su rivista]
R., DI FELICE; Bertoni, Carlo Maria; A., Catellani
abstract

Abstract: We have investigated the initial stages of formation of cubic AlN films on SiC(001) by studying the energetics of possible structures. We have considered 1X1 and p(4X1) surface reconstructions for the films, and we have allowed for different interface arrangements including atomic mixing. The results of our first-principles calculations reveal that, in N-rich conditions, no two-dimensional film structure is stable. However, in Al-rich conditions, it is possible to stabilize a thick wetting layer of cubic AlN provided the proper interface mixing is achieved. The most stable AlN film exhibits a p(4X1) surface reconstruction. (C) 1999 American Institute of Physics.

1998 - First-principle simulations of molecular processes at semiconductor surfaces [Articolo su rivista]
Bertoni, Carlo Maria; Di Felice, R; Pignedoli, Ca
abstract

We present an overview of basic adsorption and dynamical processes at semiconductor surfaces, studied by first-principle simulations. We have considered prototypical semiconductor surfaces such as GaAs(110), which is the cleavage plane of GaAs, and Si(100), which is the most common orientation for silicon from epitaxial growth. We present an adsorption path and the stable configurations of a Cl-2 molecule on GaAs(110), as well as the equilibrium state for complete and partial monolayer adsorption of H on the same surface. We also show the dynamical evolution of Sit dimers on the Si(100) face: they alternate their buckling in a correlated way producing different reconstructions.

1998 - Interaction of Cl-2 molecules with GaAs(110) surface [Articolo su rivista]
A., Marcellini; C. A., Pignedoli; Ferrario, Mauro; Bertoni, Carlo Maria
abstract

We present ab-initio molecular dynamics calculations for the chemisorption of Cl on GaAs(110), to study the adsorption sites and the presentation of surface order at the early stages of adsorption. We simulated the impact of a single Cl-2 molecule on the surface using a (2 x 2) surface cell and the repeated slab geometry. Along the dynamical path dissociation occurs, bonds are created between Cl and the atoms of Ga and As and local quenching of the buckling relaxation is found. The dynamical results are compared with the optimization geometries obtained by allowing the two Cl atoms to adsorb on Ga and As. The adsorption on both sites is the most stable arrangement. Disorder effects are seen in the dynamics. (C) 1998 Elsevier Science B.V. All rights reserved.

1997 - Ab-initio calculation of the optical properties of silicon quantum wires [Relazione in Atti di Convegno]
Ossicini, Stefano; Biagini, M.; Bertoni, Carlo Maria; Roma, G.; Bisi, Olmes
abstract

ABSTRACT We study the effect of H. O passivation and inter-wire interaction on the optical properties of nanoscale Si wires. We find that wires with diameters as small as 10-25 AA are active in the visible range. Inter-wire interaction leads to the presence of localized states which lower the band gap energy. The presence of dangling bonds generates broad features in the infrared region. O-Si bonds reduce the optical threshold. These results are important for the discussions concerning absorption and luminescence in porous silicon.

1997 - Optical properties of isolated and interacting silicon quantum wires [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; M., Biagini; A., Lugli; G., Roma; Bisi, Olmes
abstract

We have studied the effect of hydrogen passivation and inter-wire interaction on the electronic structure and optical properties of nanoscale Si wires through two first-principle techniques: linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA) and norm-conserving pseudopotential. We have considered free, partially and totally H-passivated [001] Si quantum wires with various rectangular cross-sections; moreover we have investigated the inter-wire interaction, by varying the wire density. The optical properties have been computed by evaluating the imaginary part of the dielectric function and the absorption coefficient. We find that wires with diameters as small as 10-25 Angstrom are active in the visible range. Inter-wire interaction leads to the presence of localized interface states which lower the bandgap energy. These results are important for the discussion about the dimensionality of confined Si quantum particles in porous Si and for the debate on quantum confinement models.

1996 - Confinement and passivation in isolated and coupled silicon quantum wires [Capitolo/Saggio]
Ossicini, Stefano; Bertoni, Carlo Maria; M., Biagini; G., Roma; Bisi, Olmes
abstract

Porous silicon, quantum wires, ab-initio calculation

1996 - High prevalence of adenomas and microadenomas of the duodenal papilla and periampullar region in patients with FAP [Articolo su rivista]
Bertoni, Carlo Maria; R., Sassatelli; E., Nigrisoli; M., Pennazio; A., Tansini; A., Arrigoni; PONZ DE LEON, Maurizio; F. P., Rossini; G., Bedogni
abstract

