Ultra--fast carriers relaxation in bulk silicon following photo--excitation with a short and polarized laser pulse

A novel approach based on the merging of the out--of--equilibrium Green's function method with the ab-initio, Density--Functional--Theory is used to describe the ultra--fast carriers relaxation in Silicon. The results are compared with recent two photon photo--emission measurements. We show that the interpretation of the carrier relaxation in terms of L -> X inter--valley scattering is not correct. The ultra--fast dynamics measured experimentally is, instead, due to the scattering between degenerate $L$ states that is activated by the non symmetric population of the conduction bands induced by the laser field. This ultra--fast relaxation is, then, entirely due to the specific experimental setup and it can be interpreted by introducing a novel definition of the quasi--particle lifetimes in an out--of--equilibrium context.Comment: 4 page, 2 figure

Road pricing as a citizen-candidate game

We construct a political economy model to analyze the political acceptability of road pricing policies. We use a citizen-candidate framework with a population composed by three groups differing for their income level. We show that road pricing policies are never applied when there is no redistribution of the resources in favour of other modes of transport or when the congestion of these types of transport is relatively high. The results suggest that the efficiency of the redistribution of resources from road to the alternative types of transport as well as the fraction of the population that uses the road transport are key factors in explaining the adoption of road pricing schemes

An ab-initio approach to describe coherent and non-coherent exciton dynamics

The use of ultra-short laser pulses to pump and probe materials activates a wealth of processes which involve the coherent and non coherent dynamics of interacting electrons out of equilibrium. Non equilibrium (NEQ) many body perturbation theory (MBPT) offers an equation of motion for the density-matrix of the system which well describes both coherent and non coherent processes. In the non correlated case there is a clear relation between these two regimes and the matrix elements of the density-matrix. The same is not true for the correlated case, where the potential binding of electrons and holes in excitonic states need to be considered. In the present work we discuss how NEQ-MBPT can be used to describe the dynamics of both coherent and non-coherent excitons in the low density regime. The approach presented is well suited for an ab initio implementation

Optimal Incentives in a Principal-Agent Model with Endogenous Technology

One of the standard predictions of the agency theory is that more incentives can be given to agents with lower risk aversion. In this paper, we show that this relationship may be absent or reversed when the technology is endogenous and projects with a higher efficiency are also riskier. Using a modified version of the Holmstrom and Milgrom’s framework, we obtain that lower agent’s risk aversion unambiguously leads to higher incentives when the technology function linking efficiency and riskiness is elastic, while the risk aversion–incentive relationship can be positive when this function is rigid

Non equilibrium optical properties in semiconductors from first--principles: a combined theoretical and experimental study of bulk silicon

The calculation of the equilibrium optical properties of bulk silicon by using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a cornerstone in the development of an ab--initio approach to the optical and electronic properties of materials. Nevertheless calculations of the {\em transient} optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter--free calculation based on the non--equilibrium Bethe--Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter--free interpretation of concepts like bleaching, photo--induced absorption and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.Comment: 14 pages, 13 figure

Evaluation of feasibility, effectiveness, and sustainability of school-based physical activity “active break” interventions in pre-adolescent and adolescent students: a systematic review

