35,602 research outputs found
Optimal paths on the road network as directed polymers
We analyze the statistics of the shortest and fastest paths on the road
network between randomly sampled end points. To a good approximation, these
optimal paths are found to be directed in that their lengths (at large scales)
are linearly proportional to the absolute distance between them. This motivates
comparisons to universal features of directed polymers in random media. There
are similarities in scalings of fluctuations in length/time and transverse
wanderings, but also important distinctions in the scaling exponents, likely
due to long-range correlations in geographic and man-made features. At short
scales the optimal paths are not directed due to circuitous excursions governed
by a fat-tailed (power-law) probability distribution.Comment: 5 pages, 7 figure
Solidification mechanism of highly undercooled metal alloys
Experiments were conducted on metal droplet undercooling, using Sn-25wt%Pb and Ni-34wt%Sn alloys. To achieve the high degree of undercooling, emulsification treatments were employed. Results show the fraction of supersaturated primary phase is a function of the amount of undercooling, as is the fineness of the structures. The solidification behavior of the tin-lead droplets during recalescence was analyzed using three different hypotheses; (1) solid forming throughout recalescence is of the maximum thermodynamically stable composition; (2) partitionless solidification below the T sub o temperature, and solid forming thereafter is of the maximum thermodynamically stable composition; and (3) partitionless solidification below the T sub o temperature with solid forming thereafter that is of the maximum thermodynamically metastable composition that is possible. The T sub o temperature is calculated from the equal molar free energies of the liquid solid using the regular solution approximation
Characterization of the Noise in Secondary Ion Mass Spectrometry Depth Profiles
The noise in the depth profiles of secondary ion mass spectrometry (SIMS) is
studied using different samples under various experimental conditions. Despite
the noise contributions from various parts of the dynamic SIMS process, its
overall character agrees very well with the Poissonian rather than the Gaussian
distribution in all circumstances. The Poissonian relation between the measured
mean-square error (MSE) and mean can be used to describe our data in the range
of four orders. The departure from this relation at high counts is analyzed and
found to be due to the saturation of the channeltron used. Once saturated, the
detector was found to exhibit hysteresis between rising and falling input flux
and output counts.Comment: 14 pages, 4 postscript figures, to appear on J. Appl. Phy
VLT observations of the highly ionized nebula around Brey2
We present the first high resolution HeII 4686 images of the high excitation
nebula around the WR star Brey 2 in the LMC. This nebula presents a striking
morphology: a small arc-like feature some 3.6pc in radius is particularly
prominent in the HeII 4686 line. We further discover a previously unknown faint
HeII emission that extends over an area of 22*17 pc^2. An even fainter HeII
emission is apparently associated with the interstellar bubble blown by the
progenitor of Brey2. The total HeII flux corresponds to an ionizing flux of
4*10^{47} photons/s. Halpha, [OIII], and HeI 5876 images and long-slit spectra
are also examined in this letter, enabling us to investigate the detailed
physical properties at various locations of the nebula.Comment: 4 pages, 3 figures (2 in jpg), accepted by A&A Letters, also
available from http://vela.astro.ulg.ac.be/Preprints/P80/index.htm
Probing the Region of Massless Quarks in Quenched Lattice QCD using Wilson Fermions
We study the spectrum of with being the
Wilson-Dirac operator on the lattice with bare mass equal to . The
background gauge fields are generated using the SU(3) Wilson action at
on an lattice. We find evidence that the spectrum of
is gapless for , implying that the physical quark is
massless in this whole region.Comment: 22 pages, LaTeX file, uses elsart.sty, includes 11 figures A
typographical error in one reference has been fixe
Collisionless energy transfer in kinetic turbulence: field-particle correlations in Fourier space
Turbulence is ubiquitously observed in nearly collisionless heliospheric
plasmas, including the solar wind and corona and the Earth's magnetosphere.
