Foto personale

Giorgio SANTORO

Department of Education and Humanities

G. Santoro ( 2012 ) - Il contributo di Maria Famigletti alla Formazione degli Insegnanti - RIVISTA DELL'ISTRUZIONE - n. volume 3 - pp. da 31 a 32 ISSN: 0394-8447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Per moltissimi anni la Prof. Maria Famiglietti ha partecipato alla progettazione e attuazione di percorsi di formazione degli Insegnant principalmente negli ambiti delle Tecnologie e delle Scienze nella Scuola Secondaria di Primo grado.

G. Santoro ( 2012 ) - La formazione iniziale e permanente degli Insegnanti di tecnologia - RIVISTA DELL'ISTRUZIONE - n. volume 3 - pp. da 27 a 30 ISSN: 0394-8447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Parlare di Formazione iniziale e in itinere degli insegnanti di Educazione tecnica e di Scienze nella scola di base significa parlare obbligatoriamente della figura di Maria Famiglietti e della sua ricerca didattica.

A. Franchini; V. Bortolani; G. Santoro; K. Xheka ( 2011 ) - Effects of the commensurability and disorder on friction for the system Xe/Cu - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 23 - pp. da 484004 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present molecular dynamics simulations of static friction for a monolayer of Xe deposited on a thick slab of Cu for two different geometries. The interaction potential between Xe and Cu has been derived from DFT calculations. The first geometry is the commensurate adsorption geometry (√3 ×√3)R30◦ suggested by LEED, corresponding to a coverage 1/3, where all Xe atoms are on top positions. The second one corresponds to a coverage 0.36 and is characterized by a large surface unit cell, containing 9 Xe atoms in different disordered positions. This largeunit cell mimics an incommensurate case. Our analysis points out the effect of the order/disorder in tribological properties of a realistic three-dimensional system.

A. Franchini ; V. Bortolani ; G. Santoro; M. Brigazzi ( 2010 ) - Sliding friction of N2 on Pb(111) - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 22 - pp. da 304002 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Molecular dynamics simulations of the sliding friction between two thick solid slabs are performed. The upper body is formed of light N2 particles and the substrate of heavy Pb atoms. Among the various mechanisms that are responsible for the friction, we consider the phonon–phonon interaction between the two blocks. To provide evidence of the phonon interaction, we compare two different systems. For the first we consider the substrate as formed of atoms fixed in the equilibrium (111) positions. In the second system the Pb atoms can be displaced from the ideal positions, under their mutual interactions. A comparison with recently obtained experimental data will be discussed.

V. Bortolani; A. Franchini; G. Santoro; M. Brigazzi ( 2010 ) - Theory of Friction with Applied Load - TRIBOLOGY LETTERS - n. volume 39 - pp. da 251 a 255 ISSN: 1023-8883 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a molecular dynamics simulation for the static friction under the effect of load. We chose a system formed by slabs of molecules of N2 deposited on a (111) surface of Pb. In contrary to many calculations, we assume that the Pb atoms are not kept fixed in the lattice positions, but can vibrate in their own phonon’s field. This has the important consequence that the upper and lower block can exchange energy and momentum. During the molecular dynamics simulation, the two systems can reach a thermodynamical equilibrium. When in our moleculardynamics simulation the equilibrium is reached the N2 plane at the interface reconstructs. The unit cell is still hexagonal but it contains 16 molecules in disordered positions. These positions of the N2 molecules are strongly modified by the presence of load. For small load there is a small increase of the disorder that produces a small reduction in the static friction. For larger loads the formation of clusters begins, and for high loads the size of theclusters increases and there is a tendency to the formation of vacancies. These effects are producing a large increase in the force friction. We can then distinguish different regions that characterize the behaviour of the static friction as a function of the load.

M. Brigazzi; G. Santoro; A. Franchini; I. Cigarini ( 2010 ) - Un'esperienza didattica innovativa per l'insegnamento della relatività - New Trends in Science and technology education: selected papers - CLUEB BOLOGNA ITA) - n. volume 1 - pp. da 402 a 405 ISBN: 9788849133929 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Guardando le indicazioni nazionali per l’insegnamento della fisica nelle scuole di ogni ordine e grado si può notare che la fisica moderna pur ampiamente menzionata viene confinata solonell’ultima parte dell’ultimo anno scolastico periodo critico per le attività didattiche dove impegni legati alla preparazione dell’esame finale portano ad una loro sottovalutazione.Nella pratica di classe la fisica viene quindi di fatto confinata solo ai concetti sviluppati nella prima metà dell’800 relativi alla meccanica classica ed alla elettrodinamica dei sistemi statici oin lento movimento.Nelle discipline tecniche invece vengono utilizzati concetti e teorie sviluppati nella cosiddetta epoca moderna dalla prima metà del XX secolo ad oggi; basti pensare a questo propositoall’elettronica dove vengono studiate le proprietà di conduzione nei sistemi descritti microscopicamente dalla teoria delle bande, alla tecnologia meccanica dove viene fatto cenno agli strumentidi analisi dei materiali basati sull’uso delle radiazioni ionizzanti, all’elettrotecnica dove i concetti basilari come l’effetto fotoelettrico e l’effetto Compton vengono usati per la progettazione di sistemi di produzione o utilizzazione dell’energia elettrica quali i pannelli fotovoltaicio meccanismi di apertura basati su sensori ad infrarossi usati nell’edilizia.

A Franchini; V Bortolani; G Santoro; M Brigazzi ( 2009 ) - Theoretical investigation of the anticorrugation effects on the tribological properties of the Xe/Cu interface - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 21 - pp. da 264008 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a molecular dynamics study of the slip time and static friction for a slab of Xe deposited on a slab of Cu. To put in evidence the role played by the phonon field of the two blocks, we compare results obtained with a substrate formed by fixed atoms with one formed by mobile atoms. In the last case the scattering between Xe and Cu mobile atoms is inelastic andthere is an exchange of momentum and energy between the two blocks which produces disorder in the interface plane. This disorder favors a decrease of the static friction and a consequentincrease of the slip time. We describe the interaction between Xe and Cu with a phenomenological multi-ion potential which gives rise to an anticorrugation of the charge distribution and reproduces very well the ab initio density functional calculations. Our model potential is a linear superposition of a corrugating potential and an anticorrugating one. For this reason we can study the static friction by passing from an anticorrugated to a fully corrugated system. We also investigate the slip time and we compare our results with recent experimental data measured with the quartz crystal microbalance technique.

Anna Franchini; M. Brigazzi; G. Santoro; V. Bortolani ( 2008 ) - Theory of the static friction: temperature and corrugation effects - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 20 - pp. da 224019 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a study of the static friction, as a function of temperature, between two thick solid slabs. The upper one is formed of light particles and the substrate of heavy particles. We focus our attention on the interaction between the phonon fields of the two blocks and on the interface corrugation, among the various mechanisms responsible for the friction. To give evidence of the role played by the dynamical interaction of the substrate with the upper block, we consider both a substrate formed by fixed atoms and a substrate formed by mobile atoms. To study the effect of the corrugation, we model it by changing the range parameter σ in the Lennard-Jones interaction potential. We found that in the case of the mobile substrate there is a largemomentum transfer from the substrate to the upper block. This momentum transfer increases on increasing the temperature and produces a large disorder in the upper block favouring a decreaseof the static friction with respect to the case for a rigid substrate. Reducing the corrugation, we found that with a rigid substrate the upper block becomes nearly commensurate, producing anenhancement of the static friction with respect to that with a mobile substrate.

A. Franchini; V. Bortolani; G. Santoro; M. Brigazzi ( 2008 ) - Thermal effects in static friction: thermolubricity - PHYSICAL REVIEW E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS - n. volume 78 - pp. da 046107 ISSN: 1539-3755 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a molecular dynamics analysis of the static friction between two thick slabs. The upper block is formed by N2 molecules and the lower block by Pb atoms. We study the effects of the temperature as well as the effects produced by the structure of the surface of the lower block on the static friction. To put in evidence the temperature effects we will compare the results obtained with the lower block formed by still atoms withthose obtained when the atoms are allowed to vibrate (e.g., with phonons). To investigate the importance of the geometry of the surface of the lower block we apply the external force in different directions, with respect to a chosen crystallographic direction of the substrate. We show that the interaction between the lattice dynamics of the two blocks is responsible for the strong dependence of the static friction on the temperature. The latticedynamics interaction between the two blocks strongly reduces the static friction, with respect to the case of the rigid substrate. This is due to the large momentum transfer between atoms and the N2 molecules which disorders the molecules of the interface layer. A further disorder is introduced by the temperature. We perform calculations at T=20 K which is a temperature below the melting, which for our slab is at 50 K. We found that because of the disorder the static friction becomes independent of the direction of the external applied force. The very low value of the static friction seems to indicate that we are in a regime of thermolubricity similar to that observed in dynamical friction.

Roberto Zivieri; Giorgio Santoro; Anna Franchini ( 2007 ) - Localized spin modes in ferromagnetic cylindrical dots with in-plane magnetization - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 19 - pp. da 305012 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A study of spin localized excitations in ferromagnetic tangentially magnetized dots of cylindrical shape and of nanometric size is presented. A recently formulated variational theory permits us to study the most representative localized spin modes of the spectrum. One of these, the fundamental mode, is mainly localized in the central part of the dot endfaces and is analogous to the Kittel uniform mode in ellipsoids. We also investigate the dynamical properties of spin modes localized in the lateral part of the dot endfaces along the direction of the applied magnetic field, studying the dependence of their localization on the variational parameter and the applied magnetic field. Finally, a comparisonof the calculated frequencies of some of these localized modes with available experimental data is performed.

