9,482 research outputs found
Simultaneous Heavy Ion Dissociation at Ultrarelativistic Energies
We study the simultaneous dissociation of heavy ultrarelativistic nuclei
followed by the forward-backward neutron emission in peripheral collisions at
colliders. The main contribution to this particular heavy-ion dissociation
process, which can be used as a beam luminosity monitor, is expected to be due
to the electromagnetic interaction. The Weizsacker-Williams method is extended
to the case of simultaneous excitation of collision partners which is simulated
by the RELDIS code. A contribution to the dissociation cross section due to
grazing nuclear interactions is estimated within the abrasion model and found
to be relatively small.Comment: Talk given at Bologna 2000 Conference - Structure of the Nucleus at
the Dawn of the Century, May 29 - June 3, 2000, 4 pages, 2 figure
The investigation of time dependent flame structure by ionization probes
Ionization probes were used to measure mean ionization current and frequency spectra, auto-correlations and cross-correlations in jet flames with variation in the initial Reynolds numbers and equivalence ratios. Special attention was paid to the transitional region between the burner exit plane and the plane of onset of turbulence
Electromigration in thin tunnel junctions with ferromagnetic/nonmagnetic: nanoconstrictions, local heating, and direct and wind forces
Current Induced Resistance Switching (CIS) was recently observed in thin
tunnel junctions with ferromagnetic (FM) electrodes \emph{i.e} FM/I/FM. This
effect was attributed to electromigration of metallic atoms in
nanoconstrictions in the insulating barrier (I). Here we study how the CIS
effect is influenced by a thin non-magnetic (NM) Ta layer, deposited just below
the AlO insulating barrier in tunnel junctions of the type FM/NM/I/FM
(FM=CoFe). Enhanced resistance switching occurs with increasing maximum applied
current (\Imax), until a plateau of constant CIS is reached for \Imax\sim65
mA (CIS60%) and above. However, such high electrical currents also lead
to a large (9%) irreversible resistance decrease, indicating barrier
degradation. Anomalous voltage-current characteristics with negative derivative
were also observed near \pm\Imax and this effect is here attributed to
heating in the tunnel junction. One observes that the current direction for
which resistance switches in FM/NM/I/FM (clockwise) is opposite to that of
FM/I/FM tunnel junctions (anti-clockwise). This effect will be discussed in
terms of a competition between the electromigration contributions due to the so
called direct and wind forces. It will be shown that the direct force is likely
to dominate electromigration in the Ta (NM) layers, while the wind contribution
likely dominates in the CoFe (FM) layers
PT-symmetry from Lindblad dynamics in a linearized optomechanical system
We analyze a lossy linearized optomechanical system in the red-detuned regime under the rotating wave approximation. This so-called optomechanical state transfer protocol provides effective lossy frequency converter (quantum beam-splitter-like) dynamics where the strength of the coupling between the electromagnetic and mechanical modes is controlled by the optical steady-state amplitude. By restricting to a subspace with no losses, we argue that the transition from mode-hybridization in the strong coupling regime to the damped-dynamics in the weak coupling regime, is a signature of the passive parity-time (PT) symmetry breaking transition in the underlying non-Hermitian quantum dimer. We compare the dynamics generated by the quantum open system (Langevin or Lindblad) approach to that of the PT-symmetric Hamiltonian, to characterize the cases where the two are identical. Additionally, we numerically explore the evolution of separable and correlated number states at zero temperature as well as thermal initial state evolution at room temperature. Our results provide a pathway for realizing non-Hermitian Hamiltonians in optomechanical systems at a quantum level
Multidimensional study of urban squares through perimetral analysis: three Portuguese case studies
This paper addresses one of the most symbolically and socially meaningful elements of the public open space: the urban square (Portuguese: praça). Besides their urban centrality, these spaces’ potential for liveliness depends on multiple factors and their identity as a place may only be grasped by formal methods that embrace that latent complexity and address the multi?scale and multivariate correlations of factors that defy human cognitive capabilities. This paper will present a synchronic multidimensional analysis of three Portuguese historic squares: Praça da Oliveira, Praça de Santiago (Guimarães) and Praça do Giraldo (Évora), representative of the national historic heritage.info:eu-repo/semantics/acceptedVersio
Cirrus high-definition optical coherence tomography compared with Stratus optical coherence tomography in glaucoma diagnosis
To compare the retinal nerve fiber layer (RNFL) evaluation using Cirrus
optical coherence tomography (OCT) and Stratus OCT in glaucoma diagnosis.