BACKGROUND: Patients with familial adenomatous polyposis (FAP) are traditionally considered to be at high risk for duodenal-papillary and periampullary adenomas and cancer. AIM: To evaluate prospectively the prevalence, histology and clinical significance of ampullary and periampullary macroscopic and microscopic lesions in our population of affected patients. SETTING: Three gastroenterological departments of northern Italian hospitals. PATIENT AND METHODS: Twenty-five affected patients were carefully investigated over a 24-month period by end-viewing and side-viewing upper panendoscopy. Biopsies were performed on representative macroscopic lesions and randomly on normal-appearing papillary and periampullary mucosa. RESULTS: Seven patients had macroscopic adenomas of the duodenal papilla, three of the periampullary region and five at both sites (cumulative prevalence 40%). An additional six patients had macroadenomas in the rest of the duodenum (overall prevalence 64%). Microscopic adenomas were identified in nine and two patients in the papilla and periampullary region, respectively, and in three at both sites (overall prevalence 44%). Thus, a total of 17 (68%) patients presented macro- or microadenomas at these locations. The prevalence rose to 72%, when a further patient with macroadenomas in the rest of the duodenum only was included. Malignancy was not encountered and severe dysplasia was observed only in a macroadenoma of the second duodenal portion. A higher frequency of macroadenomas in the papilla and periampullary region was significantly correlated with the presence and number of such lesions in the rest of the duodenum (P = 0.04). No other significant association was detected either between micro- or macroadenomas at different sites or with the demographic, clinical and pathological features. CONCLUSION: This study confirms that the duodenal papilla and periampullary region are sites with high prevalence of macro- and microscopic adenomas in patients with FAP. However, our data do not seem to support a higher frequency and malignancy potential of such lesions as compared to polyps in the rest of the duodenum. Nevertheless, these findings warrant a periodic, careful examination of the duodenum with either end-viewing or side-viewing endoscopy, the need for random biopsies of the papilla and periampullary region and the removal of any larger or rapidly growing lesions detected.

1996 - The phonon spectra of LiH and LiD from density-functional perturbation theory [Articolo su rivista]
G., Roma; Bertoni, Carlo Maria; S., Baroni
abstract

We present a self-consistent linear response calculation of phonon dispersion relations in LiH and LiD. Our calculations are performed within density-functional perturbation theory using the local-density approximation and the plane-wave pseudopotential method with the inclusion of the non-linear core correction for exchange and correlation. We accurately estimate zero-point effects on the structural properties at zero temperature. The calculated lattice parameter is a little too small with respect to experiment. The non linear core correction and the inclusion of zero-point effects reduce the discrepancy from 5.1% to 2.5%. Good agreement is obtained between calculated and observed phonon dispersions.

1994 - Electronic Structure of cubic GaN with Self-Energy Corrections [Articolo su rivista]
M., Palummo; L., Reining; R. W., Godby; Bertoni, Carlo Maria; N., Bornsen
abstract

Abstract: We present the results of a calculation for the bulk electronic structure of gallium nitride in the zincblende phase. We determine the equilibrium lattice constant, the cohesive energy and the bulk modulus in the Density Functional approach within the Local Density Approximation (DFT-LDA). The one-particle eigenvalues of the DFT Kohn-Sham equation do in principle not agree with the experimental band structure. Therefore, we calculate the quasi-particle energies by including self-energy corrections to the DFT-LDA exchange correlation potential, with the GW approximation for the electron self-energy. We use norm-conserving pseudopotentials and a large plane-wave basis set (100 Ry cut-off) for a converged calculation in the DFT-LDA. The LDA band gap turns out to be very sensitive to the crystal volume. We find that GW corrections to the LDA band gap are significant. A detailed comparison with other DFT-LDA results and approximate GW calculations and with existing experimental data is given.

1993 - CHEMICAL ORDER IN AMORPHOUS COVALENT ALLOYS - A THEORETICAL-STUDY OF A-SiC [Articolo su rivista]
F., Finocchi; G., Galli; M., Parrinello; Bertoni, Carlo Maria
abstract

The microscopic structure and electronic properties of amorphous silicon carbide at stoichiometric composition have been investigated with ab-initio molecular dynamics simulations. Our results show that a-SiC can be classified neither as chemically ordered nor as completely random and has a structure much more complex than commonly believed. Our data also indicate that a detailed analysis of each atomic species is crucial to the understanding of the material properties.

