2,029 research outputs found
Ab initio study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: diameter- and registration-dependent atomic configurations and electronic properties
We present a first-principles study of semiconducting carbon nanotubes
adsorbed on the unpassivated Si(100) surface. We have found metallicity for the
combined system caused by n-doping of the silicon slab representing the surface
by the SWNT. We confirm this metallicity for nanotubes of different diameters
and chiral angles, and find the effect to be independent of the orientation of
the nanotubes on the surface. We also present adsorption energetics and
configurations which show semiconducting SWNTs farther apart from the surface
and transferring less charge, in comparison with metallic SWNTs of similar
diameter.Comment: Replaces old (Jan 2006) version; more supporting material. 11 pages,
8 figures, 7 table
Carbon nanotubes on partially depassivated n-doped Si(100)-(2x1):H substrates
We present a study on the mechanical configuration and the electronic
properties of semiconducting carbon nanotubes supported by partially
depassivated silicon substrates, as inferred from topographic and spectroscopic
data acquired with a room-temperature ultrahigh vacuum scanning tunneling
microscope and density-functional theory calculations. A mechanical distortion
and doping for semiconducting carbon nanotubes on Si(100)-(2x1):H with
hydrogen-depassivated stripes up to 100 Angstrom wide are ascertained from both
experiment and theory. The results presented here point towards novel and local
functionalities of nanotube-semiconductor interfaces.Comment: 5 pages, 3 Figures, 1 Table. Accepted for publication in Physical
Review
A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams
The following article appeared in Applied Physics Letters Volume 99, Issue 21, 21 November 2011, Article number 211907 and may be found at http://scitation.aip.org/content/aip/journal/apl/99/21/10.1063/1.3663866We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H 2 beams (23 and 7 of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopyWork performed in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which issupported by the U.S. Department of Energy, Office of Basic
Energy Sciences, under Contract No. DE-AC02-98CH10886. Work supported by the Ministerio de Educacion y Ciencia through projects CONSOLIDER-INGENIO 2010 on Molecular Nanoscience (CSD 2007-00010), FIS2010-18847, and byComunidad de Madrid through the program NANOBIOMAGNET (S2009/MAT-1726)
Die Bedeutung einer Ausfallbedrohtheit von Versicherungskontrakten - ein Beitrag zur Behavioral Insurance
Kahneman/Tversky 1979 haben das theoretische Konstrukt der Probabilistic Insurance Kontrakte in die Literatur eingefĂŒhrt. Hiermit werden VersicherungsvertrĂ€ge bezeichnet, deren ErfĂŒllung im Leistungsfalle aufgrund einer möglichen Insolvenz des Versicherungsunternehmens nicht gewĂ€hrleistet ist. In Ausweitung einer Studie von Wakker/Thaler/Tversky 1997 wird in der vorliegenden Arbeit eine experimentelle Untersuchung durchgefĂŒhrt, wobei die Zahlungsbereitschaft potentieller Versicherungsnehmer in AbhĂ€ngigkeit des Ratings des den Versicherungskontrakt anbietenden Unternehmens festgestellt wird. Dabei zeigt sich, daĂ diese ausfallbedrohte Versicherungsprodukte relativ zu ausfallfreien VertrĂ€gen mit erheblichen PrĂ€mienabschlĂ€gen sanktionieren. Der Preisabschlag nimmt dabei mit sinkender UnternehmensbonitĂ€t (erhöhter Ausfallgefahr) zu. Die Befragungsergebnisse zeigen zudem das neuartige PhĂ€nomen, daĂ mit zunehmender Ausfallbedrohtheit immer weniger Personen bereit sind, ausfallbedrohte Versicherungsprodukte ĂŒberhaupt zu akzeptieren. SchlieĂlich werden SchluĂfolgerungen fĂŒr die Steuerung von Versicherungsunternehmen diskutiert
Ab initio Green's function formalism for band structures
Using the Green's function formalism, an ab initio theory for band structures
of crystals is derived starting from the Hartree-Fock approximation. It is
based on the algebraic diagrammatic construction scheme for the self-energy
which is formulated for crystal orbitals (CO-ADC). In this approach, the poles
of the Green's function are determined by solving a suitable Hermitian
eigenvalue problem. The method is not only applicable to the outer valence and
conduction bands, it is also stable for inner valence bands where strong
electron correlations are effective. The key to the proposed scheme is to
evaluate the self-energy in terms of Wannier orbitals before transforming it to
a crystal momentum representation. Exploiting the fact that electron
correlations are mainly local, one can truncate the lattice summations by an
appropriate configuration selection scheme. This yields a flat configuration
space; i.e., its size scales only linearly with the number of atoms per unit
cell for large systems and, under certain conditions, the computational effort
to determine band structures also scales linearly. As a first application of
