133,969 research outputs found
Diffuse-interface model for nanopatterning induced by self-sustained ion etch masking
We construct a simple phenomenological diffuse-interface model for
composition-induced nanopatterning during ion sputtering of alloys. In
simulations, this model reproduces without difficulties the high-aspect ratio
structures and tilted pillars observed in experiments. We investigate the time
evolution of the pillar height, both by simulations and by {\it in situ}
ellipsometry. The analysis of the simulation results yields a good
understanding of the transitions between different growth regimes and supports
the role of segregation in the pattern-formation process.Comment: 10 pages, 3 figures; minor revisions with respect to first version;
figures nicened; journal ref. adde
Weinberg power counting and the quark determinant at small chemical potential
We construct an effective action for QCD by expanding the quark determinant
in powers of the chemical potential at finite temperature in the case of
massless quarks. To cut the infinite series we adopt the Weinberg power
counting criteria. We compute the minimal effective action (~p^4), expanding in
the external momentum, which implies the use of the hard thermal loop
approximation. Our main result is a gauge invariant expression for the phase
theta of the functional determinant in QCD, and recovers dimensional reduction
in the high-temperature limit. We compute, analytically, in the range
of p << 2 pi T, including perturbative and nonperturbative contributions, the
latter treated within the mean field approximation. Implications for lattice
simulations are briefly discussed.Comment: 5 pages, 2 figures. v2: title changed, expanded discussion and added
example (calculation of at high temperature). Published in PR
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
Experimental Study of Diamond Like Carbon (DLC) Coated Electrodes for Pulsed High Gradient Electron Gun
For the SwissFEL Free Electron Laser project at the Paul Scherrer Institute,
a pulsed High Gradient (HG) electron gun was used to study low emittance
electron sources. Different metals and surface treatments for the cathode and
anode were studied for their HG suitability. Diamond Like Carbon (DLC) coatings
are found to perform exceptionally well for vacuum gap insulation. A set of DLC
coated electrodes with different coating parameters were tested for both vacuum
breakdown and photo electron emission. Surface electric fields over 250MV/m
(350 - 400kV, pulsed) were achieved without breakdown. From the same surface,
it was possible to photo-emit an electron beam at gradients up to 150MV/m. The
test setup and the experimental results are presentedComment: 4 pages, 14 figures, IPMHVC 2010 : IEEE International Power Modulator
and High Voltage Conferenc
A relativistic non-relativistic Goldstone theorem: gapped Goldstones at finite charge density
We adapt the Goldstone theorem to study spontaneous symmetry breaking in
relativistic theo- ries at finite charge density. It is customary to treat
systems at finite density via non-relativistic Hamiltonians. Here we highlight
the importance of the underlying relativistic dynamics. This leads to seemingly
new results whenever the charge in question is spontaneously broken and does
not commute with other broken charges. We find that that the latter interpolate
gapped excitations. In contrast, all existing versions of the Goldstone theorem
predict the existence of gapless modes. We derive exact non-perturbative
expressions for their gaps, in terms of the chemical potential and of the
symmetry algebra.Comment: 5 pages. v2: minor modifications, matches the PRL versio
Thermodynamic dislocation theory of high-temperature deformation in aluminum and steel
The statistical-thermodynamic dislocation theory developed in previous papers
is used here in an analysis of high-temperature deformation of aluminum and
steel. Using physics-based parameters that we expect theoretically to be
independent of strain rate and temperature, we are able to fit experimental
stress-strain curves for three different strain rates and three different
temperatures for each of these two materials. Our theoretical curves include
yielding transitions at zero strain in agreement with experiment. We find that
thermal softening effects are important even at the lowest temperatures and
smallest strain rates.Comment: 7 pages, 8 figure
An Agent-Based Simulation for Water Sharing Between Different Users
Water sharing has become a serious problem in France. One of the objectives of 1992 and 2000 directives proposed by the European Union was to reduce both the frequency and the extent of water conflicts through the establishment of multilateral negotiations, where different public and private interests can be represented in a structured institutional environment. In France, many negotiations take place at local level between farmers, water supplies, public services and environmental lists to allocate water resources between users. We suggest that Agent-Based Modelling (ABM) using a multi-agent approach could help negotiations between different players by showing the consequences of water allocation rules and taking in consideration the players' respective attitudes and their ability to change their behaviour.Multiagent-based simulation, user and agent modelling, conflict resolution and negotiation, irrigation application, Resource /Energy Economics and Policy,
Nanofiber Fabry-Perot microresonator for non-linear optics and cavity quantum electrodynamics
We experimentally realize a Fabry-Perot-type optical microresonator near the
cesium D2 line wavelength based on a tapered optical fiber, equipped with two
fiber Bragg gratings which enclose a sub-wavelength diameter waist. Owing to
the very low taper losses, the finesse of the resonator reaches F = 86 while
the on-resonance transmission is T = 11 %. The characteristics of our resonator
fulfill the requirements of non-linear optics and cavity quantum
electrodynamics in the strong coupling regime. In combination with its
demonstrated ease of use and its advantageous mode geometry, it thus opens a
realm of applications.Comment: 4 pages, 3 figure
The impact of motor symptoms on self-reported anxiety in Parkinson's disease
OBJECTIVE: Anxiety is commonly endorsed in Parkinson's disease (PD) and significantly affects quality of life. The Beck Anxiety Inventory (BAI) is often used but contains items that overlap with common PD motor symptoms (e.g., “hands trembling”). Because of these overlapping items, we hypothesized that PD motor symptoms would significantly affect BAI scores.
METHODS: One hundred non-demented individuals with PD and 74 healthy control participants completed the BAI. PD motor symptoms were assessed by the Unified Parkinson's Disease Rating Scale (UPDRS). Factor analysis of the BAI assessed for a PD motor factor, and further analyses assessed how this factor affected BAI scores.
RESULTS: BAI scores were significantly higher for PD than NC. A five-item PD motor factor correlated with UPDRS observer-rated motor severity and mediated the PD-control difference on BAI total scores. An interaction occurred, whereby removal of the PD motor factor resulted in a significant reduction in BAI scores for PD relative to NC. The correlation between the BAI and UPDRS significantly declined when controlling for the PD motor factor.
CONCLUSIONS: The results indicate that commonly endorsed BAI items may reflect motor symptoms such as tremor instead of, or in addition to, genuine mood symptoms. These findings highlight the importance of considering motor symptoms in the assessment of anxiety in PD and point to the need for selecting anxiety measures that are less subject to contamination by the motor effects of movement disorders.Published versio
Weak local rules for planar octagonal tilings
We provide an effective characterization of the planar octagonal tilings
which admit weak local rules. As a corollary, we show that they are all based
on quadratic irrationalities, as conjectured by Thang Le in the 90s.Comment: 23 pages, 6 figure
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