Ab initio complex band structure of conjugated polymers: Effects of hydrid DFT and GW schemes

The non-resonant tunneling regime for charge transfer across nanojunctions is critically dependent on the so-called \beta{} parameter, governing the exponential decay of the current as the length of the junction increases. For periodic materials, this parameter can be theoretically evaluated by computing the complex band structure (CBS) -- or evanescent states -- of the material forming the tunneling junction. In this work we present the calculation of the CBS for organic polymers using a variety of computational schemes, including standard local, semilocal, and hybrid-exchange density functionals, and many-body perturbation theory within the GW approximation. We compare the description of localization and \beta{} parameters among the adopted methods and with experimental data. We show that local and semilocal density functionals systematically underestimate the \beta{} parameter, while hybrid-exchange schemes partially correct for this discrepancy, resulting in a much better agreement with GW calculations and experiments. Self-consistency effects and self-energy representation issues of the GW corrections are discussed together with the use of Wannier functions to interpolate the electronic band-structure.Comment: Accepted for publication on Physical Review B v2: fixed some typo

Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials

Quantum ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn Source Package for Research in Electronic Structure, Simulation, and Optimization". It is freely available to researchers around the world under the terms of the GNU General Public License. Quantum ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. Quantum ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte

Etude ab-initio des auto-défauts et des mécanismes d'auto-diffusion dans un verre de silice

La silice et en général les verres silicatés sont des matériaux très utilisés dans des domaines très variés allant de la microélectronique à l'industrie nucléaire. Dans ces domaines l'un des principaux problèmes est le vieillissement de ces matériaux sous irradiation. Due à la complexité des effets irradiation-matière, la compréhension du veillissement nécessite la connaissance des mécanismes de diffusion à l'échelle atomique. Dans ce contexte, la modélisation numérique est un outil clef. Dans ce travail, nous présentons une étude ab-initio des auto-défauts et de l'auto-diffusion dans une silice modèle. Nous montrons que, contrairement au cas cristallin, les énergies de formation ainsi que les énergies de migration sont distribuées. Nous prouvons également que la distribution des energies de formation est liée à celle de la contrainte locale. En ce qui concerne les propriétés d'équilibre, nous discutons du rôle de la forme des distributions ainsi que du rôle des niveaux d'impuretés dans le gap sur le défaut majoritairement présent. Enfin, nous présentons le principal mécanisme d'auto-diffusion de l'oxygène en régime de formation homogène et hétérogène.SiO2 and silicia based compounds are key materials in a variety of scientific and technological fields as for instance, in microelectronics or nuclear technology. In all these fields, one of the still open questions is their long term aging in a radioactive environment. Due to the complexity of radiation-matter effects, the understanding of the long term aging needs the knowledge of diffusion mechanisms at the atomic scale. In that context, numerical modelling appears as a way to access this scale. We present a first principles study on self defects and self diffusion in a silica model. As expected, at variance with SiO2 crystalline phases, the defects formation energies are distributed, due to the non-equivalence of defects sites. We prove that the formation energy dispersion is correlated to the local stress. Concerning the equilibrium concentrations and oxygen diffusion mechanism, we discuss how the shape of the distribution,as well as impurity levels within the gap, play a main role in the dominance of defects types. Finally we present the main oxygen diffusion mechanism in homogeneous defect formation regime.CERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF