2,455 research outputs found
High energy collision cascades in tungsten: dislocation loops structure and clustering scaling laws
Recent experiments on in-situ high-energy self-ion irradiation of tungsten
(W) show the occurrence of unusual cascade damage effects resulting from single
ion impacts, shedding light on the nature of radiation damage expected in the
tungsten components of a fusion reactor. In this paper, we investigate the
dynamics of defect production in 150 keV collision cascades in W at atomic
resolution, using molecular dynamics simulations and comparing predictions with
experimental observations. We show that cascades in W exhibit no subcascade
break-up even at high energies, producing a massive, unbroken molten area,
which facilitates the formation of large defect clusters. Simulations show
evidence of the formation of both 1/2 and interstitial-type
dislocation loops, as well as the occurrence of cascade collapse resulting in
vacancy-type dislocation loops, in excellent agreement with experimental
observations. The fractal nature of the cascades gives rise to a scale-less
power law type size distribution of defect clusters.Comment: 6 pages, 3 figure
Kondo physics in tunable semiconductor nanowire quantum dots
We have observed the Kondo effect in strongly coupled semiconducting nanowire
quantum dots. The devices are made from indium arsenide nanowires, grown by
molecular beam epitaxy, and contacted by titanium leads. The device
transparency can be tuned by changing the potential on a gate electrode, and
for increasing transparencies the effects dominating the transport changes from
Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics
appearing in the intermediate region.Comment: 4 pages, 4 figure
Numerically Modeling the First Peak of the Type IIb SN 2016gkg
Many Type IIb supernovae (SNe) show a prominent additional early peak in
their light curves, which is generally thought to be due to the shock cooling
of extended hydrogen-rich material surrounding the helium core of the exploding
star. The recent SN 2016gkg was a nearby Type IIb SN discovered shortly after
explosion, which makes it an excellent candidate for studying this first peak.
We numerically explode a large grid of extended envelope models and compare
these to SN 2016gkg to investigate what constraints can be derived from its
light curve. This includes exploring density profiles for both a convective
envelope and an optically thick steady-state wind, the latter of which has not
typically been considered for Type IIb SNe models. We find that roughly
of extended material with a radius of
reproduces the photometric light curve data,
consistent with pre-explosion imaging. These values are independent of the
assumed density profile of this material, although a convective profile
provides a somewhat better fit. We infer from our modeling that the explosion
must have occurred within of the first observed data
point, demonstrating that this event was caught very close to the moment of
explosion. Nevertheless, our best-fitting one-dimensional models overpredict
the earliest velocity measurements, which suggests that the hydrogen-rich
material is not distributed in a spherically symmetric manner. We compare this
to the asymmetries seen in the SN IIb remnant Cas A, and we discuss the
implications of this for Type IIb SN progenitors and explosion models.Comment: 8 pages, 8 figures, updated version accepted for publication in The
Astrophysical Journa
Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V
We present a chemodynamical analysis of the Leo~V dwarf galaxy, based on Keck
II DEIMOS spectra of 8 member stars. We find a systemic velocity for the system
of kms, and barely resolve a
velocity dispersion for the system, with kms, consistent with previous studies of Leo~V. The
poorly resolved dispersion means we are unable to adequately constrain the dark
matter content of Leo~V. We find an average metallicity for the dwarf of
[Fe/H], and measure a significant spread in the iron abundance
of its member stars, with [Fe/H] dex, which cleanly
identifies Leo~V as a dwarf galaxy that has been able to self-enrich its
stellar population through extended star formation. Owing to the tentative
photometric evidence for tidal substructure around Leo~V, we also investigate
whether there is any evidence for tidal stripping or shocking of the system
within its dynamics. We measure a significant velocity gradient across the
system, of kms per
arcmin (or kms~kpc), which points almost directly
toward the Galactic centre. We argue that Leo~V is likely a dwarf on the brink
of dissolution, having just barely survived a past encounter with the centre of
the Milky Way.Comment: 14 pages, 12 figures, accepted for publication in MNRAS. Updated to
include minor revisions from referee proces
Processing techniques development
There are no author-identified significant results in this report
Direct observation of size scaling and elastic interaction between nano-scale defects in collision cascades
Using in-situ transmission electron microscopy, we have directly observed
nano-scale defects formed in ultra-high purity tungsten by low-dose high energy
self-ion irradiation at 30K. At cryogenic temperature lattice defects have
reduced mobility, so these microscope observations offer a window on the
initial, primary damage caused by individual collision cascade events. Electron
microscope images provide direct evidence for a power-law size distribution of
nano-scale defects formed in high-energy cascades, with an upper size limit
independent of the incident ion energy, as predicted by Sand et al. [Eur. Phys.
Lett., 103:46003, (2013)]. Furthermore, the analysis of pair distribution
functions of defects observed in the micrographs shows significant
intra-cascade spatial correlations consistent with strong elastic interaction
between the defects
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