826 research outputs found
Designing electronic properties of two-dimensional crystals through optimization of deformations
One of the enticing features common to most of the two-dimensional electronic
systems that are currently at the forefront of materials science research is
the ability to easily introduce a combination of planar deformations and
bending in the system. Since the electronic properties are ultimately
determined by the details of atomic orbital overlap, such mechanical
manipulations translate into modified electronic properties. Here, we present a
general-purpose optimization framework for tailoring physical properties of
two-dimensional electronic systems by manipulating the state of local strain,
allowing a one-step route from their design to experimental implementation. A
definite example, chosen for its relevance in light of current experiments in
graphene nanostructures, is the optimization of the experimental parameters
that generate a prescribed spatial profile of pseudomagnetic fields in
graphene. But the method is general enough to accommodate a multitude of
possible experimental parameters and conditions whereby deformations can be
imparted to the graphene lattice, and complies, by design, with graphene's
elastic equilibrium and elastic compatibility constraints. As a result, it
efficiently answers the inverse problem of determining the optimal values of a
set of external or control parameters that result in a graphene deformation
whose associated pseudomagnetic field profile best matches a prescribed target.
The ability to address this inverse problem in an expedited way is one key step
for practical implementations of the concept of two-dimensional systems with
electronic properties strain-engineered to order. The general-purpose nature of
this calculation strategy means that it can be easily applied to the
optimization of other relevant physical quantities which directly depend on the
local strain field, not just in graphene but in other two-dimensional
electronic membranes.Comment: 37 pages, 9 figures. This submission contains low-resolution bitmap
images; high-resolution images can be found in version 1, which is ~13.5 M
A Dynamical Model of the Inner Galaxy
An extension of Schwarzschild's galaxy-building technique is presented that,
for the first time, enables one to build Schwarzschild models with known
distribution functions (DFs). The new extension makes it possible to combine a
DF that depends only on classical integrals with orbits that respect
non-classical integrals. With such a combination, Schwarzschild's orbits are
used only to represent the difference between the true galaxy DF and an
approximating classical DF. The new method is used to construct a dynamical
model of the inner Galaxy. The model is based on an orbit library that contains
22168 regular orbits. The model aims to reproduce the three-dimensional mass
density of Binney, Gerhard & Spergel (1997), which was obtained through
deprojection of the COBE surface photometry, and to reproduce the observed
kinematics in three windows - namely Baade's Window and two off-axis fields.
The model fits essentially all the available data within the innermost 3 kpc.
The axis ratio and the morphology of the projected density contours of the COBE
bar are recovered to good accuracy within corotation. The kinematic quantities
- the line-of-sight streaming velocity and velocity dispersion, as well as the
proper motions when available - are recovered, not merely for the fitted
fields, but also for three new fields. The dynamical model deviates most from
the input density close to the Galactic plane just outside corotation, where
the deprojection of the surface photometry is suspect. The dynamical model does
not reproduce the kinematics at the most distant window, where disk
contamination may be severe.Comment: 20 pages, 5 gif figures, 11 postscript figures, submitted to MNRAS.
Zipped postscript available at
http://www-thphys.physics.ox.ac.uk/users/RalfHafner/paper.ps.g
The mass of dwarf spheroidal galaxies and the missing satellite problem
We present the results from a suite of N-body simulations of the tidal
stripping of two-component dwarf galaxies comprising some stars and dark
matter. We show that recent kinematic data from the local group dwarf
spheroidal (dSph) galaxies suggests that dSph galaxies must be sufficiently
massive (M) that tidal stripping is of little
importance for the stars. We discuss the implications of these massive dSph
galaxies for cosmology and galaxy formation.Comment: 4 pages, 1 figure, to appear in the proceedings of the IAUC198
"Near-Field Cosmology with Dwarf Elliptical Galaxies", H. Jerjen & B.
