1,202 research outputs found
Dynamic melting of confined vortex matter
We study {\em dynamic} melting of confined vortex matter moving in
disordered, mesoscopic channels by mode-locking experiments. The dynamic
melting transition, characterized by a collapse of the mode-locking effect,
strongly depends on the frequency, i.e. on the average velocity of the
vortices. The associated dynamic ordering velocity diverges upon approaching
the equilibrium melting line as . The
data provide the first direct evidence for velocity dependent melting and show
that the phenomenon also takes place in a system under disordered confinement.
\pacs{74.25.Qt,83.50.Ha,64.70.Dv,64.60.Ht}Comment: Some small changes have been made. 4 pages, 4 figures included.
Accepted for publication in Phys. Rev. Let
Vortex lattice dynamics in a-NbGe detected by mode-locking experiments
We observed mode-locking (ML) of rf-dc driven vortex arrays in a
superconducting weak pinning a-NbGe film. The ML voltage shows the expected
scaling with the rf-frequency and the magnetic
field. For large dc-velocity (corresponding to a large ML frequency), the ML
current step width exhibits a squared Bessel function dependence on the
rf-amplitude as predicted for ML of a lattice moving elastically through a
random potential.Comment: 2 pages, 2 figures. Contribution to M2S-HTSC Ri
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
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
Angular Momentum Accretion onto a Gas Giant Planet
We investigate the accretion of angular momentum onto a protoplanet system
using three-dimensional hydrodynamical simulations. We consider a local region
around a protoplanet in a protoplanetary disk with sufficient spatial
resolution. We describe the structure of the gas flow onto and around the
protoplanet in detail. We find that the gas flows onto the protoplanet system
in the vertical direction crossing the shock front near the Hill radius of the
protoplanet, which is qualitatively different from the picture established by
two-dimensional simulations. The specific angular momentum of the gas accreted
by the protoplanet system increases with the protoplanet mass. At Jovian orbit,
when the protoplanet mass M_p is M_p < 1 M_J, where M_J is Jovian mass, the
specific angular momentum increases as j \propto M_p. On the other hand, it
increases as j \propto M_p^2/3 when the protoplanet mass is M_p > 1 M_J. The
stronger dependence of the specific angular momentum on the protoplanet mass
for M_p < 1 M_J is due to thermal pressure of the gas. The estimated total
angular momentum of a system of a gas giant planet and a circumplanetary disk
is two-orders of magnitude larger than those of the present gas giant planets
in the solar system. A large fraction of the total angular momentum contributes
to the formation of the circumplanetary disk. We also discuss the satellite
formation from the circumplanetary disk.Comment: 39 pages,13 figures, Submitted to ApJ, For high resolution figures
see http://www2.scphys.kyoto-u.ac.jp/~machidam/jupiter2/ms08jan22.pd
The formation of Uranus and Neptune among Jupiter and Saturn
The outer giant planets, Uranus and Neptune, pose a challenge to theories of
planet formation. They exist in a region of the Solar System where long
dynamical timescales and a low primordial density of material would have
conspired to make the formation of such large bodies ( 15 and 17 times as
massive as the Earth, respectively) very difficult. Previously, we proposed a
model which addresses this problem: Instead of forming in the trans-Saturnian
region, Uranus and Neptune underwent most of their growth among proto-Jupiter
and -Saturn, were scattered outward when Jupiter acquired its massive gas
envelope, and subsequently evolved toward their present orbits. We present the
results of additional numerical simulations, which further demonstrate that the
model readily produces analogues to our Solar System for a wide range of
initial conditions. We also find that this mechanism may partly account for the
high orbital inclinations observed in the Kuiper belt.Comment: Submitted to AJ; 38 pages, 16 figure
Toward a Deterministic Model of Planetary Formation VI: Dynamical Interaction and Coagulation of Multiple Rocky Embryos and Super-Earth Systems around Solar Type Stars
Radial velocity and transit surveys indicate that solar-type stars bear
super-Earths, with mass and period up to ~ 20 M_E and a few months, are more
common than those with Jupiter-mass gas giants. In many cases, these
super-Earths are members of multiple-planet systems in which their mutual
dynamical interaction has influenced their formation and evolution. In this
paper, we modify an existing numerical population synthesis scheme to take into
account protoplanetary embryos' interaction with their evolving natal gaseous
disk, as well as their close scatterings and resonant interaction with each
other. We show that it is possible for a group of compact embryos to emerge
interior to the ice line, grow, migrate, and congregate into closely-packed
convoys which stall in the proximity of their host stars. After the disk-gas
depletion, they undergo orbit crossing, close scattering, and giant impacts to
form multiple rocky Earths or super-Earths in non-resonant orbits around ~
0.1AU with moderate eccentricities of ~0.01-0.1. We suggest that most
refractory super-Earths with period in the range of a few days to weeks may
have formed through this process. These super-Earths differ from Neptune-like
ice giants by their compact sizes and lack of a substantial gaseous envelope.Comment: 37 pages, 10 figures, accepted for publication in Ap
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
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