9,905 research outputs found
A simple model of quantum trajectories
Quantum trajectory theory, developed largely in the quantum optics community
to describe open quantum systems subjected to continuous monitoring, has
applications in many areas of quantum physics. In this paper I present a simple
model, using two-level quantum systems (q-bits), to illustrate the essential
physics of quantum trajectories and how different monitoring schemes correspond
to different ``unravelings'' of a mixed state master equation. I also comment
briefly on the relationship of the theory to the Consistent Histories formalism
and to spontaneous collapse models.Comment: 42 pages RevTeX including four figures in encapsulated postscript.
Submitted to special issue of American Journal of Physic
Influence of the Tachocline on Solar Evolution
Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition from
differential rotation in the convection zone to almost uniform rotation in the
radiative interior, the tachocline. At present, this layer extends only over a
few percent of the solar radius and no definitive explanations have been given
for this thiness. Following Spiegel and Zahn (1992, Astron. Astrophys.), who
invoke anisotropic turbulence to stop the spread of the tachocline deeper in
the radiative zone as the Sun evolves, we give some justifications for their
hypothesis by taking into account recent results on rotating shear instability
(Richard and Zahn 1999, Astron. Astrophys.). We study the impact of the
macroscopic motions present in this layer on the Sun's structure and evolution
by introducing a macroscopic diffusivity in updated solar models. We find
that a time dependent treatment of the tachocline significantly improves the
agreement between computed and observed surface chemical species, such as the
Li and modify the internal structure of the Sun (Brun, Turck-Chi\`eze and
Zahn, 1999, in Astrophys. J.).Comment: to appear in Annals of the New York Academy of Sciences, vol 898.
Postscript file, 9 pages and 5 figures New Email Address for A. S. Brun:
[email protected]
The Architecture of MEG Simulation and Analysis Software
MEG (Mu to Electron Gamma) is an experiment dedicated to search for the
decay that is strongly suppressed in the Standard
Model but predicted in several Super Symmetric extensions of it at an
accessible rate. MEG is a small-size experiment ( physicists at
any time) with a life span of about 10 years. The limited human resource
available, in particular in the core offline group, emphasized the importance
of reusing software and exploiting existing expertise. Great care has been
devoted to provide a simple system that hides implementation details to the
average programmer. That allowed many members of the collaboration to
contribute to the development of the software of the experiment with limited
programming skill. The offline software is based on two frameworks: {\bf REM}
in FORTRAN 77 used for the event generation and detector simulation package
{\bf GEM}, based on GEANT 3, and {\bf ROME} in C++ used in the readout
simulation {\bf Bartender} and in the reconstruction and analysis program {\bf
Analyzer}. Event display in the simulation is based on GEANT 3 graphic
libraries and in the reconstruction on ROOT graphic libraries. Data are stored
in different formats in various stage of the processing. The frameworks include
utilities for input/output, database handling and format conversion transparent
to the user.Comment: Presented at the IEEE NSS Knoxville, 2010 Revised according to
referee's remarks Accepted by European Physical Journal Plu
Rossby and Magnetic Prandtl Number Scaling of Stellar Dynamos
Rotational scaling relationships are examined for the degree of equipartition
between magnetic and kinetic energies in stellar convection zones. These
scaling relationships are approached from two paradigms, with first a glance at
scaling relationship built upon an energy-balance argument and second a look at
a force-based scaling. The latter implies a transition between a
nearly-constant inertial scaling when in the asymptotic limit of minimal
diffusion and magnetostrophy, whereas the former implies a weaker scaling with
convective Rossby number. Both scaling relationships are then compared to a
suite of 3D convective dynamo simulations with a wide variety of domain
geometries, stratifications, and range of convective Rossby numbers.Comment: 15 pages, 6 figures, accepted in Ap
Dirac model of electronic transport in graphene antidot barriers
