240 research outputs found
Hysteresis loops of magnetic thin films with perpendicular anisotropy
We model the magnetization of quasi two-dimensional systems with easy
perpendicular (z-)axis anisotropy upon change of external magnetic field along
z. The model is derived from the Landau-Lifshitz-Gilbert equation for
magnetization evolution, written in closed form in terms of the z component of
the magnetization only. The model includes--in addition to the external
field--magnetic exchange, dipolar interactions and structural disorder. The
phase diagram in the disorder/interaction strength plane is presented, and the
different qualitative regimes are analyzed. The results compare very well with
observed experimental hysteresis loops and spatial magnetization patterns, as
for instance for the case of Co-Pt multilayers.Comment: 8 pages, 8 figure
Longitudinal and transverse dissipation in a simple model for the vortex lattice with screening
Transport properties of the vortex lattice in high temperature
superconductors are studied using numerical simulations in the case in which
the non-local interactions between vortex lines are dismissed. The results
obtained for the longitudinal and transverse resistivities in the presence of
quenched disorder are compared with the results of experimental measurements
and other numerical simulations where the full interaction is considered. This
work shows that the dependence on temperature of the resistivities is well
described by the model without interactions, thus indicating that many of the
transport characteristics of the vortex structure in real materials are mainly
a consequence of the topological configuration of the vortex structure only. In
addition, for highly anisotropic samples, a regime is obtained where
longitudinal coherence is lost at temperatures where transverse coherence is
still finite. I discuss the possibility of observing this regime in real
samples.Comment: 9 pages, 7 figures included using epsf.st
Effect of disorder on the vortex-lattice melting transition
We use a three dimensional stacked triangular network of Josephson junctions
as a model for the study of vortex structure in the mixed state of high Tc
superconductors. We show that the addition of disorder destroys the first order
melting transition occurring for clean samples. The melting transition splits
in two different (continuous) transitions, ocurring at temperatures Ti and Tp
(>Ti). At Ti the perpendicular-to-field superconductivity is lost, and at Tp
the parallel-to-field superconductivity is lost. These results agree well with
recent experiments in YBaCuO.Comment: 4 pages + 2 figure
The phase diagram of high-Tc's: Influence of anisotropy and disorder
We propose a phase diagram for the vortex structure of high temperature
superconductors which incorporates the effects of anisotropy and disorder. It
is based on numerical simulations using the three-dimensional Josephson
junction array model. We support the results with an estimation of the internal
energy and configurational entropy of the system. Our results give a unified
picture of the behavior of the vortex lattice, covering from the very
anysotropic BiSrCaCuO to the less anisotropic YBaCuO, and from the first order
melting ocurring in clean samples to the continuous transitions observed in
samples with defects.Comment: 8 pages with 7 figure
Some exact results for the velocity of cracks propagating in non-linear elastic models
We analyze a piece-wise linear elastic model for the propagation of a crack
in a stripe geometry under mode III conditions, in the absence of dissipation.
The model is continuous in the propagation direction and discrete in the
perpendicular direction. The velocity of the crack is a function of the value
of the applied strain. We find analytically the value of the propagation
velocity close to the Griffith threshold, and close to the strain of uniform
breakdown. Contrary to the case of perfectly harmonic behavior up to the
fracture point, in the piece-wise linear elastic model the crack velocity is
lower than the sound velocity, reaching this limiting value at the strain of
uniform breakdown. We complement the analytical results with numerical
simulations and find excellent agreement.Comment: 9 pages, 13 figure
Supersonic crack propagation in a class of lattice models of Mode III brittle fracture
We study a lattice model for mode III crack propagation in brittle materials
in a stripe geometry at constant applied stretching. Stiffening of the material
at large deformation produces supersonic crack propagation. For large
stretching the propagation is guided by well developed soliton waves. For low
stretching, the crack-tip velocity has a universal dependence on stretching
that can be obtained using a simple geometrical argument.Comment: 4 pages, 3 figure
Observation of coasting beam at the HERA Proton--Ring
We present data collected with the HERA-B wire target which prove the
existence of coasting beam at the HERA proton storage ring. The coasting beam
is inherently produced by the proton machine operation and is not dominated by
target effects.Comment: 17 pages (Latex), 12 figures (Enc. Postscript
Stable propagation of an ordered array of cracks during directional drying
We study the appearance and evolution of an array of parallel cracks in a
thin slab of material that is directionally dried, and show that the cracks
penetrate the material uniformly if the drying front is sufficiently sharp. We
also show that cracks have a tendency to become evenly spaced during the
penetration. The typical distance between cracks is mainly governed by the
typical distance of the pattern at the surface, and it is not modified during
the penetration. Our results agree with recent experimental work, and can be
extended to three dimensions to describe the properties of columnar polygonal
patterns observed in some geological formations.Comment: 8 pages, 4 figures, to appear in PR
Pressure-induced amorphization, crystal-crystal transformations and the memory glass effect in interacting particles in two dimensions
We study a model of interacting particles in two dimensions to address the
relation between crystal-crystal transformations and pressure-induced
amorphization. On increasing pressure at very low temperature, our model
undergoes a martensitic crystal-crystal transformation. The characteristics of
the resulting polycrystalline structure depend on defect density, compression
rate, and nucleation and growth barriers. We find two different limiting cases.
In one of them the martensite crystals, once nucleated, grow easily
perpendicularly to the invariant interface, and the final structure contains
large crystals of the different martensite variants. Upon decompression almost
every atom returns to its original position, and the original crystal is fully
recovered. In the second limiting case, after nucleation the growth of
martensite crystals is inhibited by energetic barriers. The final morphology in
this case is that of a polycrystal with a very small crystal size. This may be
taken to be amorphous if we have only access (as experimentally may be the
case) to the angularly averaged structure factor. However, this `X-ray
amorphous' material is anisotropic, and this shows up upon decompression, when
it recovers the original crystalline structure with an orientation correlated
with the one it had prior to compression. The memory effect of this X-ray
amorphous material is a natural consequence of the memory effect associated to
the underlying martensitic transformation. We suggest that this kind of
mechanism is present in many of the experimental observations of the memory
glass effect, in which a crystal with the original orientation is recovered
from an apparently amorphous sample when pressure is released.Comment: 13 pages, 13 figures, to be published in Phys. Rev.
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