M. Brigazzi; G. Santoro; A. Franchini; V. Bortolani ( 2007 ) - Simulations of the temperature dependence of static friction at the N2/Pb interface - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 19 - pp. da 305014 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A molecular dynamics approach for studying the static friction between two bodies, an insulator and a metal, as a function of the temperature is presented. The upper block is formed by N2 molecules and the lower block by Pb atoms. In both slabs the atoms are mobile. The interaction potential in each blockdescribes properly the lattice dynamics of the system. We show that the lattice vibrations and the structural disorder are responsible for the behaviour of the static friction as a function of the temperature. We found that a large momentumtransfer from the Pb atoms to the N2 molecules misplaces the N2 planes in the proximity of the interface. Around T = 20 K this effect produces the formation of an hcp stacking at the interface. By increasing the temperature, the hcp stacking propagates into the slab, toward the surface. Above T = 25 K, our analysis shows a sharp, rapid drop of more than three order of magnitudein the static friction force due to the misplacing of planes in the stacking of the fcc(111) layers, which are no longer in the minimum energy configuration. Above T = 35 K, we also observe a tendency for the splitting of planes and the formation of steps near the surface. By increasing the temperature we obtain thesubsequent melting of the N2 slab interface layer at T = 50 K. The temperature behaviour of the calculated static friction is in good agreement with recent measurements made with the quartz crystal microbalance (QCM) method on the same system.

V. BORTOLANI; G. SANTORO; A. FRANCHINI ( 2006 ) - PROGETTO NAZIONALE: Proprietà nonlineari del nanoattrito / Nonlinear properties of nanofrictionPROGETTO LOCALE : Simulazioni numeriche di nanoattrito: isolanti su metalli / Computer simulations of friction at the nanoscale: insulators on metals [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

Vanossi A; Santoro G; Bortolani V ( 2004 ) - Hysteretic behaviour in driven Frenkel-Kontorova chains on irregular substrates - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 16 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The hysteretic dynamics of a Frenkel-Kontorova chain subject to irregular substrate potentials and driven by an external dc force is studied both in the underdamped and in the overdamped regime at zero temperature. The choice of a rigid external potential defined by the sum of two sinusoidal functions with different periodicity allows us to simulate microscopic sliding over quasiperiodic and multiple-well periodic substrates. We analyse, for different parameter values of the model, the behaviour of the centre of mass average velocity of the chain as a function of an adiabatic increase and decrease of the applied driving. For small damping coefficients (negligible dissipative forces), at a fixed value of the substrate potential amplitude, the width of the hysteresis region is markedly influenced by the chain stiffness. As expected, in the overdamped dynamical regime no hysteresis is observed. We comment on the nature of the dynamical states displayed during the chain motion at different strengths of the dc driving.

Virginio Bortolani; Mauro Ferrario; Anna Franchini; Giorgio Santoro; Sergio Valeri ( 2004 ) - progetto locale: Simulazioni numeriche dell'attrito microscopico e proprietà tribologiche di superfici nanostrutturate [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

V. BORTOLANI; A. FRANCHINI; G. SANTORO; A. VANOSSI ( 2003 ) - MICROSCOPIC FRICTION: FROM SIMPLE MODELS TO REAL SYSTEMS [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

A. Franchini; G. Santoro ( 2002 ) - Single-phonon scattering from clean flat metal surfaces - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 14 - pp. da 5881 a 5901 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we review the recent advances of the He atom scattering experiments and the theoretical interpretation of the time-of-flight (TOF) spectra relative to clean flat metal surfaces in the one-phonon regime. We discuss the atom-surface scattering mechanism, including the anomalies in the metal surface phonon spectrum and the details of the atom-surface interaction potential. In particular we focus the discussion on the anticorrugating effects in the TOF intensities.

A. FRANCHINI; C. MAGHERINI; G. SANTORO ( 2002 ) - Surface thermal expansion of Ag(110) - SURFACE SCIENCE - n. volume 502-503 - pp. da 443 a 448 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we calculate the surface thermal expansion of Ag(1 1 0). applying the perturbation theory to treat the effects of the anharmonic part of the interatomic potential. Results relative to the relaxation of the interlayer distance are presented for different temperatures. These results confirm the alternate contraction and expansion at room temperature and the enhancement of the anharmonicity in the surface region. (C) 2002 Elsevier Science B.V. All rights reserved.

G. Santoro; A. Franchini; V. Bortolani; DL Mills; RF Wallis ( 2001 ) - Interpretation of inelastic scattering of He from Cu surfaces in the anticorrugated potential model - SURFACE SCIENCE - n. volume 478 - pp. da 99 a 112 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we demonstrate that inclusion of the anticorrugation effects in the pseudopotential describing the dynamic interaction of neutral He atoms with phonons of Cu(1 1 1) and Cu(1 0 0) surfaces can resolve the long standing discrepancy between the inelastic scattering intensities inferred from HAS-TOF and from the electron energy loss spectroscopy data.

G. Santoro; A. Franchini; V. Bortolani; A. Vanossi ( 2001 ) - Nanoattrito / Nanofriction [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

A. FRANCHINI; G. SANTORO; V. BORTOLANI; ( 2001 ) - progetto nazionale: Interazione molecola-superficie: dall'Elio ai film di molecole organiche progetto locale: Diffusione atomo-superficie e proprieta' anarmoniche di atomi, molecole e monostrati depositati su metalli / Atom-surface scattering and anharmonic properties of atoms, molecules and overlayers deposited on metals [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

R. Zivieri; G. Santoro; V. Bortolani ( 2000 ) - Premelting of the Al(110) surface from a local perspective - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 62 - pp. da 9985 a 9988 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A molecular-dynamics study of premelting on the Al(110) surface has been performed. The surface atoms interact through a semiempirical many-body potential built up inside the embedded-atom scheme. The pre melting process has been studied by calculating the Debye-Waller factor, which shows anisotropic behavior above 800 K for a range of temperatures up to the melting point. Theoretical results have been compared to recent x-ray absorption fine-structure measurements. The calculations show that bonds between Al atoms are broken along the direction normal to the surface, while they remain intact on the surface. A decrease of coordination number for surface atoms is present as evidence of structural disorder for temperatures near melting.

R. ZIVIERI; G. SANTORO; V. BORTOLANI ( 1999 ) - Anharmonicity on Al(100) and Al(111) surfaces - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 59 - pp. da 15959 a 15965 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a molecular-dynamics analysis of the stable nonmelting (100) and (111) surfaces of Al. A many-body potential derived from first-principles calculations is used. The molecular-dynamics method includes anharmonic effects of all orders. We study static and dynamical properties of the surface. An expansion of the (111) surface and a contraction of the (100) surface results from the calculations. At low temperature, the vertical mean-square vibrational amplitude is larger than the in-plane component, while at higher temperature the in-plane component approaches the vertical one. Both components are at least twice as large as the bulk value. The interactions due to the surface decay very rapidly going into the crystal from the surface, as indicated by the analysis of the Debye-Waller factor. The evaluated linewidths for the Rayleigh surface phonon reproduce quite well the temperature dependence of the He-surface scattering data. The experimental behavior of the energy shifts, as a function of the scattering momentum transfer, presents a minimum inside the Brillouin zone, which is also found by our calculations. The surface energy shifts are about 30% larger than the bulk ones at the same temperature. The effect of surface anharmonicity is much larger for the static properties than for the dynamical properties. [S0163-1829(99)05924-X].

A. FRANCHINI; G. SANTORO; V. BORTOLANI ( 1999 ) - progetto nazionale: STRUTTURA E DINAMICA DELLE SUPERFICI E ADSORBIMENTO / SURFACES: STRUCTURE, DYNAMICS AND ADSORPTION progetto locale: STUDIO DELLE PROPRIETA' ANARMONICHE DI SUPERFICI METALLICHE AD ALTA TEMPERATURA / STUDY OF ANHARMONIC PROPERTIES OF METALLIC SURFACES AT HIGH TEMPERATURES [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

M.I. Trioni; S. Marcotullio; G. Santoro; V. Bortolani; G. Palumbo; G.P. Brivio ( 1998 ) - Ab initio adiabatic He and Ne interaction on Ag: An all-electron calculation - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 58 - pp. da 11043 a 11051 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We report an ab initio calculation of the adiabatic electronic properties of He and Ne atoms interacting with an Ag-like jellium metal surface, using the embedding method and the density functional theory in the local density approximation (LDA). Differently from previous results obtained in this framework, the noble atoms are described with their full potential. The linearized augmented plane-wave basis set is introduced to tackle the Kohn-Sham equation. Attention is focused on the atom-surface potential in the repulsive regime, i.e., at distances from the jellium edge smaller then the adsorption one, where the LDA has been shown to work well, and which are relevant in elastic and inelastic scattering experiments of He and Ne on metals. For incident atoms with initial kinetic energies in the experimental range of interest, it is shown that Ne gets closer to the metal than He as previously found with an Al substrate. An analysis of the shift of the atomic core levels by varying the atom-metal distance is also presented. [S0163-1829(98)01539-2].

G. SANTORO; A. FRANCHINI; V. BORTOLANI ( 1998 ) - Anticorrugation in inelastic He-atom scattering from Rh(111) surfaces - PHYSICAL REVIEW LETTERS - n. volume 80 - pp. da 2378 a 2381 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Recent measurements supported by nb initio calculations have shown that the interaction of He atoms with transition metal surfaces is anticorrugating, i.e., the electron density seen by He is higher in the bridge positions than in the on-top positions. By shifting the electron charge density away from the atomic positions and using the Esbjerg-Norskov approach to determine the interaction potential, we evaluate the inelastic reflection coefficients of He with Rh(lll). The structure factor introduced by the anticorrugation explains, in a simple way, the high intensity observed for the resonant mode.

Santoro G.; A. Franchini; Bortolani V. ( 1998 ) - Dynamical properties of Rh(111):H(1x1) surface - Societa Italiana di Fisica:Via Saragozza 12 Subs Service, I 40123 Bologna Italy:011 39 051 331554, EMAIL: sif@sif.it, INTERNET: http://www.sif.it, Fax: 011 39 051 581340 ) - NUOVO CIMENTO DELLA SOCIETÀ ITALIANA DI FISICA. D CONDENSED MATTER, ATOMIC, MOLECULAR AND CHEMICAL PHYSICS, BIOPHYSICS - n. volume 20 - pp. da 1055 a 1058 ISSN: 0392-6737 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In the experimental He atom scattering time-of-flight spectra of rhodium covered with hydrogen there is no evidence of the anomalous high intensity of the resonant mode present in the clean surface. In this paper we explain the different behaviour of clean and covered rhodium (111) surface in terms of the "corrugating" effect of the electron density of hydrogen atoms. For clean surface the surface electronic density is shifted with respect to the lattice atomic positions, i.e. there is an "anticorrugating" effect that enhances the intensity of the longitudinal resonant mode which becomes dominant for some scattering geometries. For the covered surface the hydrogen atoms remove the anticorrugation, the charge profile becomes corrugated and the intensity of the longitudinal resonant vibrational mode remains very weak with respect to the Rayleigh wave intensity.

