391 research outputs found
Structural models for the Si(553)-Au atomic chain reconstruction
Recent photoemission experiments on the Si(553)-Au reconstruction show a
one-dimensional band with a peculiar ~1/4 filling. This band could provide an
opportunity for observing large spin-charge separation if electron-electron
interactions could be increased. To this end, it is necessary to understand in
detail the origin of this surface band. A first step is the determination of
the structure of the reconstruction. We present here a study of several
structural models using first-principles density functional calculations. Our
models are based on a plausible analogy with the similar and better known
Si(557)-Au surface, and compared against the sole structure proposed to date
for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ].
Results for the energetics and the band structures are given. Lines for the
future investigation are also sketched
Distance dependence of force and dissipation in non-contact atomic force microscopy on Cu(100) and Al(111)
The dynamic characteristics of a tip oscillating in the nc-AFM mode in close
vicinity to a Cu(100)-surface are investigated by means of phase variation
experiments in the constant amplitude mode. The change of the quality factor
upon approaching the surface deduced from both frequency shift and excitation
versus phase curves yield to consistent values. The optimum phase is found to
be independent of distance. The dependence of the quality factor on distance is
related to 'true' damping, because artefacts related to phase misadjustment can
be excluded. The experimental results, as well as on-resonance measurements at
different bias voltages on an Al(111) surface, are compared to Joule
dissipation and to a model of dissipation in which long-range forces lead to
viscoelastic deformations
Real-Time Predictive Modeling and Robust Avoidance of Pedestrians with Uncertain, Changing Intentions
To plan safe trajectories in urban environments, autonomous vehicles must be
able to quickly assess the future intentions of dynamic agents. Pedestrians are
particularly challenging to model, as their motion patterns are often uncertain
and/or unknown a priori. This paper presents a novel changepoint detection and
clustering algorithm that, when coupled with offline unsupervised learning of a
Gaussian process mixture model (DPGP), enables quick detection of changes in
intent and online learning of motion patterns not seen in prior training data.
The resulting long-term movement predictions demonstrate improved accuracy
relative to offline learning alone, in terms of both intent and trajectory
prediction. By embedding these predictions within a chance-constrained motion
planner, trajectories which are probabilistically safe to pedestrian motions
can be identified in real-time. Hardware experiments demonstrate that this
approach can accurately predict pedestrian motion patterns from onboard
sensor/perception data and facilitate robust navigation within a dynamic
environment.Comment: Submitted to 2014 International Workshop on the Algorithmic
Foundations of Robotic
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Role of Hair Coverage and Sweating for Textile Friction on the Forearm
Friction of textiles on the human forearm is an important factor in comfort sensations of garments. We built an experiment to measure friction for textiles sliding on the forearm under loading conditions which are characteristic for wearing shirts or jackets. The hair coverage of the participants’ forearm was quantified by image analysis of photographs of the arm in the region of contact. Friction results for five standard textiles suggest to treat hair coverage in two classes. Sweating after physical activity leads to an increase of friction by factors of 2 to 5 for participants with less hairy forearms, while an increase by a factor of 1 to 1.7 only was found for participants with more hairy forearms. We introduce a method of wetting the forearm of study participants in a controlled way with water, which results in similar friction as for the sweating forearm after physical activity. The method allows for efficient studies of the role of skin moisture for friction including varying hair coverage of the skin
Observation of individual molecules trapped on a nanostructured insulator
For the first time, ordered polar molecules confined in monolayer-deep
rectangular pits produced on an alkali halide surface by electron irradiation
have been resolved at room temperature by non-contact atomic force microscopy.
Molecules self-assemble in a specific fashion inside pits of width smaller than
15 nm. By contrast no ordered aggregates of molecules are observed on flat
terraces. Conclusions regarding nucleation and ordering mechanisms are drawn.
Trapping in pits as small as 2 nm opens a route to address single molecules
Invited review: Genome-wide association analysis for quantitative traits in livestock – a selective review of statistical models and experimental designs
Quantitative or complex traits
are controlled by many genes and environmental factors. Most traits in
livestock breeding are quantitative traits. Mapping genes and causative
mutations generating the genetic variance of these traits is still a very
active area of research in livestock genetics. Since genome-wide and dense
SNP panels are available for most livestock species, genome-wide association
studies (GWASs) have become the method of choice in mapping experiments.
Different statistical models are used for GWASs. We will review the
frequently used single-marker models and additionally describe Bayesian
multi-marker models. The importance of nonadditive genetic and
genotype-by-environment effects along with GWAS methods to detect them will
be briefly discussed. Different mapping populations are used and will also be
reviewed. Whenever possible, our own real-data examples are included to
illustrate the reviewed methods and designs. Future research directions\ud
including post-GWAS strategies are outlined
Self-Doping of Gold Chains on Silicon: A New Structural Model for Si(111)5x2-Au
A new structural model for the Si(111)5x2-Au reconstruction is proposed and
analyzed using first-principles calculations. The basic model consists of a
"double honeycomb chain" decorated by Si adatoms. The 5x1 periodicity of the
honeycomb chains is doubled by the presence of a half-occupied row of Si atoms
that partially rebonds the chains. Additional adatoms supply electrons that
dope the parent band structure and stabilize the period doubling; the optimal
doping corresponds to one adatom per four 5x2 cells, in agreement with
experiment. All the main features observed in scanning tunneling microscopy and
photoemission are well reproduced.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett. (preprint with high
quality figures available at
http://cst-www.nrl.navy.mil/~erwin/papers/ausi111
Ageing of a Microscopic Sliding Gold Contact at Low Temperatures
Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact
Role of friction-induced torque in stick-slip motion
We present a minimal quasistatic 1D model describing the kinematics of the
transition from static friction to stick-slip motion of a linear elastic block
on a rigid plane. We show how the kinematics of both the precursors to
frictional sliding and the periodic stick-slip motion are controlled by the
amount of friction-induced torque at the interface. Our model provides a
general framework to understand and relate a series of recent experimental
observations, in particular the nucleation location of micro-slip instabilities
and the build up of an asymmetric field of real contact area.Comment: 6 pages, 5 figure
Atomic Scale Memory at a Silicon Surface
The limits of pushing storage density to the atomic scale are explored with a
memory that stores a bit by the presence or absence of one silicon atom. These
atoms are positioned at lattice sites along self-assembled tracks with a pitch
of 5 atom rows. The writing process involves removal of Si atoms with the tip
of a scanning tunneling microscope. The memory can be reformatted by controlled
deposition of silicon. The constraints on speed and reliability are compared
with data storage in magnetic hard disks and DNA.Comment: 13 pages, 5 figures, accepted by Nanotechnolog
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