1,501 research outputs found
Peak effect and dynamic melting of vortex matter in NbSe crystals
We present a mode locking (ML) phenomenon of vortex matter observed around
the peak effect regime of 2H-NbSe pure single crystals. The ML features
allow us not only to trace how the shear rigidity of driven vortices persists
on approaching the second critical field, but also to demonstrate a dynamic
melting transition of driven vortices at a given velocity. We observe the
velocity dependent melting signatures in the peak effect regime, which reveal a
crossover between the disorder-induced transition at small velocity and the
thermally induced transition at large velocity. This uncovers the relationship
between the peak effect and the thermal melting.Comment: To appear in Physical Review Lette
The dynamics of spiral arms in pure stellar disks
It has been believed that spirals in pure stellar disks, especially the ones
spontaneously formed, decay in several galactic rotations due to the increase
of stellar velocity dispersions. Therefore, some cooling mechanism, for example
dissipational effects of the interstellar medium, was assumed to be necessary
to keep the spiral arms. Here we show that stellar disks can maintain spiral
features for several tens of rotations without the help of cooling, using a
series of high-resolution three-dimensional -body simulations of pure
stellar disks. We found that if the number of particles is sufficiently large,
e.g., , multi-arm spirals developed in an isolated disk can
survive for more than 10 Gyrs. We confirmed that there is a self-regulating
mechanism that maintains the amplitude of the spiral arms. Spiral arms increase
Toomre's of the disk, and the heating rate correlates with the squared
amplitude of the spirals. Since the amplitude itself is limited by the value of
, this makes the dynamical heating less effective in the later phase of
evolution. A simple analytical argument suggests that the heating is caused by
gravitational scattering of stars by spiral arms, and that the self-regulating
mechanism in pure-stellar disks can effectively maintain spiral arms on a
cosmological timescale. In the case of a smaller number of particles, e.g.,
, spiral arms grow faster in the beginning of the simulation
(while is small) and they cause a rapid increase of . As a result, the
spiral arms become faint in several Gyrs.Comment: 18 pages, 19 figures, accepted for Ap
Dynamic ordering of driven vortex matter in the peak effect regime of amorphous MoGe films and 2H-NbSe2 crystals
Dynamic ordering of driven vortex matter has been investigated in the peak
effect regime of both amorphous MoGe films and 2H-NbSe2 crystals by mode
locking (ML) and dc transport measurements. ML features allow us to trace how
the shear rigidity of driven vortices evolves with the average velocity.
Determining the onset of ML resonance in different magnetic fields and/or
temperatures, we find that the dynamic ordering frequency (velocity) exhibits a
striking divergence in the higher part of the peak effect regime.
Interestingly, this phenomenon is accompanied by a pronounced peak of dynamic
critical current. Mapping out field-temperature phase diagrams, we find that
divergent points follow well the thermodynamic melting curve of the ideal
vortex lattice over wide field and/or temperature ranges. These findings
provide a link between the dynamic and static melting phenomena which can be
distinguished from the disorder induced peak effect.Comment: 9 pages, 6 figure
Formation of close-in super-Earths in evolving protoplanetary disks due to disk winds
Planets with masses larger than about 0.1 Earth-masses undergo rapid inward
migration (type I migration) in a standard protoplanetary disk. Recent
magnetohydrodynamical simulations revealed the presence of magnetically driven
disk winds, which would alter the disk profile and the type I migration in the
close-in region. We investigate orbital evolution of planetary embryos in disks
that viscously evolve under the effects of disk winds. The aim is to discuss
effects of altered disk profiles on type I migration. In addition, we aim to
examine whether observed distributions of close-in super-Earths can be
reproduced by simulations that include effects of disk winds. We perform N-body
simulations of super-Earth formation from planetary embryos, in which a recent
model for disk evolution is used. We explore a wide range of parameters and
draw general trends. We also carry out N-body simulations of close-in
super-Earth formation from embryos in such disks under various conditions. We
find that the type I migration is significantly suppressed in many cases. Even
in cases in which inward migration occurs, the migration timescale is
lengthened to 1 Myr, which mitigates the type I migration problem. This is
because the gas surface density is decreased and has a flatter profile in the
close-in region due to disk winds. We find that when the type I migration is
