Non-equilibrium Green's functions in density functional tight binding: method and applications

We present a detailed description of the implementation of the non-equilibrium Green's function (NEGF) technique on the density-functional-based tight-binding (gDFTB) simulation tool. This approach can be used to compute electronic transport in organic and inorganic molecular-scale devices. The DFTB tight-binding formulation gives an efficient computational tool that is able to handle a large number of atoms. NEGFs are used to compute the electronic density self-consistently with the open-boundary conditions naturally encountered in quantum transport problems and the boundary conditions imposed by the potentials at the contacts. The efficient block-iterative algorithm used to compute the Green's functions is illustrated. The Hartree potential of the density-functional Hamiltonian is obtained by solving the three-dimensional Poisson equation. A scheme to treat geometrically complex boundary conditions is discussed, including the possibility of including multiterminal calculations

QuantumATK: An integrated platform of electronic and atomic-scale modelling tools

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte

Indagine conoscitiva, per l’analisi di vulnerabilità, di due centri storici liguri

The present paper describes the application of a multidisciplinary procedure aimed at an articulated knowledge both of the materials, the morphology, the constructive details of the masonry elements, and of the mechanical behaviour and the possible collapse mechanisms of buildings placed in some historic centres of western Liguria. Western Liguria is a greatly interesting area, considering both its seismic history, and the characteristics of its buildings. The historic centres of Bajardo, Taggia (Imperia) and other are still displaying the signs of damage suffered during the earthquake of 1887, to which the problems due to the lack of maintenance, and in many cases of abandon, have overlapped. Particular attention has been paid to monumental buildings and to residential buildings especially interesting due to the adopted constructive techniques and/or due to a complex constructive evolution in time

Vulnerability study in seismic areas: the role of on-site and archives investigation

The seismic response of old stone masonry buildings in Western Liguria is approached by a multi-disciplinary view. The historical centers of Baiardo, Taggia and Bussana (Province of Imperia, Italy) and others, still display signs of damage due to XIX century earthquakes, to which the problems of lack of maintenance and, in many cases, of abandon have overlapped. A method for evaluating their seismic vulnerability is proposed after: (i) a direct survey of the buildings (based on a stratigraphical survey and on a survey of the crack patterns and of the wall sections), (ii) the collection of indirect information from documentary sources, (iii) the collection of data recorded through in-situ non destructive or low destructive mechanical testing and laboratory investigation aiming at the chemical, physical and mechanical characterization of masonry components. The role of historical analysis will be shown as a basic source of knowledge of the structures for the subsequent assessment of seismic vulnerability

Life prediction of hot work tool steels subjected to thermomechanical fatigue

Surface of hot work tools is damaged by complex thermomechanical-wear-reactive environment interactions. A thermomechanical fatigue (TMF) experiment using tubular specimens is developed. Tests are carried out under out-of-phase thermomechanical cycle. The mechanical strain is minimum (εm min) at upper temperature (Tmax) and it is maximum (εm max) at lower temperature (Tmin) of thermal cycle. The behaviour, the damage and the fatigue life of two tempered martensitic steels X38CrMoV5 (AISI H11) and 55NiCrMoV8 (47 HRC) are assessed. The effect of Tmin and Tmax is examined. Softening is observed for both steels. For a given mechanical strain amplitude (Δεm = εm max – Em min), a drastic dependence on Tmax is demonstrated, while Tmin has less important effect. X38CrMoV5 has a better TMF life. Ductile fatigue striations are observed on the fracture surface of X38CrMoV5 specimens. The fracture surface of 55NiCrMoV8 specimens is covered by oxidation, making difficult to reveal the fatigue striations. Both oxide cracking and spalling are observed on the external surface of specimens. A phenomenological power law uniaxial model, based on the mechanical strain amplitude and Tmin, Δεm = K(Tmax).[math], is proposed to predict the life under non-isothermal fatigue sollicitations. The life of some thermal fatigue tests is predicted within a factor of two to three. The capability of the model to predict the critical regions of an industrial hot work tool is reported. Key words : Thermomechanical fatigue, thermal fatigue, tool steels, crack initiation, crack propagation, high temperature fatigue

Tipologie murarie e costruttive nei centri storici

Nell’ambito del progetto GNDT sul tema “Analisi di scenario nella Liguria occidentale e soluzioni per la conservazione dei centri storici“ si descrive l’applicazione di una procedura multidisciplinare, indirizzata ad una conoscenza articolata sia dei materiali, della morfologia, degli aspetti costruttivi degli elementi murari, sia del comportamento meccanico e dei possibili meccanismi di collasso di edifici posti in alcuni centri storici della Liguria occidentale. La Liguria occidentale è un’area di grande interesse sia per quanto riguarda la sua storia sismica sia anche a causa delle caratteristiche dei suoi edifici (Fig. 1) che presentano un certo grado di omogeneità e formano centri storici di rilevante valore storico e culturale. Scopo della ricerca è suggerire delle linee-guida per una procedura di indagine che possa orientare gli interventi sugli edifici storici verso il duplice obiettivo della sicurezza e della compatibilità

Un approccio multidisciplinare alla prevenzione dei danni in zona sismica

Viene presentato un approccio multidisciplinare rivolto all’analisi della risposta sismica di edifici in muratura prevalentemente di pietra ed alla valutazione dell'efficacia di tecniche di riparazione, basato su: a) una raccolta di informazioni indirette dalle fonti documentarie, b) uno studio in sito (con un rilievo diretto dell’edificio, delle sue discontinuità e del suo quadro fessurativo, della morfologia della muratura, con un particolare risalto alle caratteristiche delle sezioni murarie), c) una raccolta di dati ottenuti sia tramite prove in sito leggermente o non distruttive. sia tramite prove di laboratorio, finalizzate ad una caratterizzazione chimica, fisica e meccanica dei componenti della muratura. La procedura è volta a definire i possibili meccanismi di danno e collasso sotto l'azione del sisma allo scopo di applicare modelli meccanici appropriati e di scegliere le tecniche più adeguate per la prevenzione del danneggiamento o la riparazione delle struttur