Quantum Coherence of Image-Potential States

The quantum dynamics of the two-dimensional image-potential states in front of the Cu(100) surface is measured by scanning tunneling microscopy (STM) and spectroscopy (STS). The dispersion relation and the momentum resolved phase-relaxation time of the first image-potential state are determined from the quantum interference patterns in the local density of states (LDOS) at step edges. It is demonstrated that the tip-induced Stark shift does not affect the motion of the electrons parallel to the surface.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 figures; corrected typos, minor change

Scanning tunneling microscopy and kinetic Monte Carlo investigation of Cesium superlattices on Ag(111)

Cesium adsorption structures on Ag(111) were characterized in a low-temperature scanning tunneling microscopy experiment. At low coverages, atomic resolution of individual Cs atoms is occasionally suppressed in regions of an otherwise hexagonally ordered adsorbate film on terraces. Close to step edges Cs atoms appear as elongated protrusions along the step edge direction. At higher coverages, Cs superstructures with atomically resolved hexagonal lattices are observed. Kinetic Monte Carlo simulations model the observed adsorbate structures on a qualitative level.Comment: 8 pages, 7 figure

One-dimensional potential for image-potential states on graphene

In the framework of dielectric theory the static non-local self-energy of an electron near an ultra-thin polarizable layer has been calculated and applied to study binding energies of image-states near free-standing graphene. The corresponding series of eigenvalues and eigenfunctions have been obtained by solving numerically the one-dimensional Schr{\"o}dinger equation. Image-potential-state wave functions accumulate most of their probability outside the slab. We find that a Random Phase Approximation (RPA) for the non-local dielectric function yields a superior description for the potential inside the slab, but a simple Fermi-Thomas theory can be used to get a reasonable quasi-analytical approximation to the full RPA result that can be computed very economically. Binding energies of the image-potential states follow a pattern close to the Rydberg series for a perfect metal with the addition of intermediate states due to the added symmetry of the potential. The formalism only requires a minimal set of free parameters; the slab width and the electronic density. The theoretical calculations are compared to experimental results for work function and image-potential states obtained by two-photon photoemission.Comment: 24 pages; 10 figures. arXiv admin note: text overlap with arXiv:1301.448

Influence of the leaf position within the beech crown (Fagus sylvatica LINNÉ) on the infestation rate and developmental success of Rhynchaenus fagi LINNÉ (Col., Curculionidae)

Der Buchenspringrüssler Rhynchaenus fagi L. (Coleoptera: Curculionidae) ist ein 2 bis 3 mm großer, schwarz bis schwarzbraun gefärbter, fein grau behaarter Käfer. Ein besonderes Merkmal der Tiere sind die stark verdickten Hinterbeine, wodurch sie über das namensgebende große Sprungvermögen verfügen. Des Weiteren zeichnet er sich durch ein gutes Flugvermögen aus. R. fagi überwintert in der Bodenstreu bzw. in Rindenritzen. Die Imagines erscheinen im April/Mai und führen zunächst einen charakteristischen Lochfraß an Buchenblättern durch. Befressen werden dabei alle Altersstufen vom Sämling bis zum Altholz. Im Mai legen die Weibchen 30 bis 35 Eier einzeln in die Mittelrippe an der Unterseite von Blättern ab. Die Altkäfer sterben Anfang Juni. Die Larve erzeugt durch endophytischen Fraß zunächst eine Gangmine, welche sich in der Peripherie des Blattes zur Platzmine erweitert. Die Verpuppung findet nach 2- bis 3-wöchigem Fraß innerhalb der Platzmine in einem Kokon statt. Nach 10 Tagen bis 3 Wochen Puppenruhe schlüpfen ab Mitte Juni Jungkäfer, die bis zum Herbst an Blättern, Blattstielen und Fruchtansätzen der Buche fressen (SCHWERDTFEGER 1981). Vor allem bei Massenvermehrung verursachen Käfer und Larven durch ihren Fraß Zuwachsverluste sowie teilweise erhebliche Einbußen an der Bucheckernmast (ESCHERICH 1923). Neben Buche können sich adulte R. fagi auch von den Blättern zahlreicher anderer Baumarten (Erle, Birke, Obstbäume, etc.) ernähren, die Larvalentwicklung ist jedoch nur an Fagus-Arten möglich. Der Buchenspringrüssler ist im gesamten europäischen Raum verbreitet (SCHWENKE 1974). In Abhängigkeit von der Position in der Krone weisen Buchenblätter eine unterschiedliche Blattanatomie auf. Lichtblätter besitzen ein dickeres Schwammparenchym und insbesondere ein mehrschichtiges Palisadenparenchym. Letzteres ist in Schattenblätter einschichtig (SCHÜTT et al. 1992). Licht- undSchattenblätter unterscheiden sich auch in ihren Inhaltsstoffen. So konzentrieren sich aufgrund der höheren Photosyntheseleistung Kohlenhydrate und phenolische Substanzen in Blättern der Lichtkrone und vermindern sich tendenziell Stickstoffverbindungen (JOHNSON et al. 1997, YAMASAKI & KIKUZAWA 2003). Diese physiologischen Bedingungen können die Verwertbarkeit der Blätter für Insekten herabsetzen.The study investigated the effect of leaf position (sun or shade crown) on the infestation by Rhynchaenus fagi and the developmental success of the beetles. After hibernation adults of R. fagi prefer leaves of the shade crown for regeneration feeding and oviposition. In both crown positions, shade and sun crown, the egg mortality reached about 60%. The mean density of mines with successfully completed development amounted to 0.12 and 0.22 mines/leaf in sun and shade, respectively. Emerging adults of the next generation exhibited a pronounced feeding preference for leaves of the sun crown. Larvae developed in sun exposed leaves consumed more dry matter than conspecifics in shaded leaves and resulting pupae reached a significant higher weight. Light microscopy of semi-thin sections stained with toluidine blue and Lugol solution demonstrated the response of beech leaves to damage caused by mining larvae. Within a 100 – 150 μm range alongside the mine (1) cells of the palisade tissue and the mesophyll underwent cell divisions, (2) cells appeared very close to each other with a significantly reduced intercellular space, and (3) the disappearance of starch granula paralleled an increase in phenolic cell constituents

