65 research outputs found
Polar magneto-optical Kerr effect for low-symmetric ferromagnets
The polar magneto-optical Kerr effect (MOKE) for low-symmetric ferromagnetic
crystals is investigated theoretically based on first-principle calculations of
optical conductivities and a transfer matrix approach for the electrodynamics
part of the problem. Exact average magneto-optical properties of polycrystals
are described, taking into account realistic models for the distribution of
domain orientations. It is shown that for low-symmetric ferromagnetic single
crystals the MOKE is determined by an interplay of crystallographic
birefringence and magnetic effects. Calculations for single and bi-crystal of
hcp 11-20 Co and for a polycrystal of CrO_2 are performed, with results being
in good agreement with experimental data.Comment: 14 pages, 7 figures, accepted for publication in Phys. Rev.
Probabilities in the inflationary multiverse
Inflationary cosmology leads to the picture of a "multiverse," involving an
infinite number of (spatially infinite) post-inflationary thermalized regions,
called pocket universes. In the context of theories with many vacua, such as
the landscape of string theory, the effective constants of Nature are
randomized by quantum processes during inflation. We discuss an analytic
estimate for the volume distribution of the constants within each pocket
universe. This is based on the conjecture that the field distribution is
approximately ergodic in the diffusion regime, when the dynamics of the fields
is dominated by quantum fluctuations (rather than by the classical drift). We
then propose a method for determining the relative abundances of different
types of pocket universes. Both ingredients are combined into an expression for
the distribution of the constants in pocket universes of all types.Comment: 18 pages, RevTeX 4, 2 figures. Discussion of the full probability in
Sec.VI is sharpened; the conclusions are strengthened. Note added explaining
the relation to recent work by Easther, Lim and Martin. Some references adde
SURVEY OF THE DEPENDENCE ON TEMPERATURE OF THE COERCIVITY OF GARNET-FILMS
The temperature dependence of the domain-wall coercive field of epitaxial magnetic garnets films
has been investigated in the entire temperature range of the ferrimagnetic phase, and has been found
to be described by a set of parametric exponents. In subsequent temperature regions different slopes
were observed, with breaking points whose position was found to be sample dependent. A survey
ba.ed on literature Data as well as on a large number of our own samples shows the general
existence of this piecewise exponential dependence and the presence of the breaking points. This
type of domain-wall coercive field temperature dependence was found in all samples in the large
family of the epitaxial garnets (about 30 specimens of more than ten chemical compositionsj and
also in another strongly anisotropic material (TbFeCo)
A Stochastic Measure for Eternal Inflation
We use the stochastic approach to investigate the measure for slow roll
eternal inflation. The probability for the universe of a given Hubble radius
can be calculated in this framework. In a solvable model, it is shown that the
probability for the universe to evolve from a state with a smaller Hubble
radius to that of a larger Hubble radius is dominated by the classical
probability without the stochastic source. While the probability for the
universe to evolve from a larger Hubble radius to a smaller one is suppressed
by , where the de Sitter entropy arises naturally in this
stochastic approach.Comment: 11 pages, 1 figur
Sinks in the Landscape, Boltzmann Brains, and the Cosmological Constant Problem
This paper extends the recent investigation of the string theory landscape in
hep-th/0605266, where it was found that the decay rate of dS vacua to a
collapsing space with a negative vacuum energy can be quite large. The parts of
space that experience a decay to a collapsing space, or to a Minkowski vacuum,
never return back to dS space. The channels of irreversible vacuum decay serve
as sinks for the probability flow. The existence of such sinks is a
distinguishing feature of the string theory landscape. We describe relations
between several different probability measures for eternal inflation taking
into account the existence of the sinks. The local (comoving) description of
the inflationary multiverse suffers from the so-called Boltzmann brain (BB)
problem unless the probability of the decay to the sinks is sufficiently large.
We show that some versions of the global (volume-weighted) description do not
have this problem even if one ignores the existence of the sinks. We argue that
if the number of different vacua in the landscape is large enough, the
anthropic solution of the cosmological constant problem in the string landscape
scenario should be valid for a broad class of the probability measures which
solve the BB problem. If this is correct, the solution of the cosmological
constant problem may be essentially measure-independent. Finally, we describe a
simplified approach to the calculations of anthropic probabilities in the
landscape, which is less ambitious but also less ambiguous than other methods.Comment: 42 pages, 5 figures, the paper is substantially extended, a section
on the cosmological constant is addeed; the version published in JCA
A status report on the observability of cosmic bubble collisions
In the picture of eternal inflation as driven by a scalar potential with
multiple minima, our observable universe resides inside one of many bubbles
formed from transitions out of a false vacuum. These bubbles necessarily
collide, upsetting the homogeneity and isotropy of our bubble interior, and
possibly leading to detectable signatures in the observable portion of our
bubble, potentially in the Cosmic Microwave Background or other precision
cosmological probes. This constitutes a direct experimental test of eternal
inflation and the landscape of string theory vacua. Assessing this possibility
roughly splits into answering three questions: What happens in a generic bubble
collision? What observational effects might be expected? How likely are we to
observe a collision? In this review we report the current progress on each of
these questions, improve upon a few of the existing results, and attempt to lay
out directions for future work.Comment: Review article; comments very welcome. 24 pages + 4 appendices; 19
color figures. (Revised version adds two figures, minor edits.