Binggeli (eds.). Comments welcom
A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy
The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most dark
matter dominated galaxies known, with a central mass to light ratio roughly
equal to 70. Somewhat anomalously, it appears to contain morphological
substructure in the form of a second peak in the stellar number density. It is
often argued that this substructure must be transient because it could not
survive for the > 10 Gyr age of the system, given the crossing time implied by
UMi's 8.8 km/s internal velocity dispersion. In this paper, however, we present
evidence that the substructure has a cold kinematical signature, and argue that
UMi's clumpiness could indeed be a primordial artefact. Using numerical
simulations, we demonstrate that substructure is incompatible with the cusped
dark matter haloes predicted by the prevailing Cold Dark Matter (CDM) paradigm,
but is consistent with an unbound stellar cluster sloshing back and forth
within the nearly harmonic potential of a cored dark matter halo. Thus CDM
appears to disagree with observation at the least massive, most dark matter
dominated end of the galaxy mass spectrum.Comment: Astrophysical Journal (Letters), in pres
Is the Large Magellanic Cloud a Large Microlensing Cloud?
An expression is provided for the self-lensing optical depth of the thin LMC
disk surrounded by a shroud of stars at larger scale heights. The formula is
written in terms of the vertical velocity dispersion of the thin disk
population. If tidal forcing causes 1-5 % of the disk mass to have a height
larger than 6 kpc and 10-15 % to have a height above 3 kpc, then the
self-lensing optical depth of the LMC is , which is
within the observational uncertainties. The shroud may be composed of bright
stars provided they are not in stellar hydrodynamical equilibrium.
Alternatively, the shroud may be built from low mass stars or compact objects,
though then the self-lensing optical depths are overestimates of the true
optical depth by a factor of roughly 3. The distributions of timescales of the
events and their spatial variation across the face of the LMC disk offer
possibilities of identifying the dominant lens population. In propitious
circumstances, an experiment lifetime of less than 5 years is sufficient to
decide between the competing claims of Milky Way halos and LMC lenses. However,
LMC disks can sometimes mimic the microlensing properties of Galactic halos for
many years and then decades of survey work are needed. In this case
observations of parallax or binary caustic events offer the best hope for
current experiments to deduce the lens population. The difficult models to
distinguish are Milky Way halos in which the lens fraction is low (< 10 %) and
fattened LMC disks composed of lenses with a typical mass of low luminosity
stars or greater. A next-generation wide-area microlensing survey, such as the
proposed ``SuperMACHO'' experiment, will be able to distinguish even these
difficult models with just a year or two of data.Comment: 25 pages, 4 figures, The Astrophysical Journal (in press
Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidals
We present projected velocity dispersion profiles for the Draco and Ursa
Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar
velocities, respectively. Both profiles show a sharp decline in the velocity
dispersion outside ~30 arcmin (Draco) and ~40 arcmin (UMi). New, deep
photometry of Draco reveals a break in the light profile at ~25 arcmin. These
data imply the existence of a kinematically cold population in the outer parts
of both galaxies. Possible explanations of both the photometric and kinematic
data in terms of both equilibrium and non-equilibrium models are discussed in
detail. We conclude that these data challenge the picture of dSphs as simple,
isolated stellar systems.Comment: 5 pages, accepted for publication in ApJ Letter
Benefit of triple-frequency on cycle-slip detection
At the time of writing, all the Global Navigation Satellite Systems (GNSS) support or are designed to support triple- or multi- frequency, which is expected to have advantages over single- and dual- frequency. This paper will conduct research on how triple-frequency can benefit the cycle-slip detection process. Correctly detecting and repairing cycle slips can help extend the latency of the fixed ambiguities, estimate the ionospheric delay, reduce the measurement noise and finally improve the positioning precision of the carrier phase. This paper will firstly review the widely used cycle-slip detection methods, including high-order phase differencing, Doppler integration and the ionospheric residual. For applying triple-frequency in cycle-slip detection, we will modify the Hatch-Melbourne-WĂŒbbena combination to eliminate the effect of the ionospheric bias and reduce the measurement noise on the detection value. The triple-frequency method can detect and correct cycle slips instantaneously. All the mentioned methods will be tested using triple-frequency Galileo data observed in static condition. The results show that the performance of the triple-frequency method has a higher success rate and a lower missed detection compared to those using single-frequency, especially in detecting small cycle slips in observation with large intervals. Although the ionospehric residual provides higher success rates at low elevation angles, the triple-frequency method is more advanced than the ionospheric residual, which cannot decide the magnitude of the cycle slips easily
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