the new formalism, a lithium fluoride crystal has been chosen. A minimal basis
set description is studied, and a satisfactory agreement with previous
theoretical and experimental results for the fundamental band gap and the width
of the F 2p valence band complex is obtained.Comment: 20 pages, 3 figures, 1 table, RevTeX4, new section on lithium
fluorid
Evidence for electronically-driven ferroelectricity in the family of strongly correlated dimerized BEDT-TTF molecular conductors
By applying measurements of the dielectric constants and relative length
changes to the dimerized molecular conductor
-(BEDT-TTF)Hg(SCN)Cl, we provide evidence for order-disorder
type electronic ferroelectricity which is driven by charge order within the
(BEDT-TTF) dimers and stabilized by a coupling to the anions. According to
our density functional theory calculations, this material is characterized by a
moderate strength of dimerization. This system thus bridges the gap between
strongly dimerized materials, often approximated as dimer-Mott systems at 1/2
filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge
order. Our results indicate that intra-dimer charge degrees of freedom are of
particular importance in correlated -(BEDT-TTF)X salts and can
create novel states, such as electronically-driven multiferroicity or
charge-order-induced quasi-1D spin liquids.Comment: 6 pages, 4 figures + Supplementary Information (8 pages, 8 figures
1S and MSbar Bottom Quark Masses from Upsilon Sum Rules
The bottom quark 1S mass, , is determined using sum rules which
relate the masses and the electronic decay widths of the mesons to
moments of the vacuum polarization function. The 1S mass is defined as half the
perturbative mass of a fictitious bottom-antibottom quark bound
state, and is free of the ambiguity of order which plagues the
pole mass definition. Compared to an earlier analysis by the same author, which
had been carried out in the pole mass scheme, the 1S mass scheme leads to a
much better behaved perturbative series of the moments, smaller uncertainties
in the mass extraction and to a reduced correlation of the mass and the strong
coupling. We arrive at GeV taking
as an input. From that we determine the
mass as GeV. The error in can be reduced if the three-loop corrections to the relation of
pole and mass are known and if the error in the strong coupling is
decreased.Comment: 20 pages, latex; numbers in Tabs. 2,3,4 corrected, a reference and a
comment on the fitting procedure added, typos in Eqs. 2 and 23 eliminate
Dilatonic current-carrying cosmic strings
We investigate the nature of ordinary cosmic vortices in some scalar-tensor
extensions of gravity. We find solutions for which the dilaton field condenses
inside the vortex core. These solutions can be interpreted as raising the
degeneracy between the eigenvalues of the effective stress-energy tensor,
namely the energy per unit length U and the tension T, by picking a privileged
spacelike or timelike coordinate direction; in the latter case, a phase
frequency threshold occurs that is similar to what is found in ordinary neutral
current-carrying cosmic strings. We find that the dilaton contribution for the
equation of state, once averaged along the string worldsheet, vanishes, leading
to an effective Nambu-Goto behavior of such a string network in cosmology, i.e.
on very large scales. It is found also that on small scales, the energy per
unit length and tension depend on the string internal coordinates in such a way
as to permit the existence of centrifugally supported equilibrium
configuration, also known as vortons, whose stability, depending on the very
short distance (unknown) physics, can lead to catastrophic consequences on the
evolution of the Universe.Comment: 10 pages, ReVTeX, 2 figures, minor typos corrected. This version to
appear in Phys. Rev.
Another Source of Baryons in Meson Decays
It is usually assumed that the production of baryons in meson decays is
induced primarily by the quark level process , where the charm
quark hadronizes into a charmed baryon. With this assumption, the
momentum spectrum would indicate that the transition is
dominated by multi-body decays. However, a closer examination of the
momentum spectrum reveals that the mass against which the is
recoiling almost always satisfies m_X\agt m_{\Xi_c}. This fact leads us to
examine the hypothesis that the production of charmed baryons in decays is
in fact dominated by the underlying transition , and is seen
primarily in modes with two charmed baryons in the final state. We propose a
number of tests of this hypothesis. If this mechanism is indeed important in
baryon production, then there are interesting consequences and applications,
including potentially important implications for the ``charm deficit'' in
decays.Comment: 9 pages, 3 uuencoded figures included, uses REVTeX, preprint nos.
CALT-68-1935, FERMILAB-PUB-94-132-T, JHU-TIPAC-940006, UCSD/PTH 94-0
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