Publications (28)37.54 Total impact
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Article: Spin-Dependent On-Site Electron Correlations and Localization in Itinerant Ferromagnets
Physical Review Letters 01/2013; 109:126401. · 7.37 Impact Factor -
Article: Revisiting the Yb electronic structure with low-energy photoemission spectroscopy
Physical Review B 01/2012; 85:-. · 3.69 Impact Factor -
Article: Insight on hole-hole interaction and magnetic order from dichroic auger-photoelectron coincidence spectra.
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ABSTRACT: The absence of sharp structures in the Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. By a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the photoelectron, Auger, and Auger-photoelectron coincidence spectra (APECS) of the antiferromagnetic CoO. A recently discovered dichroic effect in angle resolved (DEAR) APECS reveals a complex pattern in the Auger line shape, which is here explained in detail, labeling the final states by their total spin. Since the dichroic effect exists in the antiferromagnetic state but vanishes at the Néel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations where the usual methods are not applicable.Physical Review Letters 11/2011; 107(21):217602. · 7.37 Impact Factor -
Article: Evidence for the collapse of short-range magnetic order in CoO at the Néel temperature
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ABSTRACT: Auger-photoelectron coincidence spectroscopy is used for investigating the electronic properties of a CoO thin film above and below the magnetic transition temperature (TN). By using the dichroic effect in angle-resolved measurements, we identify and assign well-defined high-spin and low-spin structures in spite of the otherwise featureless Auger singles spectra, typically found for open-band systems. The disappearance of the dichroism for temperatures just above TN indicates a collapse of the surface short-range magnetic order, presumably due to a strongly reduced exchange field in the surface compared to that in the bulk.EPL (Europhysics Letters) 04/2011; 94(3):37008. · 2.17 Impact Factor -
Article: The attenuation length of low energy electrons in Yb.
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ABSTRACT: Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV are measured for in situ grown polycrystalline Yb films. By comparing bulk and surface core level shifted 4f components we give an estimation of the effective attenuation length (EAL) for low energy (6-20 eV) electrons in Yb, establishing a moderate increase of the EAL upon electron energy decrease. The experimental EAL data are found to be a factor of four smaller than those predicted from the so-called 'universal curve'.Journal of Physics Condensed Matter 08/2010; 22(30):305002. · 2.55 Impact Factor -
Article: Electronic states of CuPc chains on the Au(110) surface.
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ABSTRACT: The electronic properties of Cu-phthalocyanine (CuPc) molecules flat lying along the channels of the Au(110) reconstructed surface have been investigated by means of ultraviolet and x-ray photoelectron spectroscopy. The ordered chains give rise to a highly ordered single-layer structure with a (5x3) symmetry. Although from the core-level analysis not any significant charge transfer between the molecules and the underlying Au surface is observed, the valence band photoemission data bring to light CuPc-induced features localized at the interface. In particular, energy versus momentum dispersion of an interface state reveals a bandwidth of about 90 meV along the enlarged Au channels, where the CuPc chains are formed, with a defined fivefold symmetry well fitting the CuPc intermolecular distance.The Journal of chemical physics 11/2009; 131(17):174710. · 3.09 Impact Factor -
Article: Spin selectivity by auger-photoelectron coincidence spectroscopy
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ABSTRACT: The M3M4,5M4,5 Auger transition from a Cu(111) surface is studied using Angular Resolved Auger-PhotoElectron Coincidence Spectroscopy (AR-APECS). In the experiment two different geometrical configurations of the electron analyzers allow us to sample different emission angles of the ejected electrons leading to different weights of the singlet and triplet contributions in the studied transition. The experimental spectra are modeled within a two-step approach using the Cini theory for the closed band case so as to properly consider the spin-orbit interaction and the hole-hole correlation energy. Ingredients for the theory, like density of states, are obtained fully ab-initio in the framework of density functional theory by performing all-electron calculations. The obtained results confirm the recently discovered selectivity of AR-APECS in the final spin-state.Journal of Physics Conference Series 03/2008; 100(7):072020. -
Article: Evidence of Charge Transfer at the Cu-phthalocyanine/Al(100) Interface†
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ABSTRACT: Electronic and structural properties of the CuPc/Al(100) organic−inorganic interface were investigated by means of a multitechnique experimental approach based on synchrotron radiation. The chemical selectivity of X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of copper-phthalocyanine (CuPc) as a function of the molecular thickness ranging from the submonolayer to 40 Å. Photoemission from core levels shows a dramatic alteration of the electronic structure of molecules localized at the interface. At the lowest CuPc coverages, the complete reduction of the oxidation state of copper was observed, while C 1s and N 1s shake-up satellites were no longer visible. Both findings are explained with a sizable charge transfer from the substrate to the molecule involving the b1g (Cu 3d-derived) and the LUMO hybridized with the substrate conduction band. The linear polarization of the synchrotron light was employed in X-ray absorption near-edge spectroscopy (XANES) to determine the orientation of CuPc molecules. Molecular planes oriented almost perpendicular with respect to the metal surface were observed from the second layer on.01/2008; -
