Papers

AVS2001 Session SS2-ThM: Electronic Structure II

Thursday, November 1, 2001 8:40 AM in Room 122

Thursday Morning

Start	Invited?	Item
8:40 AM		SS2-ThM-2 Fermi Surface Evolution of Ag(111) Films Grown onto Si(111) Surfaces V. Perez-Dieste (LURE, France); J.F. Sanchez-Royo (LURE and ICMUV, France); J. Avila (LURE and ICMM, France); M. Izquierdo, L. Roca, A. Tejeda (LURE, France); M.C. Asensio (LURE and ICMM, France) Growth of metal films on semiconductor substrates has been the subject of extensive experimental and theoretical studies over the last decades. The determination of the metallization onset at the semiconductor interfaces and the obtention of thin single-crystal metal films, only a few atomic layers thick with atomically flat surfaces, are important goals because of their consequences on the manufacture of integrated circuits and nanosized devices. In this work, we investigate the epitaxial growth of silver overlayers on reconstructed Si(111) surfaces studied by LEED and Photoelectron Diffraction (PhD). The electronic properties of these films have been investigated by high energy resolution Angle-Resolved Photoemission (ARPES) with a synchrotron radiation source. Particular attention has been paid to the determination, by ARPES, of the spectral weight at the Fermi level along large extensions of the reciprocal space of the investigated films, from which the Fermi surface (FS) can be extracted. The evolution of the FS and the valence-band structure as a function of the silver coverage could be measured at several metal coverages. In the submonolayer regime, very localized interface-derived spectral features dominate the density of states at the Fermi level, whereas in the intermediate regime, a complex mixture of states from both the interface and the metallic silver film defines the incipient FS contours. A well defined bulk-like silver FS could be identified already at interfaces of a few Ag monolayers. However, the symmetry of these bulk-like FS contours showed sixfold symmetry rather than the threefold symmetry, characteristic of a Ag(111) single crystal. By PhD, it has been demonstrated that is due to the existence of two domains rotated 60Âº silver metallic overlayers.
9:00 AM		SS2-ThM-3 Ultrathin Epitaxial Mg Films on Si(111): Quantum Size Effects L. Aballe, C. Rogero, K. Horn (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) We demonstrate, using angle-resolved photoelectron spectroscopy and LEED, that highly perfect ultrathin epitaxial Mg(0001) films can be grown on Si(111) using low temperature deposition and annealing. This is in contrast to films grown at room temperature which present an interfacial silicide and subsequent growth of a disordered Mg metallic film. The wave-vector dependent electronic structure of the well-ordered films is investigated in detail with photoelectron spectroscopy, as a function of overlayer thickness. The spectra exhibit a number of thickness-dependent discrete peaks in the region of the magnesium s-p band for films up to 40 monolayers thick. These are caused by electron confinement within the Mg overlayer, and can be identified as quantum well resonances derived from the magnesium s-p band. These quantum well resonances (QWR) are interpreted in terms of the phase-accumulation model, and the Mg band structure is found to account for all the main features in the spectra. An estimation of the decay length of the Mg(0001) surface state wave function is obtained from its dependence of binding energy on film thickness. The in-plane dispersion of the QWR for films of different thicknesses is measured and analyzed along the surface Brillouin zone. The data point to a strain-driven thickness-dependent structural transition at a critical thickness of about 20 Mg monolayers. The dependence of spectral intensity on photon energy in the range of the Mg bulk and multipole plasmon energies demonstrates the effect of field enhancement in the Mg film.
9:20 AM		SS2-ThM-4 Resonant Photoemission Investigation of the Electronic Structure of Plutonium J.G. Tobin, D.A. Arena (Lawrence Livermore National Laboratory); J. Terry, R.K. Schulze, J.D. Farr, T. Zocco (Los Alamos National Laboratory); K. Heinzelman, E. Rotenberg, D. Shuh (Lawrence Berkeley National Laboratory); G. van der Laan (Daresbury Lab, UK) The valence electronic structures of the actinide metals and alloys in general and plutonium (Pu) in particular remain mired in controversy. Interestingly, the various phases of Pu metal provide a mirocosm of the metallic actinides as a whole. Thus, unravelling the nuances of the interplay of electronic and geometric structures in Pu will illuminate the properties of all transuranic metals. In a sense, the behavior of the Pu 5f electrons is completely counter-intuitive. The dense phase, a, has some semblance of delocalization in the 5f valence bands and can be treated theoretically within single electron models such as the Local Density Approximation (LDA). The a phase is monoclinic, which is a low symmetry ordering. The less dense d-phase is fcc and exhibits evidence of localized and/or correlated electronic behavior. Experimental Resonant Photoemission (ResPes) results for a- Pu and d- Pu bulk samples will be presented and compared to the results of an atomic model calculation. Both Pu samples exhibit limited agreement with the atomic model calculations. As expected, a- Pu appears to have more 5f valence band delocalization than d-Pu. Evidence of an enhanced sensitivity to surface corruption, by using synchrotron radiation as the excitation, will be presented. This work was performed under the auspices of the U.S Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. Los Alamos National Laboratory is operated by the University of California under Contract No. W-7405- ENG-36. Experiments were carried out at the Spectromicroscopy Facility (Beamline 7.0) at the Advanced Light Source, built and supported by the U.S. Department of Energy. The Advanced Light Source and DKS are supported by the Director, Office of Science, Office of Basic Energy Sciences, Matl. and Chem. Sciences Divisions, of the U.S. DOE under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory.
