11,305 research outputs found
Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110)
We observe using low-energy electron microscopy the self-assembly of
monolayer-thick stripes of Au on W(110) near the transition temperature between
stripes and the non-patterned (homogeneous) phase. We demonstrate that the
amplitude of this Au stripe phase decreases with increasing temperature and
vanishes at the order-disorder transition (ODT). The wavelength varies much
more slowly with temperature and coverage than theories of stress-domain
patterns with sharp phase boundaries would predict, and maintains a finite
value of about 100 nm at the ODT. We argue that such nanometer-scale stripes
should often appear near the ODT.Comment: 5 page
Determining the structure of Ru(0001) from low-energy electron diffraction of a single terrace
While a perfect hcp (0001) surface has three-fold symmetry, the diffraction
patterns commonly obtained are six-fold symmetric. This apparent change in
symmetry occurs because on a stepped surface, the atomic layers on adjacent
terraces are rotated by 180 degrees. Here we use a Low-Energy Electron
Microscope to acquire the three-fold diffraction pattern from a single hcp Ru
terrace and measure the intensity-vs-energy curves for several diffracted
beams. By means of multiple scattering calculations fitted to the experimental
data with a Pendry R-factor of 0.077, we find that the surface is contracted by
3.5(+-0.9) at 456 K.Comment: 10 pages, 4 figures. Corrected some typos, added more details.
Accepted for publication in Surface Science (Letters
Real Space Observations of Magnesium Hydride Formation and Decomposition
The mechanisms of magnesium hydride formation and thermal decomposition are
directly examined using in-situ imaging.Comment: 3 pages, 4 figure
How metal films de-wet substrates - identifying the kinetic pathways and energetic driving forces
We study how single-crystal chromium films of uniform thickness on W(110)
substrates are converted to arrays of three-dimensional (3D) Cr islands during
annealing. We use low-energy electron microscopy (LEEM) to directly observe a
kinetic pathway that produces trenches that expose the wetting layer. Adjacent
film steps move simultaneously uphill and downhill relative to the staircase of
atomic steps on the substrate. This step motion thickens the film regions where
steps advance. Where film steps retract, the film thins, eventually exposing
the stable wetting layer. Since our analysis shows that thick Cr films have a
lattice constant close to bulk Cr, we propose that surface and interface stress
provide a possible driving force for the observed morphological instability.
Atomistic simulations and analytic elastic models show that surface and
interface stress can cause a dependence of film energy on thickness that leads
to an instability to simultaneous thinning and thickening. We observe that
de-wetting is also initiated at bunches of substrate steps in two other
systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are
converted into patterns of unidirectional stripes as the trenches that expose
the wetting layer lengthen along the W[001] direction. Finally, we observe how
3D Cr islands form directly during film growth at elevated temperature. The Cr
mesas (wedges) form as Cr film steps advance down the staircase of substrate
steps, another example of the critical role that substrate steps play in 3D
island formation
Kinetics and thermodynamics of carbon segregation and graphene growth on Ru(0001)
We measure the concentration of carbon adatoms on the Ru(0001) surface that
are in equilibrium with C atoms in the crystal's bulk by monitoring the
electron reflectivity of the surface while imaging. During cooling from high
temperature, C atoms segregate to the Ru surface, causing graphene islands to
nucleate. Using low-energy electron microscopy (LEEM), we measure the growth
rate of individual graphene islands and, simultaneously, the local
concentration of C adatoms on the surface. We find that graphene growth is fed
by the supersaturated, two-dimensional gas of C adatoms rather than by direct
exchange between the bulk C and the graphene. At long times, the rate at which
C diffuses from the bulk to the surface controls the graphene growth rate. The
competition among C in three states - dissolved in Ru, as an adatom, and in
graphene - is quantified and discussed. The adatom segregation enthalpy
determined by applying the simple Langmuir-McLean model to the
temperature-dependent equilibrium concentration seriously disagrees with the
value calculated from first-principles. This discrepancy suggests that the
assumption in the model of non-interacting C is not valid
Should We Learn Probabilistic Models for Model Checking? A New Approach and An Empirical Study
Many automated system analysis techniques (e.g., model checking, model-based
testing) rely on first obtaining a model of the system under analysis. System
modeling is often done manually, which is often considered as a hindrance to
adopt model-based system analysis and development techniques. To overcome this
problem, researchers have proposed to automatically "learn" models based on
sample system executions and shown that the learned models can be useful
sometimes. There are however many questions to be answered. For instance, how
much shall we generalize from the observed samples and how fast would learning
converge? Or, would the analysis result based on the learned model be more
accurate than the estimation we could have obtained by sampling many system
executions within the same amount of time? In this work, we investigate
existing algorithms for learning probabilistic models for model checking,
propose an evolution-based approach for better controlling the degree of
generalization and conduct an empirical study in order to answer the questions.
