114 research outputs found
Mounting Materials for Automated Image Analysis of Coals Using Backscattered Electron Imaging
In order to apply SEM-based automated image analysis (AIA) to the characterization of not only minerals in coal but to the coal itself, sample preparation methods need to be developed beyond common practice. A significant consideration is the degree of contrast achievable between the mount media chosen and the coal. Four low-atomic number materials (epoxy, polyethylene, polystyrene and carnauba wax) were compared for their potential as suitable mounting media for coal samples. Epoxy is satisfactory only for characterization of mineral particles since the contrast between epoxy and coal particles is negligible. Polyethylene or polystyrene have marginal application for use as mounting material for coal characterization due to limited contrast and sample preparation artifacts. Carnauba wax appears satisfactory as a mounting material since it provides good contrast with coal particles with minimal artifacts
Energy Gap Evolution Across the Superconductivity Dome in Single Crystals of (BaK)FeAs
The mechanism of unconventional superconductivity in iron-based
superconductors (IBSs) is one of the most intriguing questions in current
materials research. Among non-oxide IBSs, (BaK)FeAs has
been intensively studied because of its high superconducting transition
temperature and fascinating evolution of the superconducting gap structure from
being fully isotropic at optimal doping (0.4) to becoming nodal at
0.8. Although this marked evolution was identified in several independent
experiments, there are no details of the gap evolution to date because of the
lack of high-quality single crystals covering the entire K-doping range of the
superconducting dome. We conducted a systematic study of the London penetration
depth, , across the full phase diagram for different
concentrations of point-like defects introduced by 2.5 MeV electron
irradiation. Fitting the low-temperature variation with the power law, , we find that the exponent is the highest and
suppression rate with disorder is the smallest at optimal doping, and they
evolve with doping being away from optimal, which is consistent with increasing
gap anisotropy, including an abrupt change around , indicating the
onset of nodal behavior. Our analysis using a self-consistent -matrix
approach suggests the ubiquitous and robust nature of s pairing in IBSs
and argues against a previously suggested transition to a wave state near
in this system
Canted Antiferromagnetic phases in the layered candidate Weyl material EuMnSb
EuMnSb is a candidate topological material which can be tuned towards a
Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM)
phases. The coupling of bands dominated by pure Sb layers hosting topological
fermions to Mn and Eu magnetic states provides a potential path to tune the
topological properties. We present a detailed analysis of the magnetic
structure on three AFM phases based on single-crystal neutron diffraction,
magnetization, and heat capacity data as well as polycrystalline Eu
M\"ossbauer data. The Mn magnetic sublattice orders into a C-type AFM structure
below ~K with the ordered Mn magnetic moment lying
perpendicular to the layers. AFM ordering of the Eu sublattice occurs below
~K with the ordered Eu magnetic moment canted away
from the layer normal and retaining its higher-temperature
order. is ferromagnetically aligned within each Eu layer but
exhibits a complicated AFM layer stacking. Both of these higher-temperature
phases are described by magnetic space group (MSG)
with the chemical and magnetic unit cells
having the same dimensions. Cooling below ~K reveals a third AFM phase
where remains unchanged but develops an
additional in-plane canting. This phase has MSG . We
additionally find evidence of short-range magnetic correlations associated with
the Eu between . Using the
determined magnetic structures, we postulate the signs of nearest-neighbor
intralayer and interlayer exchange constants and the magnetic anisotropy within
a general Heisenberg-model. We then discuss implications of the various AFM
states in EuMnSb and its topological properties.Comment: Submitted to Physical Review
Limits on the high-energy gamma and neutrino fluxes from the SGR 1806-20 giant flare of December 27th, 2004 with the AMANDA-II detector
On December 27th 2004, a giant gamma flare from the Soft Gamma-ray Repeater
1806-20 saturated many satellite gamma-ray detectors. This event was by more
than two orders of magnitude the brightest cosmic transient ever observed. If
the gamma emission extends up to TeV energies with a hard power law energy
spectrum, photo-produced muons could be observed in surface and underground
arrays. Moreover, high-energy neutrinos could have been produced during the SGR
giant flare if there were substantial baryonic outflow from the magnetar. These
high-energy neutrinos would have also produced muons in an underground array.
AMANDA-II was used to search for downgoing muons indicative of high-energy
gammas and/or neutrinos. The data revealed no significant signal. The upper
limit on the gamma flux at 90% CL is dN/dE < 0.05 (0.5) TeV^-1 m^-2 s^-1 for
gamma=-1.47 (-2). Similarly, we set limits on the normalization constant of the
high-energy neutrino emission of 0.4 (6.1) TeV^-1 m^-2 s^-1 for gamma=-1.47
(-2).Comment: 14 pages, 3 figure
Calibration and Characterization of the IceCube Photomultiplier Tube
Over 5,000 PMTs are being deployed at the South Pole to compose the IceCube
neutrino observatory. Many are placed deep in the ice to detect Cherenkov light
emitted by the products of high-energy neutrino interactions, and others are
frozen into tanks on the surface to detect particles from atmospheric cosmic
ray showers. IceCube is using the 10-inch diameter R7081-02 made by Hamamatsu
Photonics. This paper describes the laboratory characterization and calibration
of these PMTs before deployment. PMTs were illuminated with pulses ranging from
single photons to saturation level. Parameterizations are given for the single
photoelectron charge spectrum and the saturation behavior. Time resolution,
late pulses and afterpulses are characterized. Because the PMTs are relatively
large, the cathode sensitivity uniformity was measured. The absolute photon
detection efficiency was calibrated using Rayleigh-scattered photons from a
nitrogen laser. Measured characteristics are discussed in the context of their
relevance to IceCube event reconstruction and simulation efforts.Comment: 40 pages, 12 figure
Detection of Atmospheric Muon Neutrinos with the IceCube 9-String Detector
The IceCube neutrino detector is a cubic kilometer TeV to PeV neutrino
detector under construction at the geographic South Pole. The dominant
population of neutrinos detected in IceCube is due to meson decay in cosmic-ray
air showers. These atmospheric neutrinos are relatively well-understood and
serve as a calibration and verification tool for the new detector. In 2006, the
detector was approximately 10% completed, and we report on data acquired from
the detector in this configuration. We observe an atmospheric neutrino signal
consistent with expectations, demonstrating that the IceCube detector is
capable of identifying neutrino events. In the first 137.4 days of livetime,
234 neutrino candidates were selected with an expectation of 211 +/-
76.1(syst.) +/- 14.5(stat.) events from atmospheric neutrinos
Anisotropic magnetism, resistivity, London penetration depth and magneto-optical imaging of superconducting K0.80Fe1.76Se2 single crystals
Single crystals of K0.80Fe1.76Se2.02 were suscessfully grown from a ternary
solution. We show that although crystals form when cooling a near
stoichiometric melt, crystals are actually growing out of a ternary solution
that remains liquid to at least 850 C. We investigated their chemical
composition, anisotropic magnetic susceptibility and resistivity, specific
heat, thermoelectric power, London penetration depth and flux penetration via
magneto-optical imaging. Whereas the samples appear to be homogeneously
superconducting at low temperatures, there appears to be a broadened transtion
range close to Tc ~ 30 K that may be associated with small variations in
stociometry.Comment: 7 pages, 7 figure
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