9:40 AM		SS2-ThM-5 Visualization of the Electronic Structure of Metal Surfaces with Scanning Tunneling Spectroscopy J.I. Pascual (Fritz-Haber-Institut der Max-Planck Gesellschaft, Germany); Z. Song (Dalian Institute of Chemical Physics. China); J.J. Jackiw (Pennsylvania State University); M. Hansmann, G. Ceballos, H. Conrad, K. Horn, H.-P. Rust (Fritz-Haber-Institut der Max-Planck Gesellschaft, Germany) In this presentation we analyze the electronic structure of several metal surfaces with an in-creasing level of complexity: from the (111) sur-faces of noble metals, with isotropic s-p states, to anisotropic alloy surfaces like NiAl(110), where the influence of Ni d-states dominates its elec-tronic structure in the proximity of the Fermi level. The measurements are done in a low temperature scanning tunneling microscope, where the energy resolution and stability are greatly improved. The spatial dependence of the differential tunnel con-ductance in the proximity of scattering potentials like defect sites or step edges offers a direct way to access structural information on the electronic states of the surface. There, the electron wave phase is fixed at the defect position, producing oscillations in the spatial shape of the density of states in the defect vicinity. The wavelength of these oscillations in real space can be easily transformed to reciprocal space information by means of a Fourier transformation.¹ We are going to show the capabilities of this transformation by analyzing surface states on both, the isotropic and anisotropic surface states. Band gap edges also may produce oscillations in the density of states, and therefore, are also ac-cessible. By measuring the differential conduc-tance in a large range of energies we reconstruct the states’ topology in the reciprocal space. The energy range is not limited to the proximity of the Fermi energy: we probe states up to the vacuum level. Above this point we also resolve information about the surface density of states by analyzing the shape of the field emission resonances. . ¹L. Petersen et al. Phys. Rev. B 57, R5868 (1998).
10:00 AM		SS2-ThM-6 Temperature-Dependent Fermi Gap Opening in the c(6X4)-C₆₀/Ag(100) Two-Dimensional Superstructure M. Sancrotti, C. Cepek, I. Vobornik (Laboratorio Nazionale TASC-INFM, Italy); A. Goldoni (Sincrotrone Trieste, Italy); E. Magnano (Laboratorio Nazionale TASC-INFM, Italy); G. Selvaggi (Universita' di Modena, Italy); J. Kröger (Zürich Universität, Switzerland); G. Panaccione, G. Rossi (Laboratorio Nazionale TASC-INFM, Italy) The interest in fullerene-based films, their surfaces, and related low-dimensional systems has been recently renewed for example by the discovery that superconductivity persists at surfaces of A₃C₆₀ films¹ and by the possibility of achieving a critical temperature as high as 52 K in hole-doped C₆₀ single crystals.² The possibility of controlling at a fine scale the charge state of single C₆₀ molecules and the buckyball-buckyball distance makes the fullerene-based films extremely charming for a wide range of applications. In addition, chemical and physical properties of low-dimensional C₆₀-based layers may be considered superior to the bulk materials, prepared by means of standard intercalation methods and plagued by the presence of multi-phases. Here we report on a high-resolution angle integrated photoemission study³ of one monolayer of C₆₀ chemisorbed on Ag(100). The results show the reversible opening of a gap at the Fermi level at temperatures 25 ≤ T ≤ 300 K. The gap reaches a maximum value of 10 meV at T ≤ 70 K. This finding is a first evidence of an electronic phase transition in C₆₀ monolayers and has implications on the ongoing debate about surface superconductivity in C₆₀-based bulk materials. ¹ ¹ R. Hesper, L.H. Tjeng, A. Heeres, and G.A. Sawatzky, Phys. Rev. Lett. 85 (2000) 1970 ² J.H. Schön, Ch. Kloc, R.C. Haddon, and B. Batlogg, Science 288 (2000) 656 ³ C. Cepek, I. Vobornik, A. Goldoni, E. Magnano, G. Selvaggi, J. Kröger, G. Panaccione, G. Rossi, and M. Sancrotti, Phys. Rev. Lett. 86 (2001) 3100