1993 - STRUCTURAL AND DYNAMIC PROPERTIES OF SURFACES FROM AB-INITIO MOLECULAR-DYNAMICS [Articolo su rivista]
Bertoni, Carlo Maria; A. I., Shkrebtii; R., Difelice; F., Finocchi
abstract

The recent application to the semiconductor surfaces of the powerful ab-initio molecular dynamics (Car-Parrinello) method [R. Car,M. Parrinello,Phys. Rev. Lett. 55,2471 (1985)] will be reviewed. The essential features of the method will be addressed first,then we will review a number of recent sudies on the properties of the clean and adsorbate covered surfaces of Si,Ge and GaAs,obtained through Car-Parrinello simulations. Finally an original application of the ab-initio molecular dynamics for the clean and hydrogenated GaAs(110) surfaces is presented.

1993 - Surface vibration at clean and hydrogenated GaAs(110) from ab-intio molecular-dynamics [Articolo su rivista]
DI FELICE, Rosa; A. I., Shkrebtii; F., Finocchi; Bertoni, Carlo Maria; G., Onida
abstract

Abstract: We present results of finite-temperature Car-Parrinello molecular dynamics simulations of the atomic geometry and the vibrations at clean and hydrogenated (1 and 0.25 ML coverages) GaAs(110) surfaces. Through an accurate analysis of the atomic motion based on signal processing techniques the phonon modes have been investigated. Some surface vibrational modes at high-symmetry k points, with layer-by-layer resolution of the eigenvectors, have been found. The results are compared with available experimental data and previous calculations.

1993 - THE ELECTRONIC-STRUCTURE OF GALLIUM NITRIDE [Articolo su rivista]
M., Palummo; Bertoni, Carlo Maria; L., Reining; F., Finocchi
abstract

The results of a density functional calculation on gallium nitride are given. We use norm-conserving pseudopotentials with sufficiently extended sets of plane waves to investigate the ground-state properties and the electronic band structure for the zincblende phase of GaN and compare them with the corresponding results for the wurtzite structure. A comparison with the outcomes of other calculations and with the existing experimental data is also given.

1992 - Hydrogen covered Si(111) surfaces [Articolo su rivista]
BUONGIORNO NARDELLI, M.; Finocchi, F.; Palummo, M.; DI FELICE, R.; Bertoni, Carlo Maria; Bernardini, F.; Ossicini, Stefano
abstract

The recently discovered method for the production of an ideally H-terminated, stable and easily transferable Si(111)1 × 1 surface renews the interest for this prototypical system. Through a density functional description of the electronic structure based on pseudopotential and LMTO methods, we discuss in detail spectroscopical information, bond geometry, stretching frequency and the energetics of this surface. Further attention is devoted to the chemisorption of atomic hydrogen on the Si(111)2 × 1 surface and to the removal of the reconstruction, which leads to a less perfect 1 × 1 surface.

1991 - Electronic and structural properties of semiconductor surfaces with chemisorbed oxygen [Articolo su rivista]
Finocchi, F.; BUONGIORNO NARDELLI, M.; Bernardini, F.; Palummo, M.; DE FELICE, R.; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

Electronic calculations

1991 - HYDROGEN ON SEMICONDUCTOR SURFACES - THEORY OF THE ELECTRONIC-STRUCTURE [Articolo su rivista]
Bertoni, Carlo Maria; F., Finocchi; F., Bernardini; M., BUONGIORNO NARDELLI
abstract

The atomic geometry and the electronic structure of GaAs(110) and Si(111) with full coverage of chemisorbed hydrogen is described in the scheme of the density functional theory and using norm-conserving pseudopotentials, as ideal prototypes of semiconductor surfaces interacting with hydrogen. The removal of relaxation or reconstruction, the bond geometry and the stretching frequencies can be described in a full ab initio approach.

1991 - STRUCTURAL-PROPERTIES OF AMORPHOUS SiC VIA AB-INITIO MOLECULAR-DYNAMICS [Articolo su rivista]
F., Finocchi; G., Galli; M., Parrinello; Bertoni, Carlo Maria
abstract

We have investigated a-SiC at stoichiometric composition at room temperature using a first principles molecular dynamics technique. This method has proven to be very successful in predicting a variety of properties of disordered systems, amongst them a-C and a-Si. From our results, we analyzed the short range order of the system, in particular whether it is chemically ordered or has a random distribution of Si-C bonds and the different kind of bonding configurations attained by the two species.

1990 - Calculation of the electronic properties of simple metal surfaces through lateral averaged pseudopotential [Articolo su rivista]
Finocchi, F.; Bertoni, Carlo Maria; Ossicini, Stefano; Solonovich, V. K.
abstract

Metal surfaces, jellium models

1990 - Chapter 6 - Basic Structural and Electronic Properties of Semiconductor Surfaces. [Capitolo/Saggio]
Bertoni, Carlo Maria
abstract

Starting from simple models, the description is extended to the surfaces delimiting 3D crystals, with the use of localized and plane wave basis set. A review of the methods is given, from semi-empirical to ab-initio DFT calculations. A report on different cases of ideal and reconstructed surfaces is given, with reference to most relevant and intriguing examples of semiconductor surfaces.

