25,259 research outputs found
Heterogeneous spin state in the field-induced phase of volborthite as seen via 51V nuclear magnetic resonance
We report results of 51V NMR in the field-induced phase of volborthite
Cu3V2O7(OH)dot2H2O, a spin-1/2 antiferromagnet on a distorted kagome lattice.
In magnetic fields above 4.5 T, two types of V sites with different spin-echo
decay rates are observed. The hyperfine field at the fast decaying sites has a
distribution, while it is more homogeneous at the slowly decaying sites. Our
results indicate a heterogeneous state consisting of two spatially alternating
Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting
with the other showing a conventional static order.Comment: 5 pages, 4 figure
Early X-ray/UV Line Signatures of GRB Progenitors and Hypernovae
We calculate the X-ray/UV spectral line signatures expected from the
interaction of a gamma-ray burst afterglow and a dense pre-burst environment
produced by the progenitor. We explore the conditions under which Fe line and
edge equivalent widths of 1 keV can arise, and discuss the possibility
of gaining information about possible progenitor scenarios using X-ray metal
line spectra in the first few days of a burst. A wind or supernova shell around
the burst produces an X-ray absorption line spectrum and later emission lines,
while a hypernova funnel model produces mainly emission lines. The Fe \ked can
in some cases be more prominent than the Fe \kal line. Under simple assumptions
for the input continuum luminosity, current reports of observed Fe line
luminosities are compatible with an Fe-enriched funnel model, while lower
values are expected in shell models.Comment: revisions to ApJ ms first submitted 8/21/99; uses a higher and
flatter input spectrum, with modified implications suggesting preference for
funnel model
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A culture of silence: modes of objectification and the silencing of disabled bodies
Throughout history different practices have attempted to silence the experiences of disabled people. In this paper we explore some of these practices including the medical, familial, and self-subjugating practices English-speaking Canadian polio survivors experienced throughout their lives. We analyze participantâs experiences of silence and silencing through a Foucauldian lens, drawing on the three modes of objectification to explain the institutional and cultural discourses around polio subjects that acted upon and through the polio body to silence it. Participantsâ oral history accounts demonstrate how sociocultural and medical practices effectively silenced survivors from speaking about their polio experiences. However, the trope of silence is also uprooted within oral history traditions. We will demonstrate how participants broke their silence and shifted their perspectives on polio and disability, and how this process contributed to their resistance of hegemonic conceptualizations of disability as defective
Matter Mixing in Aspherical Core-collapse Supernovae: Three-dimensional Simulations with Single Star and Binary Merger Progenitor Models for SN 1987A
We perform three-dimensional hydrodynamic simulations of aspherical core-collapse supernovae focusing on the matter mixing in SN 1987A. The impacts of four progenitor (pre-supernova) models and parameterized aspherical explosions are investigated. The four pre-supernova models include a blue supergiant (BSG) model based on a slow merger scenario developed recently for the progenitor of SN 1987A (Urushibata et al. 2018). The others are a BSG model based on a single star evolution and two red supergiant (RSG) models. Among the investigated explosion (simulation) models, a model with the binary merger progenitor model and with an asymmetric bipolar-like explosion, which invokes a jetlike explosion, best reproduces constraints on the mass of high velocity Ni, as inferred from the observed [Fe II] line profiles. The advantage of the binary merger progenitor model for the matter mixing is the flat and less extended profile of the C+O core and the helium layer, which may be characterized by the small helium core mass. From the best explosion model, the direction of the bipolar explosion axis (the strongest explosion direction), the neutron star (NS) kick velocity, and its direction are predicted. Other related implications and future prospects are also given
On the ergoregion instability in rotating gravastars
The ergoregion instability is known to affect very compact objects that
rotate very rapidly and do not possess a horizon. We present here a detailed
analysis on the relevance of the ergoregion instability for the viability of
gravastars. Expanding on some recent results, we show that not all rotating
gravastars are unstable. Rather, stable models can be constructed also with
J/M^2 ~ 1, where J and M are the angular momentum and mass of the gravastar,
respectively. The genesis of gravastars is still highly speculative and
fundamentally unclear if not dubious. Yet, their existence cannot be ruled out
by invoking the ergoregion instability. For the same reason, not all
ultra-compact astrophysical objects rotating with J/M^2 <~ 1 are to be
considered necessarily black holes.Comment: 10 pages, 7 figure
Phase Diagram and Spin Dynamics in Volborthite with a Distorted Kagome Lattice
We report 51V-NMR study on a high-quality powder sample of volborthite
Cu3V2O7(OH)2/2H2O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagome
lattice formed by isosceles triangles. In the magnetic fields below 4.5 T, a
sharp peak in the nuclear spin-lattice relaxation rate 1/T1 accompanied with
line broadening revealed a magnetic transition near 1 K. The low temperature
phase shows anomalies such as a Lorentzian line shape, a 1/T1 propto T behavior
indicating dense low energy excitations, and a large spin-echo decay rate 1/T2
pointing to unusually slow fluctuations. Another magnetic phase appears above
4.5 T with less anomalous spectral shape and dynamics.Comment: 4 pages, 4 figure
Role of low- component in deformed wave functions near the continuum threshold
The structure of deformed single-particle wave functions in the vicinity of
zero energy limit is studied using a schematic model with a quadrupole deformed
finite square-well potential. For this purpose, we expand the single-particle
wave functions in multipoles and seek for the bound state and the Gamow
resonance solutions. We find that, for the states, where is
the -component of the orbital angular momentum, the probability of each
multipole components in the deformed wave function is connected between the
negative energy and the positive energy regions asymptotically, although it has
a discontinuity around the threshold. This implies that the
resonant level exists physically unless the component is inherently large
when extrapolated to the well bound region. The dependence of the multipole
components on deformation is also discussed
Fuel-cell performance of multiply-crosslinked polymer electrolyte membranes prepared by two-step radiation technique
A multiply-crosslinked polymer electrolyte membrane was
prepared by the radiation-induced co-grafting of styrene and a
bis(vinyl phenyl)ethane (BVPE) crosslinker into a
radiation-crosslinked polytetrafluoroethylene (cPTFE) film. We
then investigated its H2/O2 fuel-cell performance at 60 and 80ÂșC in
terms of the effect of radiation and chemical crosslinking. At 60ÂșC,
all the membranes initially exhibited similar performance, but only
the cPTFE-based membranes were durable at 80ÂșC, indicating the
necessity of radiation crosslinking in the PTFE main chains.
Importantly, cell performance of the multiply-crosslinked
membrane was found high enough to reach that of a Nafion112
membrane. This is probably because the BVPE crosslinks in the
graft component improved the membrane-electrode interface in
addition to membrane durability. After severe OCV hold tests at 80
and 95ÂșC, the performance deteriorated, while no significant
change was observed in ohmic resistivity. Accordingly, our
membranes seemed so chemically stable that an influence on
overall performance loss could be negligible
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