10,885 research outputs found
Numerical modeling study of the momentum deposition of small amplitude gravity waves in the thermosphere
We study the momentum deposition in the thermosphere from the dissipation of
small amplitude gravity waves (GWs) within a wave packet using a fully
nonlinear two-dimensional compressible numerical model. The model solves the
nonlinear propagation and dissipation of a GW packet from the stratosphere
into the thermosphere with realistic molecular viscosity and thermal
diffusivity for various Prandtl numbers. The numerical simulations are
performed for GW packets with initial vertical wavelengths (λ<sub><i>z</i></sub>)
ranging from 5 to 50 km. We show that λ<sub><i>z</i></sub> decreases in
time as a GW packet dissipates in the thermosphere, in agreement with the
ray trace results of Vadas and Fritts (2005) (VF05). We also find good
agreement for the peak height of the momentum flux (<i>z</i><sub>diss</sub>) between our
simulations and VF05 for GWs with initial λ<sub><i>z</i></sub> ≤ 2π <i>H</i> in
an isothermal, windless background, where <i>H</i> is the density scale height. We
also confirm that <i>z</i><sub>diss</sub> increases with increasing Prandtl number. We
include eddy diffusion in the model, and find that the momentum deposition
occurs at lower altitudes and has two separate peaks for GW packets with
small initial λ<sub><i>z</i></sub>. We also simulate GW packets in a
non-isothermal atmosphere. The net λ<sub><i>z</i></sub> profile is a competition
between its decrease from viscosity and its increase from the increasing
background temperature. We find that the wave packet disperses more in the
non-isothermal atmosphere, and causes changes to the momentum flux and
λ<sub><i>z</i></sub> spectra at both early and late times for GW packets with
initial λ<sub><i>z</i></sub> ≥ 10 km. These effects are caused by the
increase in <i>T</i> in the thermosphere, and the decrease in <i>T</i> near the mesopause
Refinement and growth enhancement of Al2Cu phase during magnetic field assisting directional solidification of hypereutectic Al-Cu alloy.
International audienceUnderstanding how the magnetic fields affect the formation of reinforced phase during solidification is crucial to tailor the structure and therefor the performance of metal matrix in situ composites. In this study, a hypereutectic Al-40 wt.% Cu alloy has been directionally solidified under various axial magnetic fields and the morphology of Al2Cu phase was quantified in 3D by means of high resolution synchrotron X-ray tomography. With rising magnetic fields, both increase of Al2Cu phase's total volume and decrease of each column's transverse section area were found. These results respectively indicate the growth enhancement and refinement of the primary Al2Cu phase in the magnetic field assisting directional solidification. The thermoelectric magnetic forces (TEMF) causing torque and dislocation multiplication in the faceted primary phases were thought dedicate to respectively the refinement and growth enhancement. To verify this, a real structure based 3D simulation of TEMF in Al2Cu column was carried out, and the dislocations in the Al2Cu phase obtained without and with a 10T high magnetic field were analysed by the transmission electron microscope
Identification of the white dwarf companion to millisecond pulsar J2317+1439
We report identification of the optical counterpart to the companion of the
millisecond pulsar J2317+1439. At the timing position of the pulsar, we find an
object with , and . The
magnitudes and colors of the object are consistent with it being a white dwarf.