1990 - Chemisorption of H on GaAs(110): a first-principle calculation. [Articolo su rivista]
Bertoni, Carlo Maria; M., BUONGIORNO NARDELLI; F., Bernardini; F., Finocchi; Molinari, Elisa
abstract

We calculate the equilibrium structure of a monolayer of H chemisorbed on GaAs(110), and show that is characterized by the full removal of the substrate relaxation and by a counterrelaxation of about -5 degrees.The corresponding vibration frequencies of the H-subtrate bonds and chemisorption induced changes in the electronic structure are also calculated, and found in good agreementwith the available experimental data.

1990 - Electronic structure of Si(111)-NiSi2(111) A-type and B-type interfaces [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Bertoni, Carlo Maria
abstract

The self-consistent electronic properties of the epitaxial Si(111)-NiSi2(111) interfaces are computed for the experimentally observed A- and B-type interface structures. The densities of states projected at the different atomic sites and the two-dimensional band structure provide a detailed analysis of the electronic properties of the silicon-silicide interface. The Schottky-barrier height turns out to be dependent not only on the interface structure, but also on the interface relaxation distance. A critical analysis of existing results is also presented.

1990 - Hydrogen adsorption on compound semiconductor surfaces [Articolo su rivista]
Bertoni, Carlo Maria; M., Buongiorno Nardelli; Molinari, Elisa
abstract

We present the results of the total-energy calculations for different adsorption geometries of atomic hydrogen on GaAs (110) and the corresponding changes in the surface electronic states. In particular we discuss the problem of the quenching of the substrate relaxation.

1990 - SIMPLE METAL-SURFACES AND IMAGE POTENTIAL STATES [Articolo su rivista]
F., Finocchi; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

The surfaces of Al and Na are simple systems, but they show several important features like surface states and image resonances. Using the weighted density approximation within the density functional theory we are able to account self-consistently for image effects in the description of their electronic properties.

1990 - STRUCTURAL MODELS OF RECONSTRUCTED Si(110) SURFACE PHASES [Articolo su rivista]
A. I., Shkrebtii; Bertoni, Carlo Maria; R., Delsole; B. A., Nesterenko
abstract

Structural models are proposed for the wide class of experimentally observed superstructures on the Si(110) surface: both for the case of Si(110) "16 X 2" reconstruction and for the superstructures stabilized by Ni or Cu contamination, like (4 X 5), (2 X 1) and (5 X 1). These models are based on semi-empirical tight-binding total energy calculations for model subunits, on surface lattice dynamics experiments and also on scanning tunnelling microscope measurements. These models consist of a variety of building blocks, namely: new type of pi-bonded dimers, adatoms, atoms with unsaturated dangling bonds, regular sequences of surface steps and missing rows. Surface stresses due to the pi-dimer or adatom formation and Ni contamination are considered to be the driving force of the Si(110) reconstructions.

1990 - THE ELECTRONIC-PROPERTIES OF SI-NISI2(111) EPITAXIAL INTERFACES [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Bertoni, Carlo Maria
abstract

Abstract The densities of states and the two-dimensional band structure provide a detailed investigation of the electronic structure of the A-type and B-type epitaxial Si---NiSi2(111) interfaces. We find that the Schottky barrier height is dependent not only on the type of interface growth, but also on the interface relaxation distance.

1990 - Temperature effects on the surface properties of metals [Articolo su rivista]
Solonovich, V. K.; Kukharenco, L. V.; Finocchi, F.; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

Metal surfaces, theory and experiment.

1989 - Azimuthal Dependence of the Vibrational Excitation in Si(111)(2x1) [Articolo su rivista]
DEL PENNINO, Umberto; M. G., Betti; C., Mariani; Nannarone, Stefano; Bertoni, Carlo Maria; I., Abbati; A., Rizzi
abstract

In the (2x1) reconstructed Si(111) the surface vibration at 56 meV shows an azimuthal dependence, has shown by HREELS at very low primary beam energies.

1989 - Selfconsistent calculations of the electronic properties of simple metal surfaces [Articolo su rivista]
Finocchi, F.; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

Jellium calculations

1989 - Sviluppi nel Campo della Struttura Elettronica delle Superfici [Capitolo/Saggio]
G., Chiarotti; Bertoni, Carlo Maria
abstract

Si ripercorrono gli esperimenti importanti e gli sviluppi teorici e interpretativi nelo studio della struttura elettronica delle superfici e delle proprietà ottiche di superficie.

1987 - ADSORPTION OF MONO-VALENT METALS ON THE GaAs(110) SURFACE [Articolo su rivista]
Bertoni, Carlo Maria; C., Calandra; C., Mariani; Valeri, Sergio
abstract

We present a synchrotron radiation investigation on the electronic properties of the GaAs(110) surface covered with alkali metals (Cs, K). As and Ga core level spectra for the clean surface are compared with those obtained after metal deposition. Satellites structures at about 4 and 6 eV from the main peak are found, which can be interpreted in terms of the excitation of overlayer plasma modes. We also study comparatively the effect of K and Cs on the oxidation of the GaAs surface.

1987 - Electron density profiles at charged metal surfaces in the weighted density approximation [Articolo su rivista]
Ossicini, Stefano; Finocchi, F.; Bertoni, Carlo Maria
abstract

We present a selfconsistent calculation of the electron distribution at metal surfaces in thepresence of a static electric field in the semi-infinite jellium model. The calculation is based on theHohenberg-Kohn-Sham theory with a nonlocal approximation for the exchange-correlationenergy and potential. For different metallic density r s = 2, 3, 4 we present numerical results for themean position and the spread of the induced charge density. The results for the centroid of thecharge induced by a small field are discussed and compared with the image plane position valuesobtained from the exchange-correlation potential.

1987 - HIGH-RESOLUTION ELECTRON-ENERGY LOSS INVESTIGATION OF THE AZIMUTHAL DEPENDENCE OF THE SI(111)(2X1) SURFACE EXCITATIONS [Articolo su rivista]
DEL PENNINO, Umberto; M. G., Betti; C., Mariani; Bertoni, Carlo Maria; S., Nannarone; I., Abbati; L., Braicovich; A., Rizzi
abstract

Electron energy loss spectroscopy has been used, with low energy of the primary beam and azimuthal resolution, to study the anisotropy of the surface vibrational and electronic properties of the cleavage surface of silicon. The analysis of the loss function has been performed through the subtraction of the kinematic prefactor accounting for the finite angular acceptance of the spectrometer. The angular dependence of the phonon excitation, even at very low primary energies, is not so high as expected in previous works. The loss features associated with the transition between surface electronic bands is discussed in terms of a theoretical model of the surface dielectric function.

1987 - Image force effects in the barrier height for metal-metal tunneling electrons [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

The density-functional theory is applied to calculate the interface barrier height between two metals as a function of the metal separation. Image-force effects are considered, since a nonlocal exchange-correlation energy functional is used. It is shown that the inclusion of nonlocality in the description of the exchange-correlation potential is important for a correct description of the barrier height, which turns out to be considerably lowered for metal-metal separations greater than a few angstroms.

1987 - Image potential at metal surfaces [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

The density-functional theory is applied to calculate the self-consistent electron density distribution and the surface potential at metal surfaces with different electron densities. The surface potential yields the correct long-range image potential, since a nonlocal exchange-correlation energy functional is used. It is shown that within this scheme it is possible to calculate the position of the image plane as a function of the bulk density, the influence of the image potential on the binding energy of the image potential states, and the correct behavior of the interface barrier height for metal–metal tunneling electrons.

1987 - Image potential effects on the metal-metal barrier height [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

Jellium non local calculation

1986 - Azimuthal Dependence of Reflection High Resolution Electron Energy Loss of Si(111) (2x1) [Articolo su rivista]
DEL PENNINO, Umberto; Betti, Maria Grazia; Mariani, Carlo; Bertoni, Carlo Maria; Nannarone, Stefano; I., Abbati; L., Braicovich; Rizzi, Angela
abstract

The changes in the energy of the surface states transitions in Si(111)(2x1) surface have been investigated by HREELS.

1986 - Image plane for surface potential [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; Gies, P.
abstract

A method of describing the nonlocality of exchange and correlation energy and potential, which gives the correct long-range imagelike behaviour, is adopted in a self-consistentcalculation of a solid surface in the framework of the jellium model. The exchange-correlation potential Vxc(x) whithin the approximation proposed by Gunnarson and Jones, is written for asystem with translational invariance in the plane parallel to the surface. The electron density n(z) and the total effective potential V seen by an electron are self-consistently calculated.We show that within this scheme it is possible to calculate the position of the image plane, as a function of the bulk density.

1986 - Image plane position for metal surfaces [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; Gies, P.
abstract

Metal surfaces.

1986 - Non-local exchange and correlation in the jellium model of surfaces [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; Gies, P.
abstract

A method of describing the non-locality of exchange and correlation energy and potential. used by the authors in a density-functional calculation of atomic structure, is now adopted in a self-consistent calculation of a solid surface in the framework of the jellium model. The exchange-correlation potential VXC(z), within the approximation proposed by Gunnarsson and Jones, is written for a system with translational invariance in the plane parallel to the surface. The electron density n(z) and the total effective potential Veff(z) seen by an electron are self-consistently calculated for different electron densities (rs varying from 2 to 6). The behaviour of the different contributions to the surface potential, the intensity of the Friedel oscillations, the surface energies, and the image-plane position z0 are presented, discussed and compared with the results of the local density approximation.

1985 - Band structure calculation for GaAs and Si beyond the local density approximation [Articolo su rivista]
Manghi, Franca; G., Riegler; Bertoni, Carlo Maria; G. B., Bachelet
abstract

We have performed pseudopotential self-consistent band-structure calculations for bulk solids in the framework of the Kohn-Sham density-functional theory, applying the nonlocal exchange-correlation functional of Gunnarsson and Jones to valence electrons with no further approximation. We present our method together with representative results for GaAs and Si, the main interest being focused on single-particle energy eigenvalues and gaps. An accurate comparison of the results obtained with this approach and the local-density-approximation results is given. The dependence of such a comparison on the choice of the basis set (localized Gaussian orbitals or plane waves) is also examined, and found to be unimportant. The reliability of the method, the changes in band structure, their k→ dependence, and the behavior of the exchange-correlation potential throughout the unit cell are discussed. We find that the inclusion of nonlocality in the description of exchange and correlation does not change valence-band states significantly and increases by a very small amount the energy difference between some conduction-band states and the top of the valence band. The increase (≲0.2 eV) is, however, insufficient to solve the problem of the local-density-approximation minimum band gaps, which remain much smaller than the measured energy gaps.

1985 - Density functional calculation of atomic structure with non-local exchange and correlation [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

A method of describing the nonlocality of the exchange and correlation energy and potential is used in a self-consistent spinless atomic calculation. The exchange and correlation potential V is analytically written and used in the calculation of the total energies and orbitals eigenvalues of the self-consistent Kohn-Sham equations for the atomic case. The results for the lighter atoms and for other examples are discussed and compared with the results obtained within the framework of the local-density approximation (LDA). We also present an analysis of the shape and asymptotic behavior of the potentials. A comparison of the outcome of the method with the results of LDA, Hartree-Fock calculations, and other approximate descriptions of nonlocality effects clearly indicates the power of the method.

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 - 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 - Density of states at the Si(111)-(2×1) surface: A study of the clean and H-covered surface [Articolo su rivista]
A., Selloni; Bertoni, Carlo Maria
abstract

The electronic structure of the Si (111)-(2×1) surface is analyzed by comparing different reconstruction models. The changes of state density induced by chemisorption of hydrogen are investigated for the different surface geometries.

1983 - ELECTRON-STATES AT Zn(0001) SURFACE [Articolo su rivista]
Manghi, Franca; Bertoni, Carlo Maria
abstract

The electronic structure of the Zn(0001) surface is analyzed through a realistic slab calculation with an accurate tight-binding sp-d model hamiltonian. The results are compared with a recent photoemission investigation of this surface. The comparison with noble and near-noble metal surfaces, with reference to the surface states in the s-p gap at the centre of the surface Brillouin zone, is also discussed.

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 - 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 - DIRECT EVIDENCE FOR D-BAND INVOLVEMENT IN THE TISE2 PHASE-TRANSITION [Articolo su rivista]
N. G., Stoffel; F., Levy; Bertoni, Carlo Maria; G., Margaritondo
abstract

We have investigated the Ti d-electronic states of stoichiometric TiSe2 for k-vectors in the neighborhood of the L-point at temperatures above and below the critical temperature using angle-resolved photoemission. Strong changes observed in the spectra upon lowering the temperature definitely demostrate that the lattice-instability phase transition involves Ti d-electrons as required by an excitonic-insulator mechanism. The hypothesis that TiSe2 at low temperatures is the first and only known example of an exitonic insulator, therefore, remains plausible after this crucial test.

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.

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 - Density-of-states changes near the Fermi level and the lattice instability in TiSe_2 [Articolo su rivista]
G., Margaritondo; Bertoni, Carlo Maria; J. H., Weaver; F., Levy; N. G., Stoffel; A. D., Katnani
abstract

We report the direct observation of the density-of-states changes near the Fermi level associated with the charge- density-wave-like lattice instability in TiSe,. Synchrotron-radiation photoemission spectra have been taken below and above the critical temperature on highly stoichiometric TiSe,. The changes in the spectra due to the phase transition have been analyzed in terms of several different models of the corresponding changes in the electronic structure. The best agreement with the experimental results is given by models in which a large gap is created upon interaction between overlapping p and d bands at the Fermi level.

1981 - EVIDENCE FOR SEMICONDUCTOR-SEMICONDUCTOR INTERFACE STATES - SI(111) (2X1)-GE [Articolo su rivista]
P., Perfetti; N. G., Stoffel; A. D., Katnani; G., Margaritondo; C., Quaresima; F., Patella; A., Savoia; Bertoni, Carlo Maria; Manghi, Franca
abstract

A direct correlation was found for the first time between theory and experiments on the localized electronic states at a semiconductor-semiconductor interface. The investigation involved synchrotron-radiation photoemission experiments and tight-binding calculations on Ge adatoms on cleaved Si substrates. The theoretically predicted interface states in a region 4-9 eV below the Fermi level were detected in the experimental spectra. These occupied states and their unoccupied counterparts have a fundamental influence on the localized one-electron transitions and on relevant heterojunction parameters in transport processes.

1981 - PHOTOEMISSION STUDIES OF HETEROJUNCTION INTERFACE FORMATION - GE-GAAS(110) AND GE-SI(111) [Articolo su rivista]
G., Margaritondo; N. G., Stoffel; A. D., Katnani; H. S., Edelman; Bertoni, Carlo Maria
abstract

A comparison between the formation mechanism of Ge-GaAs(110) and Ge-Si(111) is presented. The localized valence band states and core-level states were detected by photoemission spectroscopy with synchrotron radiation. Both interfaces are sharp and Ge forms smooth overlayers at room temperature. The measured band discontinuities sharply disagree with the prediction of 'linear' interface models. More sophisticated interface models give band discontinuities in agreement with our experimental results.

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 - 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 - 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.

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 - Linear-Combination-of-Atomic-Orbital description of the electron states at the (0001) surfaces of hexagonal-close-packed metals [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; Manghi, Franca
abstract

The tight-binding method for calculating the electronic structure of a slab of regulary stacked atomic planes has been extended to treat the case of a (0001) surface of a hexagonal-close-packed (hcp) transition metal. The symmetry of the Hamiltonian is discussed and the basis functions of the representations along high-symmetry lines of the two-dimensional Brillouin zone are given. The theory is applied to the study of a 13-layer slab of hcp scandium. We find that several surface states and resonances arise in the d-band region and we discuss the effects of the self-consistency on these surface features. Possible experiments to test the theoretical results are suggested.

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

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 - Absence of filled surface states in the s-p gap of clean (111) surface of Ag [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; Manghi, Franca
abstract

The nature and location of the surface states, present at the (111) face of Ag, are investigated through a theoretical calculation of the band structure and local density of states based on a LCAO method. Previous assignments of surface structures in the photoemission spectra are critically discussed in the light of our results.

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 - 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 - 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.

1974 - Dielectric screening matrix and lattice dynamics of Si [Articolo su rivista]
Bertoni, Carlo Maria; Bortolani, Virginio; C., Calandra; E., Tosatti
abstract

The dynamical matrix of a semiconductor is set up by evaluating the full dielectric matrix to the lowest order in a Brillouin-Wigner perturbation scheme. The introduction in the dynamical matrix of the nondiagonal elements of the dielectric matrix, which account for the lattice effects on the electron screening, gives rise to noncentral forces between the atoms and restores the correct q ~0 limit for the longitudinal acoustic frequencies through the fulfillment of the acoustic sum rule. A calculation of the phonon frequencies of Si is presented. The results, which are in reasonable agreement with experimental data, show that the off-diagonal elements are responsible for the stability of the crystal against shear and give real values for transverse acoustic frequencies. It is also shown that a third-order Rayleigh-Schrodinger perturbation theory, with some correction to include higher-order effects, can provide a good description of phonon spectrum of Si, but poorly reproduces the experimental elastic constants.

1974 - Role of three-body interactions in the calculator of the total energy and the shear modulus of hexagonal metals [Articolo su rivista]
Bertoni, Carlo Maria; Bisi, Olmes; F., Nizzoli
abstract

By calculating the total energy and the shear modulus of hexagonal metals we show that the three-body interactions are not very important in the determination of the stable structure, but must be taken into account when studying the dynamical properties.

1974 - The Vacancy Formation Energy for Zirconium. [Articolo su rivista]
M., Antonini; V., Bortolani; Bertoni, Carlo Maria
abstract

The vacancy formation energy of zirconium has been calculated by means of a microscopic model which takes into account the electron-electron and electron-ion interaction. The result obtained is in reasonable agreement with the presently available experimental data. The connection between the vacancy formation energy and the basic mechanism for irradiation creep in this materials are discussed.