In order to use graphene for semiconductor applications, such as transistors
with high on/off ratios, a band gap must be introduced into this otherwise
semimetallic material. A promising method of achieving a band gap is by
introducing nanoscale perforations (antidots) in a periodic pattern, known as a
graphene antidot lattice (GAL). A graphene antidot barrier (GAB) can be made by
introducing a 1D GAL strip in an otherwise pristine sheet of graphene. In this
paper, we will use the Dirac equation (DE) with a spatially varying mass term
to calculate the electronic transport through such structures. Our approach is
much more general than previous attempts to use the Dirac equation to calculate
scattering of Dirac electrons on antidots. The advantage of using the DE is
that the computational time is scale invariant and our method may therefore be
used to calculate properties of arbitrarily large structures. We show that the
results of our Dirac model are in quantitative agreement with tight-binding for
hexagonal antidots with armchair edges. Furthermore, for a wide range of
structures, we verify that a relatively narrow GAB, with only a few antidots in
the unit cell, is sufficient to give rise to a transport gap
Electronic and optical properties of graphene antidot lattices: Comparison of Dirac and tight-binding models
The electronic properties of graphene may be changed from semimetallic to
semiconducting by introducing perforations (antidots) in a periodic pattern.
The properties of such graphene antidot lattices (GALs) have previously been
studied using atomistic models, which are very time consuming for large
structures. We present a continuum model that uses the Dirac equation (DE) to
describe the electronic and optical properties of GALs. The advantages of the
Dirac model are that the calculation time does not depend on the size of the
structures and that the results are scalable. In addition, an approximation of
the band gap using the DE is presented. The Dirac model is compared with
nearest-neighbour tight-binding (TB) in order to assess its accuracy. Extended
zigzag regions give rise to localized edge states, whereas armchair edges do
not. We find that the Dirac model is in quantitative agreement with TB for GALs
without edge states, but deviates for antidots with large zigzag regions.Comment: 15 pages, 7 figures. Accepted by Journal of Physics: Condensed matte
Impingement of Water Droplets on an NACA 65(sub 1) -212 Airfoil at an Angle of Attack of 4 Deg
The trajectories of droplets in the air flowing past an NACA 651-212 airfoil at an angle of attack of 40 were determined. The collection efficiency, the area of droplet impingement, and the rate of droplet impingement were calculated from the trajectories and are presented herein
Seismic tests for solar models with tachocline mixing
We have computed accurate 1-D solar models including both a macroscopic
mixing process in the solar tachocline as well as up-to-date microscopic
physical ingredients. Using sound speed and density profiles inferred through
primary inversion of the solar oscillation frequencies coupled with the
equation of thermal equilibrium, we have extracted the temperature and hydrogen
abundance profiles. These inferred quantities place strong constraints on our
theoretical models in terms of the extent and strength of our macroscopic
mixing, on the photospheric heavy elements abundance, on the nuclear reaction
rates such as and and on the efficiency of the microscopic
diffusion. We find a good overall agreement between the seismic Sun and our
models if we introduce a macroscopic mixing in the tachocline and allow for
variation within their uncertainties of the main physical ingredients. From our
study we deduce that the solar hydrogen abundance at the solar age is and that based on the Be photospheric depletion, the
maximum extent of mixing in the tachocline is 5% of the solar radius. The
nuclear reaction rate for the fundamental reaction is found to be
MeV barns, i.e., 1.5% higher than the
present theoretical determination. The predicted solar neutrino fluxes are
discussed in the light of the new SNO/SuperKamiokande results.Comment: 16 pages, 12 figures, A&A in press (1) JILA, University of Colorado,
Boulder, CO 80309-0440, USA, (2) LUTH, Observatoire de Paris-Meudon, 92195
Meudon, France, (3) Tata Institute of Fundamental Research, Homi Bhabha road,
Mumbai 400005, India, (4) Department of Physics, University of Mumbai, Mumbai
400098, Indi
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