20,977 research outputs found

### The Pierre Auger Observatory: Results on Ultra-High Energy Cosmic Rays

The focus of this article is on recent results on ultra-high energy cosmic
rays obtained with the Pierre Auger Observatory. The world's largest instrument
of this type and its performance are described. The observations presented here
include the energy spectrum, the primary particle composition, limits on the
fluxes of photons and neutrinos and a discussion of the anisotropic
distribution of the arrival directions of the most energetic particles.
Finally, plans for the construction of a Northern Auger Observatory in
Colorado, USA, are discussed.Comment: Proceedings of the International Workshop on Advances in Cosmic Ray
Science, Waseda University, Shinjuku, Tokyo, Japan, March 2008; to be
published in the Journal of the Physical Society of Japan (JPSJ) supplemen

### Reionization of Hydrogen and Helium by Early Stars and Quasars

We compute the reionization histories of hydrogen and helium due to the
ionizing radiation fields produced by stars and quasars. For the quasars we use
a model based on halo-merger rates that reproduces all known properties of the
quasar luminosity function at high redshifts. The less constrained properties
of the ionizing radiation produced by stars are modeled with two free
parameters: (i) a transition redshift, z_tran, above which the stellar
population is dominated by massive, zero-metallicity stars and below which it
is dominated by a Scalo mass function; (ii) the product of the escape fraction
of stellar ionizing photons from their host galaxies and the star-formation
efficiency, f_esc f_*. We constrain the allowed range of these free parameters
at high redshifts based on the lack of the HI Gunn-Peterson trough at z<6 and
the upper limit on the total intergalactic optical depth for electron
scattering, tau_es<0.18, from recent cosmic microwave background (CMB)
experiments. We find that quasars ionize helium by a redshift z~4, but cannot
reionize hydrogen by themselves before z~6. A major fraction of the allowed
combinations of f_esc f_* and z_tran lead to an early peak in the ionized
fraction due to metal-free stars at high redshifts. This sometimes results in
two reionization epochs, namely an early HII or HeIII overlap phase followed by
recombination and a second overlap phase. Even if early overlap is not
achieved, the peak in the visibility function for scattering of the CMB often
coincides with the early ionization phase rather than with the actual
reionization epoch. Consequently, tau_es does not correspond directly to the
reionization redshift. We generically find values of tau_es>7%, that should be
detectable by the MAP satellite.Comment: 33 pages, 10 figures, Accepted for publication in Ap

### Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films

Ferromagnetic resonance has been used to study the magnetic properties and
magnetization dynamics of polycrystalline Fe$_{1-x}$V$_{x}$ alloy films with
$0\leq x < 0.7$. Films were produced by co-sputtering from separate Fe and V
targets, leading to a composition gradient across a Si substrate. FMR studies
were conducted at room temperature with a broadband coplanar waveguide at
frequencies up to 50 GHz using the flip-chip method. The effective
demagnetization field $4 \pi M_{\mathrm{eff}}$ and the Gilbert damping
parameter $\alpha$ have been determined as a function of V concentration. The
results are compared to those of epitaxial FeV films

### Apparent first-order wetting and anomalous scaling in the two-dimensional Ising model

The global phase diagram of wetting in the two-dimensional (2d) Ising model
is obtained through exact calculation of the surface excess free energy.
Besides a surface field for inducing wetting, a surface-coupling enhancement is
included. The wetting transition is critical (second order) for any finite
ratio of surface coupling J_s to bulk coupling J, and turns first order in the
limit J_s/J to infinity. However, for J_s/J much larger than 1 the critical
region is exponentially small and practically invisible to numerical studies. A
distinct pre-asymptotic regime exists in which the transition displays
first-order character. Surprisingly, in this regime the surface susceptibility
and surface specific heat develop a divergence and show anomalous scaling with
an exponent equal to 3/2.Comment: This new version presents the exact solution and its properties
whereas the older version was based on an approximate numerical study of the
mode

### Non-equilibrium Dynamics of Finite Interfaces

We present an exact solution to an interface model representing the dynamics
of a domain wall in a two-phase Ising system. The model is microscopically
motivated, yet we find that in the scaling regime our results are consistent
with those obtained previously from a phenomenological, coarse-grained Langevin
approach.Comment: 12 pages LATEX (figures available on request), Oxford preprint
OUTP-94-07

### Quantum corrections to the Larmor radiation formula in scalar electrodynamics

We use the semi-classical approximation in perturbative scalar quantum
electrodynamics to calculate the quantum correction to the Larmor radiation
formula to first order in Planck's constant in the non-relativistic
approximation, choosing the initial state of the charged particle to be a
momentum eigenstate. We calculate this correction in two cases: in the first
case the charged particle is accelerated by a time-dependent but
space-independent vector potential whereas in the second case it is accelerated
by a time-independent vector potential which is a function of one spatial
coordinate. We find that the corrections in these two cases are different even
for a charged particle with the same classical motion. The correction in each
case turns out to be non-local in time in contrast to the classical
approximation.Comment: 19 page

### The kinetics of homogeneous melting beyond the limit of superheating

Molecular dynamics simulation is used to study the time-scales involved in
the homogeneous melting of a superheated crystal. The interaction model used is
an embedded-atom model for Fe developed in previous work, and the melting
process is simulated in the microcanonical $(N, V, E)$ ensemble. We study
periodically repeated systems containing from 96 to 7776 atoms, and the initial
system is always the perfect crystal without free surfaces or other defects.
For each chosen total energy $E$ and number of atoms $N$, we perform several
hundred statistically independent simulations, with each simulation lasting for
between 500 ps and 10 ns, in order to gather statistics for the waiting time
$\tau_{\rm w}$ before melting occurs. We find that the probability distribution
of $\tau_{\rm w}$ is roughly exponential, and that the mean value $<\tau_{\rm
w} >$ depends strongly on the excess of the initial steady temperature of the
crystal above the superheating limit identified by other researchers. The mean
$$ also depends strongly on system size in a way that we have
quantified. For very small systems of $\sim 100$ atoms, we observe a persistent
alternation between the solid and liquid states, and we explain why this
happens. Our results allow us to draw conclusions about the reliability of the
recently proposed Z method for determining the melting properties of simulated
materials, and to suggest ways of correcting for the errors of the method.Comment: 19 pages, 8 figure

### The Role of Perceived Uncertainty, Ego Identity, and Perceived Behavioral Control in Predicting Patient's Attitude Toward Medical Surgery

Medical surgery has sometimes become the only best choice for a patient's well-being. Unfortunately, not all patients have the willingness to live it. Often, therapeutic failure is caused by uncooperative attitudes of the patients which originate from their negative attitudes toward the surgery. This research is aimed at finding a theoretical model to explain psychological factors forming the patient's attitudes. This predictive correlational research was conducted on 99 patients suffering heart disease and cancer continuum who require medical surgery in DKI Jakarta, Indonesia. Research results showed that a commitment aspect of ego identity is able to indirectly predict attitude toward medical surgery through mediation of perceived uncertainty. Perceived behavioral control directly predicts the attitude in a negative direction. This research concludes that patients' commitment towards their identity plays a significant role as they deal with medical surgery

- â€¦