359 research outputs found
Intelligent legged climbing service robot for remote maintenance applications in hazardous environments
Imide and isatin derivatives as β-lactam mimics of β-lactam antibiotics
Activated γ-lactams, which are derivatives of succinimide, phthalimide and isatin with suitable
elements of molecular recognition, have been synthesised as mimics of the ß-lactam antibiotics
and their chemical and biological reactivity determined
Plans for phase coherent long baseline interferometry for geophysical applications using the Anik-B communications satellite
A pilot project to establish an operational phase stable very long baseline interferometer (VLBI) for geophysical studies is described. Methods for implementation as well as practical applications are presented
Detecting sterile dark matter in space
Space-based instruments provide new and, in some cases, unique opportunities
to search for dark matter. In particular, if dark matter comprises sterile
neutrinos, the x ray detection of their decay line is the most promising
strategy for discovery. Sterile neutrinos with masses in the keV range could
solve several long-standing astrophysical puzzles, from supernova asymmetries
and the pulsar kicks to star formation, reionization, and baryogenesis. The
best current limits on sterile neutrinos come from Chandra and XMM-Newton.
Future advances can be achieved with a high-resolution x-ray spectrometry in
space.Comment: 11 pages, 1 figure, to appear in proceedings "From Quantum to Cosmos:
fundametal physics research in space", Washington, DC, May 22-24, 200
A Quench Detection and Monitoring System for Superconducting Magnets at Fermilab
A quench detection system was developed for protecting and monitoring the
superconducting solenoids for the Muon-to-Electron Conversion Experiment (Mu2e)
at Fermilab. The quench system was designed for a high level of dependability
and long-term continuous operation. It is based on three tiers: Tier-I,
FPGA-based Digital Quench Detection (DQD); Tier-II, Analog Quench Detection
(AQD); and Tier-3, the quench controls and data management system. The Tier-I
and Tier-II are completely independent and fully redundant systems. The Tier-3
system is based on National Instruments (NI) C-RIO and provides the user
interface for quench controls and data management. It is independent from Tiers
I & II. The DQD provides both quench detection and quench characterization
(monitoring) capability. Both DQD and AQD have built-in high voltage isolation
and user programmable gains and attenuations. The DQD and AQD also includes
user configured current dependent thresholding and validation times.
A 1st article of the three-tier system was fully implemented on the new
Fermilab magnet test stand for the HL-LHC Accelerator Up-grade Project (AUP).
It successfully provided quench protection and monitoring (QPM) for a cold
superconducting bus test in November 2020. The Mu2e quench detection design has
since been implemented for production testing of the AUP magnets. A detailed
description of the system along with results from the AUP superconducting bus
test will be presented
Dynamics of a ferromagnetic domain wall and the Barkhausen effect
We derive an equation of motion for the the dynamics of a ferromagnetic
domain wall driven by an external magnetic field through a disordered medium
and we study the associated depinning transition. The long-range dipolar
interactions set the upper critical dimension to be , so we suggest that
mean-field exponents describe the Barkhausen effect for three-dimensional soft
ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a
function of the field driving rate and the intensity of the demagnetizing
field, and find results in quantitative agreement with experiments on
crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde
Phase coherent VLBI using a satellite link
The results from experiments using the Hermes (formerly CTS) communications satellite to provide a local oscillator and data link between two antennas are presented. The techniques used to compensate for the satellite motion and translation oscillator are described. Plans for a series of three station experiments to measure UT and polar motion using the ANIK-B satellite to synchronize the local oscillators are discussed
First direct observation of Dirac fermions in graphite
Originating from relativistic quantum field theory, Dirac fermions have been
recently applied to study various peculiar phenomena in condensed matter
physics, including the novel quantum Hall effect in graphene, magnetic field
driven metal-insulator-like transition in graphite, superfluid in 3He, and the
exotic pseudogap phase of high temperature superconductors. Although Dirac
fermions are proposed to play a key role in these systems, so far direct
experimental evidence of Dirac fermions has been limited. Here we report the
first direct observation of massless Dirac fermions with linear dispersion near
the Brillouin zone (BZ) corner H in graphite, coexisting with quasiparticles
with parabolic dispersion near another BZ corner K. In addition, we report a
large electron pocket which we attribute to defect-induced localized states.
Thus, graphite presents a novel system where massless Dirac fermions,
quasiparticles with finite effective mass, and defect states all contribute to
the low energy electronic dynamics.Comment: Nature Physics, in pres
Dynamics of a ferromagnetic domain wall: avalanches, depinning transition and the Barkhausen effect
We study the dynamics of a ferromagnetic domain wall driven by an external
magnetic field through a disordered medium. The avalanche-like motion of the
domain walls between pinned configurations produces a noise known as the
Barkhausen effect. We discuss experimental results on soft ferromagnetic
materials, with reference to the domain structure and the sample geometry, and
report Barkhausen noise measurements on FeCoB amorphous
alloy. We construct an equation of motion for a flexible domain wall, which
displays a depinning transition as the field is increased. The long-range
dipolar interactions are shown to set the upper critical dimension to ,
which implies that mean-field exponents (with possible logarithmic correction)
are expected to describe the Barkhausen effect. We introduce a mean-field
infinite-range model and show that it is equivalent to a previously introduced
single-degree-of-freedom model, known to reproduce several experimental
results. We numerically simulate the equation in , confirming the
theoretical predictions. We compute the avalanche distributions as a function
of the field driving rate and the intensity of the demagnetizing field. The
scaling exponents change linearly with the driving rate, while the cutoff of
the distribution is determined by the demagnetizing field, in remarkable
agreement with experiments.Comment: 17 RevTeX pages, 19 embedded ps figures + 1 extra figure, submitted
to Phys. Rev.
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