Direct resolution of unoccupied states in solids via two photon photoemission

Non-linear effects in photoemission are shown to open a new access to the band structure of unoccupied states in solids, totally different from hitherto used photoemission spectroscopy. Despite its second-order nature, strong resonant transitions occur, obeying exact selection rules of energy, crystal symmetry, and momentum. Ab-initio calculations are used to demonstrate that such structures are present in low-energy laser spectroscopy experimental measurements on Si previously published. Similar resonances are expected in ultraviolet angle-resolved photoemission spectra, as shown in a model calculation on Al.Comment: 12 pages, including 4 figure

Self-energy and lifetime of Shockley and image states on Cu(100) and Cu(111): Beyond the GW approximation of many-body theory

We report many-body calculations of the self-energy and lifetime of Shockley and image states on the (100) and (111) surfaces of Cu that go beyond the $GW$ approximation of many-body theory. The self-energy is computed in the framework of the GW\Gamma approximation by including short-range exchange-correlation (XC) effects both in the screened interaction W (beyond the random-phase approximation) and in the expansion of the self-energy in terms of W (beyond the GW approximation). Exchange-correlation effects are described within time-dependent density-functional theory from the knowledge of an adiabatic nonlocal XC kernel that goes beyond the local-density approximation.Comment: 8 pages, 5 figures, to appear in Phys. Rev.

Nonadiabatic dynamics of electron scattering from adsorbates in surface bands

We present a comparative study of nonadiabatic dynamics of electron scattering in quasi-two-dimensional surface band which is induced by the long-range component of the interactions with a random array of adsorbates. Using three complementary model descriptions of intraband spatiotemporal propagation of quasiparticles that go beyond the single-adsorbate scattering approach we are able to identify distinct subsequent regimes of evolution of an electron following its promotion into an unoccupied band state: (i) early quadratic or ballistic decay of the initial-state survival probability within the Heisenberg uncertainty window, (ii) preasymptotic exponential decay governed by the self-consistent Fermi golden rule scattering rate, and (iii) asymptotic decay described by a combined inverse power-law and logarithmic behavior. The developed models are applied to discuss the dynamics of intraband adsorbate-induced scattering of hot electrons excited into the n=1 image-potential band on Cu(100) surface during the first stage of a two-photon photoemission process. Estimates of crossovers between the distinct evolution regimes enable assessments of the lifespan of a standard quasiparticle behavior and thereby of the range of applicability of the widely used Fermi golden rule and optical Bloch equations approach for description of adsorbate-induced quasiparticle decay and dephasing in ultrafast experiments

Unoccupied Topological States on Bismuth Chalcogenides

The unoccupied part of the band structure of topological insulators Bi$_2$Te$_{x}$Se$_{3-x}$ ($x=0,2,3$) is studied by angle-resolved two-photon photoemission and density functional theory. For all surfaces linearly-dispersing surface states are found at the center of the surface Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical analysis shows that this feature appears in a spin-orbit-interaction induced and inverted local energy gap. This inversion is insensitive to variation of electronic and structural parameters in Bi$_2$Se$_3$ and Bi$_2$Te$_2$Se. In Bi$_2$Te$_3$ small structural variations can change the character of the local energy gap depending on which an unoccupied Dirac state does or does not exist. Circular dichroism measurements confirm the expected spin texture. From these findings we assign the observed state to an unoccupied topological surface state