R. ZIVIERI; G. SANTORO; V. BORTOLANI ( 1998 ) - Multiphonon effects in the one-phonon cross section of Al - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 58 - pp. da 5429 a 5434 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a theoretical analysis of the phonon linewidth and energy shift observed with neutron spectroscopy in the one-phonon resonances. To take full account of the anharmonicity of the crystal we use a; molecular-dynamics approach to evaluate the atomic displacement-displacement correlation functions. The atoms in the crystal interact with a semiempirical many-body potential and anharmonic effects of all orders are taken into account. The linewidths are evaluated for Al at two different temperatures, 80 and 300 K, in the high-symmetry directions of the crystal. Our results reproduce well the experimental data. The larger damping of the longitudinal phonons compared to that one of transverse phonons is fully explained in terms of anharmonic effects. Our analysis also confirms the anomalous behavior of the energy shift for longitudinal phonons in the [111] direction.

S. Ossicini; A. Franchini; G. Santoro; V. Bortolani; S. Valeri ( 1997 ) - CRESCITA E CARATTERIZZAZIONE DI SISTEMI A BASSA DIMENSIONALITA': SUPERFICI, INTERFACCE, MULTISTRATI [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

G. Bracco; L. Bruschi; R. Tatarek; A. Franchini; V. Bortolani; G. Santoro ( 1996 ) - Anomalous linewidth behavior of the S3 surface resonance on Ag(110) - EUROPHYSICS LETTERS - n. volume 34 - pp. da 687 a 692 ISSN: 0295-5075 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We report detailed He atom scattering measurements of the phonon linewidth for the Rayleigh mode S-1 and the resonant mode S-3 of Ag(110) along the whole <(Gamma) over bar><(Y) over bar> side of the surface Brillouin zone as a function of the sample temperature. The novel behaviour observed for the resonance linewidth is explained by including the mixed-mode nature into the anharmonic calculations performed within the framework of the Born-von Karman model.

V. BORTOLANI; A. FRANCHINI; G. SANTORO ( 1996 ) - INTRINSIC EFFECTS OF ANHARMONICITY IN LOW DIMENSIONALITY SYSTEMS [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

A. Franchini; Santoro G.; Bortolani V.; Bellman A.G.; Cvetko D.; Floreano F.; Morgante A.; Peloi M.; Tommasini F.; Zambelli T. ( 1994 ) - Role of the He-atom surface potential on the Cu(001) phonon determination - SURFACE REVIEW AND LETTERS - n. volume 1 - pp. da 67 a 74 ISSN: 0218-625X [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present new measurements of He atom inelastic scattering from the (001) surface of Copper. Our results confirmed the recent data obtained in Gottingen and show that at large momentum transfer in the GM direction the intensity of the longitudinal resonance is larger than that of the Rayleigh wave. We interpret these results by evaluating the reflection coefficient in the Distorted Wave Born Approximation. By taking the He surface interaction potential as a sum of nonspherical pairwise potential we are able to explain quantitatively the behavior of the experimental spectra.

A. Franchini; Bortolani V.; Santoro G.; Wallis R.F.; Maradudin A.A. ( 1994 ) - Surface phonon linewidth of Al - Patron Editore:Via Badini 12, I 40050 Quarto Inferiore Bologna Italy:011 39 051 767003, EMAIL: abbonamenti@patroneditore.com, info@patroneditore.com, INTERNET: http://www.patroneditore.com, Fax: 011 39 051 768252 ) - VUOTO - n. volume XXIII - pp. da 94 a 96 ISSN: 0391-3155 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we present a numerical calculation of the Rayleigh wave phonon linewidth based on a cubic anharmonic expansion of the interatomic potential of the crystal. The linewidth is evaluated at high symmetry points of the twodimensional Brillouin Zone (SBZ) of the Al low Miller indices surfaces for various temperatures. The results are consistent with the experimental He surface scattering measurements and show an enhanced linewidth with respect to the nulk transverse one.

WALLIS RF; MARADUDIN AA; BORTOLANI V; EGUILUZ AG; QUONG AA; FRANCHINI A; SANTORO G ( 1993 ) - COMPARISON BETWEEN PHENOMENOLOGICAL AND PSEUDOPOTENTIAL FORCE-CONSTANTS FOR THE LATTICE-DYNAMICS OF AL - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 48 - pp. da 6043 a 6053 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we present a critical comparison of,calculations of phonon-dispersion curves for aluminum, based on pseudopotential and empirical force-constant methods. The former method is.based on both perturbative and nonperturbative evaluations of the total energy of the crystal. In the perturbative approach the total energy is evaluated to second order in the electron-ion interaction with a local pseudopotential. In the nonperturbative approach the electron-ion interaction is treated exactly using a non-local ab initio pseudopotential. In the empirical force-constant method the total energy is represented by a sum of two-body and three-body terms, the latter being restricted to interactions among triplets of nearest neighbors and of nearest and next-nearest neighbors. All models include physical mechanisms for ensuring the breakdown of the Cauchy relation. We show that in each method convergence of the calculated phonon frequencies requires the inclusion of long-range interactions. The radial-force constants obtained by the empirical and perturbative pseudopotential methods are very similar for the first three shells of neighbors. On the other hand, the tangential force constants differ markedly, a reflection of the different physical mechanisms that come into play in the various models in the establishment of equilibrium.

Franchini A.; Bortolani V.; Santoro G.; Celli V.; Eguiluz A.G.; Gaspar J.A.; Gester M.; Lock A.; Toennies J.P. ( 1993 ) - Comparison of semiempirical and ab-initio pseudopotential calculations of the lattice-dynamics of aluminium surfaces with He-scattering time-of-flight spectra - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 47 - pp. da 4691 a 4704 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We report a detailed comparison of two theoretical methods for the study of the surface lattice dynamics of sp-bonded aluminum and compare the results with He-atom time-of-flight spectra for the (001), (110), and (111) surfaces of this metal. One of the theoretical methods is a semiempirical Born-von Karman scheme in which the interatomic force constants are related to central pairwise forces and also to ''angle-dependent'' interactions between triplets of atoms. The other method is an ab initio scheme in which the interatomic force constants are calculated from the electron-density response function and second-order pseudopotential perturbation theory. Both schemes require the inclusion of long-range central forces (up to ten nearest neighbors) in order to obtain converged results. In addition, the semiempirical approach must include three-body forces up to second-nearest neighbors to allow for the breakdown of the Cauchy relation, which in the ab initio method arises as a direct consequence of the volume dependence of the effective pair potential. The semiempirical approach reproduces accurately the surface phonon-dispersion curves calculated with the ab initio scheme. The leading surface force constants obtained in both methods have similar values, which provides justification for the physical significance of the semiempirical force constants. Both methods reproduce, with great accuracy, the experimental dispersion relations for the Rayleigh surface phonons, except for the case of the (110) surface. Assuming that the He atom couples only to the ion cores, the reflection coefficients calculated in both methods reproduce well the intensities of the weak resonances observed in the He time-of-flight spectra below the longitudinal edge. These structures can be explained in terms of Fano resonances.

GP BRIVIO; TB GRIMELY; V. BORTOLANI; G. SANTORO ( 1993 ) - Dynamics of adsorprion/desorption of Ne-Cu(111): stiking - CHEMICAL PHYSICS LETTERS - n. volume 208 - pp. da 93 a 96 ISSN: 0009-2614 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The sticking coefficient s of Ne on Cu at substrate temperatures T(s) between 5 and 30 K is computed, within the framework of the same theoretical model as already used for thermal desorption. At very low T(s), s is worked out taking advantage of the proportionality between sticking and desorption rates when desorption is an infrequent event. In this regime, s increases with T(s). At higher T(s) where desorption is not an infrequent event, master equations must be used, and there is no longer proportionality between sticking and desorption rates. In this regime, s decreases with increasing T(s). Calculations for two different potentials fitted to the bound states show that sticking is a much more sensitive probe of the form of the gas-solid potential than thermal desorption fluxes.

V. BORTOLANI; D. CVETKO; F. FLOREANO; A. FRANCHINI; A. LAUSI; A. MORGANTE; M. PELOI; G. SANTORO; F. TOMMASINI; T. ZAMBELLI; AF BELLMAN ( 1993 ) - Elastic and inelastic interactions of He and Ne atoms with metals surfaces - JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA - n. volume 64/65 - pp. da 671 a 675 ISSN: 0368-2048 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The experimental information accumulated in recent years on elastic and inelastic scattering of He and Ne atoms from metal surfaces is analysed by describing the atom-surface interaction as a superposition of pseudo-pairwise potentials. These are chosen according to the quite accurate potentials presently available for van der Waals pairs and take a phenomenological account for many body effects. The emerging picture is quite satisfactory and allows the physical interaction of a given atom-surface system to be predicted with excellent accuracy. In particular the coupling of He atoms to the surface vibrational modes is considered with attention focused on He inelastic scattering from Cu(001).

BRIVIO GP; BORTOLANI V; SANTORO G; CORTINA AM ( 1992 ) - ON THE INFLUENCE OF INTRABOUND STATE CASCADING IN THERMAL-DESORPTION OF LIGHT NOBLE-GASES - SURFACE SCIENCE - n. volume 262 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Following a previously developed microscopic model, we show that the time of flight (TOF) spectra and the angular resolved fluxes (ARD) of thermally desorbed Ne atoms from Cu(111) are not appreciably affected by intrabound state cascading mediated by one phonon processes, in the surface temperature range 4 less-than-or-equal-to T less-than-or-equal-to 36 K, although inclusion of such processes increases the desorption time of about a factor 2 at T = 36 K. Our system, being a model for thermal desorption of light noble gases, shows that intrabound state cascading effects are not relevant in explaining the TOF spectra and the ARD at low substrate temperatures.

