1,004,707 research outputs found
Skyrmion collapse
We investigate the topological change of a Belavin-Polyakov skyrmion under
the action of a spin-polarized current. The dynamics is described by the
Schr\"odinger equation for the electrons carrying the current coupled to the
Landau-Lifshitz equation for the evolution of the magnetic texture in a square
lattice. We show that the addition of an exchange dissipation term, tends to
smooth the transition from the skyrmion state to the ferromagnetic state. We
demonstrate that this topological change, in the continuum dissipationless
limit, can be described as a self-similar finite-time singularity by which the
skyrmion core collapses.Comment: 9 pages, 6 figures; v2. discussion added (the title of the published
version is "Skyrmion to ferromagnetic state transition: A description of the
topological change as a finite-time singularity in the skyrmion dynamics"
Collapse Models
This is a review of formalisms and models (nonrelativistic and relativistic)
which modify Schrodinger's equation so that it describes wavefunction collapse
as a dynamical physical process.Comment: 40 pages, to be published in "Open Systems and Measurement in
Relativistic Quantum Theory," F. Petruccione and H. P. Breuer eds. (Springer
Verlag, 1999
A Lagrangian Dynamical Theory for the Mass Function of Cosmic Structures: I Dynamics
A new theory for determining the mass function of cosmic structures is
presented. It relies on a realistic treatment of collapse dynamics.
Gravitational collapse is analyzed in the Lagrangian perturbative framework.
Lagrangian perturbations provide an approximation of truncated type, i.e.
small-scale structure is filtered out. The collapse time is suitably defined as
the instant at which orbit crossing takes place. The convergence of the
Lagrangian series in predicting the collapse time of a homogeneous ellipsoid is
demonstrated; it is also shown that third-order calculations are necessary in
predicting collapse. Then, the Lagrangian prediction, with a correction for
quasi-spherical perturbations, can be used to determine the collapse time of a
homogeneous ellipsoid in a fast and precise way. Furthermore, ellipsoidal
collapse can be considered as a particular truncation of the Lagrangian series.
Gaussian fields with scale-free power spectra are then considered. The
Lagrangian series for the collapse time is found to converge when the collapse
time is not large. In this case, ellipsoidal collapse gives a fast and accurate
approximation of the collapse time; spherical collapse is found to poorly
reproduce the collapse time, even in a statistical sense. Analytical fits of
the distribution functions of the inverse collapse times, as predicted by the
ellipsoid model and by third-order Lagrangian theory, are given. These will be
necessary for a determination of the mass function, which will be given in
paper II.Comment: 18 pages, Latex, uses mn.sty and psfig, 7 postscript figures (fig. 2
and 3 not complete). Revised version, stylistic changes. MNRAS, in pres
Fragmentation Instability of Molecular Clouds: Numerical Simulations
We simulate fragmentation and gravitational collapse of cold, magnetized
molecular clouds. We explore the nonlinear development of an instability
mediated by ambipolar diffusion, in which the collapse rate is intermediate to
fast gravitational collapse and slow quasistatic collapse. Initially uniform
stable clouds fragment into elongated clumps with masses largely determined by
the cloud temperature, but substantially larger than the thermal Jeans mass.
The clumps are asymmetric, with significant rotation and vorticity, and lose
magnetic flux as they collapse. The clump shapes, intermediate collapse rates,
and infall profiles may help explain observations not easily fit by
contemporary slow or rapid collapse models.Comment: 25pp, 20 small eps figures, in press ApJ, April 1, 200
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