Realistic theory of electronic correlations in nanoscopic systems

Nanostructures with open shell transition metal or molecular constituents host often strong electronic correlations and are highly sensitive to atomistic material details. This tutorial review discusses method developments and applications of theoretical approaches for the realistic description of the electronic and magnetic properties of nanostructures with correlated electrons. First, the implementation of a flexible interface between density functional theory and a variant of dynamical mean field theory (DMFT) highly suitable for the simulation of complex correlated structures is explained and illustrated. On the DMFT side, this interface is largely based on recent developments of quantum Monte Carlo and exact diagonalization techniques allowing for efficient descriptions of general four fermion Coulomb interactions, reduced symmetries and spin-orbit coupling, which are explained here. With the examples of the Cr (001) surfaces, magnetic adatoms, and molecular systems it is shown how the interplay of Hubbard U and Hund's J determines charge and spin fluctuations and how these interactions drive different sorts of correlation effects in nanosystems. Non-local interactions and correlations present a particular challenge for the theory of low dimensional systems. We present our method developments addressing these two challenges, i.e., advancements of the dynamical vertex approximation and a combination of the constrained random phase approximation with continuum medium theories. We demonstrate how non-local interaction and correlation phenomena are controlled not only by dimensionality but also by coupling to the environment which is typically important for determining the physics of nanosystems.Comment: tutorial review submitted to EPJ-ST (scientific report of research unit FOR 1346); 14 figures, 26 page

Many-body effects on Cr(001) surfaces: An LDA+DMFT study

The electronic structure of the Cr(001) surface with its sharp resonance at the Fermi level is a subject of controversial debate of many experimental and theoretical works. To date, it is unclear whether the origin of this resonance is an orbital Kondo or an electron-phonon coupling effect. We have combined ab initio density functional calculations with dynamical mean-field simulations to calculate the orbitally resolved spectral function of the Cr(001) surface. The calculated orbital character and shape of the spectrum is in agreement with data from (inverse) photoemission experiments. We find that dynamic electron correlations crucially influence the surface electronic structure and lead to a low energy resonance in the $d_{z^2}$ and $d_{xz/yz}$ orbitals. Our results help to reconvene controversial experimental results from (I)PES and STM measurements.Comment: 8 pages, 5 figure

Densification par Spark Plasma Sintering (SPS) de matériaux d’électrolytes, difficilement densifiables, pour piles à combustible

Des matériaux tels que les apatites à base d’oxydes de lanthane et de silicium ou des pérovskites conductrices protoniques, potentiellement utilisables au sein de piles à combustible, présentent une grande résistance au frittage. Celle-ci limite d’autant plus leur utilisation au sein de piles à combustible, surtout s’ils doivent être employés comme électrolytes. Plusieurs stratégies peuvent être envisagées pour remédier à ce problème parmi lesquelles l’emploi de nouvelles méthodes de frittage ou le choix d’une méthode de synthèse efficace (permettant par exemple de diminuer la taille des grains ou de limiter celle des agrégats souvent rédhibitoires au moment du frittage). Nous présentons ici les résultats de frittage par Spark Plasma Sintering (appelé par la suite SPS) en les comparants à ceux obtenus par frittage conventionnel haute température. Les matériaux étudiés ont des compositions dérivées des phases La9,330,67Si6O26 et BaZr0,9Y0,1O2,950,05 pour lesquelles des problèmes de frittage ont été rencontrés. Nous insisterons sur les particularités des matériaux obtenus par SPS en termes de structure et microstructure et des conséquences sur les propriétés de transport anionique

Influence of synthesis route and composition on electrical properties of La9.33 + xSi6O26 + 3x/2 oxy-apatite compounds

Oxy-apatite materials La9.33 + xSi6O26 + 3x/2 are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well-known difficulties related to their densification prevent them from being used as such. This paper presents strategies to obtain oxyapatite dense materials. First, freeze-drying has been optimized to obtain ultrafine and very homogeneous La9.33 + xSi6O26 + 3x/2 (0≤x≤0.67) nanopowders. From these powders, conventional and Spark Plasma Sintering (SPS) have been used leading to very dense samples obtained at temperatures rather lower than those previously reported. For instance, SPS has allowed to prepare fully dense and transparent ceramics from 1200 °C under 100 MPa. The microstructure and transport properties of such samples have been then evaluated as a function of sintering conditions and lanthanum content. It will be show that for lanthanum content higher than 9.60 per unit formula, the parasitic phase La2SiO5 appears leading to a egradation of conduction properties.We also show that grain boundaries and porosity (for conventionally-sintered materials) seem to have blocking effects on oxygen transport. The highest overall conductivity values at 700 °C, i.e. σ700 °C=7.33.10−3 S cm−1, were measured for La9.33Si6O26 material conventionally-sintered at 1500 °C which contains bigger grains' size by comparison with σ700 °C=4.77.10−3 S cm−1 for SPS-sintered materials at the same temperature but for few minutes. These values are associated with activation energies close to 0.83–0.91 eV, regardless of sintering condition, which are commonly encountered for anionic conductivity into such materials

On Evidence-based Risk Management in Requirements Engineering

Background: The sensitivity of Requirements Engineering (RE) to the context makes it difficult to efficiently control problems therein, thus, hampering an effective risk management devoted to allow for early corrective or even preventive measures. Problem: There is still little empirical knowledge about context-specific RE phenomena which would be necessary for an effective context- sensitive risk management in RE. Goal: We propose and validate an evidence-based approach to assess risks in RE using cross-company data about problems, causes and effects. Research Method: We use survey data from 228 companies and build a probabilistic network that supports the forecast of context-specific RE phenomena. We implement this approach using spreadsheets to support a light-weight risk assessment. Results: Our results from an initial validation in 6 companies strengthen our confidence that the approach increases the awareness for individual risk factors in RE, and the feedback further allows for disseminating our approach into practice.Comment: 20 pages, submitted to 10th Software Quality Days conference, 201

Pair neutron transfer in Ni 60 + Sn 116 probed via γ -particle coincidences

D. Montanari et al. ; 6 págs.; 5 figs.; 1 tab.We performed a γ-particle coincidence experiment for the Ni60+Sn116 system to investigate whether the population of the two-neutron pickup channel leading to Ni62 is mainly concentrated in the ground-state transition, as has been found in a previous work [D. Montanari et al., Phys. Rev. Lett. 113, 052501 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.052501]. The experiment has been performed by employing the PRISMA magnetic spectrometer coupled to the Advanced Gamma Tracking Array (AGATA) demonstrator. The strength distribution of excited states corresponding to the inelastic, one- and two-neutron transfer channels has been extracted. We found that in the two-neutron transfer channel the strength to excited states corresponds to a fraction (less than 24%) of the total, consistent with the previously obtained results that the 2n channel is dominated by the ground-state to ground-state transition. ©2016 American Physical SocietyThis work was partly supported by the EU FP7/2007-2013 under Grant No. 262010-ENSAR and by the Croatian Science Foundation under Project No. 7194. A.G. was partially supported by MINECO and Generalitat Valenciana, Spain, under Grants No. FPA2014-57196-C5 and No. PROMETEOII/2014/019 and the EU under the FEDER program.Peer Reviewe