A. FRANCHINI; V. BORTOLANI; G. SANTORO; JA GASPAR; AG EGUILUZ ( 1992 ) - The phonons of the Al surfaces: comparison between semi-empirical and ab-initio calculations - SURFACE SCIENCE - n. volume 269-270 - pp. da 146 a 151 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We report inelastic He-atom surface scattering data for the low-index surfaces of aluminum. The experimental data are analyzed by using a semi-empirical Born-von Karman model in which both two-body and three-body interactions are taken into account and by using an ab initio pseudopotential approach. Our analysis shows that the semi-empirical model containing long-range forces agrees with great accuracy with the pseudopotential calculations for what concerns both the range of the interactions and the dispersion of the surface vibrational modes. The calculations of the reflection coefficients allow us to study the nature of the weak resonances observed in the time-of-flight spectra. These structures are related to a picking up of modes with polarization vector normal to the surface due to the modification introduced by the surface on the entire force field. Our analysis of Al proves that for simple metals an accurate semi-empirical model can reproduce with great accuracy both the surface lattice dynamics, without introducing new free parameters in the surface interatomic potential, as well as an ab initio pseudopotential calculation.

FRANCHINI A; SANTORO G; BORTOLANI V; MARADUDIN AA; WALLIS RF ( 1992 ) - Theory of the anharmonic linewidths of surface phonons in aluminum - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 45 - pp. da 11982 a 11988 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A theoretical investigation has been made of the linewidth of the Rayleigh mode on the Al(111) surface due to cubic anharmonic interactions between atoms. The normal-mode frequencies and eigenvectors for the harmonic crystal were calculated by using a model containing central interactions extending up to tenth-nearest neighbors and three-body interactions extending up to second-nearest neighbors. This model reproduces with great accuracy the experimental bulk-phonon frequencies as well as the surface-phonon frequencies. The linewidths were evaluated along the SIGMA-BAR direction as a function of the lateral momentum transfer and at the MBAR point as a function of the temperature. The results compare favorably with recent experimental data.

M. AMIOTTI; A. BORGHESI; G. GUIZZETTI; F. NAVA; G. SANTORO ( 1991 ) - Far-infrared spectroscopy of thermally annealed tungsten silicide films - EUROPHYSICS LETTERS - n. volume 14 - pp. da 587 a 590 ISSN: 0295-5075 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The far-infrared transmittance spectrum of tungsten silicide has been observed for the first time. WSi2 polycrystalline films were prepared by coevaporation and chemical-vapour deposition on silicon wafers, and subsequently thermally annealed at different temperatures. The observed structures are interpreted, on the basis of the symmetry properties of the crystal, such as infrared-active vibrational modes. Moreover, the marked lineshape dependence on annealing temperature enables this technique to analyse the formation of the solid silicide phases.

V. CELLI; D. HIMES; V. BORTOLANI; G. SANTORO; JP TOENNIES; G. ZANG ( 1991 ) - One-phonon and multiphonon processes in atom-surface scattering - SURFACE SCIENCE - n. volume 242 - pp. da 518 a 523 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We show that previously unexplained features in the TOF spectra of He and Ne scattered from solid surfaces can be attributed to multiphonon processes. We find that a simple Gaussian distribution, obtained by Brako and Newns within the trajectory approximation, accounts rather well for these features. The average energy transfer appearing in this distribution is evaluated by the Baule formula. We obtain an interpolation formula that describes both multiphonon and one-phonon processes, and is consistent with the Distorted Wave Born Approximation for the latter.

GP BRIVIO; TB GRIMLEY; V. BORTOLANI; G. SANTORO ( 1991 ) - Theory of Thermal Desorption of Light Noble Gases from Metals: Aplication to Ne-Cu(111) - SURFACE SCIENCE - n. volume 245 - pp. da 17 a 28 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A quantum theory of thermal desorption of light noble gases, at low temperatures, in the framework of the distorted-wave Born approximation, is presented. Such a theory has been applied to the calculation of the angle-resolved thermal desorption flux, and time-of-flight spectrum for Ne-Cu(111) using a microscopic description of the phonons of Cu(111), and a realistic gas-solid potential. We show that desorption is driven by long-wavelength phonons, but the precise angular dependence results from the temperature-dependent interplay of statistical and dynamical factors. The angular distribution is narrower than cosine at 4 K, broader than cosine at 12 K, and distinctly non-cosine for temperatures greater than 20 K with the maximum away from the surface normal. He desorbing from graphite shows similar features.

BORTOLANI V.; BRIVIO G.P.; A. FRANCHINI; GRIMLEY T.B.; SANTORO G. ( 1990 ) - Angle-Resolved Thermal Desorption and Time-of-flight spectra:Ne-Cu(111) - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - CHEMICAL PHYSICS LETTERS - n. volume 170 - pp. da 1 a 4 ISSN: 0009-2614 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The angle-resolved thermal desorption flux, and time-of-flight spectrum have been computed for Ne-Cu( 111) using a microscopic description of the phonons of Cu( 1 I 1 ), and a realistic gas-solid potential. The angular distribution is narrower than cosine at 5 K, quasi-cosine at 10 K, and distinctly non-cosine at 20 and 30 K with the maximum away from the surface normal. The desorption is driven by long-wavelength phonons, but the precise angular dependence results from the temperature-dependent interplay of statistical and dynamical factors. He desorbing from graphite shows similar features.

M. ZOLI; G. SANTORO; V. BORTOLANI; A. A. MARADUDIN; R. F. WALLIS ( 1990 ) - Anharmonic Damping of Phonon Modes in the Face-Centered Cubic Metals - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 41 - pp. da 7507 a 7514 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

For the fcc metals Cu, Ag, Au, and Al, a theoretical analysis has been made of the one-phonon linewidth due to cubic anharmonic interactions. Third-order central and angular force constants have been obtained by using the symmetry properties of the lattice to relate the force constants with the experimental third-order elastic moduli. The convergence of the value of the nearest-neighbor third-order force constant has been studied as a function of the range of the cubic interactions. We have found that for metals for which the third-order Cauchy relations are violated it is necessary to introduce cubic angular forces to obtain a convergent value of the nearest-neighbor force constant. For Al and Cu the evaluated linewidths are compared with experimental data.

EGUILUZ A.G.; GASPAR J.A.; BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1990 ) - Comparison of ab-Initio and Phenomenological Calculations of Surface Phonons - Patron Editore:Via Badini 12, I 40050 Quarto Inferiore Bologna Italy:011 39 051 767003, EMAIL: abbonamenti@patroneditore.com, info@patroneditore.com, INTERNET: http://www.patroneditore.com, Fax: 011 39 051 768252 ) - VUOTO - n. volume XX - pp. da 723 a 723 ISSN: 0391-3155 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we will address to the problem of the physical meaning of the force constants by comparing the forces acting betwen atoms both in the surface region and in the bulk, calculated in the framework of a sophisticated force constant model and the corresponding forces evaluated in the framework of an a-priori pseudopotential theory, where the electron-ion system is treated perturbatively by considering the effect of the electronic screening and the roel od the electron-electron term on the electron-phono interaction.

BORTOLANI V.; ERCOLESSI F.; TOSATTI E.; A. FRANCHINI; SANTORO G. ( 1990 ) - Explanation of the Force Constant Softening in Noble Metals: the Case of Au(111) - Elsevier Science Limited:Oxford Fulfillment Center, PO Box 800, Kidlington Oxford OX5 1DX United Kingdom:011 44 1865 843000, 011 44 1865 843699, EMAIL: asianfo@elsevier.com, tcb@elsevier.co.UK, INTERNET: http://www.elsevier.com, http://www.elsevier.com/locate/shpsa/, Fax: 011 44 1865 843010 ) - VACUUM - n. volume 41 - pp. da 428 a 430 ISSN: 0042-207X [Articolo in rivista (262) - Articolo su rivista]
Abstract

A new mechanism for the well known in-plane force softening on the noble metal surfaces is presented. The mechanism is discussed in terms of a many-body force scheme and is shown to amount to an effective decrease of the lateral atomic ‘size’ of the surface atom. Its strong sensitivity to surface relaxation and reconstruction is emphasized. As a non-trivial illustration we calculate the phonon density of states of Au(111) and show that the large softening leading to the observed anomalous longitudinal resonance is directly related to our proposed mechanism. We also discuss the importance of the surface stress on the statics and dynamics of these surfaces.

A. BORGHESI; A. PIAGGI; A. FRANCHINI; G. GUIZZETTI; F. NAVA; G. SANTORO ( 1990 ) - Far Infrared Vibrational Spectroscopy in CrSi_2 - EUROPHYSICS LETTERS - n. volume 11 - pp. da 61 a 65 ISSN: 0295-5075 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present and discuss reflectance and trasmittance measurements on CrSi, polycrystalline films in the (100 + 700) cm-' wavenumber range. The spectra show that the compound has a nonmetallic behaviour. The strong and sharp structures observed are interpreted on the basis of the symmetry properties of the normal modes of CrSie, and their strength is attributed to a dynamical charge. Moreover, preliminary lattice dynamicscalculations allow to explain the relative intensity of the infrared-active modes.

CELLI V.; HIMES D.; BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1990 ) - Multiphonon Effects in Atom Surface Scattering: the Case of Pt(111) - Patron Editore:Via Badini 12, I 40050 Quarto Inferiore Bologna Italy:011 39 051 767003, EMAIL: abbonamenti@patroneditore.com, info@patroneditore.com, INTERNET: http://www.patroneditore.com, Fax: 011 39 051 768252 ) - VUOTO - n. volume XX - pp. da 721 a 722 ISSN: 0391-3155 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we show that the huge structures recently observed experimentally ith the He-surface atom scattering technique by using hot beams can be related with some confidence to multiphonon scattering events.

A. BORGHESI; A. PIAGGI; A. STELLA; G. GUIZZETTI; F. NAVA; G. SANTORO ( 1990 ) - Optical and vibrational properties of Cr and Fe disilicides - The Physics of Semiconductors - World Scientific Singapore SGP) - pp. da 2001 a 2004 ISBN: 9789810204143 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

We present and discuss reflectance and trasmittance measurements on CrSi_2 an FeSi_2 polycristalline films, The strong and sharp structures observed in the 100-700 cm^-1 wavenumber range are interpreted basin on normal modes symmetry properties. We also examine the optical response in the energy region around the band-gap for CrSi_2 and determine the dispersion of its complex refractive index.

