822 research outputs found
Excitonic and vibronic spectra of Frenkel excitons in a two-dimensional simple latice
Excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional
(2D) lattice with one molecule per unit cell have been studied and their
manifestation in the linear absorption is simulated. We use the Green function
formalism, the vibronic approach (see Lalov and Zhelyazkov [Phys. Rev. B
\textbf{75}, 245435 (2007)]), and the nearest-neighbor approximation to find
expressions of the linear absorption lineshape in closed form (in terms of the
elliptic integrals) for the following 2D models: (a) vibronic spectra of
polyacenes (naphthalene, anthracene, tetracene); (b) vibronic spectra of a
simple hexagonal lattice. The two 2D models include both linear and quadratic
FE--phonon coupling. Our simulations concern the excitonic density of state
(DOS), and also the position and lineshape of vibronic spectra (FE plus one
phonon, FE plus two phonons). The positions of many-particle (MP-unbound)
FE--phonon states, as well as the impact of the Van Hove singularities on the
linear absorption have been established by using typical values of the
excitonic and vibrational parameters. In the case of a simple hexagonal lattice
the following types of FEs have been considered: (i) non-degenerate FEs whose
transition dipole moment is perpendicular to the plane of the lattice, and (ii)
degenerate FEs with transition dipole moments parallel to the layer. We found a
cumulative impact of the linear and quadratic FE--phonon coupling on the
positions of vibronic maxima in the case (ii), and a compensating impact in the
case (i).Comment: 13 pages, 12 figure
Recognising the importance of 'family time-out' in consultations: An exploratory qualitative study
Objectives: Patients are often accompanied by family or companions during consultations, but little is known about how this might influence the process. We explored how the presence of a companion in a consultation contributes to communication and the decision-making process. Design: Observational study. Setting: A teaching hospital and a district general hospital in south-west England. Participants: 31 patients and their physicians were observed during consultations in which decisions to undergo palliative chemotherapy were made. Each patient was accompanied by at least one companion. Outcome measures: Communication patterns between physicians, patients and companions. Results: In addition to standard patient/physician interactions, patients and companions were often found to discuss medical information and exchange opinions between themselves without the physician actively participating. We called these instances 'family timeout'. On the occasion of disagreement between patients and companions about preferred treatment options, physicians and patients were able to agree the decision while acknowledging the differences in opinion. Conclusions: Instances of 'family time-out' may contribute to better consultation outcomes because they are understood and supported by the patient's social system. This study highlights the potentially important role of exchanges between patients and companions during consultations and how physicians may benefit from observation of such exchanges. We recommend testing the value of making space for family time-out during consultations. Also, we recommend further study into the medical ethics of family time-out. While the focus here is on palliative chemotherapy, this finding has implications for other consultations, particularly those involving difficult treatment decisions
Solidification behavior of intensively sheared hypoeutectic Al-Si alloy liquid
The official published version of this article can be found at the link below.The effect of the processing temperature on the microstructural and mechanical properties of Al-Si (hypoeutectic) alloy solidified from intensively sheared liquid metal has been investigated systematically. Intensive shearing gives a significant refinement in grain size and intermetallic particle size. It also is observed that the morphology of intermetallics, defect bands, and microscopic defects in high-pressure die cast components are affected by intensive shearing the liquid metal. We attempt to discuss the possible mechanism for these effects.Funded by the EPSRC
Triplet superconductivity in quasi one-dimensional systems
We study a Hubbard hamiltonian, including a quite general nearest-neighbor
interaction, parametrized by repulsion V, exchange interactions Jz, Jperp,
bond-charge interaction X and hopping of pairs W. The case of correlated
hopping, in which the hopping between nearest neighbors depends upon the
occupation of the two sites involved, is also described by the model for
sufficiently weak interactions. We study the model in one dimension with usual
continuum-limit field theory techniques, and determine the phase diagram. For
arbitrary filling, we find a very simple necessary condition for the existence
of dominant triplet superconducting correlations at large distance in the spin
SU(2) symmetric case: 4V+J<0. In the correlated hopping model, the three-body
interaction should be negative for positive V. We also compare the predictions
of this weak-coupling treatment with numerical exact results for the
correlated-hopping model obtained by diagonalizing small chains, and using
novel techniques to determine the opening of the spin gap.Comment: 8 pages, 3 figure
New results from the NA57 experiment
We report results from the experiment NA57 at CERN SPS on hyperon production
at midrapidity in Pb-Pb collisions at 158 GeV/ and 40 GeV/.
