2,507 research outputs found

    Field effect on surface states in a doped Mott-Insulator thin film

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    Surface effects of a doped thin film made of a strongly correlated material are investigated both in the absence and presence of a perpendicular electric field. We use an inhomogeneous Gutzwiller approximation for a single band Hubbard model in order to describe correlation effects. For low doping, the bulk value of the quasiparticle weight is recovered exponentially deep into the slab, but with increasing doping, additional Friedel oscillations appear near the surface. We show that the inverse correlation length has a power-law dependence on the doping level. In the presence of an electrical field, considerable changes in the quasiparticle weight can be realized throughout the system. We observe a large difference (as large as five orders of magnitude) in the quasiparticle weight near the opposite sides of the slab. This effect can be significant in switching devices that use the surface states for transport

    Low density ferromagnetism in the Hubbard model

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    A single-band Hubbard model with nearest and next-nearest neighbour hopping is studied for d=1d=1, 2, 3, using both analytical and numerical techniques. In one dimension, saturated ferromagnetism is found above a critical value of UU for a band structure with two minima and for small and intermediate densities. This is an extension of a scenario recently proposed by M\"uller--Hartmann. For three dimensions and non-pathological band structures, it is proven that such a scenario does not work.Comment: 4 pages, 3 postscript figure

    The One and the Many: Elaborating a taxonomy of Impact Assessment practices in Europe

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    Audio commons ontology: A data model for an audio content ecosystem

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    Multiple online services host repositories of audio clips of different kinds, ranging from music tracks, albums, playlists, to instrument samples and loops, to a variety of recorded or synthesized sounds. Programmatic access to these resources maybe used by client applications for tasks ranging from customized musical listening and exploration, to music/sounds creation from existing sounds and samples, to audio-based user interaction in apps and games. We designed an ontology to facilitate interoperability between repositories and clients in this domain. There was no previous comprehensive data model for our domain, however the new ontology relates to existing ontologies, such as the Functional Requirements for Bibliographic Records for the authoring and publication process of creative works, the Music Ontology for the authoring and publication of music, the EBU Core ontology to describe media files and formats and the Creative Commons Licensing ontology to describe licences. This paper documents the design of the ontology and its evaluation with respect to specific requirements gathered from stakeholders

    Potential-energy (BCS) to kinetic-energy (BEC)-driven pairing in the attractive Hubbard model

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    The BCS-BEC crossover within the two-dimensional attractive Hubbard model is studied by using the Cellular Dynamical Mean-Field Theory both in the normal and superconducting ground states. Short-range spatial correlations incorporated in this theory remove the normal-state quasiparticle peak and the first-order transition found in the Dynamical Mean-Field Theory, rendering the normal state crossover smooth. For UU smaller than the bandwidth, pairing is driven by the potential energy, while in the opposite case it is driven by the kinetic energy, resembling a recent optical conductivity experiment in cuprates. Phase coherence leads to the appearance of a collective Bogoliubov mode in the density-density correlation function and to the sharpening of the spectral function.Comment: 5 pages, 4 figure

    The Diffusion of Impact Assessment Practices in Europe

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    The diffusion of the use of various forms of impact assessments (IAs) in different political settings and legal traditions illustrates the great malleability of the tool. This diversity is not only reflected in the adoption of different models of IA across the various jurisdictions examined, but also in the way this practice is effectively implemented. Factors explaining the various types of IA implemented in various European jurisdictions include the patterns of diffusion from one country to another, the interaction of politics with expert knowledge and the prevailing “evidence eco-system” in each jurisdiction. In this study we explore diffusion patterns, not only in terms of the adoption of the tool of IA, but also in terms of the specific types of IA implemented. We do so by introducing a taxonomy developed with the purpose to describe the interaction of politics and expertise in each jurisdiction. The last part of the chapter empirically connects the diffusion process with the type of IA prevalent in a jurisdiction

    Asymmetry between the electron- and hole-doped Mott transition in the periodic Anderson model

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    We study the doping driven Mott metal-insulator transition (MIT) in the periodic Anderson model set in the Mott-Hubbard regime. A striking asymmetry for electron or hole driven transitions is found. The electron doped MIT at larger U is similar to the one found in the single band Hubbard model, with a first order character due to coexistence of solutions. The hole doped MIT, in contrast, is second order and can be described as the delocalization of Zhang-Rice singlets.Comment: 18 pages, 19 figure

    The Heisenberg antiferromagnet on a triangular lattice: topological excitations

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    We study the topological defects in the classical Heisenberg antiferromagnet in two dimensions on a triangular lattice (HAFT). While the topological analysis of the order parameter space indicates that the defects are of Z2Z_2 type, consideration of the energy leads us to a description of the low--energy stationary points of the action in terms of ±\pm vortices, as in the planar XY model. Starting with the continuum description of the HAFT, we show analytically that its partition function can be reduced to that of a 2--dimensional Coulomb gas with logarithmic interaction. Thus, at low temperatures, the correlation length is determined by the spinwaves, while at higher temperatures we expect a crossover to a Kosterlitz--Thouless type behaviour. The results of recent Monte Carlo calculations of the correlation length are consistent with such a crossover.Comment: 9 pages, revtex, preprint: ITP-UH 03/9
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