BORTOLANI V.; ERCOLESSI F.; TOSATTI E.; A. FRANCHINI; SANTORO G. ( 1990 ) - The Origin of Force Constant Softening on the Close Packed Noble Metal Surfaces - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - EUROPHYSICS LETTERS - n. volume 12 - pp. da 149 a 154 ISSN: 0295-5075 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A qualitatively new mechanism is proposed for the well-known in-plane force softening found on many noble-metal surfaces. The mechanism is discussed in terms of one of the recently developed many-body force schemes and is shown to amount to an effective decrease of the lateral atomic .sizes of the surface atom. Its strong sensitivity to the strength of the manybodyforce terms, as well as to surface relaxation and reconstruction, is emphasized. -As a nontrivial illustration, we calculate the phonons of Au(lll), and show that the exceedingly large softening leading to the recently observed anomalous longitudinal resonance is directly related to our proposed mechanism.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1990 ) - Thermal Desorption of Light Rare Gases from Metals: one-phonon Processes - Patron Editore:Via Badini 12, I 40050 Quarto Inferiore Bologna Italy:011 39 051 767003, EMAIL: abbonamenti@patroneditore.com, info@patroneditore.com, INTERNET: http://www.patroneditore.com, Fax: 011 39 051 768252 ) - VUOTO - n. volume XX - pp. da 57 a 58 ISSN: 0391-3155 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we report calculations of the ADF and of the TOF spectra of the physisorbed system Ne-Cu(111) at very low coverage: specifically we consider a one-phonon driven desorption process, which uses an accurate description of the surface lattice dynamics and a Morse gas-surface potential, whose parameters are fitted in order to obtain the experimental boun states of Ne on a copper surface.

BRIVIO G.P.; GRIMLEY T.B.; BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1990 ) - Vibrational effects in Desorption of Light Rare Gases from Metals - VACUUM - n. volume 41 - pp. da 202 a 203 ISSN: 0042-207X [Articolo in rivista (262) - Articolo su rivista]
Abstract

The angle-resolved thermal desorption flux for Ne-Cu(111) has been computed at 5, 10 and 15 K in the Distorted Wave Born Approximation using an accurate microscopic description of the surface lattice dynamics, and a realistic gas-solid potential. The angular distribution narrows as the temperature is lowered, being broader than the equilibrium theory cosine law at 15 and 10 K and narrower than it at 5 K. The Time-of-Flight spectra of the desorbed atoms are also analysed.

BORTOLANI V.; CELLI V.; A. FRANCHINI; IDIODI J.; SANTORO G.; KERN K.; POELSEMA B.; COMSA G. ( 1989 ) - Debye-Waller factor for He/Pt(111) - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 208 - pp. da 1 a 12 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We report accurate measurements of the dependence of the specular reflectivity on surface temperature and on incident angle, for He scattering off Pt(111). The usual Debye-Waller formula with a Beeby correction fits the data, provided that the effective well depth is adjusted. We show that a full calculation of the Debye-Waller exponent also reproduces the data. The inelastic atom-surface interaction and the surface phonon spectrum used in the calculation are the ones that reproduce the time-of-flight data obtained in separate experiments. No further adjustable parameters are needed.

V. BORTOLANI; G. SANTORO; A. FRANCHINI ( 1989 ) - SURFACE ATOM INTERACTION AND SURFACE LATTICE DYNAMICS OF METALS [Altro (298) - Partecipazione a progetti di ricerca]
Abstract

BORTOLANI V.; A. FRANCHINI; SANTORO G.; TOENNIES J.P.; WOLL CH.; ZHANG G. ( 1989 ) - Surface phonons on the Pt(111) surface: a comparison of He-scattering experiments with lattice dynamical calculations - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 40 - pp. da 3524 a 3545 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Angular distributions and surface-phonon dispersion curves have been measured by He-atom scattering for clean Pt between T=160 and 373 K and for Pt-H(1×1) at 160 K along the 〈110〉 and 〈112〉 azimuths. Inelastic-scattering intensities over a wide range of wave vectors are also reported for the clean surface. The results are analyzed with use of the distorted-wave Born approximation for a model potential describing the He-atom–Pt-surface potential. A set of 14 force constants adjusted to the bulk-phonon dispersion data cannot explain the measured time-of-flight spectra and the observed first-layer relaxation. However, by modifying at the surface the nearest-neighbor tangential force constants (α1’’=1.7α1b, and α1’=1.3α1b), the radial force constants (β1’’=0.39β1b), and the three-body force constant (γ1’’=0.037γ1b)— where the superscripts ∥ and ⊥ denote forces within the surface plane and between the first and second planes, respectively—a good fit to all the data is achieved. The new surface force constants are discussed in terms of electronic-charge redistribution within the first layer.

TATAREK R.; BRACCO G.; TOMMASINI F.; A. FRANCHINI; BORTOLANI V.; SANTORO G.; WALLIS R.F. ( 1989 ) - Surface Resonant Phonons of Ag(110) - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 211-212 - pp. da 314 a 322 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A study of the dynamics of the Ag(110) surface in the ḡGȲ direction is presented. The experimental TOF spectra show the existence of two low frequency modes with high scattering intensity and a dispersionless high frequency mode with a smaller intensity. The theoretical analysis based on a force constant model that contains long-range central interactions and short-range angular forces explains accurately both the dispersions and the scattering intensities of the three modes in terms of resonant and localized vibrations related to the modifications of the surface force constants caused by the different role played by the many-body forces in the surface region.

LOCK A.; TOENNIES J.P.; WOLL CH.; BORTOLANI V.; SANTORO G.; A. FRANCHINI ( 1988 ) - Phonons at the surface of the nearly-free-electron metal Al(111): realisation of an ideal surface - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 37 - pp. da 7087 a 7090 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Surface phonons of the nearly-free-electron metal Al(111) have been measured by high-resolution He-atom time-of-flight experiments for four different directions. This is the first surface which shows no evidence for an anomalous resonant mode found on the noble metals and Pt and attributed to a strong (≊50%) reduction of surface lateral force constants. By fitting the time-of-flight spectra via a lattice-dynamical simulation the surface interlayer force constant is found to agree to within ±3% with the bulk value and the lateral force constant reduction is less than 20%.

A. FRANCHINI; SANTORO G.; BORTOLANI V. ( 1988 ) - Surface Lattice Dynamics of Cu(111) - Patron Editore:Via Badini 12, I 40050 Quarto Inferiore Bologna Italy:011 39 051 767003, EMAIL: abbonamenti@patroneditore.com, info@patroneditore.com, INTERNET: http://www.patroneditore.com, Fax: 011 39 051 768252 ) - VUOTO - n. volume XVIII - pp. da 34 a 35 ISSN: 0391-3155 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We presnet a lattice dynamical calculation of the Cu(111) surface, based on the slab method in the framework of a force constant parametrization scheme that includes central and angular interactions.

A. FRANCHINI; SANTORO G. ; BORTOLANI V.; WALLIS R.F. ( 1988 ) - Theory of surface resonant phonons on the open (110) surface of Silver - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 38 - pp. da 12139 a 12143 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Recent time-of-flight measurements have been interpreted in terms of a force-constant model. We show that for phonon waves propagating along the ΓX direction, containing nearest neighbors, it is necessary to introduce a large softening of the lateral surface force constants (40%) in agreement with earlier calculations on the Ag(111) surface.

IDIODI J.; BORTOLANI V.; A. FRANCHINI; SANTORO G.; CELLI V. ( 1987 ) - Calculation of Debye-Waller factor for atom-surface scattering:He on Ag(111) - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 35 - pp. da 6029 a 6033 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

By a consistent application of the distorted-wave Born approximation, the Debye-Waller exponent for the scattering of He atoms from the Ag(111) surface is directly computed. When diffraction is negligible, as in this case, the decrease in specular intensity is simply obtained by summing the scattering due to all possible one-phonon processes. Using one-phonon computed intensities that agree with experiment along high-symmetry directions of the surface Brillouin zone, we find that the computed Debye-Waller exponent agrees with experiment and corresponds to an effective Debye temperature of 241 K. All corrections to the eikonal-type formula, 2W=4pz2〈uz2〉, are automatically included. In particular, the contribution of phonons with high parallel momentum is sharply cut off.

SANTORO G.; A. FRANCHINI; BORTOLANI V.; HARTEN U.; TOENNIES J.P.; WOLL CH. ( 1987 ) - Inelastic Phonon Scattering of Helium atoms from the reconstructed Au(111) surface - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 183 - pp. da 180 a 188 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present inelastic atom-surface phonon scattering time-of-flight spectra for the reconstructed Au(111) surface. The shapes of the spectra are compared with calculated reflection coefficients based on a simplified lattice dynamical model for the reconstructed surface. The force constants were determined from a best fit of the surface phonon dispersion curves and the lateral atom-atom interactions in the surface layer were found to be 30% of the bulk value. The theoretical spectra reproduce the overall lineshape of the experimental spectra and reveal additional features which can be attributed to the reconstruction of the surface layer with respect to the substrate.

BLACK E.; A. FRANCHINI; BORTOLANI V.; SANTORO G.; WALLIS R.F. ( 1987 ) - Surface phonon dispersion on Cu(110): a comparison of experiment and theory - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 36 - pp. da 2996 a 3001 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The surface-phonon frequencies of the open (110) surface of Cu have been investigated by means of force-constant models. Agreement with experiment is achieved by introducing a surface stress normal to the surface pointing toward the crystal. This force accounts for the charge redistribution which gives rise to the observed contraction of the surface interlayer spacing. In the ΓY direction, where the nearest neighbors do not lie on the surface, a weakening of the surface radial force constants is not necessary as for the (111) surface. The model used explains quite satisfactorily the three modes observed experimentally.

BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1987 ) - The phonon frequencies of the (111) surface of Platinum: existence of three localized modes - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 189-190 - pp. da 675 a 678 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The He surface reflection coefficients have been evaluated for the Pt(111) surface by using a long range force constants model for the bulk phonons. The lowering of the lateral surface force constants allows one to explain in a quantitative manner the existence of all the peaks observed in the experimental spectra.

BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1986 ) - Inelastic He scattering from metal surfaces - Patron Editore:Via Badini 12, I 40050 Quarto Inferiore Bologna Italy:011 39 051 767003, EMAIL: abbonamenti@patroneditore.com, info@patroneditore.com, INTERNET: http://www.patroneditore.com, Fax: 011 39 051 768252 ) - VUOTO - n. volume XVI - pp. da 54 a 54 ISSN: 0391-3155 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present inelastic aton surface phonon scattering time of flight spectra for the reconstructed Au(111) and for the Pt(111) unreconstructed surface.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1985 ) - On the detection of pseudo surface modes in Ag(111) with He-surface scattering - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 152-153 - pp. da 811 a 813 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We evaluate the surface phonon dispersion relations of Ag(111) within a force constants parametrization of the bulk dynamics. By changing the surface force constants we are able to explain all the salient features appearing in the experimental time of flight spectra. The parallel momentum Q has been considered along the direction [1-10] of the surface Brillouin zone. We show that the peaks of the time of flight spectra are related to the Rayleigh wave, to the pseudo Rayleigh wave and to a new resonance of longitudinal character. A comparison is made with the [11-2] direction.

BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1985 ) - Surface phonon calculations in metals and comparison with experimental techniques - NATO ASI Series, Series B: Physics - Plenum Press New York USA) - n. volume 121 - pp. da 401 a 449 ISBN: 9780306419126 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Surface phonon calculations in metals and comparison with experimental techniques

V. BORTOLANI; G. SANTORO ( 1985 ) - The Theory of Surface Phonons in Metals - Proceedings of the Second International Conference on Phonon Physics - World Scientific Singapore SGP) - pp. da 566 a 571 ISBN: 9971500027 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

The measurements of the surface phonon dispersion relations has been achived recently by means of He atom-surface scattering experiments. The results obtained with this thecnique present unaspected features that can only by explained by considering the role played by the electron-phonon interaction at the surface. In this paper we discuss within a phenomenological model recent measurements performed on the clese pakeed surface of noble metals. We show that resonant modes can be accounted for by a lowering of the surface interactions.

SANTORO G.; A. FRANCHINI ( 1985 ) - Theory of Reconstruction in clean and covered metallic surfaces - Phonon 1985 Proceedings of the Second International Conference on Phonon Physics - World Scientific Publishing Company Incorporated Singapore SGP) - pp. da 589 a 595 ISBN: 9789971500023 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

In this paper we discuss the microscopical origin of the reconstruction of the clean and covered metal surfaces and the crytical behaviour of the observed phase transitions. Among the models invoked to account for the formation of the reconstructed phase, we focus our attention on the electronic mechanism that seems to explain the energetics of these systems. A special attention will be done on W(100) and W(100):H, that have been extensively studied both experimentally and theoretically.

V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1984 ) - Brillouin Scattering at Interfaces and Longwavelength Acoustic Phonons - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - JOURNAL DE PHYSIQUE. COLLOQUE - n. volume 45 - pp. da 45 a 53 ISSN: 0449-1947 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present the theory of Brillouin scattering for an interface composed by a semi infinite medium and a finite slab. According to the thickness of the slab and the nature of the two systems various interesting features are present in the scattering cross section. In backward scattering experiments, to which we refer, are present structures corresponding to the Rayleigh, Sezawa and Lamb modes polarized in the sagittal plane. By considering transparent thick films of sufficiently high thickness it is possible to obtain information on the interface mode, the Stonely wave. This mode gives rise to a maximum in the Brillouin cross section. In the limit in which the thickness of the slab goes to infinity, the Lamb modes give rise to the density of states of the supported semi infinite medium.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1984 ) - Explanation of the anomalous peak observed in the He/Ag(111) atom scattering - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW LETTERS - n. volume 52 - pp. da 429 a 432 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The inelastic He/Ag(111) cross section has been evaluated in the one-phonon distorted-wave Born approximation. The authors use an atom-surface potential made up by a repulsive part derived from a superposition of atomic charges and by the attractive van der Waals interaction. The surface phonon density of states is evaluated within a force-constants parametrization of the bulk dynamics. By changing the surface force constants the authors are able to explain all the salient features appearing in the experimental time-of-flight spectra.

BORTOLANI V.; A. FRANCHINI; SANTORO G. ( 1984 ) - One phonon scattering of He atoms from the Ag(111) surface - Springer Series in Surface Sciences: Dynamical Phenomena at Surfaces, Interfaces and Superlattices - Springer Verlag Berlin DEU) - n. volume 3 - pp. da 92 a 100 ISBN: 9780387155050; 9783540155058 | 9783540155058 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

One phonon scattering of He atoms from the Ag(111) surface: calculation od the cross section in the Distorted Wave Born Approximation.

V. BORTOLANI; G. SANTORO; U. HARTEN; J. P. TOENNIES ( 1984 ) - "Surface Phonon Calculations for Noble Metals: Comparison with the He-Surface Scattering Experiments - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 148 - pp. da 82 a 89 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We evaluate the surface phonon dispersion relations of Ag(111) and Au(111) within a force constants parametrization of the bułk dynamics. By changing the surface force constants we are able to explain all the salient features appearing in the experimental time of flight spectra of a 7.9 meV He beam. The parallel momentum Q has been considered along the [11̄0] and [112̄] directions of the surface Brillouin ozne. We show that the peaks of the time of flight spectra are related to the Rayleigh wave, to the pseudo Rayleigh wave and to a new resonance of longitudinal character.

NIZZOLI F.; A. FRANCHINI; SANTORO G. ( 1984 ) - The interface polycarbonate-glass: a theoretical study of Brillouin spectra and photoelestic properties - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - JOURNAL DE PHYSIQUE. COLLOQUE - n. volume 45 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present calculations of surface waves and Brillouin scattering intensity for polycarbonate films supported by a pyrex substrate in the thickness range 0-0.6 µm. Comparison between calculated and experimental spectra shows that it is possible to obtain accurate information concerning the thickness of the film and the value and sign of the photoelastic constants.

V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1983 ) - Acoustic and Optical Surface Phonons - Ab Initio Calculation of Phonon Spectra - Plenum Publishing Corporation New York USA) - pp. da 201 a 214 ISBN: 9780306411199 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Very recently surface Brillouin scattering from opaque materials and electron energy loss spectroscopy from metallic surfaces with adsorbate were succesfully emplyed to detect surface phonons. We present calculations of surface acoustic phonons in the longwavelength limit for a semi-infinite medium in order to explain the Brillouin cross section. The case of GaAs and Al coated Si are considered. The surface optical phonons that are rapidly decaying in the medium are evaluated with the layer method and the results are compared with the electron energy loss spectra.

BORTOLANI V.; A. FRANCHINI; GARCIA N.; NIZZOLI F.; SANTORO G. ( 1983 ) - Calculation of potential cut-off for one-phonon atom-surface scattering - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 28 - pp. da 7358 a 7361 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A cluster calculation for Cu(111), Ag(111), and Au(111) shows that the lateral Fourier transform of the charge deformation due to a displacement of a single surface atom is a Gaussian exp(-Q2 / 2Qc2). This is proportional to the scattering amplitude for one-phonon exchange atom-surface scattering and produces a cutoff in the cross section. The values of Qc are evaluated for the above-mentioned surfaces, explaining very well the experimental observations. We also find a simple formula that should be useful to experimentalists in calculating the cutoff.

BORTOLANI V; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1983 ) - Calculation of the EELS spectra of the Ni(001) surface with p(2x2) and c(2x2) overlayers of Oxygen - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA - n. volume 29 - pp. da 219 a 224 ISSN: 0368-2048 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a force constant slab calculation of the nickel (001) surface phonons with p(2×2) and c(2×2) oxygen overlayers. According to very recent experimental results we assume in both configurations an equal distance of 0.9 Å between the oxygen overlayer and the nickel surface. With two adjustable parameters (the O---Ni force constants) we are able to reproduce with good accuracy the position and the lineshape of the four peaks present in the experimental EELS spectra. The nature of these peaks is investigated in detail and the noticeable difference between the spectra of the two overlayers is explained.

A. FASOLINO; G. SANTORO; E. TOSATTI ( 1983 ) - Surface Lattice-Dynamical Approach to the Reconstruction of W(100) - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 125 - pp. da 317 a 325 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We review the surface reconstruction of the bcc (100) transition metal surface, particularly W(100) and some of the results obtained, with the method of the effective surface lattice dynamics.

A. FASOLINO; G. SANTORO; E. TOSATTI ( 1983 ) - The Effective Mass Method for Surface States - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. C. SOLID STATE PHYSICS - n. volume 16 - pp. da 985 a 997 ISSN: 0022-3719 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Concepts borrowed from the effective mass theory of impurity states in semiconductors are shown to be useful in the theory of surface states. Explicit application is made to surface phonons. For a bcc (100) surface, in particular, the authors compare their approximate surface eigenvalues and eigenvectors with essentially exact results from brute force slab calculations, and find generally good agreement. Several approaches to the solution of the effective mass equation are considered, and their relative accuracies are discussed.

