2,236 research outputs found
Theoretical analysis of the electronic structure of the stable and metastable c(2x2) phases of Na on Al(001): Comparison with angle-resolved ultra-violet photoemission spectra
Using Kohn-Sham wave functions and their energy levels obtained by
density-functional-theory total-energy calculations, the electronic structure
of the two c(2x2) phases of Na on Al(001) are analysed; namely, the metastable
hollow-site structure formed when adsorption takes place at low temperature,
and the stable substitutional structure appearing when the substrate is heated
thereafter above ca. 180K or when adsorption takes place at room temperature
from the beginning. The experimentally obtained two-dimensional band structures
of the surface states or resonances are well reproduced by the calculations.
With the help of charge density maps it is found that in both phases, two
pronounced bands appear as the result of a characteristic coupling between the
valence-state band of a free c(2x2)-Na monolayer and the
surface-state/resonance band of the Al surfaces; that is, the clean (001)
surface for the metastable phase and the unstable, reconstructed "vacancy"
structure for the stable phase. The higher-lying band, being Na-derived,
remains metallic for the unstable phase, whereas it lies completely above the
Fermi level for the stable phase, leading to the formation of a
surface-state/resonance band-structure resembling the bulk band-structure of an
ionic crystal.Comment: 11 pages, 11 postscript figures, published in Phys. Rev. B 57, 15251
(1998). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Security bound of two-bases quantum key-distribution protocols using qudits
We investigate the security bounds of quantum cryptographic protocols using
-level systems. In particular, we focus on schemes that use two mutually
unbiased bases, thus extending the BB84 quantum key distribution scheme to
higher dimensions. Under the assumption of general coherent attacks, we derive
an analytic expression for the ultimate upper security bound of such quantum
cryptography schemes. This bound is well below the predictions of optimal
cloning machines. The possibility of extraction of a secret key beyond
entanglement distillation is discussed. In the case of qutrits we argue that
any eavesdropping strategy is equivalent to a symmetric one. For higher
dimensions such an equivalence is generally no longer valid.Comment: 12 pages, 2 figures, to appear in Phys. Rev.
O adsorption and incipient oxidation of the Mg(0001) surface
First principles density functional calculations are used to study the early
oxidation stages of the Mg(0001) surface for oxygen coverages 1/16 <= Theta <=
3 monolayers. It is found that at very low coverages O is incorporated below
the topmost Mg layer in tetrahedral sites. At higher oxygen-load the binding in
on-surface sites is increased but at one monolayer coverage the on-surface
binding is still about 60 meV weaker than for subsurface sites. The subsurface
octahedral sites are found to be unfavorable compared to subsurface tetrahedral
sites and to on-surface sites. At higher coverages oxygen adsorbs both under
the surface and up. Our calculations predict island formation and clustering of
incorporated and adsorbed oxygen in agreement with previous calculations. The
calculated configurations are compared with the angle-scanned x-ray
photoelectron diffraction experiment to determine the geometrical structure of
the oxidized Mg(0001) surface.Comment: 10 pages, 5 figure
The Physicist's Guide to the Orchestra
An experimental study of strings, woodwinds (organ pipe, flute, clarinet,
saxophone and recorder), and the voice was undertaken to illustrate the basic
principles of sound production in music instruments. The setup used is simple
and consists of common laboratory equipment. Although the canonical examples
(standing wave on a string, in an open and closed pipe) are easily reproduced,
they fail to explain the majority of the measurements. The reasons for these
deviations are outlined and discussed.Comment: 11 pages, 10 figures (jpg files). Submitted to European Journal of
Physic
Fullerene-based molecular nanobridges: A first-principles study
Building upon traditional quantum chemistry calculations, we have implemented
an {\em ab-initio} method to study the electrical transport in nanocontacts. We
illustrate our technique calculating the conductance of C molecules
connected in various ways to Al electrodes characterized at the atomic level.
Central to a correct estimate of the electrical current is a precise knowledge
of the local charge transfer between molecule and metal which, in turn,
guarantees the correct positioning of the Fermi level with respect to the
molecular orbitals. Contrary to our expectations, ballistic transport seems to
occur in this system.Comment: 4 pages in two-column forma
Quantum Statistics of Surface Plasmon Polaritons in Metallic Stripe Waveguides
Single surface plasmon polaritons are excited using photons generated via
spontaneous parametric down-conversion. The mean excitation rates, intensity
correlations and Fock state populations are studied. The observed dependence of
the second order coherence in our experiment is consistent with a linear
uncorrelated Markovian environment in the quantum regime. Our results provide
important information about the effect of loss for assessing the potential of
plasmonic waveguides for future nanophotonic circuitry in the quantum regime.Comment: 21 pages, 6 figures, published in Nano Letters, publication date
(web): March 27 (2012
A bank of unscented Kalman filters for multimodal human perception with mobile service robots
A new generation of mobile service robots could be ready soon to operate in human environments if they can robustly estimate position and identity of surrounding people. Researchers in this field face a number of challenging problems, among which sensor uncertainties and real-time constraints.
In this paper, we propose a novel and efficient solution for simultaneous tracking and recognition of people within the observation range of a mobile robot. Multisensor techniques for legs and face detection are fused in a robust probabilistic framework to height, clothes and face recognition algorithms. The system is based on an efficient bank of Unscented Kalman Filters that keeps a multi-hypothesis estimate of the person being tracked, including the case where the latter is unknown to the robot.
Several experiments with real mobile robots are presented to validate the proposed approach. They show that our solutions can improve the robot's perception and recognition of humans, providing a useful contribution for the future application of service robotics
Angle-scanned photoemission: Fermi surface mapping and structural determination
A brief survey of the angle-scanned photoemission technique is given. It incorporates two complementary methods in one:http://www.sciencedirect.com/science/article/B6TVX-3X82696-4F/1/e2ffd4efc660238ad8d4ad8f685991b
Chiral Biphenyldicarboxylic Acid Networks Stabilized by Hydrogen Bonding
The self-assembly of 4,4′-biphenyldicarboxylic acid on a Au(111) surface is investigated by means of in situ ultra-high-vacuum scanning tunneling microscopy. Three types of open networks with increasing interpore distance are observed. Experiments and theoretical calculations reveal that the two-dimensional porous networks are stabilized by various types of hydrogen bonds between carboxyl groups and by van der Waals interactions. Cyclic tetrameric and half-cyclic trimeric hydrogen-bonding motifs are shown to induce chirality in the open networks
Angle-scanned photoemission: Fermi surface mapping and structural determination
A brief survey of the angle-scanned photoemission technique is given. It incorporates two complementary methods in one:http://www.sciencedirect.com/science/article/B6TVX-3X82696-4F/1/e2ffd4efc660238ad8d4ad8f685991b
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