5,422 research outputs found
Magnetic domain wall propagation in a submicron spin-valve stripe: influence of the pinned layer
The propagation of a domain wall in a submicron ferromagnetic spin-valve
stripe is investigated using giant magnetoresistance. A notch in the stripe
efficiently traps an injected wall stopping the domain propagation. The authors
show that the magnetic field at which the wall is depinned displays a
stochastic nature. Moreover, the depinning statistics are significantly
different for head to head and tail-to-tail domain walls. This is attributed to
the dipolar field generated in the vicinity of the notch by the pinned layer of
the spin-valve
The chiral quark condensate and pion decay constant in nuclear matter at next-to-leading order
Making use of the recently developed chiral power counting for the physics of
nuclear matter [1,2], we evaluate the in-medium chiral quark condensate up to
next-to-leading order for both symmetric nuclear matter and neutron matter. Our
calculation includes the full in-medium iteration of the leading order local
and one-pion exchange nucleon-nucleon interactions. Interestingly, we find a
cancellation between the contributions stemming from the quark mass dependence
of the nucleon mass appearing in the in-medium nucleon-nucleon interactions.
Only the contributions originating from the explicit quark mass dependence of
the pion mass survive. This cancellation is the reason of previous observations
concerning the dominant role of the long-range pion contributions and the
suppression of short-range nucleon-nucleon interactions. We find that the
linear density contribution to the in-medium chiral quark condensate is only
slightly modified for pure neutron matter by the nucleon-nucleon interactions.
For symmetric nuclear matter the in-medium corrections are larger, although
smaller compared to other approaches due to the full iteration of the lowest
order nucleon-nucleon tree-level amplitudes. Our calculation satisfies the
Hellmann-Feynman theorem to the order worked out. Also we address the problem
of calculating the leading in-medium corrections to the pion decay constant. We
find that there are no extra in-medium corrections that violate the
Gell-Mann-Oakes-Renner relation up to next-to-leading order.Comment: 21 pages, 9 figure
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Do Engineering Students Learn Ethics From an Ethics Course?
The goal of the present research is to develop machine-assisted methods that can assist in the analysis of students’ written compositions in ethics courses. As part of this research, we analyzed Social Impact Assessment (SIA) papers submitted by engineering undergraduates in a course on engineering ethics. The SIA papers required students to identify and discuss a contemporary engineering technology (e.g., autonomous tractor trailers) and to explicitly discuss the ethical issues involved in that technology. Here we describe the ability of three machine tools to discriminate differences in the technical compared to ethical portions of the SIA papers. First, using LIWC (Language Inquiry and Word Count) we quantified differences in analytical thinking, expertise and self-confidence, disclosure, and affect, in the technical and ethical portions of the papers. Next, we applied MEH (Meaning Extraction Helper) to examine differences in critical concepts in the technical and ethical portions of the papers. Finally, we used LDA (Latent Dirichlet Allocation) to examine differences in the topics in the technical and ethical portions of the papers. The results of these three tests demonstrate the ability of machine-based tools to discriminate conceptual, affective, and motivational differences in the texts that students compose that relate to engineering technology and to engineering ethics. We discuss the utility and future directions for this research.Cockrell School of Engineerin
High resolution imaging of young M-type stars of the solar neighborhood: Probing the existence of companions down to the mass of Jupiter
Context. High contrast imaging is a powerful technique to search for gas
giant planets and brown dwarfs orbiting at separation larger than several AU.