Article: Two-step-wise interpretation of highly asymmetric, grazing angle (e,2e) on solids: A real momentum spectroscopy for surfaces and overlayers
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ABSTRACT: This paper deals with the mechanism of grazing incidence e,2e events from surfaces. Two different approaches are considered. In both cases, elastic scattering with the crystal lattice assists the inelastic collision; these two steps are coupled either coherently or incoherently. Experimental evidence is given that the "coher-ent" approach reproduces better the cross section dependence on momentum transfer in the specific case of asymmetric kinematics at moderate electron energies. This model has allowed us to map out the band disper-sion of the outermost valence states of highly oriented pyrolytic graphite and to measure the momentum distribution of -electron states without invoking the contribution of reciprocal lattice vectors in the momen-tum conservation. Agreement between theory and experiment is satisfactory, though the presence of events where crystal momentum is reconstructed cannot be ruled out. These results, obtained with a significant reduction of the experiment duration by an implemented apparatus, show that reflection e,2e can be used to build up a momentum spectroscopy with high surface sensitivity.Physical Review B 01/2008; 77:085116. · 3.69 Impact Factor -
Article: A high efficiency spectrometer for reflection (e,2e) experiments at surfaces
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ABSTRACT: To study electron momentum densities in solids by grazing angle reflection kinematics has been shown to be feasible [S. Iacobucci, S. Rioual, A. Ruocco, M. Mastropietro, G. Stefani, Surf. Sci. 454 (2000) 1026], but development of this spectroscopy has been hampered by long acquisition time; to fully exploit potentialities of this method is mandatory to reduce duration of the experiment within times comparable with clean surface lifetimes in ultra-high vacuum. This paper reports on recent developments of the reflection (e,2e) spectrometer that make a sizeable step forward in attaining this goal. It operates in asymmetric kinematics and at small grazing angle, thus allowing to enhance the surface sensitivity. A drastic reduction in acquisition time has been achieved by implementing parallel acquisition, both in energy and angle, of the detected electron pairs. To achieve parallel acquisition in energy and momentum, each of the two electron analysers is equipped with a two-dimensional position sensitive detector. A custom-made electronic hardware and software have been developed for the automatic control of the experiment and for acquisition and storage of the coincidence events. After discussing in some details the relevant features of the new spectrometer, few examples of valence band mapping and electron momentum densities measured in highly oriented pyrolitic graphite with energy and momentum resolutions of 1.3 eV and 0.1 A −1 are given.Journal of Electron Spectroscopy and Related Phenomena 01/2007; 161:140-146. · 1.96 Impact Factor -
Article: Copper−Phthalocyanine Induced Reconstruction of Au(110)†
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ABSTRACT: The structure of ultrathin Cu−phthalocyanine (Cu−Pc) film on Au(110) has been studied by means of several diffraction tecniques: helium atom scattering (HAS), low energy electron diffraction (LEED) and grazing incidence X-ray diffraction (GIXD). HAS has been used to measure the long range order of the organic overlayer, whereas LEED at 200 eV has been used to probe the corresponding substrate reconstruction. At the monolayer coverage, the Au(110) substrate displays a reconstruction with a 3-fold periodicity along the [001] direction, whose structure has been studied by out of plane GIXD and variable polarization X-ray absorption near edge spectroscopy (XANES). We found the structure of the substrate unit cell to be an asymmetric shallow (1 × 3) reconstruction with the Cu−Pc molecules tilted by an angle of 32° from the (110) surface plane.07/2004; -
Article: Copper-phthalocyanine ultra thin films grown onto Al(100) surface investigated by synchrotron radiation
Journal of Electron Spectroscopy and Related Phenomena 07/2004; 137(SI):165-169. · 1.96 Impact Factor -
Article: Relevance of the electron energy-loss spectroscopy for in situ studies of the growth mechanism of copper phthalocyanine molecules on metal surfaces: Al (100)
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ABSTRACT: Reflection electron energy-loss spectroscopy (EELS) in specular and off specular geometry has been employed to study the early stage of the copper phthalocyanine (CuPc) growth on Al(100) substrate. EEL spectroscopy has been a useful tool in order to study the electronic structure of molecular films also in the submonolayer regime. The electronic structure of the first deposited layer of CuPc is strongly influenced by charge transfer from the Al substrate to the lowest unoccupied molecular orbital (LUMO). The strong molecule-substrate interaction gives rise to a coverage dependent frequency shift of the Al surface plasmon. Successive layers have essentially the electronic structure of the molecular solid. Momentum resolved EELS measurements reveal that, in the case of the thicker film investigated (22Å), the plane of the molecule is almost perpendicular to the surface of the substrate.Phys. Rev. B. 04/2003; 67(15). -
Article: CuPc molecules adsorbed on Au(110)-(1x2): growth morphology and evolution of valence band states