Objective The present systematic review aimed to investigate the impact of school-based physical activity (PA) interventions of “active breaks” on PA levels, classroom behaviour, cognitive functions, and well-being in pre-adolescents and adolescents attending secondary and high school. Methods In March 2021, we performed a systematic research in CINAHL, Cochrane Library, Embase, MedLine, and PsycINFO databases and through grey literature. Quality assessment was performed in accordance with the Cochrane Tool for Quality Assessment for RCTs and the STROBE tool for observational studies. We included studies that investigated classroom PA interventions led by teachers such as active breaks or physically active lessons on PA levels, classroom behaviour, cognitive function, and quality of life in pre-adolescent and adolescent population attending secondary and high school. Synthesis Three studies met the inclusion criteria. Two studies showed a positive effect of active breaks on students’ classroom behaviour and quality of life. One study registered a positive effect in the increase in school PA levels; unfortunately, this effect was not found in the overall levels of PA or in the reduction of sedentary behaviour. All three studies showed the feasibility and acceptability of active breaks intervention in secondary and high school settings. Conclusion This systematic review suggests the potential benefit of this type of intervention integrated in the secondary and high school curriculum on classroom behaviour, school PA levels, and well-being.Objectif La présente revue systématique visait à étudier l’impact des interventions d’activité physique (AP) en milieu scolaire des « pauses actives » sur les niveaux d’AP, le comportement en classe, les fonctions cognitives et le bien-être des préadolescents et des adolescents fréquentant l’école secondaire et le lycée. Méthodes En mars 2021, nous avons effectué une recherche systématique dans les bases de données CINAHL, Cochrane Library, Embase, MedLine, PsycINFO et à travers la littérature grise. L’évaluation de la qualité a été réalisée conformément à l’outil Cochrane d’évaluation de la qualité pour les ECR et à l’outil STROBE pour les études observationnelles. Nous avons inclus les études portant sur les interventions d’AP en classe menées par les enseignants, telles que les pauses actives ou les leçons d’activité physique sur les niveaux d’AP, le comportement en classe, la fonction cognitive et la qualité de vie chez les préadolescents et les adolescents fréquentant l’école secondaire et le lycée. Synthèse Trois études répondaient aux critères d’inclusion. Deux études ont montré un effet positif des pauses actives sur le comportement en classe et la qualité de vie des élèves. Une étude a enregistré un effet positif dans l’augmentation des niveaux d’AP à l’école, malheureusement cet effet n’a pas été trouvé dans les niveaux globaux d’AP ou dans la réduction du comportement sédentaire. Les trois études ont montré la faisabilité et l’acceptabilité de l’intervention des pauses actives dans les écoles secondaires et les lycées. Conclusion Cette revue systématique suggère le bénéfice potentiel de ce type d’intervention intégrée dans le programme scolaire des collèges et lycées sur le comportement en classe, les niveaux d’AP à l’école et le bien-être

Drought and stand susceptibility to attacks by the European spruce bark beetle: A remote sensing approach

Several time-series analyses have demonstrated that after extreme summer droughtbark beetle damage increased. However, studies predicting stand susceptibility overlarge spatial extents are limited by technical constraints in obtaining detailed,spatially-explicit data on infestation spot occurrence.2. Using a unique dataset of georeferenced bark beetle infestation data, we testedwhether the spatial variation of local growing conditions of forest stands, topogra-phy, and landscape variables modified the local occurrence ofIps typographusinfes-tations after a severe hot drought in Central Europe.3. Bark beetle infestation occurrence depended on soil-related aridity intensity, eleva-tion, slope, and soil conditions. We showed that elevation interacted with growingconditions and topography. At low elevations, spruce forests growing on flat areasand wetter soils were more sensitive to the infestations. On the contrary, forestson steep slopes and soils with low water availability were rarely attacked. At thelandscape scale, bark beetle damage increased with host tree cover but decreasedwith compositional diversity.4. Our findings are generally consistent with the growth-differentiation balancehypothesis that predicts that trees growing under chronic dry conditions tend to bemore resistant against biotic disturbances.5. Spruce stands at low elevations located in homogeneous landscapes dominated byspruce were those more exposed to bark beetles in the initial phase of a drought-induced outbrea

Spinorial formulation of the GW-BSE equations and spin properties of excitons in two-dimensional transition metal dichalcogenides

In many paradigmatic materials, such as transition metal dichalcogenides, the role played by the spin degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the optical and electronic properties of these materials. Despite the fact that the GW-BSE approach correctly accounts for electronic correlations, the spin-orbit coupling effect is often neglected or treated perturbatively. Recently, spinorial formulations of GW-BSE have become available in different flavors in material-science codes. However, an accurate validation and comparison of different approaches is still missing. In this work, we go through the derivation of the noncollinear GW-BSE approach. The scheme is applied to transition metal dichalcogenides comparing the perturbative and full spinorial approaches. Our calculations reveal that dark-bright exciton splittings are generally improved when the spin-orbit coupling is included nonperturbatively. The exchange-driven intravalley mixing between the A and B excitons is found to play a role for Mo-based systems, being especially strong in the case of MoSe2. We finally compute the excitonic spin and use it to sharply analyze the spinorial properties of transition metal dichalcogenide excitonic states