Local and Global Casimir Energies: Divergences, Renormalization, and the Coupling to Gravity
From the beginning of the subject, calculations of quantum vacuum energies or
Casimir energies have been plagued with two types of divergences: The total
energy, which may be thought of as some sort of regularization of the
zero-point energy, , seems manifestly divergent. And
local energy densities, obtained from the vacuum expectation value of the
energy-momentum tensor, , typically diverge near
boundaries. The energy of interaction between distinct rigid bodies of whatever
type is finite, corresponding to observable forces and torques between the
bodies, which can be unambiguously calculated. The self-energy of a body is
less well-defined, and suffers divergences which may or may not be removable.
Some examples where a unique total self-stress may be evaluated include the
perfectly conducting spherical shell first considered by Boyer, a perfectly
conducting cylindrical shell, and dilute dielectric balls and cylinders. In
these cases the finite part is unique, yet there are divergent contributions
which may be subsumed in some sort of renormalization of physical parameters.
The divergences that occur in the local energy-momentum tensor near surfaces
are distinct from the divergences in the total energy, which are often
associated with energy located exactly on the surfaces. However, the local
energy-momentum tensor couples to gravity, so what is the significance of
infinite quantities here? For the classic situation of parallel plates there
are indications that the divergences in the local energy density are consistent
with divergences in Einstein's equations; correspondingly, it has been shown
that divergences in the total Casimir energy serve to precisely renormalize the
masses of the plates, in accordance with the equivalence principle.Comment: 53 pages, 1 figure, invited review paper to Lecture Notes in Physics
volume in Casimir physics edited by Diego Dalvit, Peter Milonni, David
Roberts, and Felipe da Ros
A western gray whale mitigation and monitoring program for a 3-D seismic survey, Sakhalin Island, Russia
The introduction of anthropogenic sounds into the marine environment can impact some marine mammals. Impacts can be greatly reduced if appropriate mitigation measures and monitoring are implemented. This paper concerns such measures undertaken by Exxon Neftegas Limited, as operator of the Sakhalin-1 Consortium, during the Odoptu 3-D seismic survey conducted during 17 August’ September 2001. The key environmental issue was protection of the critically endangered western gray whale (Eschrichtius robustus), which feeds in summer and fall primarily in the Piltun feeding area off northeast Sakhalin Island. Existing mitigation and monitoring practices for seismic surveys in other jurisdictions were evaluated to identify best practices for reducing impacts on feeding activity by western gray whales. Two buffer zones were established to protect whales from physical injury or undue disturbance during feeding. A 1 km buffer protected all whales from exposure to levels of sound energy potentially capable of producing physical injury. A 4’ km buffer was established to avoid displacing western gray whales from feeding areas. Trained Marine Mammal Observers (MMOs) on the seismic ship Nordic Explorer had the authority to shut down the air guns if whales were sighted within these buffers
Lateral frontal cortex volume reduction in Tourette syndrome revealed by VBM
<p>Abstract</p> <p>Background</p> <p>Structural changes have been found predominantly in the frontal cortex and in the striatum in children and adolescents with Gilles de la Tourette syndrome (GTS). The influence of comorbid symptomatology is unclear. Here we sought to address the question of gray matter abnormalities in GTS patients <it>with </it>co-morbid obsessive-compulsive disorder (OCD) and/or attention deficit hyperactivity disorder (ADHD) using voxel-based morphometry (VBM) in twenty-nine adult actually unmedicated GTS patients and twenty-five healthy control subjects.</p> <p>Results</p> <p>In GTS we detected a cluster of decreased gray matter volume in the left inferior frontal gyrus (IFG), but no regions demonstrating volume increases. By comparing subgroups of GTS with comorbid ADHD to the subgroup with comorbid OCD, we found a left-sided amygdalar volume increase.</p> <p>Conclusions</p> <p>From our results it is suggested that the left IFG may constitute a common underlying structural correlate of GTS with co-morbid OCD/ADHD. A volume reduction in this brain region that has been previously identified as a key region in OCD and was associated with the active inhibition of attentional processes may reflect the failure to control behavior. Amygdala volume increase is discussed on the background of a linkage of this structure with ADHD symptomatology. Correlations with clinical data revealed gray matter volume changes in specific brain areas that have been described in these conditions each.</p
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