V. BORTOLANI; A. MARVIN; F. NIZZOLI; G. SANTORO ( 1983 ) - Theory of Brillouin Scattering from Surface Acoustic Phonons in Supported Films - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. C. SOLID STATE PHYSICS - n. volume 16 - pp. da 1757 a 1776 ISSN: 0022-3719 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The theory of the Brillouin scattering from the surface of a semi-infinite medium, developped previously, is extended to coated surfaces of cubic materials. The Brillouin cross-section is optained for any scattering geometry and for p and s polarization of the incident and scattered light. The formulae for the cross section contain contributions from ripple and elastooptic scattering mechanisms for the film as well as for the substrate. The theory is applied to silica film deposited on crystalline silicon and the behaviour od the cross-section against the film thickbess is investigated. We find that there are strong interference effects among the various contributions to the cross-section, so that the intensity of the spectral lines dramatically oscillates by changing the thickness. Although in general the ripple scattering mechanissm is the dominant one it is found that the elastooptic coupling in the silica film is not negligeable and sometimes is even prevailing. The discrete phonon spectrum (Rayleigh, Sezawa, Lamb modes) and the continuous spectrum (resonances, mixed modes) are both studied.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G.; BENEDEK G.; CELLI V. ( 1983 ) - Theory of inelastic Ne scattering from the Ni(111) surface - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - SURFACE SCIENCE - n. volume 128 - pp. da 249 a 264 ISSN: 0039-6028 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The recent inelastic Ne scattering data for Ni(111) by Feuerbacher and Willis are theoretically analyzed in the framework of the distorted wave Born approximation. A Ne-surface soft potential model is inferred from two-body Ne-Ni interactions and the many-body effects due to the phonon-induced surface electron charge redistribution are qualitatively described by three-body forces. The surface-projected phonon densities for the Ni(111) surface are calculated from a force constant model including three-body forces. The cut-off produced by the softness of the Ne-Ni potential accounts for the observed cross-over from Rayleigh-wave to bulk phonon scattering regimes. The one-phonon picture correlates well with experiments for outgoing angles slightly apart from the specular condition (Rayleigh-wave regime) whereas for larger deviations from the specular condition (bulk phonon regime) multiphonon contributions are important.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G.; BENEDEK G.; CELLI V.; GARCIA N. ( 1983 ) - Theory of one-phonon scattering of atoms from metal surfaces: application to He:Ag(111) - Elsevier Science Limited:Oxford Fulfillment Center, PO Box 800, Kidlington Oxford OX5 1DX United Kingdom:011 44 1865 843000, 011 44 1865 843699, EMAIL: asianfo@elsevier.com, tcb@elsevier.co.UK, INTERNET: http://www.elsevier.com, http://www.elsevier.com/locate/shpsa/, Fax: 011 44 1865 843010 ) - SOLID STATE COMMUNICATIONS - n. volume 12 - pp. da 1045 a 1048 ISSN: 0038-1098 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present explicit expressions for one-phonon scattering of atoms from metal surfaces, within the distorted wave Born approximation by including Van der Waals forces. A comparison is made with the recent high-resolution time of flight spectra for He/Ag(1 1 1). We explain in quantitative terms the position and the intensity of the peaks due to scattering from the Rayleigh wave of silver. Also the bulk phonons contribute to the spectra, but the calculated intensities underestimate the data.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1982 ) - Calculation of the phonon spectrum of the Ni(111) surface covered with Oxygen - Springer Series in Chemical Physics: Dynamics of Gas-Surface Interaction - Springer-Verlag Berlin DEU) - n. volume 21 - pp. da 196 a 206 ISBN: 9780387116938; 9783540116936 | 9783540116936 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Calculation of the phonon spectrum of the Ni(111) surface covered with Oxygen: comparison of the P(2x2) and Sqrt(3)xSqrt(3)R30° overlayer geometry.

V. BORTOLANI; F. NIZZOLI; G. SANTORO; J. R. SANDERCOCK ( 1982 ) - Strong Interference Effects in Surface Brillouin Scattering from a Supported Transparent Film - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER - n. volume 25 - pp. da 3442 a 3445 ISSN: 0163-1829 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A measurement of surface Brillouin scattering from a 2250-Å film of silica grown on (001) silicon is presented. The spectrum shows many structures related to Rayleigh, Sezawa, and Lamb waves. We calculate the Brillouin cross section taking the ripple and the elasto-optic coupling mechanisms into account in the two media. Both the mechanisms are found to be important and there is evidence of strong interference effects between the ripple and the elasto-optic contributions to the scattering amplitude.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1982 ) - Surface phonon calculation of the (111) Ni surface with adsorbed Oxygen - Elsevier Science Limited:Oxford Fulfillment Center, PO Box 800, Kidlington Oxford OX5 1DX United Kingdom:011 44 1865 843000, 011 44 1865 843699, EMAIL: asianfo@elsevier.com, tcb@elsevier.co.UK, INTERNET: http://www.elsevier.com, http://www.elsevier.com/locate/shpsa/, Fax: 011 44 1865 843010 ) - SOLID STATE COMMUNICATIONS - n. volume 41 - pp. da 369 a 373 ISSN: 0038-1098 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In the framework of a central and angular force constant model we have evaluated the phonon spectrum and the loss function of the Ni (111) surface with p(2×2) and ( )R30° adsorbed lattices of Oxigen. The calculations compare well with recent EELS experimental data. We give reason of the different origin of the weaks observed in the measured spectra in terms of either the modes of the substrate or of the modes induced by the adsorbate.

A. MARVIN; V. BORTOLANI; F. NIZZOLI; G. SANTORO; V. CELLI ( 1982 ) - Theory of Three Wave Mixing of Volume-EM, Surface Acoustic Waves and Surface Polaritons - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. C. SOLID STATE PHYSICS - n. volume 15 - pp. da 3273 a 3280 ISSN: 0022-3719 [Articolo in rivista (262) - Articolo su rivista]
Abstract

This paper deals with the theory of light scattering from a thin metallic film where corrugations exists at both surfaces. The scattering cross section for backward and forward diffraction is computed by solving perturbatively the Maxwell equations to first order in the corrugation amplitude. A comparison is made with the experimental results presented in the preceding paper where the corrugations are of dynamic character (surface elastic waves). The different behaviour of the backward and forward cross sections is explained in terms of interference effects.

V. BORTOLANI; G. SANTORO ( 1981 ) - Brillouin Scattering and Surface Acoustic Phonons - Recent Developements in Condensed Matter Physics - Plenum Press New York USA) - n. volume 1 - pp. da 655 a 672 ISBN: 9780306406461 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

In this work we present the theory of the scattering of light from an anisotropic semiinfinite medium. We show that the Brillouin scatterin can occour with two different mechanisms. The first one is related to the surface corrucation induced by the thermal phonons. The ripple effect is dominant in the scattering from metalli surfaces where the penetration depth of the light is very small. The second scattering mechanism is due to the modulation of the dielectric function of the whole medium induced by the phonons. It results particularly important for opaque semiconductors (elastooptic effect). In order to investigate the effects of the anisotropy of the medium we prent results for various faces of clean semiconductors.

A. FASOLINO; G. SANTORO; E. TOSATTI ( 1981 ) - Effective Mass Approximation for Surface Phonon States - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - JOURNAL DE PHYSIQUE. COLLOQUE - n. volume 42 - pp. da 840 a 842 ISSN: 0449-1947 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We show how effective mass concepts borrowed from the theory of impurity states in semiconductors can usefully be applied to the calculation of surface phonon states

J. R. SANDERCOCK; F. NIZZOLI; V. BORTOLANI; G. SANTORO; A. MARVIN ( 1981 ) - Surface and Interface Acoustic Phonons in Al-Coated Silicon - Recent Developements in Condensed Matter Physics - Plenum Press New York USA) - n. volume 2 - pp. da 419 a 425 ISBN: 9780306406478 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Measurements are presented of the Brillouin cross-section for a thin aluminium film on a silicon substrate measured using a tandem multipass interferometer. The measured spectra are compared with the cross-section computed by assuming that the light is inelasically scattered from surface corrugation due to the thermally excited acoustic phonons. By varying the thicness of the aluminium film we observe different spectral features such as the modified Rayleigh mode, the Sezawa modes and tho continuum os states of the sbstrate. The limiting case of a thick coating is also discussed.

V. Bortolani; F. Nizzoli; G. Santoro; A. Franchini ( 1981 ) - Surface lattice dynamics of clean and covered nickel - VACUUM - n. volume 31 - pp. da 467 a 467 ISSN: 0042-207X [Articolo in rivista (262) - Articolo su rivista]
Abstract

The phonon spectrum of the (111) and (001) clean surfaces ofnickel has been evaluated by using the slab method In order todescribe the bulk fcc nickel we have introduced bond stretchingand angle bending force constants up to second nelghbours Thenumerical values of the force constants have been determined by aleast-square fit of the experimental bulk phonons The calculationshave been carried out with a 31 layer slabThe available EELS experiments on these surfaces wereperformed with covered surfaces, where some interacuon occursbetween the over-layer and the substrate In order to make acomparison between theory and experiments we have thereforeevaluated the phonon spectrum of N1 (111) covered with halfmonolayer of oxygen with the p(2 x 2) and (3 x 3)R30 ° orderedstructures The comparison between the loss spectra and oursurface local densities of phonon states turns out to be very good.

BORTOLANI V.; A. FRANCHINI; NIZZOLI F.; SANTORO G. ( 1981 ) - Surface lattice dynamics of Nickel - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - JOURNAL DE PHYSIQUE. COLLOQUE - n. volume 42 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In the framework of a central and angular force constants mode1,we have evaluated the phonon spectrum and the loss function of the Ni(111) surface covered with Oxigen. We explain quantitatively the main features of the observed electron energy loss spectra.

A. FASOLINO; G. SANTORO; E. TOSATTI ( 1981 ) - Surface Phonons and the Incommensurate Reconstruction of Clean Mo(100) - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - JOURNAL DE PHYSIQUE. COLLOQUE - n. volume 42 - pp. da 846 a 848 ISSN: 0449-1947 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a model lattice dynamical study of the Mo(100) incomensurate reconstruction. We find that both the predicted periodicity and polarization of the incommensurate distortion agree with experiment. Interesting changes of the periodicity are predicted in presence of an electric field of magnitudes such as those used in Field-Ion-Microscopy.

V. BORTOLANI; F. NIZZOLI; G. SANTORO; A. MARVIN ( 1981 ) - Theory of Acoustic Phonons at Interfaces and Brillouin Scattering - EDP Sciences:17 Ave Du Hoggar, BP112, 91944 Les Ulis Cedex A France:011 33 1 69187575, EMAIL: subscribers@edpsciences.com, INTERNET: http://www.edpsciences.com, Fax: 011 33 1 69860678 ) - JOURNAL DE PHYSIQUE. COLLOQUE - n. volume 42 - pp. da 804 a 806 ISSN: 0449-1947 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present calculations of the Brillouin cross section for a metallic thin film (Al and Au) on a Si substrate. The results for the Al coating are explained in terms of scattering from the surface corrugation due to the thermally excited acoustic phonons. In the case of Au we find an appreciable contribution from the elasto-optic coupling in the film.

J. R. SANDERCOCK; F. NIZZOLI; V. BORTOLANI; G. SANTORO; A. MARVIN ( 1980 ) - Brillouin Scattering from Interface Acoustic Phonons - Societe Francaise du Vide:19 rue du Renard, 75004 Paris France:011 33 1 42781582 ) - LE VIDE, LES COUCHES MINCES - n. volume 201 - pp. da 754 a 757 ISSN: 0223-4335 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present measurements of the Brillouin cross-section for a metallic thin film (aluminium and gold) on a silicon substrate, obtained by using a tandem multipass interferometer. The results for the aluminium coating are explained in terms of scattering from the surface corrugation due to the thermally excited acoustic phonons. In the case of gold we find an appeciable contribution from the elastooptic coupling in the film. In both cases the interface modes are experimentally resolved and theoretically explained.