1974 - Third order perturbation theory and lattice dynamics of simple metals [Articolo su rivista]
Bertoni, Carlo Maria; V., Bortolani; C., Calandra; F., Nizzoli
abstract

The dynamic matrix of simple metals is set up by evaluating the total electron energy to the third order in the electron-ion potential. In this way three body unpaired non central forces, arising from the off diagonal elements of the dielectric matrix are explicitly introduced.Since the usual on-Fermi-sphere approximation to a non local pseudopotential form factor gives a completely unreliable estimate of third order contributions and a full non local calculation is not feasible, we give a procedure to construct an average local potential, which disposes of most on the effects of non-locality and allows for a realistic contribution of the third-order corrections to lattice dynamics. We evaluate phonon frequencies and elastic constants for Li, Na, Al and Pb.

1973 - A perturbative approach to the valence charge density in tetrahedrally bonded semiconductors [Articolo su rivista]
Bertoni, Carlo Maria; V., Bortolani; C., Calandra; F., Nizzoli
abstract

An equation for the charge density in term of the dielectric matrix of the crystal is used to study the valence charge density distribution in covalent and and partially ionic semiconductors. Through a perturbation expansion of the dielectric matrix in terms of the ion pseudopotential, we can write the valence charge density as a series of powers of the pseudopoptential. The linear term arises from the diagonal elements of the dielectric matrix and represents the superposition of spherical charge distributions around each atom. The non linear term, due to the off-diagonal elements, is responsible of the heaping up of charge between the ions and describes the covalent part of the chemical bond. This term give also reason of the nonzero intensities of the 'forbidden' reflections (222) and (442).

1973 - Three-body forces in the lattice dynamics of Beryllium [Articolo su rivista]
Bertoni, Carlo Maria; V., Bortolani; C., Calandra; F., Nizzoli
abstract

We present a first-principles calculation of the lattice dynamics of Be. The dynamical matrix is set up by evaluating the total electron energy up to third-order in the electron-ion pseudopotential. In this way we are able to describe the three-body forces, which are essential in reproducing the experimental data.

1973 - Valence Charge Density in Aluminum [Articolo su rivista]
Bertoni, Carlo Maria; V., Bortolani; C., Calandra; F., Nizzoli
abstract

A comparison is made between the valence charge density obtained using a local on Fermi sphere potential and that derived from a suitable average potential which takes into account the effects due to the nonlocality of the pseudopotential. The two electron distributions present significant differences, showing that the calculation of the valence charge density, which does not take account of the nonlocality of the pseudopotential, can lead to uncertain results.

1972 - Dielectric matrix and covalent charge distribution of silicon [Articolo su rivista]
E., Tosatti; C., Calandra; V., Bortolani; Bertoni, Carlo Maria
abstract

Abstract. The valence charge density associated with the nearly free electron dielectric matrix of silicon is computed. The part arising from the off-diagonal dielectric elements is shown to contain the effects of covalency and displays the piling up of bonding charge between the ions.

1972 - Dielectric matrix and phonon frequencies in Silicon [Articolo su rivista]
Bertoni, Carlo Maria; C., Bortolani; C., Calandra; E., Tosatti
abstract

The dielectric matrix of a semiconductor is computed in the pseudopotential scheme. The result is applied to the calculation of the phonon frequencies in the direction [100] of Si. The presence of off-diagonal dielectric terms introduces long range-forces and leads to real TA frequencies, thus avoiding the necessity of introducing explicitly a bond charge model.

1972 - Surface States in Ionic Crystals: Calculation for CsBr [Articolo su rivista]
Bertoni, Carlo Maria; C., Calandra
abstract

The Green's function approach has been applied to surface electron states together a realistic model of bulk band structure of ionic crystals for the calculation of surface energy band structure of the (110) face of simple cubic CsBr. The results, which supply the first accurate test of the method, are in agreement both with the main trend of the experimental data for ionic crystals and with previous theoretical evaluations.

1972 - Thermodynamic Properties of Alkali Metals [Articolo su rivista]
Bertoni, Carlo Maria; V., Bortolani; G., Giunchi
abstract

Abstract - We have calculated the equation of state at O K for the various alkali metals starting from a theoretical description of the volume dependence of their dynamical matrix. We also obtain a detailed knowledge of the individual Grueneisen parameters which also allow us to calculate temperature effects like the thermal pressure and the thermal expansion coefficient. The agreement with the experimental data is satisfactory and it can be considered as a test of validity of the quasi-harmonic approximation.

Università degli studi di Modena e Reggio Emilia

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