17,698 research outputs found
Parallel density matrix propagation in spin dynamics simulations
Several methods for density matrix propagation in distributed computing
environments, such as clusters and graphics processing units, are proposed and
evaluated. It is demonstrated that the large communication overhead associated
with each propagation step (two-sided multiplication of the density matrix by
an exponential propagator and its conjugate) may be avoided and the simulation
recast in a form that requires virtually no inter-thread communication. Good
scaling is demonstrated on a 128-core (16 nodes, 8 cores each) cluster.Comment: Submitted for publicatio
MEXIT: Maximal un-coupling times for stochastic processes
Classical coupling constructions arrange for copies of the \emph{same} Markov
process started at two \emph{different} initial states to become equal as soon
as possible. In this paper, we consider an alternative coupling framework in
which one seeks to arrange for two \emph{different} Markov (or other
stochastic) processes to remain equal for as long as possible, when started in
the \emph{same} state. We refer to this "un-coupling" or "maximal agreement"
construction as \emph{MEXIT}, standing for "maximal exit". After highlighting
the importance of un-coupling arguments in a few key statistical and
probabilistic settings, we develop an explicit \MEXIT construction for
stochastic processes in discrete time with countable state-space. This
construction is generalized to random processes on general state-space running
in continuous time, and then exemplified by discussion of \MEXIT for Brownian
motions with two different constant drifts.Comment: 28 page
Integrability of generalized (matrix) Ernst equations in string theory
The integrability structures of the matrix generalizations of the Ernst
equation for Hermitian or complex symmetric -matrix Ernst potentials
are elucidated. These equations arise in the string theory as the equations of
motion for a truncated bosonic parts of the low-energy effective action
respectively for a dilaton and - matrix of moduli fields or for a
string gravity model with a scalar (dilaton) field, U(1) gauge vector field and
an antisymmetric 3-form field, all depending on two space-time coordinates
only. We construct the corresponding spectral problems based on the
overdetermined -linear systems with a spectral parameter and the
universal (i.e. solution independent) structures of the canonical Jordan forms
of their matrix coefficients. The additionally imposed conditions of existence
for each of these systems of two matrix integrals with appropriate symmetries
provide a specific (coset) structures of the related matrix variables. An
equivalence of these spectral problems to the original field equations is
proved and some approach for construction of multiparametric families of their
solutions is envisaged.Comment: 15 pages, no figures, LaTeX; based on the talk given at the Workshop
``Nonlinear Physics: Theory and Experiment. III'', 24 June - 3 July 2004,
Gallipoli (Lecce), Italy. Minor typos, language and references corrections.
To be published in the proceedings in Theor. Math. Phy
The orbital motion, absolute mass, and high-altitude winds of exoplanet HD209458b
For extrasolar planets discovered using the radial velocity method, the
spectral characterization of the host star leads to a mass-estimate of the star
and subsequently of the orbiting planet. In contrast, if also the orbital
velocity of the planet would be known, the masses of both star and planet could
be determined directly using Newton's law of gravity, just as in the case of
stellar double-line eclipsing binaries. Here we report on the detection of the
orbital velocity of extrasolar planet HD209458b. High dispersion ground-based
spectroscopy during a transit of this planet reveals absorption lines from
carbon monoxide produced in the planet atmosphere, which shift significantly in
wavelength due to the change in the radial component of the planet orbital
velocity. These observations result in a mass determination of the star and
planet of 1.00+-0.22 Msun and 0.64+-0.09 Mjup respectively. A ~2 km/sec
blueshift of the carbon monoxide signal with respect to the systemic velocity
of the host star suggests the presence of a strong wind flowing from the
irradiated dayside to the non-irradiated nightside of the planet within the
0.01-0.1 mbar atmospheric pressure range probed by these observations. The
strength of the carbon monoxide signal suggests a CO mixing ratio of 1-3x10-3
in this planet's upper atmosphere.Comment: 11 Pages main article and 6 pages suppl. information: A final, edited
version appears in the 24 May 2010 issue of Natur
The Computational Complexity of the Lorentz Lattice Gas
The Lorentz lattice gas is studied from the perspective of computational
complexity theory. It is shown that using massive parallelism, particle
trajectories can be simulated in a time that scales logarithmically in the
length of the trajectory. This result characterizes the ``logical depth" of the
Lorentz lattice gas and allows us to compare it to other models in statistical
physics.Comment: 9 pages, LaTeX, to appear in J. Stat. Phy
Determining parameters of the Neugebauer family of vacuum spacetimes in terms of data specified on the symmetry axis
We express the complex potential E and the metrical fields omega and gamma of
all stationary axisymmetric vacuum spacetimes that result from the application
of two successive quadruple-Neugebauer (or two double-Harrison) transformations
to Minkowski space in terms of data specified on the symmetry axis, which are
in turn easily expressed in terms of multipole moments. Moreover, we suggest
how, in future papers, we shall apply our approach to do the same thing for
those vacuum solutions that arise from the application of more than two
successive transformations, and for those electrovac solutions that have axis
data similar to that of the vacuum solutions of the Neugebauer family.
(References revised following response from referee.)Comment: 18 pages (REVTEX
Systematic reviews of complementary therapies - an annotated bibliography. Part 1: Acupuncture
Background Complementary therapies are widespread but controversial. We aim to provide a comprehensive collection and a summary of systematic reviews of clinical trials in three major complementary therapies (acupuncture, herbal medicine, homeopathy). This article is dealing with acupuncture. Potentially relevant reviews were searched through the register of the Cochrane Complementary Medicine Field, the Cochrane Library, Medline, and bibliographies of articles and books. To be included articles had to review prospective clinical trials of acupuncture; had to describe review methods explicitly; had to be published; and had to focus on treatment effects. Information on conditions, interventions, methods, results and conclusions was extracted using a pretested form and summarized descriptively. Results From a total of 48 potentially relevant reviews preselected in a screeening process 39 met the inclusion criteria. 22 were on various pain syndromes or rheumatic diseases. Other topics addressed by more than one review were addiction, nausea, asthma and tinnitus. Almost unanimously the reviews state that acupuncture trials include too few patients. Often included trials are heterogeneous regarding patients, interventions and outcome measures, are considered to have insufficient quality and contradictory results. Convincing evidence is available only for postoperative nausea, for which acupuncture appears to be of benefit, and smoking cessation, where acupuncture is no more effective than sham acupuncture. Conclusions A large number of systematic reviews on acupuncture exists. What is most obvious from these reviews is the need for (the funding of) well-designed, larger clinical trials
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