No abstract available

No abstract available

[en] The growth morphology and the electronic structures of thin metastable and epitaxial Ag films grown on Si(001)2x1 and Si(111)7x7 surfaces at low temperature were investigated by scanning tunneling microscopy and angle-resolved photoelectron spectroscopy using synchrotron radiation. The morphology of Ag films on Si(001)2x1 exhibits a strong thickness and temperature dependence, indicating an intriguing growth mechanism. The as-deposited film at ∼100 K is composed of nanoclusters with flat tops in a uniform quasi-layer-by-layer film at 2-3 ML and of homogeneous clusters having a more three-dimensional (3D) character above ∼5 ML. By subsequent annealing at 300-450 K, flat epitaxial Ag(111) films are formed at a nominal coverage larger than 5 ML, while a percolating network of 2D islands is formed at a lower coverage. For optimally annealed epitaxial films, discrete Ag 5s states are observed at binding energies of 0.3-3 eV, together with the Ag(111) surface state. The discrete electronic states are consistently interpreted by a standard description of the quantum-well states (QWSs) based on phase-shift quantization rules. The phase shift, the energy dispersion and the thickness-versus-energy relation of the QWSs of epitaxial Ag(111) films are consistently derived. The in-plane band structures of the QWSs of epitaxial Ag films grown on Si(111)7x7 and Si(001)2x1 surfaces were investigated in detail. In contrast to the expected free-electron-like behavior, the QWSs show intriguing in-plane dispersions, such as (i) a significant enhancement of the in-plane dispersion with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-normal emission. Such unexpected electronic properties of a QWS are obviously related to the substrate band structure. Further, the QWS splitting is explained by the energy-dependent phase shift of the film-substrate interface occurring at the substrate band edge

[en] Wire-type metals in nano scale are essential for nano/molecular electronics and their fundamental properties are challenging with various exotic ground states and fluctuations. As an unconventional form of such 1D metallic systems, we have investigated the self-organized metallic atomic wires on flat and vicinal Si surfaces such as In/Si(111), Au/Si(111), Au/Si(557), and Au/Si(553), and Pb/Si(557). For some of these systems, we observed Peierls-type phase transitions due to the 1D bands nested fully with electron fillings of 1/2 or 1/3. In the present talk, I review the achievements so far and the present debates on these phase transitions. A few issues related to the transitions are introduced such as (i) the impurity control over the T_c, the band gap, and the solitonic dynamic fluctuation, and (ii) the atomic-scale characterization of the embryonic charge-density wave order. I also raise the question of why some of these systems like Au/Si(111) and Pb/Si(557) are robust against Peierls instability down to fairly low temperature

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[en] In order to satisfy demands to study the electronic structure of quantum nanostructures, a VUV beamline and a high-resolution and high-throughput photoemission end-station combined with a molecular beam epitaxy (MBE) system have been constructed at the BL-1C of the Photon Factory. An angle-resolved photoemission spectrometer, having high energy- and angular-resolutions; VG Microtech ARUPS10, was installed. The total energy resolution of 31 meV at the 60 eV of photon energy is achieved. For the automated angle-scanning photoemission, the electron spectrometer mounted on a two-axis goniometer can be rotated in vacuum by the computer-controlled stepping motors. Another distinctive feature of this end-station is a connection to a MBE chamber in ultahigh vacuum (UHV). In this system, MBE-grown samples can be transferred into the photoemission chamber without breaking UHV. Photoemission spectra of MBE-grown GaAs(0 0 1) surfaces were measured with high-resolution and bulk and surface components are clearly resolved

[en] Pb-induced superstructures on Si(557) are investigated by low-energy-electron diffraction (LEED) and scanning tunneling microscopy (STM). Using an indirect heating method, we have succeeded in obtaining almost perfect single-domain LEED patterns of one-dimensional wire (chain) structures, so called αx2 and βx2 phases. Careful LEED analysis and STM investigation reveal that these phases are formed on the (223) and (112) facets, respectively. The αx2 phase has regular bundles of triple wires at low annealing temperature but wider bundles through step bunching after a higher temperature annealing. Along the wires of the αx2 phase, which was recently reported to exhibit a transition between one-dimensional (1D) metallic and 2D semiconducting conductance, a clear commensurate x2 modulation is observed at 78-120 K in contrast to the incommensurate and disordered structure reported previously. A tentative atomic structure model of the αx2 phase is proposed based on the dense Pb overlayers on (111) and (223) facets. The details of the STM images of the βx2 phase are discussed

