What about U on Surfaces? - Extended Hubbard Models for Adatom Systems from First Principles

Electronic correlations together with dimensional constraints lead to some of the most fascinating properties known in condensed matter physics. As possible candidates where these conditions are realized, semiconductor (111) surfaces and adatom systems on surfaces have been under investigation for quite some time. However, state-of-the-art theoretical studies on these materials that include many body effects beyond the band picture are rare. First principles estimates of inter-electronic Coulomb interactions for the correlated states are missing entirely, and usually these interactions are treated as adjustable parameters. In the present work, we report on calculations of the interaction parameters for the group IV surface-adatom systems in the \alpha-phase series of Si(111):\{C, Si, Sn, Pb\}. For all systems investigated, interelectronic Coulomb interactions are indeed large compared to the kinetic energy of the states in question. Moreover, our study reveals that intersite interactions cannot be disregarded. We explicitly construct an extended Hubbard model for the series of group IV surface-adatom systems on silicon, which can be used for further many-body calculations.Comment: Submitted to JPCM in April 2012; Published in February 201

Le rôle de l'Etat-Congrès et des acteurs extérieurs dans la montée du séparatisme sikh

Les Sikhs, adeptes du sikhisme, foi monothéiste, expression syncrétique de l'hindouisme et de l'islam, fondée sur les terres du Pendjab à la fin du quinzième siècle par Guru Nanak (1469-1539), ne représentent, avec quelques 17 millions de personnes, que 2 % de la population indienne. Si la place des Sikhs dans la mosaïque pluri-religieuse de l'Inde paraît de ce fait moindre, (les hindous représentant 83% de la population, les musulmans 11,3 %, les chrétiens 2,4 %, les bouddhistes 0,7 %, les j..

Claire Peltier, La médiatisation de la formation et de l’apprentissage – Mélanges offerts à Daniel Peraya

Un ouvrage, une œuvre Cet ouvrage collectif, intitulé La médiatisation de la formation et de l’apprentissage – Mélanges offerts à Daniel Peraya, a été réalisé en hommage à Daniel Peraya pour son départ en retraite. Ces Mélanges sont à l’image du parcours hétérogène de ce Professeur, au croisement de nombreuses disciplines : les sciences de la communication, la psychologie cognitive, les sciences de l’éducation, la technologie de l’éducation et la sociologie des usages. Ainsi, les différents c..

Spectral density of an interacting dot coupled indirectly to conducting leads

We study the spectral density of electrons rho in an interacting quantum dot (QD) with a hybridization lambda to a non-interacting QD, which in turn is coupled to a non-interacting conduction band. The system corresponds to an impurity Anderson model in which the conduction band has a Lorentzian density of states of width Delta2. We solved the model using perturbation theory in the Coulomb repulsion U (PTU) up to second order and a slave-boson mean-field approximation (SBMFA). The PTU works surprisingly well near the exactly solvable limit Delta2 -> 0. For fixed U and large enough lambda or small enough Delta2, the Kondo peak in rho(omega) splits into two peaks. This splitting can be understood in terms of weakly interacting quasiparticles. Before the splitting takes place the universal properties of the model in the Kondo regime are lost. Using the SBMFA, simple analytical expressions for the occurrence of split peaks are obtained. For small or moderate Delta2, the side bands of rho(omega) have the form of narrow resonances, that were missed in previous studies using the numerical renormalization group. This technique also has shortcomings for describing properly the split Kondo peaks. As the temperature is increased, the intensity of the split Kondo peaks decreases, but it is not completely suppressed at high temperatures.Comment: 13 pages, 13 figures, accepted in Phys. Rev.

Le patronat en Inde

Drawing a sociological profile of the Indian entrepreneur is an ambitious and tricky task. India is a world of diversities, and the same is true in its corporate world. Some great Indian businessmen developed worldwide activities, while others confined themselves to the local market. They may be deeply attached to the traditions of their religious or caste community, whereas others are very westernized and have been so for generations. They may be heirs to an hundred-year old family business, or self-made men whose affairs flourished within a few years. They may have studied in prestigious business schools, or given up their studies rather early. In order to have an overview of these various universes, and also to provide a practical guide of key names and concepts, this book focuses on three different levels: the socio-cultural world (family, community and value-concepts in India), the politics of business (history and strategy of five Indian industrial empires) and the employer in his enterprise (international leather shoemakers in the southern state of Tamil Nadu).Inde ; patronat ; sociologie

Low-energy models for correlated materials: bandwidth renormalization from Coulombic screening

We provide a prescription for constructing Hamiltonians representing the low energy physics of correlated electron materials with dynamically screened Coulomb interactions. The key feature is a renormalization of the hopping and hybridization parameters by the processes that lead to the dynamical screening. The renormalization is shown to be non-negligible for various classes of correlated electron materials. The bandwidth reduction effect is necessary for connecting models to materials behavior and for making quantitative predictions for low-energy properties of solids.Comment: 4 pages, 2 figure

Hubbard U and Hund's Exchange J in Transition Metal Oxides: Screening vs. Localization Trends from Constrained Random Phase Approximation

In this work, we address the question of calculating the local effective Coulomb interaction matrix in materials with strong electronic Coulomb interactions from first principles. To this purpose, we implement the constrained random phase approximation (cRPA) into a density functional code within the linearized augmented plane wave (LAPW) framework. We apply our approach to the 3d and 4d early transition metal oxides SrMO3 (M=V, Cr, Mn) and (M=Nb, Mo, Tc) in their paramagnetic phases. For these systems, we explicitly assess the differences between two physically motivated low-energy Hamiltonians: The first is the three-orbital model comprising the t2g states only, that is often used for early transition metal oxides. The second choice is a model where both, metal d- and oxygen p-states are retained in the construction of Wannier functions, but the Hubbard interactions are applied to the d-states only ("d-dp Hamiltonian"). Interestingly, since -- for a given compound -- both U and J depend on the choice of the model, so do their trends within a family of these compounds. In the 3d perovskite series SrMO3 the effective Coulomb interactions in the t2g Hamiltonian decrease along the series, due to the more efficient screening. The inverse -- generally expected -- trend, increasing interactions with increasing atomic number, is however recovered within the more localized "d-dp Hamiltonian". Similar conclusions are established in the layered 4d perovskites series Sr2MO4 (M=Mo, Tc, Ru, Rh). Compared to their isoelectronic and isostructural 3d analogues, the 4d 113 perovskite oxides SrMO3 (M=Nb, Mo, Tc) exhibit weaker screening effects. Interestingly, this leads to an effectively larger U on 4d shells than on 3d when a t2g model is constructed.Comment: 21 pages, 7 figure