METHODS: One hundred thirty normal and 86 patients with glaucoma were included in
this prospective study. The signal strengths of the OCTs were evaluated. The
sensitivities and specificities of global RNFL average thickness were compared in
the four quadrants and in each clock hour sector. Receiver operating
characteristic (ROC) curves, areas under the ROC (AUC), and the likelihood ratio
(LR) were plotted for RNFL thickness. Agreement between the OCTs was calculated
by using the Bland-Altman method and kappa (kappa) coefficient. RESULTS:
Twenty-three percent of all cases examined with Stratus OCT and 1.9% examined
with Cirrus OCT had a signal strength below 6 (P = 0.01). In cases with signal
strengths > or =6, the mean signal strength was higher with Cirrus OCT than with
Stratus OCT (P = 0.01). The RNFL measurements by Cirrus were thicker than those
of Stratus OCT (P < 0.05). The AUCs were 0.829 for Stratus and 0.837 for Cirrus
OCT (P = 0.706) for global RNFL average. LRs were similar in both OCTs in global
RNFL classification but varied in quadrants. The widths of the limits of
agreement varied between 42.16 and 97.79 microm. There was almost perfect
agreement (kappa = 0.82) in the average RNFL classification. CONCLUSIONS: Cirrus
OCT has better scan quality than Stratus OCT, especially in glaucomatous eyes. In
cases with good-quality scans, the sensitivity and specificity, and AUCs were
similar. The best agreement was in the global average RNFL classification. The
widths of limits of agreements exceed the limits of resolution of the OCTs
Accretion-Induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aur
It is widely accepted that much of the stochastic variability of T Tauri
stars is due to accretion by a circumstellar disk. The emission line spectrum
as well as the excess continuum emission are common probes of this process. In
this communication, we present additional probes of the circumstellar
environment in the form of resonance lines of low ionization potential
elements. Using a set of 14 high resolution echelle observations of the
classical T Tauri star (CTTS), RW Aur, taken between 1986 and 1996, we
carefully measure the continuum veiling at each epoch by comparing more than
500 absorption lines with those of an appropriate template. This allows us to
accurately subtract out the continuum emission and to recover the underlying
photospheric spectrum. In doing so, we find that selected photospheric lines
are enhanced by the accretion process, namely the resonance lines of LiI and
KI. A resonance line of TiI and a low excitation potential line of CaI also
show weak enhancements. Simple slab models and computed line bisectors lead us
to propose that these line enhancements are markers of cool gas at the
beginning of the accretion flow which provides an additional source of line
opacity. These results suggest that published values of surface lithium
abundances of classical T Tauri stars are likely to be overestimated. This
would account for the various reports of surface lithium abundances in excess
of meteoritic values among the extreme CTTS. Computing LTE lithium abundances
of RW Aur in a low and then high accretion state yields abundances which vary
by one order of magnitude. The low accretion state lithium abundance is
consistent with theoretical predictions for a star of this age and mass while
the high accretion state spectrum yields a super-meteoritic lithium abundance.Comment: 28 pages, 8 figures, accepted by Ap
The gamma-ray burst monitor for Lobster-ISS
Lobster-ISS is an X-ray all-sky monitor experiment selected by ESA two years
ago for a Phase A study (now almost completed) for a future flight (2009)
aboard the Columbus Exposed Payload Facility of the International Space
Station. The main instrument, based on MCP optics with Lobster-eye geometry,
has an energy passband from 0.1 to 3.5 keV, an unprecedented daily sensitivity
of 2x10^{-12} erg cm^{-2}s$^{-1}, and it is capable to scan, during each orbit,
the entire sky with an angular resolution of 4--6 arcmin. This X-ray telescope
is flanked by a Gamma Ray Burst Monitor, with the minimum requirement of
recognizing true GRBs from other transient events. In this paper we describe
the GRBM. In addition to the minimum requirement, the instrument proposed is
capable to roughly localize GRBs which occur in the Lobster FOV (162x22.5
degrees) and to significantly extend the scientific capabilities of the main
instrument for the study of GRBs and X-ray transients. The combination of the
two instruments will allow an unprecedented spectral coverage (from 0.1 up to
300/700 keV) for a sensitive study of the GRB prompt emission in the passband
where GRBs and X-Ray Flashes emit most of their energy. The low-energy spectral
band (0.1-10 keV) is of key importance for the study of the GRB environment and
the search of transient absorption and emission features from GRBs, both goals
being crucial for unveiling the GRB phenomenon. The entire energy band of
Lobster-ISS is not covered by either the Swift satellite or other GRB missions
foreseen in the next decade.Comment: 6 pages, 4 figures. Paper presented at the COSPAR 2004 General
Assembly (Paris), accepted for publication in Advances in Space Research in
June 2005 and available on-line at the Journal site
(http://www.sciencedirect.com/science/journal/02731177), section "Articles in
press
On algebraic models of relativistic scattering
In this paper we develop an algebraic technique for building relativistic
models in the framework of direct-interaction theories. The interacting mass
operator M in the Bakamjian-Thomas construction is related to a quadratic
Casimir operator C of a non-compact group G. As a consequence, the S matrix can
be gained from an intertwining relation between Weyl-equivalent representations
of G. The method is illustrated by explicit application to a model with SO(3,1)
dynamical symmetry.Comment: 10 pages, to appear in J. Phys. A : Math. Theo
- …