42,440 research outputs found
First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra
We show that any {\em quasi-particle} or GW approximation to the self-energy
does not capture excitonic features in time-resolved (TR) photoemission
spectroscopy. In this work we put forward a first-principles approach and
propose a feasible diagrammatic approximation to solve this problem. We also
derive an alternative formula for the TR photocurrent which involves a single
time-integral of the lesser Green's function. The diagrammatic approximation
applies to the {\em relaxed} regime characterized by the presence of
quasi-stationary excitons and vanishing polarization. The main distinctive
feature of the theory is that the diagrams must be evaluated using {\em
excited} Green's functions. As this is not standard the analytic derivation is
presented in detail. The final result is an expression for the lesser Green's
function in terms of quantities that can all be calculated {\em ab initio}. The
validity of the proposed theory is illustrated in a one-dimensional model
system with a direct gap. We discuss possible scenarios and highlight some
universal features of the exciton peaks. Our results indicate that the exciton
dispersion can be observed in TR {\em and} angle-resolved photoemission.Comment: 15 pages, 8 figure
From Bound Duties to Actual Protection: Industrial Liberalisation in the Doha Round. ENEPRI Working Paper No. 41, 2005
In the background of the Doha Round of trade negotiations, this study proposes a CGE assessment of multilateral liberalisation of market access for non-agricultural products. The scenarios considered include the so-called âGirard proposalâ (with alternative choices for the coefficient involved), the removal of tariff peaks and complete liberalisation. This study is the first to take into account the difference between bound and applied tariffs, while considering all the enforced preferential trade arrangements and computing tariff cuts at the detailed product level (HS-6 classification). Although the liberalisation of market access for non-agricultural products is found to be welfare-enhancing at the world level, cross-country distributive effects prove significant. A soft liberalisation would not significantly reduce applied duties in developing countries, owing to their considerable binding overhang. By contrast, a deep liberalisation would entail fierce price competition among those developing countries that are largely specialised in similar sectors and in the same product quality range
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On the need for bump event correction in vibration test profiles representing road excitations in automobiles
This paper presents an approach to the synthesis of compressed vibration test profiles
representing much longer time histories obtained in road testing of ground vehicles. Vibration test
profiles are defined as those related directly to operational testing on specific road surfaces and
which summarise the input to the vehicle in the given conditions. The method extends classical
Fourier transform technique by means of bump event correction in the background Fourier signal
where the bump event term implies a high-amplitude transient event of the shock type. The
orthogonal wavelet decomposition was used as a specific filtering tool facilitating bump event
identification. Examples of seat guide vertical acceleration have been considered. Calculated
probability density functions suggest the ability of the bump correction method to improve the
statistical accuracy of the final vibration test profile with respect to the original road data. Test
profiles obtained by means of Fourier transform synthesis with subsequent reinsertion of bump
events from separated frequency bands were more accurate than those obtained by Fourier synthesis
alone. Further developments led to advanced bump reinsertion with synchronisation of events
occurring in different frequency bands at the same moment of time. Test profiles generated in this
way have provided better accuracy compared to the non-synchronised algorithm
Study of payload sensor contamination
Epoxy/glass test specimens representing Scout fourth and fifth stage motor casings were prepared and studied in a test program designed to determine the relative contamination contributed to payload sensors located near the motor casings. The relative contamination detected in the test program was dependent on several factors, including: (1) distance of test specimen to sensor, (2) sensor temperature, (3) geometric position of the sensor (relative to test specimen), (4)test specimen temperature, (5) chemical composition of the test specimen, (6) sensor function, and (7) additional materials (such as adhesive-backed aluminum tape) added to the test specimens. The thermal characteristics and the decomposition and outgassing products were determined. These data were used to better understand the relative contamination data from the remote sensor testing
Reactive Molecular Dynamics Simulations of Shock Through a Single Crystal of Pentaerythritol Tetranitrate
Large-scale molecular dynamics simulations and the reactive force field ReaxFF were used to study shock-induced initiation in crystalline pentaerythritol tetranitrate (PETN). In the calculations, a PETN single crystal was impacted against a wall, driving a shockwave back through the crystal in the [100] direction. Two impact speeds (4 and 3 km/s) were used to compare strong and moderate shock behavior. The primary difference between the two shock strengths is the time required to exhibit the same qualitative behaviors with the lower impact speed lagging behind the faster impact speed. For both systems, the shock velocity exhibits an initial deceleration due to onset of endothermic reactions followed by acceleration due to the onset of exothermic reactions. At long times, the shock velocity reaches a steady value. After the initial deceleration period, peaks are observed in the profiles of the density and axial stress with the strongly shocked system having sharp peaks while the weakly shocked system developed broad peaks due to the slower shock velocity acceleration. The dominant initiation reactions in both systems lead to the formation of NO_2 with lesser quantities of NO_3 and formaldehyde also produced
Fragment size correlations in finite systems - application to nuclear multifragmentation
We present a new method for the calculation of fragment size correlations in
a discrete finite system in which correlations explicitly due to the finite
extent of the system are suppressed. To this end, we introduce a combinatorial
model, which describes the fragmentation of a finite system as a sequence of
independent random emissions of fragments. The sequence is accepted when the
sum of the sizes is equal to the total size. The parameters of the model, which
may be used to calculate all partition probabilities, are the intrinsic
probabilities associated with the fragments. Any fragment size correlation
function can be built by calculating the ratio between the partition
probabilities in the data sample (resulting from an experiment or from a Monte
Carlo simulation) and the 'independent emission' model partition probabilities.
This technique is applied to charge correlations introduced by Moretto and
collaborators. It is shown that the percolation and the nuclear statistical
multifragmentaion model ({\sc smm}) are almost independent emission models
whereas the nuclear spinodal decomposition model ({\sc bob}) shows strong
correlations corresponding to the break-up of the hot dilute nucleus into
nearly equal size fragments
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