By comparing with white dwarf cooling models, we estimate that it has a mass of
M, an effective temperature of
K and a cooling age of Gyr. Combining our
results with published constraints on the orbital parameters obtained through
pulsar timing, we estimate the pulsar mass to be
M. Although the constraint on the pulsar mass is still weak, there is
a significant possibility that the pulsar could be more massive than two solar
mass.Comment: 7 pages, 6 figures, accepted for publication in Ap
Delay-dependent robust stability of stochastic delay systems with Markovian switching
In recent years, stability of hybrid stochastic delay systems, one of the important issues in the study of stochastic systems, has received considerable attention. However, the existing results do not deal with the structure of the diffusion but estimate its upper bound, which induces conservatism. This paper studies delay-dependent robust stability of hybrid stochastic delay systems. A delay-dependent criterion for robust exponential stability of hybrid stochastic delay systems is presented in terms of linear matrix inequalities (LMIs), which exploits the structure of the diffusion. Numerical examples are given to verify the effectiveness and less conservativeness of the proposed method
Probing neutral top-pion via a flavor-changing process
In the framework of topcolor-assisted-technicolor model(TC2), we study a
flavor-changing neutral top-pion production process . The study shows that there exists a resonance effect
which can enhance the cross section up to a few fb even tens fb. For a yearly
luminosity 100 at future linear colliders, there might be hundreds
even thousands events to be produced. On the other hand, the background of such
flavor-changing process is very clean due to the GIM mechanism in SM . With
such sufficient events and clean background, neutral toppion could be detected
at future linear colliders with high center of energy and luminosity. Our study
provides a possible way to test TC2 model.Comment: 10 pages, 4 figures,has been accepted by Phys.Rev.
Semimetal-semiconductor transition and giant linear magnetoresistances in three-dimensional Dirac semimetal Bi0.96Sb0.04 single crystals
Three-dimensional (3D) Dirac semimetals are new quantum materials and can be
viewed as 3D analogues of graphene. Many fascinating electronic properties have
been proposed and realized in 3D Dirac semimetals, which demonstrates their
potential applications in next generation quantum devices. Bismuth-antimony
Bi1-xSbx can be tuned from a topological insulator to a band insulator through
a quantum critical point at x ~ 4%, where 3D Dirac fermions appear. Here, we
report on a magnetotransport study of Bi1-xSbx at such a quantum critical
point. An unusual magnetic-field induced semimetal-semiconductor phase
transition was observed in the Bi0.96Sb0.04 single crystals. In a magnetic
field of 8 T, Bi0.96Sb0.04 single crystals show giant magnetoresistances of up
to 6000% at low-temperature, 5 K, and 300% at room-temperature, 300 K. The
observed magnetoresistances keep linear down to approximate zero-field when the
temperature is below 200 K. Our experimental results are not only interesting
for the fundamental physics of 3D Dirac semimetals, but also for potential
applications of 3D Dirac semimetals in magnetoelectronic devices.Comment: 9 pages, 3 figure
Strain-induced magnetic phase transition in SrCoO thin films
It has been well established that both in bulk at ambient pressure and for
films under modest strains, cubic SrCoO () is a
ferromagnetic metal. Recent theoretical work, however, indicates that a
magnetic phase transition to an antiferromagnetic structure could occur under
large strain accompanied by a metal-insulator transition. We have observed a
strain-induced ferromagnetic to antiferromagnetic phase transition in
SrCoO films grown on DyScO substrates, which provide a large
tensile epitaxial strain, as compared to ferromagnetic films under lower
tensile strain on SrTiO substrates. Magnetometry results demonstrate the
existence of antiferromagnetic spin correlations and neutron diffraction
experiments provide a direct evidence for a G-type antiferromagnetic structure
with Ne\'el temperatures between and depending on the oxygen content of the samples. Therefore, our
data experimentally confirm the predicted strain-induced magnetic phase
transition to an antiferromagnetic state for SrCoO thin films
under large epitaxial strain.Comment: 6 pages, 4 figure
Topological quantum phase transition in an extended Kitaev spin model
We study the quantum phase transition between Abelian and non-Abelian phases
in an extended Kitaev spin model on the honeycomb lattice, where the periodic
boundary condition is applied by placing the lattice on a torus. Our analytical
results show that this spin model exhibits a continuous quantum phase
transition. Also, we reveal the relationship between bipartite entanglement and
the ground-state energy. Our approach directly shows that both the entanglement
and the ground-state energy can be used to characterize the topological quantum
phase transition in the extended Kitaev spin model.Comment: 9 Pages, 4 figure
- âŠ