, and yields are compared with those from the STAR
experiment at the higher energy of the BNL RHIC. , , \
and preliminary transverse mass spectra are presented and interpreted
within the framework of a hydro-dynamical blast wave model.Comment: 8 pages, 3 figures, contribution to the proceedings of The XXXVIIIth
Rencontres de Moriond "QCD and High Energy Hadronic Interactions
Expansion dynamics of Pb-Pb collisions at 40 A GeV/c viewed by negatively charged hadrons
In this paper we present results on transverse mass spectra and Hanbury-Brown
and Twiss correlation functions of negatively charged hadrons, which are
expected to be mostly negative pions, measured in Pb-Pb collisions at 40 A
GeV/c beam momentum. Based on these data, the collision dynamics and the
space-time extent of the system at the thermal freeze-out are studied over a
centrality range corresponding to the most central 53% of the Pb--Pb inelastic
cross section. Comparisons with freeze-out conditions of strange particles and
HBT results from other experiments are discussed.Comment: 29 pages, 18 figure
Results on hyperon production from the NA57 experiment
Recent results on hyperon production in Pb--Pb collisions from the NA57
experiment are reported. Strangeness enhancement and the transverse mass
spectra properties at 158 GeV per nucleon are described.Comment: submitted to Acta Phys. Hung. A (Heavy Ion Physics
Strange particle production in 158 and 40 GeV/ Pb-Pb and p-Be collisions
Results on strange particle production in Pb-Pb collisions at 158 and 40
GeV/ beam momentum from the NA57 experiment at CERN SPS are presented.
Particle yields and ratios are compared with those measured at RHIC.
Strangeness enhancements with respect to p-Be reactions at the same beam
momenta have been also measured: results about their dependence on centrality
and collision energy are reported and discussed.Comment: Contribution to the proceedings of the "Hot Quarks 2004" Conference,
July 18-24 2004, New Mexico, USA, submitted to Journal of Physics G 7 pages,
5 figure
Jupiter's X-ray Emission During Solar Minimum
The 2007â2009 solar minimum was the longest of the space age. We present the first of two companion papers on Chandra and XMMâNewton Xâray campaigns of Jupiter through FebruaryâMarch 2007. We find that low solar Xâray flux during solar minimum causes Jupiter's equatorial regions to be exceptionally Xâray dim (0.21 GW at minimum; 0.76 GW at maximum). While the Jovian equatorial emission varies with solar cycle, the aurorae have comparably bright intervals at solar minimum and maximum. We apply atomic charge exchange models to auroral spectra and find that iogenic plasma of sulphur and oxygen ions provides excellent fits for XMMâNewton observations. The fitted spectral S:O ratios of 0.4â1.3 are in good agreement with in situ magnetospheric S:O measurements of 0.3â1.5, suggesting that the ions that produce Jupiter's Xâray aurora predominantly originate inside the magnetosphere. The aurorae were particularly bright on 24â25 February and 8â9 March, but these two observations exhibit very different spatial, spectral, and temporal behavior; 24â25 February was the only observation in this campaign with significant hard Xâray bremsstrahlung from precipitating electrons, suggesting this may be rare. For 8â9 March, a bremsstrahlung component was absent, but bright oxygen O6+ lines and bestâfit models containing carbon, point to contributions from solar wind ions. This contribution is absent in the other observations. Comparing simultaneous Chandra ACIS and XMMâNewton EPIC spectra showed that ACIS systematically underreported 0.45â to 0.6âkeV Jovian emission, suggesting quenching may be less important for Jupiter's atmosphere than previously thought. We therefore recommend XMMâNewton for spectral analyses and quantifying opacity/quenching effects
On Physical Equivalence between Nonlinear Gravity Theories
We argue that in a nonlinear gravity theory, which according to well-known
results is dynamically equivalent to a self-gravitating scalar field in General
Relativity, the true physical variables are exactly those which describe the
equivalent general-relativistic model (these variables are known as Einstein
frame). Whenever such variables cannot be defined, there are strong indications
that the original theory is unphysical. We explicitly show how to map, in the
presence of matter, the Jordan frame to the Einstein one and backwards. We
study energetics for asymptotically flat solutions. This is based on the
second-order dynamics obtained, without changing the metric, by the use of a
Helmholtz Lagrangian. We prove for a large class of these Lagrangians that the
ADM energy is positive for solutions close to flat space. The proof of this
Positive Energy Theorem relies on the existence of the Einstein frame, since in
the (Helmholtz--)Jordan frame the Dominant Energy Condition does not hold and
the field variables are unrelated to the total energy of the system.Comment: 37 pp., TO-JLL-P 3/93 Dec 199
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