2,730 research outputs found
Nanofriction behavior of cluster-assembled carbon films
We have characterized the frictional properties of nanostructured (ns) carbon
films grown by Supersonic Cluster Beam Deposition (SCBD) via an Atomic
Force-Friction Force Microscope (AFM-FFM). The experimental data are discussed
on the basis of a modified Amonton's law for friction, stating a linear
dependence of friction on load plus an adhesive offset accounting for a finite
friction force in the limit of null total applied load. Molecular Dynamics
simulations of the interaction of the AFM tip with the nanostructured carbon
confirm the validity of the friction model used for this system. Experimental
results show that the friction coefficient is not influenced by the
nanostructure of the films nor by the relative humidity. On the other hand the
adhesion coefficient depends on these parameters.Comment: 22 pages, 6 figures, RevTex
Dynamics of plasma blobs in a shear flow
The global dynamic of plasma blobs in a shear flow is investigated in a simple magnetized torus using
the spatial Fourier harmonics (k-space) framework. Direct experimental evidence of a linear drift in
k space of the density fluctuation energy synchronized with blob events is presented. During this drift, an
increase of the fluctuation energy and a production of the kinetic energy associated with blobs are observed.
The energy source of the blob is analyzed using an advection-dissipation-type equation that includes
blob-flow exchange energy, linear drift in k space, nonlinear processes, and viscous dissipations.
We show that blobs tap their energy from the dominant E B vertical background flow during the linear
drift stage. The exchange of energy is unidirectional as there is no evidence that blobs return energy to the
flow
Scale-dependent angle of alignment between velocity and magnetic field fluctuations in solar wind turbulence
Under certain conditions, freely decaying magnetohydrodynamic (MHD) turbulence evolves in such a way that velocity and magnetic field fluctuations delta v and delta B approach a state of alignment in which delta v proportional to delta B. This process is called dynamic alignment. Boldyrev has suggested that a similar kind of alignment process occurs as energy cascades from large to small scales through the inertial range in strong incompressible MHD turbulence. In this study, plasma and magnetic field data from the Wind spacecraft, data acquired in the ecliptic plane near 1 AU, are employed to investigate the angle theta(tau) between velocity and magnetic field fluctuations in the solar wind as a function of the time scale tau of the fluctuations and to look for the scaling relation similar to tau(1/4) predicted by Boldyrev. We find that the angle appears to scale like a power law at large inertial range scales, but then deviates from power law behavior at medium to small inertial range scales. We also find that small errors in the velocity vector measurements can lead to large errors in the angle measurements at small time scales. As a result, we cannot rule out the possibility that the observed deviations from power law behavior arise from errors in the velocity measurements. When we fit the data from 2 x 10(3) s to 2 x 10(4) s with a power law of the form proportional to tau(p), our best fit values for p are in the range 0.27-0.36
Theory of incompressible MHD turbulence with scale-dependent alignment and cross-helicity
(Abridged) An anisotropic theory of MHD turbulence with nonvanishing
cross-helicity is constructed based on Boldyrev's alignment hypothesis and
probabilities p and q for fluctuations v and b to be positively or negatively
aligned. Guided by observations suggesting that the normalized cross-helicity
and the probability p are approximately constant in the inertial range, a
generalization of Boldyrev's theory is derived that depends on the three ratios
w+/w-, epsilon+/epsilon-, and p/q. The theory reduces to Boldyrev's original
theory when w+ = w-, epsilon+ = epsilon-, and p = q.Comment: Submitted to Phys. Plasma
Certifying and removing disparate impact
What does it mean for an algorithm to be biased? In U.S. law, unintentional
bias is encoded via disparate impact, which occurs when a selection process has
widely different outcomes for different groups, even as it appears to be
neutral. This legal determination hinges on a definition of a protected class
(ethnicity, gender, religious practice) and an explicit description of the
process.
When the process is implemented using computers, determining disparate impact
(and hence bias) is harder. It might not be possible to disclose the process.
In addition, even if the process is open, it might be hard to elucidate in a
legal setting how the algorithm makes its decisions. Instead of requiring
access to the algorithm, we propose making inferences based on the data the
algorithm uses.
We make four contributions to this problem. First, we link the legal notion
of disparate impact to a measure of classification accuracy that while known,
has received relatively little attention. Second, we propose a test for
disparate impact based on analyzing the information leakage of the protected
class from the other data attributes. Third, we describe methods by which data
might be made unbiased. Finally, we present empirical evidence supporting the
effectiveness of our test for disparate impact and our approach for both
masking bias and preserving relevant information in the data. Interestingly,
our approach resembles some actual selection practices that have recently
received legal scrutiny.Comment: Extended version of paper accepted at 2015 ACM SIGKDD Conference on
Knowledge Discovery and Data Minin
Retroperitoneal tumours
Primary retro-peritoneal tumours, though uncommon, are not great rarities and should not be neglected. They merit more notice than is generally given them in the odd half page of the standard surgical text book. They pose a difficult and fascinating problem to the pathologist and to the physician. To the practicing surgeon they are a challenge that may tax his resource. Most workers agree that the diagnosis is not easy. Enlargements of the kidney, adrenal, pancreas, spleen and liver have to be excluded, as also such lesions as aortic aneurysms. The tumours may arise from anywhere from the diaphragm to the pelvic floor, and from a wide variety of tissue such as fat, areolar tissue, connective tissue, fascia, muscle, vascular tissue, nerve tissue, somatic and autonomic, lymphatic vessels and lymphatic nodes. Five cases of retroperitoneal tumours are described, focusing on the respective diagnosis, surgical treatments and therapy.peer-reviewe
Paradoxes in Fair Computer-Aided Decision Making
Computer-aided decision making--where a human decision-maker is aided by a
computational classifier in making a decision--is becoming increasingly
prevalent. For instance, judges in at least nine states make use of algorithmic
tools meant to determine "recidivism risk scores" for criminal defendants in
sentencing, parole, or bail decisions. A subject of much recent debate is
whether such algorithmic tools are "fair" in the sense that they do not
discriminate against certain groups (e.g., races) of people.
Our main result shows that for "non-trivial" computer-aided decision making,
either the classifier must be discriminatory, or a rational decision-maker
using the output of the classifier is forced to be discriminatory. We further
provide a complete characterization of situations where fair computer-aided
decision making is possible
A new Fermi smearing approach for scattering of multi-GeV electrons by nuclei
The cross section for electron scattering by nuclei at high momentum
transfers is calculated within the Fermi smearing approximation (FSA), where
binding effects on the struck nucleon are introduced via the relativistic
Hartree approximation (RHA). The model naturally preserves current
conservation, since the response tensor for an off-shell nucleon conserves the
same form that for a free one but with an effective mass. Different
parameterizations for the inelastic nucleon structure function, are analyzed.
The smearing at the Fermi surface is introduced through a momentum distribution
obtained from a perturbative nuclear matter calculation. Recent CEBAF data on
inclusive scattering of 4.05 GeV electrons on Fe are well reproduced for
all measured geometries for the first time, as is evident from the comparison
with previous calculations.Comment: 8 pages in Revtex4 style, 6 eps figures, to appear in Physical Review
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