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ABSTRACT: We present the growth morphology, the long range ordering, and the evolution of the valence band electronic states of ultra-thin films of copper phthalocyanine (CuPc) deposited on the Au(110)-(1x2) reconstructed surface, as a function of the organic molecule coverage. The Low Energy Electron Diffraction (LEED) patterns present a (5x3) reconstruction from the early adsorption stages. High-Resolution UV photoelectron spectroscopy (HR-UPS) data show the disappearance of the Au surface states related to the (1x2) reconstruction, and the presence of new electronic features related to the molecule-substrate interaction and to the CuPc molecular states. The CuPc highest occupied molecular orbital (HOMO) gradually emerges in the valence band, while the interface electronic states are quenched, upon increasing the coverage. Comment: pdf 15 pages, 4 figures, accepted for publication: Surface Science03/2003; -
Article: Relevance of the EEL spectroscopy for in-situ studies of the growth mechanism of copper-phthalocyanine molecules on metal surfaces: Al(100)
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ABSTRACT: Reflection electron energy loss spectroscopy (EELS) in specular and off specular geometry has been employed to study the early stage of the copper phthalocyanine (CuPc) growth on Al (100) substrate. EEL spectroscopy has been a useful tool in order to study electronic structure of molecular films also in the submonolayer regime. The electronic structure of the first deposited layer of CuPc is strongly influenced by charge transfer from the Al substrate to the lowest unoccupied molecular orbital (LUMO). The strong molecule-substrate interaction gives rise to a coverage dependent frequency shift of the Al surface plasmon. Successive layers have essentially the electronic structure of the molecular solid. Momentum resolved EELS measurements reveal that, in the case of the thicker film investigated (22 \AA), the plane of the molecule is almost perpendicular to the surface of the substrate. Comment: 8 pages, 5 figures, accepted for publication: Phys. Rev. B03/2003; -
Article: Angle-resolved Auger-photoelectron coincidence spectroscopy (AR-APECS) of the Ge(100) surface
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ABSTRACT: We have measured the angular distribution of Ge L3M45M45 Auger electrons in coincidence with Ge 2p3/2 core photoelectrons along the (001) azimuth of the Ge(100) surface. Intensity modulations arising from diffraction effects are suppressed in the coincidence Auger angular distribution and, when specific emission angles of the photoelectrons are considered, new features appear. We attribute the former effect to enhanced surface specificity of the coincidence technique and the latter to sensitivity of the coincidence measurement to alignment of the core hole state.Phys. Rev. B. 01/2003; 67(3). -
Article: The ALOISA end station at Elettra:: a novel multicoincidence spectrometer for angle resolved APECS
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ABSTRACT: Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra, by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved – Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude. # 2001 Elsevier Science B.V. All rights reserved.01/2001; 468:1468-1472. -
Article: Scattering Mechanism of electrons interacting with surfaces in specular reflection geometry: graphite
Physical Review B 01/1999; · 3.69 Impact Factor -
Article: Electronic Properties and Orbital-Filling Mechanism in Rb-Intercalated Copper Phthalocyanine
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ABSTRACT: The evolution of the electronic properties of a thin film of copper phthalocyanine deposited on Al(100) and progressively intercalated with rubidium atoms was followed by photoemission and X-ray absorption spectroscopies. Electron donation from the Rb atoms to the C32H16N8Cu molecules results in the lifting of the degeneracy in the eg ligand-derived molecular orbital and the lowering of the molecular symmetry. For Rb~2C32H16N8Cu, spectral evidence indicates that both donated electrons reside in the first split-off eg-derived level, thus creating an electronic inequivalence between the C atoms in the benzene rings. For higher Rb concentrations, a reduction of the Cu oxidation state is observed, together with a new Cu-derived state in valence-band photoemission spectra, testifying to the filling of the b1g orbital. Thus, even though b1g is the lowest unoccupied orbital of the neutral molecule, in the film, the Cu-derived b1g-derived states are occupied only after a partial filling of the eg-derived band has taken place. Despite the fact that the eg-derived spectral weight becomes larger as the rubidium content in the RbxC32H16N8Cu compound increases, no spectral density was observed at the Fermi level, showing that the film remains insulating for all of the investigated stoichiometries. -
Article: New developments in surface (e,2e) experiments
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ABSTRACT: This work presents the implementation of a position-sensitive detector in the Montelibretti surface (e,2e) apparatus that allows to fully define the direction of the ejected electron from the surface, and hence to improve the momentum resolution. The coincidence electronic chain and the acquisition software have been upgraded in order to achieve high data collection speed, storage of large dimension data files and fast visualisation of the results. Preliminary angle resolved (e,2e) data relative to the ionisation of the Å-1 band of highly oriented pyrolitic graphite show that a momentum resolution better than 0.2 Å-1 has been achieved without degrading the overall speed of acquisition.http://dx.doi.org/10.1051/jp4:1999632.
Top co-authors
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Institutions
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1999–2012
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Università Degli Studi Roma Tre
- Department of Mathematics and Physics
Roma, Latium, Italy
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2011
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University of Rome Tor Vergata
- Dipartimento di Fisica
Roma, Latium, Italy
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