V. BORTOLANI; F. NIZZOLI; G. SANTORO; A. MARVIN ( 1980 ) - Effects of Anisotropy on Surface Brillouin Scattering from Cubic Crystals - Proceedings of the VIIth International Conference on Raman Spectroscopy - North-Holland Publishing Company Amsterdam NLD) - pp. da 440 a 441 ISBN: 9780444860385 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

The paper deals with the theoretical interpretation of the Brillouin scattering from different surfaces of cubic crystals. Both the ripple and the elastooptic mechanisms are considered. The anysotropy plais an important role in determining the leading scattering mechanism and the pattern of the spectra.

A. FASOLINO; G. SANTORO; E. TOSATTI ( 1980 ) - Instability, Distortion and Dynamics of the W(100) Surface - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW LETTERS - n. volume 44 - pp. da 1684 a 1687 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A model for the reconstructions of the W(100) surface is presented. Four types of distortions are obtained, along 〈011〉, 〈001〉, 〈100〉 (commensurate), and (× 10) (incommensurate). The 〈011〉 distortion, corresponding to the observed (√2×√2)45° clean surface, is worked out in detail. A distortion magnitude of ∼ 0.35 Å is found, and a surface coherence length ξ≃15° Å close to experiment. The full vibrational spectrum, the distortion penetration, and the anisotropic surface anharmonic energies are also obtained.

A. FASOLINO; G. SANTORO; E. TOSATTI ( 1980 ) - Reconstruction and Dynamics of the W(100), Mo(100) and Cr(100) Surfaces - Societe Francaise du Vide:19 rue du Renard, 75004 Paris France:011 33 1 42781582 ) - LE VIDE, LES COUCHES MINCES - n. volume 201 - pp. da 679 a 682 ISSN: 0223-4335 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A model has been developed that is capable of describing the structural and lattice dynamical properties of the reconstructed W(100), and the closely related Mo(100) and Cr(100), surfaces. In particular the different reconstructions (both commensurate and incommensurate) that are seen on the clean neutral surface, on a high-field FIM tip and on H-covered surfaces, are found to belong to the same T equals O surface ″phase diagram″ .

V. BORTOLANI; F. NIZZOLI; G. SANTORO; J. R. SANDERCOCK; A. MARVIN ( 1980 ) - Ripple and Elasto-Optic Coupling in Surface Brillouin Scattering from a Gold Thin Film - Proceedings of the VIIth International Conference on Raman Spectroscopy - North-Holland Publishing Company Amsterdam NLD) - pp. da 442 a 443 ISBN: 9780444860385 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

The measured Brillouin spectrum of a thin film of gold deposited on silicon shows surface and interface waves. A calculation of the cross-section gives evidence of a remarkable elastooptic coupling in the film, in addition to the ripple contribution characteristic of scattering from metal surfaces.

A. MARVIN; V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1980 ) - Surface Brillouin Scattering from Acoustic Phonons: II. Application to Semiconductors - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. C. SOLID STATE PHYSICS - n. volume 13 - pp. da 1607 a 1616 ISSN: 0022-3719 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We have applied to semiconductors the thory of Brillouin scattering from corrugated surfaces developed previously. We have fitted the available spectra measured with 5145 A light in order to evaluate the elastooptic constants of the medium. For GaAs we found k_11=42.5 k_12=51, k_44=25.5. This is the first determination of these constants in GaAs at this wavelenght where other techniques are not applicable. For silicon we found k_11=53 k_12=25, k_44=23. It is also shown that in opaque semiconductors the elastooptic coupling depends strongly on the surface orentation.

V. BORTOLANI; F. NIZZOLI; G. SANTORO; A. MARVIN; J. R. SANDERCOCK ( 1979 ) - Brillouin Scattering from Surface Phonons in Al-Coated Semiconductors - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW LETTERS - n. volume 43 - pp. da 224 a 227 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Brillouin spectra from thermally excited surface acoustic phonons in GaAs and Si surfaces coated with a thin Al film are presented. The measurements show features different from the Al and the semiconductor surfaces. We are able to give a theoretical explanation of the experimental results in terms of the coupling between the normal modes of the film and of the substrate.

F. NIZZOLI; G. SANTORO; V. BORTOLANI ( 1979 ) - Surface Lattice Dynamics of Trigonal Se and Te - The Physics of Selenium and Tellurium - Springer-Verlag Berlino DEU) - pp. da 81 a 83 ISBN: 9783540096924 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Very recently Brillouin scattering from thermally excited phonons has proved to be the most succesfull technique to directly measure surface vibrations, both in the discrete and in the continuos part of the spectrum.

V. BORTOLANI; V. CELLI; A. MARVIN; G. SANTORO ( 1978 ) - A Method for Determining Surface States by Complex Extension of Tight-Binding - Bologna ) - NUOVO CIMENTO DELLA SOCIETÀ ITALIANA DI FISICA. B - n. volume 46 - pp. da 121 a 135 ISSN: 1124-187X [Articolo in rivista (262) - Articolo su rivista]
Abstract

A method is developed for calculating surface states and the composition of the surface wave functions. The formulation is made for tight-binding Hamiltonians. The present scheme is based on the extension of the bulk bands in the complexk-plane. The surface is viewed as a boundary condition of the Schrödinger difference equation of the infinite crystal. At points of high symmetry of the two-dimensional Brillouin zone, this approach gives analytic expressions for surface states and allows one to determine the composition of real and evanescent waves which make up the wave function of the semi-infinite crystal. It provides an attractive computational method that can also be used to study reconstructed surfaces.

V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1978 ) - Acoustic Phonons at the (111) Surface of Chromium - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS F. METAL PHYSICS - n. volume 10 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Recent Brillouin scattering data for the (111) surface of Cr (Sandercock 1978) are compared with the theoretical cross section computed for a semi-infinite anisotropic elastic medium undergoing free-surface boundary conditions. The surface ripple mechanism is found to be responsible for the continuum of phonon states localised between the transverse and longitudinal thresholds.

V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1978 ) - Brillouin Scattering from Surface Ripples in GaAs - Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: nlinfo-f@elsevier.nl, INTERNET: http://www.elsevier.nl, Fax: 011 31 20 4853598 ) - PHYSICS LETTERS A - n. volume 68 - pp. da 342 a 344 ISSN: 0375-9601 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We compute the power spectrum of the surface displacements normal to the boundary for the faces (001), (110) and (111) of GaAs in the long wavelength limit. Recent experimental results of Brillouin scattering of reflected light are interpreted.

V. BORTOLANI; F. NIZZOLI; G. SANTORO; E. TOSATTI ( 1978 ) - Long-Wavelength Surface Phonon Spectrum in W(001) - Elsevier Science Limited:Oxford Fulfillment Center, PO Box 800, Kidlington Oxford OX5 1DX United Kingdom:011 44 1865 843000, 011 44 1865 843699, EMAIL: asianfo@elsevier.com, tcb@elsevier.co.UK, INTERNET: http://www.elsevier.com, http://www.elsevier.com/locate/shpsa/, Fax: 011 44 1865 843010 ) - SOLID STATE COMMUNICATIONS - n. volume 26 - pp. da 507 a 510 ISSN: 0038-1098 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We calculate eigenvectors and local densities of long-wavelength phonon states for a (001) semi-infinite crystal of W, by a microscopic force constant approach. The results are discussed in relation to surface structure (relaxation) and to surface-sensitive spectroscopies.

V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1978 ) - Surface Density of Acoustic Phonons in GaAs - American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW LETTERS - n. volume 41 - pp. da 39 a 42 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We calculate the surface density of acoustic phonons for GaAs(110) and relate it to the Brillouin elasto-optic cross section. A continuum of surfacelike excitations is found in good agreement with the position and the shape of the surface structure observed in the measured Brillouin spectrum. The role played by the optical absorption coefficient in determining different scattering mechanisms is also discussed.

V. BORTOLANI; F. NIZZOLI; G. SANTORO ( 1978 ) - Surface Lattice Dynamics in Semiinfinite Transition Metals - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - INSTITUTE OF PHYSICS CONFERENCE SERIES - n. volume 39 - pp. da 326 a 329 ISSN: 0951-3248 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present calculations of the surface phonon spectra in semiinfinite crystals of Iron and Thungsten,bound by an ideal (001) face,based on a force-constant parametrization of the bulk dynamics. The method allows to treat equally well stongly localized and deeply penetrating atomic vibrations. Local densities of phonon states are shown for the surface and for the first layer underneath,both for the localized modes and for the travelling waves,which are considerably modified by the presence of the surface.

C. Calandra; G. Santoro ( 1976 ) - Electronic structure of nonpolar surfaces of II-VI compounds - THE JOURNAL OF VACUUM SCIENCE AND TECHNOLOGY - n. volume 13 - pp. da 773 a 778 ISSN: 0022-5355 [Articolo in rivista (262) - Articolo su rivista]
Abstract

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

C. CALANDRA; G. SANTORO ( 1976 ) - Intrinsic Surface States in III-V Compounds - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. C. SOLID STATE PHYSICS - n. volume 9 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The electronic structure of the (110) face of GaAs, GaP and InSb is investigated using a realistic tight-binding model. Calculations of the surface band structure are presented and the nature of the various surface bands is discussed. The results are in agreement with the experimental data and provide a quantitative description of the cation-derived surface states lying in the bulk band gap.

C.M. BERTONI; O. BISI; C. CALANDRA; F. NIZZOLI; G. SANTORO ( 1976 ) - sp-d hybridization effects on the electronic structure of the (100) surface in Copper - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS F. METAL PHYSICS - n. volume 6 [Articolo in rivista (262) - Articolo su rivista]
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.

C. CALANDRA; G. SANTORO ( 1975 ) - Surface States of the (110) Surface of GaAs - IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. C. SOLID STATE PHYSICS - n. volume 8 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Results are presented of a new calculation of the surface states at the (110) surface of GaAs. By using Koster-Slater parameters fitted to bulk optical and photoemission data, the surface band structure has been calculated with a LCAO method. The main conclusion is the existence of two surface bands, one inside the band gap at about 1 eV from the top of the valence band and the other a few tenths of an eV below it. The results show good agreement with the available experimental data.