significantly suppressed, planets undergo late orbital instability during the
gas depletion, leading to a non-resonant configuration. We also find that
observed distributions of close-in super-Earths (e.g., period ratio, mass
ratio) can be reproduced. In addition, we show that in some results of
simulations, systems with a chain of resonant planets, like the TRAPPIST-1
system, form.Comment: 18 pages, 19 figures, accepted for publication in A&
Colloidal Composite of Hydroxylated Fullerenes and Gold Nanoparticles
Since bare gold nanoparticles are unstable, they have to be stabilized by protecting with ligands, stabilizing with polymers or immobilizing on solids. Properties of gold nanoparticles depend on the design of their protecting ligands
Functionalization of different polymers with sulfonic groups as a way to coat them with a biomimetic apatite layer
Covalent coupling of sulfonic group (–SO3H)
was attempted on different polymers to evaluate efficacy of
this functional group in inducing nucleation of apatite in
body environment, and thereupon to design a simple biomimetic
process for preparing bonelike apatite-polymer
composites. Substrates of polyethylene terephthalate
(PET), polycaprolactam (Nylon 6), high molecular weight
polyethylene (HMWPE) and ethylene-vinyl alcohol copolymer
(EVOH) were subjected to sulfonation by being
soaked in sulfuric acid (H2SO4) or chlorosulfonic acid
(ClSO3H) with different concentrations. In order to incorporate
calcium ions, the sulfonated substrates were soaked
in saturated solution of calcium hydroxide (Ca(OH)2). The
treated substrates were soaked in a simulated body fluid
(SBF). Fourier transformed infrared spectroscopy, thin-film
X-ray diffraction, and scanning electron microscopy
showed that the sulfonation and subsequent Ca(OH)2
treatments allowed formation of –SO3H groups binding
Ca2+ ions on the surface of HMWPE and EVOH, but not on
PET and Nylon 6. The HMWPE and EVOH could thus
form bonelike apatite layer on their surfaces in SBF within
7 d. These results indicate that the –SO3H groups are
effective for inducing apatite nucleation, and thereby that
surface sulfonation of polymers are effective pre-treatment
method for preparing biomimetic apatite on their surfaces
Effects of hydroxyapatite and PDGF concentrations on osteoblast growth in a nanohydroxyapatite-polylactic acid composite for guided tissue regeneration
The technique of guided tissue regeneration (GTR) has evolved over recent years in an attempt to achieve periodontal tissue regeneration by the use of a barrier membrane. However, there are significant limitations in the currently available membranes and overall outcomes may be limited. A degradable composite material was investigated as a potential GTR membrane material. Polylactic acid (PLA) and nanohydroxyapatite (nHA) composite was analysed, its bioactive potential and suitability as a carrier system for growth factors were assessed. The effect of nHA concentrations and the addition of platelet derived growth factor (PDGF) on osteoblast proliferation and differentiation was investigated. The bioactivity was dependent on the nHA concentration in the films, with more apatite deposited on films containing higher nHA content. Osteoblasts proliferated well on samples containing low nHA content and differentiated on films with higher nHA content. The composite films were able to deliver PDGF and cell proliferation increased on samples that were pre absorbed with the growth factor. nHA–PLA composite films are able to deliver active PDGF. In addition the bioactivity and cell differentiation was higher on films containing more nHA. The use of a nHA–PLA composite material containing a high concentration of nHA may be a useful material for GTR membrane as it will not only act as a barrier, but may also be able to enhance bone regeneration by delivery of biologically active molecules
The Effect of an Early Planetesimal-Driven Migration of the Giant Planets on Terrestrial Planet Formation
The migration of the giant planets due to the scattering of planetesimals
causes powerful resonances to move through the asteroid belt and the
terrestrial planet region. Exactly when and how the giant planets migrated is
not well known. In this paper we present results of an investigation of the
formation of the terrestrial planets during and after the migration of the
giant planets. The latter is assumed to have occurred immediately after the
dissipation of the nebular disk -- i.e. "early" with respect to the timing of
the Late Heavy Bombardment (LHB). The presumed cause of our modeled early
migration of the giant planets is angular mometum transfer between the planets
and scattered planetesimals.Comment: Accepted for publication in Astronomy and Astrophysic
PO-0807: 3D and 4D dose calculations for tumour-tracking irradiation of lung/liver tumours using gimbaled linac
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