[en] The growth morphology and the electronic structures of thin metastable Ag films grown on Si(001)2 x 1 surfaces at low temperatures are investigated by scanning tunneling microscopy and angle-resolved photoemission spectroscopy using synchrotron radiation. The morphology of Ag films exhibits a strong thickness and substrate temperature dependence indicating an intriguing growth mechanism. At a nominal coverage larger than 5 ML, the as-deposited film is composed of homogeneous clusters having 3-dimensional character at the substrate temperatures of ∼100K and of flat epitaxial Ag(111) films by a subsequent annealing at 300-450K. Discrete Ag 5 s states are observed at binding energies of 0.3-3 eV together with the surface state. The discrete electronic states can be interpreted in terms of the quantum-well states (QWS) based on the phase-shift quantization. The phase shift, the energy dispersion and the thickness-versus-energy relation (Structure Plot) of the QWS are consistently derived. On the other hand, for the in-plane dispersion, in contrast to the free-electron-like behavior expected, these QWS show (1) a significant enhancement of the inplane effective mass with decreasing binding energy and (2) a splitting of a QWS into two electronic states with different dispersions at off-Γ-bar point. Such unexpected electronic properties of QWS are investigated in detail and found obviously related to the semiconductor substrate band structure. Furthermore, the QWS splitting is explained in terms of the energy dependence of phase shift at film-substrate interface occurring at the substrate band edge. (author)

[en] The electronic structures and the thermal reaction of chemisorbed C₂H₂ and C₂H₄ on the Si(001)2x1 surface have been investigated by carbon K-edge near-edge x-ray absorption fine structure (NEXAFS) and ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation. The bonding and antibonding states due to the interaction of the molecules and the Si surface atoms are identified by detailed polarization-dependent UPS and NEXAFS measurements, respectively. These bonding and antibonding states are shown to originate from the hybridization between the occupied Si dangling bonds and the lowest unoccupied molecular orbitals (π_C-C^*) of C₂H₂ and C₂H₄ double-σ-bonded on the top of the Si dimer. The thermal evolution of mainly C₂H₂ is investigated in detail for a wide temperature range of 60-1500 K from the condensation to the surface alloy formation. The coexistence of the physisorbatelike and the chemisorbed molecular species is observed at 70-90 K for C₂H₂ and C₂H₄, for the coverages greater than ∼0.25 monolayer (ML). The π_C-C^* resonance of those physisorbatelike C₂H₄ species in NEXAFS exhibits an unusual polarization dependence indicating adsorption with their molecular planes aligned perpendicular to the surface. The dissociation of C₂H₂ chemisorbates is shown to occur at 600-700 K as observed by UPS. After the dissociation of molecules, the atomic hydrogen adsorbates are identified by the monohydridelike surface resonance states in the UP spectra at 800-950 K. Most of the Si dangling bonds are passivated by, at least partly, the hydrogen adsorbates at this stage. At ∼1000 K, the desorption of hydrogen occurs, which accompanies the appearance of a broad SiC-like feature in the UP spectra at ∼3 eV below Fermi level

[en] The geometric symmetry of the surface plays an important role for the spin–orbit-induced spin texture of two-dimensional electronic states. This article reviews the peculiar Rashba spins induced by a C_3 symmetry, including the completely spin polarized surface states with the polarization vector oriented perpendicular to the surface, i.e. a direction that is not expected in a typical Rashba system. This review also describes that this peculiar Rashba situation has possibility to suppress backscattering and therefore to greatly improve the efficiency of spin transport, which is an essential issue in the development of high-performance semiconductor spintronic devices