274,862 research outputs found
Moduli Spaces of Parabolic Higgs Bundles and Parabolic K(D) Pairs over Smooth Curves: I
This paper concerns the moduli spaces of rank two parabolic Higgs bundles and
parabolic K(D) pairs over a smooth curve. Precisely which parabolic bundles
occur in stable K(D), pairs and stable Higgs bundles is determined. Using Morse
theory, the moduli space of parabolic Higgs bundles is shown to be a
non-compact, connected, simply connected manifold, and a computation of its
Poincar\'e polynomial is given.Comment: 25 pages, figure correcte
Rationality of Moduli Spaces of Parabolic Bundles
The moduli space of parabolic bundles with fixed determinant over a smooth
curve of genus greater than one is proved to be rational whenever one of the
multiplicities associated to the quasi-parabolic structure is equal to one. It
follows that if rank and degree are coprime, the moduli space of vector bundles
is stably rational, and the bound obtained on the level is strong enough to
conclude rationality in many cases.Comment: latex2
Micromagnetic Simulation of Nanoscale Films with Perpendicular Anisotropy
A model is studied for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains with Ising-like behavior,
interacting via exchange as well as via dipolar forces. Additionally, the model
contains an energy barrier and a coupling to an external magnetic field.
Disorder is taken into account in order to describe realistic domain and domain
wall structures. The influence of a finite temperature as well as the dynamics
can be modeled by a Monte Carlo simulation.
Many of the experimental findings can be investigated and at least partly
understood by the model introduced above. For thin films the magnetisation
reversal is driven by domain wall motion. The results for the field and
temperature dependence of the domain wall velocity suggest that for thin films
hysteresis can be described as a depinning transition of the domain walls
rounded by thermal activation for finite temperatures.Comment: Revtex, Postscript Figures, to be published in J. Appl.Phy
Identification of Coulomb blockade and macroscopic quantum tunneling by noise
The effects of Macroscopic Quantum Tunneling (MQT) and Coulomb Blockade (CB)
in Josephson junctions are of considerable significance both for the
manifestations of quantum mechanics on the macroscopic scale and potential
technological applications. These two complementary effects are shown to be
clearly distinguishable from the associated noise spectra. The current noise is
determined exactly and a rather sharp crossover between flux noise in the MQT
and charge noise in the CB regions is found as the applied voltage is changed.
Related results hold for the voltage noise in current-biased junctions.Comment: 6 pages, 3 figures, epl.cls include
Resummations in Hot Scalar Electrodynamics
The gauge-boson sector of perturbative scalar electrodynamics is investigated
in detail as a testing ground for resummation methods in hot gauge theories. It
also serves as a simple non-trivial reference system for the non-Abelian gluon
plasma. The complete next-to-leading order contributions to the polarization
tensor are obtained within the resummation scheme of Braaten and Pisarski. The
simpler scheme proposed recently by Arnold and Espinosa is shown to apply to
static quantities only, whereas Braaten-Pisarski resummation turns out to need
modification for collective phenomena close to the light-cone. Finally, a
recently proposed resummation of quasi-particle damping contributions is
assessed critically.Comment: 53 p. LaTeX, 7 figs. (2 in LaTeX, 5 EPS appended as uu-encoded file),
ITP-UH-01/94 & DESY 94-03
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