2,446 research outputs found
S-4B orbital workshop attitude control system study
Saturn S-4B orbital workshop attitude control system analysi
Radio Emission from 3D Relativistic Hydrodynamic Jets: Observational Evidence of Jet Stratification
We present the first radio emission simulations from high resolution three
dimensional relativistic hydrodynamic jets, which allow for a study of the
observational implications of the interaction between the jet and external
medium. This interaction gives rise to a stratification of the jet where a fast
spine is surrounded by a slow high energy shear layer. The stratification, and
in particular the large specific internal energy and slow flow in the shear
layer largely determines the emission from the jet. If the magnetic field in
the shear layer becomes helical (e.g., resulting from an initial toroidal field
and an aligned field component generated by shear) the emission shows a cross
section asymmetry, in which either the top or the bottom of the jet dominates
the emission. This, as well as limb or spine brightening, is a function of the
viewing angle and flow velocity, and the top/bottom jet emission predominance
can be reversed if the jet changes direction with respect to the observer, or
presents a change in velocity. The asymmetry is more prominent in the polarized
flux, because of field cancellation (or amplification) along the line of sight.
Recent observations of jet cross section emission asymmetries in the blazar
1055+018 can be explained assuming the existence of a shear layer with a
helical magnetic field.Comment: 6 pages, 5 figures, 1 latex style file, ApJL accepte
Enhancement of Wigner crystallization in quasi low-dimensional solids
The crystallization of electrons in quasi low-dimensional solids is studied
in a model which retains the full three-dimensional nature of the Coulomb
interactions. We show that restricting the electron motion to layers (or
chains) gives rise to a rich sequence of structural transitions upon varying
the particle density. In addition, the concurrence of low-dimensional electron
motion and isotropic Coulomb interactions leads to a sizeable stabilization of
the Wigner crystal, which could be one of the mechanisms at the origin of the
charge ordered phases frequently observed in such compounds
Two-Dimensional Wigner Crystal in Anisotropic Semiconductor
We investigate the effect of mass anisotropy on the Wigner crystallization
transition in a two-dimensional (2D) electron gas. The static and dynamical
properties of a 2D Wigner crystal have been calculated for arbitrary 2D Bravais
lattices in the presence of anisotropic mass, as may be obtainable in Si
MOSFETs with (110) surface. By studying the stability of all possible lattices,
we find significant change in the crystal structure and melting density of the
electron lattice with the lowest ground state energy.Comment: 4 pages, revtex, 4 figure
Laughlin-Jastrow-correlated Wigner crystal in a strong magnetic field
We propose a new ground state trial wavefunction for a two-dimensional Wigner
crystal in a strong perpendicular magnetic field. The wavefunction includes
Laughlin-Jastrow correlations between electron pairs, and may be interpreted as
a crystal state of composite fermions or composite bosons. Treating the power
of the Laughlin-Jastrow factor as a variational parameter, we use quantum
Monte Carlo simulations to compute the energy of these new states. We find that
our wavefunctions have lower energy than existing crystalline wavefunctions in
the lowest Landau level. Our results are consistent with experimental
observations of the filling factor at which the transition between the
fractional quantum Hall liquid and the Wigner crystal occurs for electron
systems. Exchange contributions to the wavefunctions are estimated
quantitatively and shown to be negligible for sufficiently small filling
factors
Pricing and hedging of Asian options: Quasi-explicit solutions via Malliavin calculus
We use Malliavin calculus and the Clark-Ocone formula to derive the hedging strategy of an arithmetic Asian Call option in general terms. Furthermore we derive an expression for the density of the integral over time of a geometric Brownian motion, which allows us to express hedging strategy and price of the Asian option as an analytic expression. Numerical computations which are based on this expression are provided
Interstitials, Vacancies, and Supersolid Order in Vortex Crystals
Interstitials and vacancies in the Abrikosov phase of clean Type II
superconductors are line imperfections, which cannot extend across macroscopic
equilibrated samples at low temperatures. We argue that the entropy associated
with line wandering nevertheless can cause these defects to proliferate at a
sharp transition which will exist if this occurs below the temperature at which
the crystal actually melts. Vortices are both entangled and crystalline in the
resulting ``supersolid'' phase, which in a dual ``boson'' analog system is
closely related to a two-dimensional quantum crystal of He with
interstitials or vacancies in its ground state. The supersolid {\it must} occur
for , where is the decoupling field above which
vortices begin to behave two-dimensionally. Numerical calculations show that
interstitials, rather than vacancies, are the preferred defect for , and allow us to estimate whether proliferation also
occurs for B\,\lot\,B_\times.The implications of the supersolid phase for
transport measurements, dislocation configurations and neutron diffraction are
discussed.Comment: 53 pages and 15 figures, available upon request, written in plain TE
Spatiotemporal Response of Crystals in X-ray Bragg Diffraction
The spatiotemporal response of crystals in x-ray Bragg diffraction resulting
from excitation by an ultra-short, laterally confined x-ray pulse is studied
theoretically. The theory presents an extension of the analysis in symmetric
reflection geometry [1] to the generic case, which includes Bragg diffraction
both in reflection (Bragg) and transmission (Laue) asymmetric scattering
geometries. The spatiotemporal response is presented as a product of a
crystal-intrinsic plane wave spatiotemporal response function and an envelope
function defined by the crystal-independent transverse profile of the incident
beam and the scattering geometry. The diffracted wavefields exhibit amplitude
modulation perpendicular to the propagation direction due to both angular
dispersion and the dispersion due to Bragg's law. The characteristic measure of
the spatiotemporal response is expressed in terms of a few parameters: the
extinction length, crystal thickness, Bragg angle, asymmetry angle, and the
speed of light. Applications to self-seeding of hard x-ray free electron lasers
are discussed, with particular emphasis on the relative advantages of using
either the Bragg or Laue scattering geometries. Intensity front inclination in
asymmetric diffraction can be used to make snapshots of ultra-fast processes
with femtosecond resolution
Experimental setup for studying an ultracold mixture of trapped Yb-Li
We describe and characterize an experimental apparatus that has been used to
study interactions between ultracold lithium atoms and ytterbium ions. The
preparation of ultracold clouds of Li atoms is described as well as their
subsequent transport and overlap with Yb ions trapped in a Paul trap. We
show how the kinetic energy of the ion after interacting with the atoms can be
obtained by laser spectroscopy. From analyzing the dynamics of the ion in the
absence of atoms, we conclude that background heating, due to electric field
noise, limits attainable buffer gas cooling temperatures. We suspect that this
effect can be mitigated by noise reduction and by increasing the density of the
Li gas, in order to improve its cooling power. Imperfections in the Paul trap
lead to so-called excess micromotion, which poses another limitation to the
buffer gas cooling. We describe in detail how we measure and subsequently
minimize excess micromotion in our setup. We measure the effect of excess
micromotion on attainable ion temperatures after buffer gas cooling and compare
this to molecular dynamics simulations which describe the observed data very
well.Comment: 11 pages and 11 figure
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