4,669 research outputs found
Dynamics of molecular nanomagnets in time-dependent external magnetic fields: Beyond the Landau-Zener-St\"{u}ckelberg model
The time evolution of the magnetization of a magnetic molecular crystal is
obtained in an external time-dependent magnetic field, with sweep rates in the
kT/s range. We present the 'exact numerical' solution of the time dependent
Schr\"{o}dinger equation, and show that the steps in the hysteresis curve can
be described as a sequence of two-level transitions between adiabatic states.
The multilevel nature of the problem causes the transition probabilities to
deviate significantly from the predictions of the Landau-Zener-St\"{u}ckelberg
model. These calculations allow the introduction of an efficient approximation
method that accurately reproduces the exact results. When including phase
relaxation by means of an appropriate master equation, we observe an interplay
between coherent dynamics and decoherence. This decreases the size of the
magnetization steps at the transitions, but does not modify qualitatively the
physical picture obtained without relaxation.Comment: 8 pages, 7 figure
Early Stage of Superradiance from Bose-Einstein Condensates
We investigate the dynamics of matter and optical waves at the early stage of
superradiant Rayleigh scattering from Bose-Einstein Condensates. Our analysis
is within a spatially dependent quantum model which is capable of providing
analytic solutions for the operators of interest. The predictions of the
present model are compared to the predictions of a closely related mean field
model, and we provide a procedure that allows one to calculate quantum
expectation values by averaging over semiclassical solutions. The coherence
properties of the outgoing scattered light are also analyzed, and it is shown
that the corresponding correlation functions may provide detailed information
about the internal dynamics of the system.Comment: 27 pages, 8 figure
A Technique to Derive Improved Proper Motions for Kepler Objects of Interest
We outline an approach yielding proper motions with higher precision than
exists in present catalogs for a sample of stars in the Kepler field. To
increase proper motion precision we combine first moment centroids of Kepler
pixel data from a single Season with existing catalog positions and proper
motions. We use this astrometry to produce improved reduced proper motion
diagrams, analogous to a Hertzsprung-Russell diagram, for stars identified as
Kepler Objects of Interest. The more precise the relative proper motions, the
better the discrimination between stellar luminosity classes. With UCAC4 and
PPMXL epoch 2000 positions (and proper motions from those catalogs as
quasi-bayesian priors) astrometry for a single test Channel (21) and Season (0)
spanning two years yields proper motions with an average per-coordinate proper
motion error of 1.0 millisecond of arc per year, over a factor of three better
than existing catalogs. We apply a mapping between a reduced proper motion
diagram and an HR diagram, both constructed using HST parallaxes and proper
motions, to estimate Kepler Object of Interest K-band absolute magnitudes. The
techniques discussed apply to any future small-field astrometry as well as the
rest of the Kepler field.Comment: Accepted to The Astronomical Journal 15 August 201
Passage-time statistics of superradiant light pulses from Bose-Einstein condensates
We discuss the passage-time statistics of superradiant light pulses generated
during the scattering of laser light from an elongated atomic Bose-Einstein
condensate. Focusing on the early-stage of the phenomenon, we analyze the
corresponding probability distributions and their scaling behaviour with
respect to the threshold photon number and the coupling strength. With respect
to these parameters, we find quantities which only vary significantly during
the transition between the Kapitza Dirac and the Bragg regimes. A possible
connection of the present observations to Brownian motion is also discussed.Comment: Close to the version published in J. Phys.
Interferometric Astrometry of the Low-mass Binary Gl 791.2 (= HU Del) Using Hubble Space Telescope Fine Guidance Sensor 3: Parallax and Component Masses
With fourteen epochs of fringe tracking data spanning 1.7y from Fine Guidance
Sensor 3 we have obtained a parallax (pi_abs=113.1 +- 0.3 mas) and perturbation
orbit for Gl 791.2A. Contemporaneous fringe scanning observations yield only
three clear detections of the secondary on both interferometer axes. They
provide a mean component magnitude difference, Delta V = 3.27 +- 0.10. The
period (P = 1.4731 yr) from the perturbation orbit and the semi-major axis (a =
0.963 +- 0.007 AU) from the measured component separations with our parallax
provide a total system mass M_A + M_B = 0.412 +- 0.009 M_sun. Component masses
are M_A=0.286 +- 0.006 M_sun and M_B = 0.126 +- 0.003 M_sun. Gl 791.2A and B
are placed in a sparsely populated region of the lower main sequence
mass-luminosity relation where they help define the relation because the masses
have been determined to high accuracy, with errors of only 2%.Comment: 19 pages, 5 figures. The paper is to appear in August 2000 A
Global entanglement and coherent states in an N-partite system
We consider a quantum system consisting of N parts, each of which is a
"quKit" described by a K dimensional Hilbert space. We prove that in the
symmetric subspace, S, a pure state is not globally entangled, if and only if
it is a coherent state. It is also shown that in the orthogonal complement all
states are globally entangled
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