1,627 research outputs found
Barkhausen noise from zigzag domain walls
We investigate the Barkhausen noise in ferromagnetic thin films with zigzag
domain walls. We use a cellular automaton model that describes the motion of a
zigzag domain wall in an impure ferromagnetic quasi-two dimensional sample with
in-plane uniaxial magnetization at zero temperature, driven by an external
magnetic field. The main ingredients of this model are the dipolar spin-spin
interactions and the anisotropy energy. A power law behavior with a cutoff is
found for the probability distributions of size, duration and correlation
length of the Barkhausen avalanches, and the critical exponents are in
agreement with the available experiments. The link between the size and the
duration of the avalanches is analyzed too, and a power law behavior is found
for the average size of an avalanche as a function of its duration.Comment: 11 pages, 12 figure
Lyapunov exponents from geodesic spread in configuration space
The exact form of the Jacobi -- Levi-Civita (JLC) equation for geodesic
spread is here explicitly worked out at arbitrary dimension for the
configuration space manifold M_E = {q in R^N | V(q) < E} of a standard
Hamiltonian system, equipped with the Jacobi (or kinetic energy) metric g_J. As
the Hamiltonian flow corresponds to a geodesic flow on (M_E,g_J), the JLC
equation can be used to study the degree of instability of the Hamiltonian
flow. It is found that the solutions of the JLC equation are closely resembling
the solutions of the standard tangent dynamics equation which is used to
compute Lyapunov exponents. Therefore the instability exponents obtained
through the JLC equation are in perfect quantitative agreement with usual
Lyapunov exponents. This work completes a previous investigation that was
limited only to two-degrees of freedom systems.Comment: REVTEX file, 10 pages, 2 figure
Lyapunov exponents from geodesic spread in configuration space
The exact form of the Jacobi–Levi-Civita (JLC) equation for geodesic spread is here explicitly worked out at arbitrary dimension for the configuration space manifold [Formula Presented] of a standard Hamiltonian system, equipped with the Jacobi (or kinetic energy) metric [Formula Presented] As the Hamiltonian flow corresponds to a geodesic flow on [Formula Presented] the JLC equation can be used to study the degree of instability of the Hamiltonian flow. It is found that the solutions of the JLC equation are closely resembling the solutions of the standard tangent dynamics equation which is used to compute Lyapunov exponents. Therefore the instability exponents obtained through the JLC equation are in perfect quantitative agreement with usual Lyapunov exponents. This work completes a previous investigation that was limited only to two degrees of freedom systems. © 1997 The American Physical Society
Dynamic hysteresis from zigzag domain walls
We investigate dynamic hysteresis in ferromagnetic thin films with zigzag
domain walls. We introduce a discrete model describing the motion of a wall in
a disordered ferromagnet with in-plane magnetization, driven by an external
magnetic field, considering the effects of dipolar interactions and anisotropy.
We analyze the effects of external field frequency and temperature on the
coercive field by Monte Carlo simulations, and find a good agreement with the
experimental data reported in literature for Fe/GaAs films. This implies that
dynamic hysteresis in this case can be explained by a single propagating domain
wall model without invoking domain nucleation.Comment: 10 pages, 13 figures; minor modifications and two figures adde
Analyzing intramolecular vibrational energy redistribution via the overlap intensity-level velocity correlator
Numerous experimental and theoretical studies have established that
intramolecular vibrational energy redistribution (IVR) in isolated molecules
has a heirarchical tier structure. The tier structure implies strong
correlations between the energy level motions of a quantum system and its
intensity-weighted spectrum. A measure, which explicitly accounts for this
correaltion, was first introduced by one of us as a sensitive probe of phase
space localization. It correlates eigenlevel velocities with the overlap
intensities between the eigenstates and some localized state of interest. A
semiclassical theory for the correlation is developed for systems that are
classically integrable and complements earlier work focusing exclusively on the
chaotic case. Application to a model two dimensional effective spectroscopic
Hamiltonian shows that the correlation measure can provide information about
the terms in the molecular Hamiltonian which play an important role in an
energy range of interest and the character of the dynamics. Moreover, the
correlation function is capable of highlighting relevant phase space structures
including the local resonance features associated with a specific bright state.
In addition to being ideally suited for multidimensional systems with a large
density of states, the measure can also be used to gain insights into the phase
space transport and localization. It is argued that the overlap intensity-level
velocity correlation function provides a novel way of studying vibrational
energy redistribution in isolated molecules. The correlation function is
ideally suited to analyzing the parametric spectra of molecules in external
fields.Comment: 16 pages, 13 figures (low resolution
New AGNs discovered by H.E.S.S
During the last year, six new Active Galactic Nuclei (AGN) have been
discovered and studied by H.E.S.S. at Very High Energies (VHE). Some of these
recent discoveries have been made thanks to new enhanced analysis methods and
are presented at this conference for the first time. The three blazars 1ES
0414+009, SHBL J001355.9-185406 and 1RXS J101015.9-311909 have been targeted
for observation due to their high levels of radio and X-ray fluxes, while the
Fermi/LAT catalogue of bright sources triggered the observation of PKS 0447-439
and AP Librae. Additionally, the BL Lac 1ES 1312-423 was discovered in the
field-of-view (FoV) of Centaurus A thanks to the large exposure dedicated by
H.E.S.S. to this particularly interesting source. The newly-discovered sources
are presented here and in three companion presentations at this conference.Comment: 8 pages, 3 figures, proceeding from the 25th Texas Symposium on
Relativistic Astrophysics (Heidelberg, Germany, 2010
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