Non-equilibrium Bethe-Salpeter equation for transient photo-absorption spectroscopy

In this work we propose an accurate first-principle approach to calculate the transient photo--absorption spectrum measured in Pump\&\,Probe experiments. We formulate a condition of {\em adiabaticity} and thoroughly analyze the simplifications brought about by the fulfillment of this condition in the non--equilibrium Green's function (NEGF) framework. Starting from the Kadanoff-Baym equations we derive a non--equilibrium Bethe--Salpeter equation (BSE) for the response function that can be implemented in most of the already existing {\em ab--initio} codes. In addition, the {\em adiabatic} approximation is benchmarked against full NEGF simulations in simple model hamiltonians, even under extreme, nonadiabatic conditions where it is expected to fail. We find that the non--equilibrium BSE is very robust and captures important spectral features in a wide range of experimental configurations.Comment: 13 pages, 5 captioned figure

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

Anomalous temperature dependence of the band-gap in Black Phosphorus

Black Phosphorus (BP) has gained renewed attention due to its singular anisotropic electronic and optical properties that might be exploited for a wide range of technological applications. In this respect, the thermal properties are particularly important both to predict its room temperature operation and to determine its thermoelectric potential. From this point of view, one of the most spectacular and poorly understood phenomena is, indeed, the BP temperature-induced band-gap opening: when temperature is increased the fundamental band-gap increases instead of decreasing. This anomalous thermal dependence has also been observed, recently, in its monolayer counterpart. In this work, based on \textit{ab-initio} calculations, we present an explanation for this long known, and yet not fully explained, effect. We show that it arises from a combination of harmonic and lattice thermal expansion contributions, which are, in fact, highly interwined. We clearly narrow down the mechanisms that cause this gap opening by identifying the peculiar atomic vibrations that drive the anomaly. The final picture we give explains both the BP anomalous band-gap opening and the frequency increase with increasing volume (tension effect).Comment: Published in Nano Letter

Experiences from the Northern Italy 2004 earthquake: Vulnerability assessment and strengthening of historic churches

This work is about the lesson learnt from the analysis of the damages caused to some churches by the strong earthquake, which struck Northern Italy in the year 2004. Focus is paid to some mechanisms which are not well known in the literature: the excessive and differential rocking of neighbouring transverse arches, the differential deflection of single leaf vault rings, the tie over-tension induced by the transverse arch rocking. In the paper, the structural solutions appositely studied to repair and to reduce the structure seismic vulnerability of these buildings are presented. Among these solutions: internal perimeter ties, lightweight wooden roof box structure, lightweight spandrel ribs are illustrated

How does collegiality survive managerially led universities? Evidence from a European Survey

Today's universities are, accordingly to Clark's entrepreneurial model, sustained by managerialism, whereas collegialism may remain in contrast or work in a different way. More recent literature suggests the clash such as the potential for coexistence between managerialism and collegialism. The study analyses data from a survey of 26 universities in 8 European countries, focusing on middle managers (MMs). The results show that at the level of the individual institutions, there are notable positive correlations between the presence of collegial and of managerial cultures. Multilevel regressions at institutional level are analysed, to ‘predict’ collegiality in light of the universities' managerial culture and other factors affecting organizational change: accountability; distribution of discretional power; funding; impact of quality assurance (QA) and evaluation. The results illustrate that in more managerial universities, collegial culture increases above all when MMs believe that distance-steering tools (QA and evaluation) have had positive impacts. We find that collegiality can indeed thrive, even when ‘managerially led’

Wooden Roof Box Structure for the Anti-Seismic Strengthening of HistoricBuildings

A wooden roof strengthening technique aimed at transforming the roof pitches into anti-seismic shear resisting diaphragms is presented in this paper. Shear diaphragms gather and transfer the seismic loads to the shear resisting walls. Diaphragms are built on top of the existing structures without significantly modifying the roof overall layout. The proposed strengthening technique is mainly reversible, minimally impairing the building integrity, and can be easily applied for the construction of anti-seismic wooden roofs in new buildings. A simplified design approach is presented, which allows to identify the static role of each element. An analytical method for the estimate of the box structure displacements, as well as a short digression on the displacement control requirements is also illustrated. The diaphragm technique was recently applied for the anti-seismic retrofit of some monumental buildings in Italy. A few case studies, as well as the basic design criteria for applying this technique are presented in this paper

Dynamical excitonic effects in metals and semiconductors

The dynamics of an electron--hole pair induced by the time--dependent screened Coulomb interaction is discussed. In contrast to the case where the static electron--hole interaction is considered we demonstrate the occurrence of important dynamical excitonic effects in the solution of the Bethe--Salpeter equation.This is illustrated in the calculated absorption spectra of noble metals (copper and silver) and silicon. Dynamical corrections strongly affect the spectra, partially canceling dynamical self--energy effects and leading to good agreement with experiment.Comment: Accepted for publication on Phys. Rev. Let

La valutazione e i suoi effetti sull'università: una comparazione fra Italia e Francia

The paper presents the results of a comparative study on four universities in France and in Italy, which investigates how evaluation helps to strengthen the university as autonomous and professional organization, able to formulating strategies and/or tactics of adaptation in relation to regulatory changes and context, overcoming the traditional loosely coupled feature. The hypothesis is that the academic organizations show different effects of resilience, linked to intended and unintended changes

Analysis of Reinforced Concrete Elements Including Shear Effects

This paper deals with the analysis of reinforced concrete (RC)plane frames under monotonic and cyclic loading, including axial,bending, and shear effects. A force-based two-dimensional (2D)element based on the Timoshenko beam theory is introduced. Theelement formulation is general and yields the exact solution withinthe Timoshenko beam theory. A simple, nonlinear, shear force-shear deformation law is used at the section level, together with aclassical fiber section for the axial and bending effects. Sheardeformations are thus uncoupled from axial and bending effects inthe section stiffness, but shear and bending forces become coupledat the element level because equilibrium is enforced along thebeam element. The element is validated through comparisons withexperimental data on the shear performance of bridge columns.The seismic analysis of a viaduct that collapsed during the 1995 Kobe earthquake is presented