10:20 AM		SS2-ThM-7 Electronic Transport through Surface-state Bands S. Hasegawa (University of Tokyo, Japan) Surface states that are inherent in surface superstructures, provide unique platforms for studying low-dimensional electron systems, exhibiting some aspects of many-body effects, in phase transitions, for example. Electronic transport is a key for such physics, too. First, I present direct detections of electrical transport through the surface-state bands on silicon, by in-situ measurements in UHV with macroscopic four-point probes,¹ microscopic ones,^2,3 and four-tip STM.⁴ The influences of atomic steps and domain boundaries on the conductivity were directly measured. Next, I present characteristic changes in surface conductivity at a surface phase transition.⁵ A Si(111)-8x2-In surface at 100K is believed to be a charge-density-wave (CDW) phase.⁶ By adding small amounts of impurity atoms on it, the CDW phase was destroyed, accompanied with steep increases in conductivity. This means that the metalicity of the surface is recovered by this change; the impurity atoms act as electronic disturbers. ¹ S. Hasegawa, et al., Prog. Surf. Scie. 60 (1999) 89. ² I. Shiraki, et al., Surf. Rev. Lett. 7 (2000) 533. ³ C. L. Petersen, et al., Appl. Phys. Lett. 77 (2000) 3782. ⁴ I. Shiraki, et al., Surf. Scie. (2001) in press. ⁵ S. V. Ryjkov, et al., Surf. Scie (2001) in press. ⁶ H.-W. Yeom, et al., Phys. Rev. Lett. 82 (1999) 4898.
11:00 AM		SS2-ThM-9 Electronic Structure of the Alkali Halide-metal Interfaces: LiCl(100)/ Cu(100) M. Kiguchi, H. Inoue, K. Saiki, A. Koma (The University of Tokyo, Japan) When an insulator film is prepared in the close vicinity of a metal surface, a novel phase is formed whose property may differ from the bulk one. It is suggested that the band gap is reduced for an insulating thin film on a metal substrate, due to the presence of the dielectric boundary and the overlayer reduced dimensionally. However, the large difference of chemical bond between metals and insulators makes it difficult to form a well ordered interface. The electronic structure of the insulator-metal interface has been little studied. Recently, we have succeeded in growing a single-crystalline LiCl film on Cu(100) in a layer-by-layer fashion. In the present work, we have studied the electronic structure of LiCl film on Cu(100) using EELS (electron energy loss spectroscopy) and UPS (ultraviolet photoelectron spectroscopy), as a model system of alkali halide-metal interface. The EEL spectrum(Ep=60 eV) shows clear band gap region from 0 eV to 7 eV. The band gap energy did not change for the LiCl thickness from 1 ML to 20 ML. In addition, the 61 eV loss peak observed in the EELS (Ep=200eV) did not change with the thickness. The result of EELS indicates that relative position of the conduction band to the valence band and to the Li 1s core band was unchanged. In the result of UPS, on the other hand, the valence band showed upward shift with decreasing film thickness. The results of EELS and UPS suggest the occurrence of the bend bending in ionic layers.
11:20 AM		SS2-ThM-10 Theoretical Analysis of Field Emission from Atomically Sharp Al Tips Y. Gohda, S. Watanabe (The University of Tokyo, and CREST, Japan Science and Technology Corp.) Well-conditioned tips, which end with a single atom, are of current interest, because they produce self-collimated, coherent electron beams, total energy distribution (TED) of which can have multiple peaks (MP).^1,2 However, the origin of the MP in TED has been controversial and thus has not been understood well: Binh et al. claimed that they had observed MP from pure W(111) surface with single-atom protrusion,¹ while Yu et al. claimed that carburization was essential in observing MP from metal-carbide protrusions.² In the present work, field emission from Al(100) surface having single-atom protrusion without any impurity is analyzed employing the method newly developed by Gohda et al.,³ which is based on the self-consistent density functional theory including scattering states. We have found that MP in TED of field emission current becomes remarkable as the local potential barrier in front of the topmost Al atom dissapears with increase in applied field strength. We have also clarified that the peak below the Fermi energy is attributed to localized sates at the topmost Al atom, while the peak at the Fermi energy comes from delocalized states. ¹ V. T. Binh et al., Phys. Rev. Lett. 69, 2527 (1992). ² M. L. Yu et al., Phys. Rev. Lett. 77, 1636 (1996). ³ Y. Gohda et al., Phys. Rev. Lett. 85, 1750 (2000).