Around solar-type stars, giant planets are expected to form by core accretion
or by gravitational instability, but since core accretion is increasingly
difficult as the primary star becomes lighter, gravitational instability would
be the a probable formation scenario for yet-to-be-found distant giant planets
around a low-mass star. A systematic survey for such planets around M dwarfs
would therefore provide a direct test of the efficiency of gravitational
instability. Aims. We search for gas giant planets orbiting around late-type
stars and brown dwarfs of the solar neighborhood. Methods. We obtained deep
high resolution images of 16 targets with the adaptive optic system of VLT-NACO
in the Lp band, using direct imaging and angular differential imaging. This is
currently the largest and deepest survey for Jupiter-mass planets around
Mdwarfs. We developed and used an integrated reduction and analysis pipeline to
reduce the images and derive our 2D detection limits for each target. The
typical contrast achieved is about 9 magnitudes at 0.5" and 11 magnitudes
beyond 1". For each target we also determine the probability of detecting a
planet of a given mass at a given separation in our images. Results. We derived
accurate detection probabilities for planetary companions, taking into account
orbital projection effects, with in average more than 50% probability to detect
a 3MJup companion at 10AU and a 1.5MJup companion at 20AU, bringing strong
constraints on the existence of Jupiter-mass planets around this sample of
young M-dwarfs.Comment: Accepted for publication in A&
Thermal Effects on the Magnetic Field Dependence of Spin Transfer Induced Magnetization Reversal
We have developed a self-aligned, high-yield process to fabricate CPP
(current perpendicular to the plane) magnetic sensors of sub 100 nm dimensions.
A pinned synthetic antiferromagnet (SAF) is used as the reference layer which
minimizes dipole coupling to the free layer and field induced rotation of the
reference layer. We find that the critical currents for spin transfer induced
magnetization reversal of the free layer vary dramatically with relatively
small changes the in-plane magnetic field, in contrast to theoretical
predictions based on stability analysis of the Gilbert equations of
magnetization dynamics including Slonczewski-type spin-torque terms. The
discrepancy is believed due to thermal fluctuations over the time scale of the
measurements. Once thermal fluctuations are taken into account, we find good
quantitative agreement between our experimental results and numerical
simulations.Comment: 14 pages, 4 figures, Submitted to Appl. Phys. Lett., Comparison of
some of these results with a model described by N. Smith in cond-mat/040648
Sparse aperture masking at the VLT II. Detection limits for the eight debris disks stars Pic, AU Mic, 49 Cet, Tel, Fomalhaut, g Lup, HD181327 and HR8799
Context. The formation of planetary systems is a common, yet complex
mechanism. Numerous stars have been identified to possess a debris disk, a
proto-planetary disk or a planetary system. The understanding of such formation
process requires the study of debris disks. These targets are substantial and
particularly suitable for optical and infrared observations. Sparse Aperture
masking (SAM) is a high angular resolution technique strongly contributing to
probe the region from 30 to 200 mas around the stars. This area is usually
unreachable with classical imaging, and the technique also remains highly
competitive compared to vortex coronagraphy. Aims. We aim to study debris disks
with aperture masking to probe the close environment of the stars. Our goal is
either to find low mass companions, or to set detection limits. Methods. We
observed eight stars presenting debris disks ( Pictoris, AU
Microscopii, 49 Ceti, Telescopii, Fomalhaut, g Lupi, HD181327 and
HR8799) with SAM technique on the NaCo instrument at the VLT. Results. No close
companions were detected using closure phase information under 0.5 of
separation from the parent stars. We obtained magnitude detection limits that
we converted to Jupiter masses detection limits using theoretical isochrones
from evolutionary models. Conclusions. We derived upper mass limits on the
presence of companions in the area of few times the diffraction limit of the
telescope around each target star.Comment: 7 pages, All magnitude detection limits maps are only available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5
Chiral nature of magnetic monopoles in artificial spin ice
Micromagnetic properties of monopoles in artificial kagome spin ice systems
are investigated using numerical simulations. We show that micromagnetics
brings additional complexity into the physics of these monopoles that is, by
essence, absent in spin models: besides a fractionalized classical magnetic
charge, monopoles in the artificial kagome ice are chiral at remanence. Our
simulations predict that the chirality of these monopoles can be controlled
without altering their charge state. This chirality breaks the vertex symmetry
and triggers a directional motion of the monopole under an applied magnetic
field. Our results also show that the choice of the geometrical features of the
lattice can be used to turn on and off this chirality, thus allowing the
investigation of chiral and achiral monopoles.Comment: 10 pages, 4 figure
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