326 research outputs found
Atom loss and the formation of a molecular Bose-Einstein condensate by Feshbach resonance
In experiments conducted recently at MIT on Na Bose-Einstein condensates [S.
Inouye et al, Nature 392, 151 (1998); J. Stenger et al, Phys. Rev. Lett. 82,
2422 (1999)], large loss rates were observed when a time-varying magnetic field
was used to tune a molecular Feshbach resonance state near the state of a pair
of atoms in the condensate. A collisional deactivation mechanism affecting a
temporarily formed molecular condensate [see V. A. Yurovsky, A. Ben-Reuven, P.
S. Julienne and C. J. Williams, Phys. Rev. A 60, R765 (1999)], studied here in
more detail, accounts for the results of the slow-sweep experiments. A best fit
to the MIT data yields a rate coefficient for deactivating atom-molecule
collisions of 1.6e-10 cm**3/s. In the case of the fast sweep experiment, a
study is carried out of the combined effect of two competing mechanisms, the
three-atom (atom-molecule) or four-atom (molecule-molecule) collisional
deactivation vs. a process of two-atom trap-state excitation by curve crossing
[F. H. Mies, P. S. Julienne, and E. Tiesinga, Phys. Rev. A 61, 022721 (2000)].
It is shown that both mechanisms contribute to the loss comparably and
nonadditively.Comment: LaTeX, 14 pages, 12 PostScript figures, uses REVTeX and psfig,
submitted to Physical Review
Anisotropic Superparamagnetism of Monodispersive Cobalt-Platinum Nanocrystals
Based on the high-temperature organometallic route (Sun et al. Science 287,
1989 (2000)), we have synthesized powders containing CoPt_3 single crystals
with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths
sigma=0.15(1). In the entire temperature range from 5 K to 400 K, the
zero-field cooled susceptibility chi(T) displays significant deviations from
ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the
deviations arise from the (mean-field) type reduction of the ferromagnetic
moments, while below the blocking temperature T_b, chi(T) is suppressed by the
presence of energy barriers, the distributions of which scale with the particle
volumes obtained from transmission electron microscopy (TEM). This indication
for volume anisotropy is supported by scaling analyses of the shape of the
magnetic absorption chi''(T,omega) which reveal distribution functions for the
barriers being also consistent with the volume distributions observed by TEM.
Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior
providing 2.5(1) mu_B per CoPt_3 in agreement with reports on bulk and thin
film CoPt_3. The non-Langevin shape of the magnetization curves at lower
temperatures is for the first time interpreted as anisotropic
superparamagnetism by taking into account an anisotropy energy of the
nanoparticles E_A(T). Using the magnitude and temperature variation of E_A(T),
the mean energy barriers and 'unphysical' small switching times of the
particles obtained from the analyses of chi''(T,omega) are explained. Below T_b
hysteresis loops appear and are quantitatively described by a blocking model,
which also ignores particle interactions, but takes the size distributions from
TEM and the conventional field dependence of E_A into account.Comment: 12 pages with 10 figures and 1 table. Version accepted for
publication in Phys. Rev. B . Two-column layou
Temporal fluctuations of waves in weakly nonlinear disordered media
We consider the multiple scattering of a scalar wave in a disordered medium
with a weak nonlinearity of Kerr type. The perturbation theory, developed to
calculate the temporal autocorrelation function of scattered wave, fails at
short correlation times. A self-consistent calculation shows that for
nonlinearities exceeding a certain threshold value, the multiple-scattering
speckle pattern becomes unstable and exhibits spontaneous fluctuations even in
the absence of scatterer motion. The instability is due to a distributed
feedback in the system "coherent wave + nonlinear disordered medium". The
feedback is provided by the multiple scattering. The development of instability
is independent of the sign of nonlinearity.Comment: RevTeX, 15 pages (including 5 figures), accepted for publication in
Phys. Rev.
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Nuclear Alpha-Particle Condensates
The -particle condensate in nuclei is a novel state described by a
product state of 's, all with their c.o.m. in the lowest 0S orbit. We
demonstrate that a typical -particle condensate is the Hoyle state
( MeV, state in C), which plays a crucial role for
the synthesis of C in the universe. The influence of antisymmentrization
in the Hoyle state on the bosonic character of the particle is
discussed in detail. It is shown to be weak. The bosonic aspects in the Hoyle
state, therefore, are predominant. It is conjectured that -particle
condensate states also exist in heavier nuclei, like O,
Ne, etc. For instance the state of O at MeV
is identified from a theoretical analysis as being a strong candidate of a
condensate. The calculated small width (34 keV) of ,
consistent with data, lends credit to the existence of heavier Hoyle-analogue
states. In non-self-conjugated nuclei such as B and C, we discuss
candidates for the product states of clusters, composed of 's,
triton's, and neutrons etc. The relationship of -particle condensation
in finite nuclei to quartetting in symmetric nuclear matter is investigated
with the help of an in-medium modified four-nucleon equation. A nonlinear order
parameter equation for quartet condensation is derived and solved for
particle condensation in infinite nuclear matter. The strong qualitative
difference with the pairing case is pointed out.Comment: 71 pages, 41 figures, review article, to be published in "Cluster in
Nuclei (Lecture Notes in Physics) - Vol.2 -", ed. by C. Beck,
(Springer-Verlag, Berlin, 2011
Categorizing Different Approaches to the Cosmological Constant Problem
We have found that proposals addressing the old cosmological constant problem
come in various categories. The aim of this paper is to identify as many
different, credible mechanisms as possible and to provide them with a code for
future reference. We find that they all can be classified into five different
schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to
imaginary spacetime.Comment: updated version, accepted for publicatio
Predicting oral anticoagulant response using a pharmacodynamic model
We developed a pharmacokinetic and pharmacodynamic model of warfarin absorption, metabolism, and anticoagulant action appropriate for guiding anticoagulant therapy. The model requires only two independently adjustable parameters to describe warfarin's effect on individual patients. For any given individual, these parameters are rapidly and inexpensively identified using a computer program based on the model. Test data were generated by superimposing Gaussian noise on dose-response curves calculated with the model. Then the computer program was applied to the test data. Future prothrombin complex activities (PCA's) and maintenance doses were predicted accurately early in the course of drug administration. In addition, the program accurately predicted PCA response in two groups of normal volunteers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44006/1/10439_2006_Article_BF02363455.pd
RELATIONAL GOODS, MONITORING AND NON-PECUNIARY COMPENSATIONS IN THE NONPROFIT SECTOR: THE CASE OF THE ITALIAN SOCIAL SERVICES
Bounded analytic functions in the Dirichlet space
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46275/1/209_2005_Article_BF01163287.pd
- …