One of our findings is that the effectiveness of learning may sometimes be
limited.Comment: 15 pages, plus 2 reference pages, accepted by FASE 2017 in ETAP
Structure and magnetism in ultrathin iron oxides characterized by low energy electron microscopy
We have grown epitaxial films a few atomic layers thick of iron oxides on
ruthenium. We characterize the growth by low energy electron microscopy. Using
selected area diffraction and intensity vs. voltage spectroscopy, we detect two
distinct phases which are assigned to wustite and magnetite. Spin polarized low
energy electron microscopy shows magnetic domain patterns in the magnetite
phase at room temperature.Comment: 21 pages, 10 figures, for J. Phys Cond Matt special LEEM/PEEM issue
in honor of E. Baue
Flux and spectral variations in the Circinus Galaxy
We report on a dramatic flux (50 % increase in the LECS and MECS band) and
spectral variation between two BeppoSAX observations of the Circinus Galaxy
performed almost three years apart. Through the analysis of all Chandra
observations available in the archive, including a new DDT observation on May
2001, we show that a high flux state of an extremely variable Ultra Luminous
X-ray source (CG X-1: Bauer et al. 2001}, which is within the adopted BeppoSAX
source extraction region of 2", is the most likely explanation for most of the
observed variation. However, the presence of a high flux 6.7 keV line and the
spectral variation of the PDS in the new BeppoSAX data could be partly due to
intrinsic variation of the nucleus. Comparing the longest Chandra observation
and the BeppoSAX one, we find that the long-term flux variability of CG X-1 is
not accompanied by a significant spectral variability. We also re-analysed the
Chandra HEG nuclear spectra and report on the presence of a Compton shoulder
with a flux of about 20% the line core, in agreement with theoretical
expectations for Compton-thick matter.Comment: 7 pages, 3 figures, accepted for publication on A&
The XMM deep survey in the CDF-S II. a 9-20 keV selection of heavily obscured active galaxies at z>1.7
We present results on a search of heavily obscured active galaxies z>1.7
using the rest-frame 9-20 keV excess for X-ray sources detected in the deep
XMM-CDFS survey. Out of 176 sources selected with the conservative detection
criteria (>8 sigma) in the first source catalogue of Ranalli et al., 46 objects
lie in the redshift range of interest with the median redshift z~2.5. Their
typical rest-frame 10-20 keV luminosity is 1e+44 erg/s, as observed. Among
optically faint objects that lack spectroscopic redshift, four were found to be
strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption
features in their X-ray spectra were used to obtain X-ray spectroscopic
redshifts. Using the X-ray colour-colour diagram based on the rest-frame 3-5
keV, 5-9 keV, and 9-20 keV bands, seven objects were selected for their 9-20
keV excess and were found to be strongly absorbed X-ray sources with column
density of nH > 0.6e+24 cm-2, including two possible Compton thick sources.
While they are emitting at quasar luminosity, ~3/4 of the sample objects are
found to be absorbed by nH > 1e+22 cm-2. A comparison with local AGN at the
matched luminosity suggests an increasing trend of the absorbed source fraction
for high-luminosity AGN towards high redshifts.Comment: 9 pages, 7 figures. Accepted for publication in A&
A Cross-correlation method to search for gravitational wave bursts with AURIGA and Virgo
We present a method to search for transient GWs using a network of detectors
with different spectral and directional sensitivities: the interferometer Virgo
and the bar detector AURIGA. The data analysis method is based on the
measurements of the correlated energy in the network by means of a weighted
cross-correlation. To limit the computational load, this coherent analysis step
is performed around time-frequency coincident triggers selected by an excess
power event trigger generator tuned at low thresholds. The final selection of
GW candidates is performed by a combined cut on the correlated energy and on
the significance as measured by the event trigger generator. The method has
been tested on one day of data of AURIGA and Virgo during September 2005. The
outcomes are compared to the results of a stand-alone time-frequency
coincidence search. We discuss the advantages and the limits of this approach,
in view of a possible future joint search between AURIGA and one
interferometric detector.Comment: 11 pages, 6 figures, submitted to CQG special issue for Amaldi 7
Proceeding
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