2,990 research outputs found
Coherent field emission image of graphene predicted with a microscopic theory
Electrons in the mono-layer atomic sheet of graphene have a long coherence
length of the order of micrometers. We will show that this coherence is
transmitted into the vacuum via electric field assisted electron emission from
the graphene edge. The emission current density is given analytically. The
parity of the carbon pi-electrons leads to an image whose center is dark as a
result of interference. A dragonfly pattern with a dark body perpendicular to
the edge is predicted for the armchair edge whose emission current density is
vanishing with the mixing angle of the pseudo-spin. The interference pattern
may be observed up to temperatures of thousand Kelvin as evidence of coherent
field emission. Moreover, this phenomenon leads to a novel coherent electron
line source that can produce interference patterns of extended objects with
linear sizes comparable to the length of the graphene edge.Comment: 6 pages, 3 figure
Physisorption of an electron in deep surface potentials off a dielectric surface
We study phonon-mediated adsorption and desorption of an electron at
dielectric surfaces with deep polarization-induced surface potentials where
multi-phonon transitions are responsible for electron energy relaxation.
Focusing on multi-phonon processes due to the nonlinearity of the coupling
between the external electron and the acoustic bulk phonon triggering the
transitions between surface states, we calculate electron desorption times for
graphite, MgO, CaO, (\text{Al}_2\text{O}_3), and (\text{SiO}_2) and electron
sticking coefficients for (\text{Al}_2\text{O}_3), CaO, and (\text{SiO}_2). To
reveal the kinetic stages of electron physisorption, we moreover study the time
evolution of the image state occupancy and the energy-resolved desorption flux.
Depending on the potential depth and the surface temperature we identify two
generic scenarios: (i)adsorption via trapping in shallow image states followed
by relaxation to the lowest image state and desorption from that state via a
cascade through the second strongly bound image state in not too deep
potentials and (ii)adsorption via trapping in shallow image states but followed
by a relaxation bottleneck retarding the transition to the lowest image state
and desorption from that state via a one step process to the continuum in deep
potentials.Comment: 12 pages, 7 figure
Homalg: A meta-package for homological algebra
The central notion of this work is that of a functor between categories of
finitely presented modules over so-called computable rings, i.e. rings R where
one can algorithmically solve inhomogeneous linear equations with coefficients
in R. The paper describes a way allowing one to realize such functors, e.g.
Hom, tensor product, Ext, Tor, as a mathematical object in a computer algebra
system. Once this is achieved, one can compose and derive functors and even
iterate this process without the need of any specific knowledge of these
functors. These ideas are realized in the ring independent package homalg. It
is designed to extend any computer algebra software implementing the
arithmetics of a computable ring R, as soon as the latter contains algorithms
to solve inhomogeneous linear equations with coefficients in R. Beside
explaining how this suffices, the paper describes the nature of the extensions
provided by homalg.Comment: clarified some points, added references and more interesting example
Toric Construction of Global F-Theory GUTs
We systematically construct a large number of compact Calabi-Yau fourfolds
which are suitable for F-theory model building. These elliptically fibered
Calabi-Yaus are complete intersections of two hypersurfaces in a six
dimensional ambient space. We first construct three-dimensional base manifolds
that are hypersurfaces in a toric ambient space. We search for divisors which
can support an F-theory GUT. The fourfolds are obtained as elliptic fibrations
over these base manifolds. We find that elementary conditions which are
motivated by F-theory GUTs lead to strong constraints on the geometry, which
significantly reduce the number of suitable models. The complete database of
models is available at http://hep.itp.tuwien.ac.at/f-theory/. We work out
several examples in more detail.Comment: 35 pages, references adde
Excitation Enhancement of a Quantum Dot Coupled to a Plasmonic Antenna
Plasmonic antennas are key elements to control the luminescence of quantum
emitters. However, the antenna's influence is often hidden by quenching losses.
Here, the luminescence of a quantum dot coupled to a gold dimer antenna is
investigated. Detailed analysis of the multiply excited states quantifies the
antenna's influence on the excitation intensity and the luminescence quantum
yield separately
Nonequilibrium fluctuation-dissipation relations for one- and two-particle correlation functions in steady-state quantum transport
We study the non-equilibrium (NE) fluctuation-dissipation (FD) relations in
the context of quantum thermoelectric transport through a two-terminal
nanodevice in the steady-state. The FD relations for the one- and two-particle
correlation functions are derived for a model of the central region consisting
of a single electron level. Explicit expressions for the FD relations of the
Green's functions (one-particle correlations) are provided. The FD relations
for the current-current and charge-charge (two-particle) correlations are
calculated numerically. We use self-consistent NE Green's functions
calculations to treat the system in the absence and in the presence of
interaction (electron-phonon) in the central region. We show that, for this
model, there is no single universal FD theorem for the NE steady state. There
are different FD relations for each different class of problems. We find that
the FD relations for the one-particle correlation function are strongly
dependent on both the NE conditions and the interactions, while the FD
relations of the current-current correlation function are much less dependent
on the interaction. The latter property suggests interesting applications for
single-molecule and other nanoscale transport experiments.Comment: This revised version is now accepted for publication in the Journal
of Chemical Physics (March 2014). arXiv admin note: text overlap with
arXiv:1305.507
Thermal field desorption spectroscopy of chemisorbed hydrogen for a single step site
The steady state molecular hydrogen ion yield from a single atomic step site of a [110]-oriented tungsten and of a [100]-oriented rhodium crystal is determined as a function of surface temperature using mass and energy resolved probe hole field ion microscopy. A second order kinetic model is developed to fit the experimental data thus obtaining the hydrogen binding energy. For local fields of about 3 V/Ă… the data are close to values obtained from thermal desorption spectroscopy. A comparison is made with calculations of the field-adsorption binding energy of atomic hydrogen on a jellium surface based on density functional theory
Three particles in a finite volume: The breakdown of spherical symmetry
Lattice simulations of light nuclei necessarily take place in finite volumes,
thus affecting their infrared properties. These effects can be addressed in a
model-independent manner using Effective Field Theories. We study the model
case of three identical bosons (mass m) with resonant two-body interactions in
a cubic box with periodic boundary conditions, which can also be generalized to
the three-nucleon system in a straightforward manner. Our results allow for the
removal of finite volume effects from lattice results as well as the
determination of infinite volume scattering parameters from the volume
dependence of the spectrum. We study the volume dependence of several states
below the break-up threshold, spanning one order of magnitude in the binding
energy in the infinite volume, for box side lengths L between the two-body
scattering length a and L = 0.25a. For example, a state with a three-body
energy of -3/(ma^2) in the infinite volume has been shifted to -10/(ma^2) at L
= a. Special emphasis is put on the consequences of the breakdown of spherical
symmetry and several ways to perturbatively treat the ensuing partial wave
admixtures. We find their contributions to be on the sub-percent level compared
to the strong volume dependence of the S-wave component. For shallow bound
states, we find a transition to boson-diboson scattering behavior when
decreasing the size of the finite volume.Comment: 21 pages, 4 figures, 2 table
Surface states and the charge of a dust particle in a plasma
We investigate electron and ion surface states of a negatively charged dust
particle in a gas discharge and identify the charge of the particle with the
electron surface density bound in the polarization-induced short-range part of
the particle potential. On that scale, ions do not affect the charge. They are
trapped in the shallow states of the Coulomb tail of the potential and act only
as screening charges. Using orbital-motion limited electron charging fluxes and
the particle temperature as an adjustable parameter, we obtain excellent
agreement with experimental data.Comment: 4 pages, 3 figures, slightly revised manuscript including radius
dependence of the particle charg
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