Understanding the collisionless mechanisms responsible for the energy transfer
from the turbulent fluctuations to the particles is a frontier in kinetic
turbulence research. Collisionless energy transfer from the turbulence to the
particles can take place reversibly, resulting in non-thermal energy in the
particle velocity distribution functions (VDFs) before eventual collisional
thermalization is realized. Exploiting the information contained in the
fluctuations in the VDFs is valuable. Here we apply a recently developed method
based on VDFs, the field-particle correlation technique, to a ,
solar-wind-like, low-frequency Alfv\'enic turbulence simulation with well
resolved phase space to identify the field-particle energy transfer in velocity
space. The field-particle correlations reveal that the energy transfer,
mediated by the parallel electric field, results in significant structuring of
the ion and electron VDFs in the direction parallel to the magnetic field.
Fourier modes representing the length scales between the ion and electron
gyroradii show that energy transfer is resonant in nature, localized in
velocity space to the Landau resonances for each Fourier mode. The energy
transfer closely follows the Landau resonant velocities with varying
perpendicular wavenumber and plasma . This resonant signature,
consistent with Landau damping, is observed in all diagnosed Fourier modes that
cover the dissipation range of the simulation.Comment: 31 pages, accepted by JPP, minor improvements compared to v
Generalised Spin Projection for Fermion Actions
The majority of compute time doing lattice QCD is spent inverting the fermion
matrix. The time that this takes increases with the condition number of the
matrix. The FLIC(Fat Link Irrelevant Clover) action displays, among other
properties, an improved condition number compared to standard actions and hence
is of interest due to potential compute time savings. However, due to its two
different link sets there is a factor of two cost in floating point
multiplications compared to the Wilson action. An additional factor of two has
been attributed due to the loss of the so-called spin projection trick. We show
that any split-link action may be written in terms of spin projectors, reducing
the additional cost to at most a factor of two. Also, we review an efficient
means of evaluating the clover term, which is additional expense not present in
the Wilson action.Comment: 4 page
Cosmological Constraints on Radion Evolution in the Universal Extra Dimension Model
The constraints on the radion evolution in the Universal Extra Dimension
(UED) model from Cosmic Microwave Background (CMB) and Type Ia supernovae (SNe
Ia) data are studied. In the UED model, where both the gravity and standard
model fields can propagate in the extra dimensions, the evolution of the extra
dimensional volume, the radion, induces variation of fundamental constants. We
discuss the effects of variation of the relevant constants in the context of
UED for CMB power spectrum and SNe Ia data. We then use the three-year WMAP
data to constrain the radion evolution at z \sim 1100, and the 2 \sigma
constraint on \dot{\rho} / \rho_0 (\rho is a function of the radion, to be
defined in the text) is [ -8.8, 6.6] \times 10 ^{-13} yr^-1. The SNe Ia gold
sample yields a constraint on \dot{\rho} / \rho_0, for redshift between 0 and
1, to be [-4.7, 14] \times 10^{-13} yr^-1. Furthermore, the constraints from
SNe Ia can be interpreted as bounds on the evolution QCD scale parameter,
\dot{\Lambda}_{QCD} / \Lambda_{QCD, 0}, [-1.4, 2.8] \times 10^{-11} yr^-1,
without reference to the UED model.Comment: 8 pages, 3 figures, comments added, to appear in Phys. Rev.
Aharonov-Bohm Radiation of Fermions
We analyze Aharonov-Bohm radiation of charged fermions from oscillating
solenoids and cosmic strings. We find that the angular pattern of the radiation
has features that differ significantly from that for bosons. For example,
fermionic radiation in the lowest harmonic is approximately isotropically
distributed around an oscillating solenoid, whereas for bosons the radiation is
dipolar. We also investigate the spin polarization of the emitted
fermion-antifermion pair. Fermionic radiation from kinks and cusps on cosmic
strings is shown to depend linearly on the ultraviolet cut-off, suggesting
strong emission at an energy scale comparable to the string energy scale.Comment: 14 pages, 6 figures. Version 2: Expanded discussion on boundary
conditions obeyed by Dirac equation mode functions (in Section V B).
Acknowledgements and references added. Version 3: Minor changes made in
response to referee's comment
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