17,085 research outputs found
Addendum: Ultrahigh-energy cosmic-ray bounds on nonbirefringent modified-Maxwell theory
Nonbirefringent modified-Maxwell theory, coupled to standard Dirac particles,
involves nine dimensionless parameters, which can be bounded by the inferred
absence of vacuum Cherenkov radiation for ultrahigh-energy cosmic rays
(UHECRs). With selected UHECR events, two-sided bounds on the eight
nonisotropic parameters are obtained at the 10^{-18} level, together with an
improved one-sided bound on the single isotropic parameter at the 10^{-19}
level.Comment: 5 pages with revtex
Description of Atmospheric Conditions at the Pierre Auger Observatory Using Meteorological Measurements and Models
Atmospheric conditions at the site of a cosmic ray observatory must be known
well for reconstructing observed extensive air showers, especially when
measured using the fluorescence technique. For the Pierre Auger Observatory, a
sophisticated network of atmospheric monitoring devices has been conceived.
Part of this monitoring was a weather balloon program to measure atmospheric
state variables above the Observatory. To use the data in reconstructions of
air showers, monthly models have been constructed. Scheduled balloon launches
were abandoned and replaced with launches triggered by high-energetic air
showers as part of a rapid monitoring system. Currently, the balloon launch
program is halted and atmospheric data from numerical weather prediction models
are used. A description of the balloon measurements, the monthly models as well
as the data from the numerical weather prediction are presented
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
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 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 and number of atoms , 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
before melting occurs. We find that the probability distribution
of is roughly exponential, and that the mean value 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 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
Zeolite-dye micro lasers
We present a new class of micro lasers based on nanoporous molecular sieve
host-guest systems. Organic dye guest molecules of
1-Ethyl-4-(4-(p-Dimethylaminophenyl)-1,3-butadienyl)-pyridinium Perchlorat were
inserted into the 0.73-nm-wide channel pores of a zeolite AlPO-5 host. The
zeolitic micro crystal compounds where hydrothermally synthesized according to
a particular host-guest chemical process. The dye molecules are found not only
to be aligned along the host channel axis, but to be oriented as well. Single
mode laser emission at 687 nm was obtained from a whispering gallery mode
oscillating in a 8-m-diameter monolithic micro resonator, in which the
field is confined by total internal reflection at the natural hexagonal
boundaries inside the zeolitic microcrystals.Comment: Accepted for publication in Phys. Rev. Let
Glassy dynamics, metastability limit and crystal growth in a lattice spin model
We introduce a lattice spin model where frustration is due to multibody
interactions rather than quenched disorder in the Hamiltonian. The system has a
crystalline ground state and below the melting temperature displays a dynamic
behaviour typical of fragile glasses. However, the supercooled phase loses
stability at an effective spinodal temperature, and thanks to this the Kauzmann
paradox is resolved. Below the spinodal the system enters an off-equilibrium
regime corresponding to fast crystal nucleation followed by slow activated
crystal growth. In this phase and in a time region which is longer the lower
the temperature we observe a violation of the fluctuation-dissipation theorem
analogous to structural glasses. Moreover, we show that in this system there is
no qualitative difference between a locally stable glassy configuration and a
highly disordered polycrystal
Nonthermal THz to TeV Emission from Stellar Wind Shocks in the Galactic Center
The central parsec of the Galaxy contains dozens of massive stars with a
cumulative mass loss rate of ~ 10^{-3} solar masses per year. Shocks among
these stellar winds produce the hot plasma that pervades the central part of
the galaxy. We argue that these stellar wind shocks also efficiently accelerate
electrons and protons to relativistic energies. The relativistic electrons
inverse Compton scatter the ambient ultraviolet and far infrared radiation
field, producing high energy gamma-rays with a roughly constant luminosity from
\~ GeV to ~ 10 TeV. This can account for the TeV source seen by HESS in the
Galactic Center. Our model predicts a GLAST counterpart to the HESS source with
a luminosity of ~ 10^{35} ergs/s and cooling break at ~ 4 GeV. Synchrotron
radiation from the same relativistic electrons should produce detectable
emission at lower energies, with a surface brightness ~ 10^{32} B^2_{-3}
ergs/s/arcsec^2 from ~ THz to ~ keV, where B_{-3} is the magnetic field
strength in units of mG. The observed level of diffuse thermal X-ray emission
in the central parsec requires B < 300 micro-G in our models. Future detection
of the diffuse synchrotron background in the central parsec can directly
constrain the magnetic field strength, providing an important boundary
condition for models of accretion onto Sgr A*.Comment: submitted to ApJ Letter
Critical Exponents of the Metal-Insulator Transition in the Two-Dimensional Hubbard Model
We study the filling-controlled metal-insulator transition in the
two-dimensional Hubbard model near half-filling with the use of zero
temperature quantum Monte Carlo methods. In the metallic phase, the
compressibility behaves as where
is the critical chemical potential. In the insulating phase, the
localization length follows with . Under the assumption of hyperscaling, the compressibility
data leads to a correlation length exponent . Our
results show that the exponents and agree within
statistical uncertainty. This confirms the assumption of hyperscaling with
correlation length exponent and dynamical exponent . In
contrast the metal-insulator transition in the generic band insulators in all
dimensions as well as in the one-dimensional Hubbard model satisfy the
hyperscaling assumption with exponents and .Comment: Two references added. The DVI file and PS figure files are also
available at http://www.issp.u-tokyo.ac.jp/labs/riron/imada/furukawa/; to
appear in J. Phys. Soc. Jpn 65 (1996) No.
Matching Conditions in Atomistic-Continuum Modeling of Materials
A new class of matching condition between the atomistic and continuum regions
is presented for the multi-scale modeling of crystals. They ensure the accurate
passage of large scale information between the atomistic and continuum regions
and at the same time minimize the reflection of phonons at the interface. These
matching conditions can be made adaptive if we choose appropriate weight
functions. Applications to dislocation dynamics and friction between
two-dimensional atomically flat crystal surfaces are described.Comment: 6 pages, 4 figure
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