1,680 research outputs found

    Hierarchical Parallelisation of Functional Renormalisation Group Calculations -- hp-fRG

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    The functional renormalisation group (fRG) has evolved into a versatile tool in condensed matter theory for studying important aspects of correlated electron systems. Practical applications of the method often involve a high numerical effort, motivating the question in how far High Performance Computing (HPC) can leverage the approach. In this work we report on a multi-level parallelisation of the underlying computational machinery and show that this can speed up the code by several orders of magnitude. This in turn can extend the applicability of the method to otherwise inaccessible cases. We exploit three levels of parallelisation: Distributed computing by means of Message Passing (MPI), shared-memory computing using OpenMP, and vectorisation by means of SIMD units (single-instruction-multiple-data). Results are provided for two distinct High Performance Computing (HPC) platforms, namely the IBM-based BlueGene/Q system JUQUEEN and an Intel Sandy-Bridge-based development cluster. We discuss how certain issues and obstacles were overcome in the course of adapting the code. Most importantly, we conclude that this vast improvement can actually be accomplished by introducing only moderate changes to the code, such that this strategy may serve as a guideline for other researcher to likewise improve the efficiency of their codes

    Quasi-particle functional Renormalisation Group calculations in the two-dimensional t-t'-Hubbard model

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    We extend and apply a recently introduced quasi-particle functional renormalisation group scheme to the two-dimensional Hubbard model with next-nearest-neighbour hopping and away from half filling. We confirm the generation of superconducting correlations in some regions of the phase diagram, but also find that the inclusion of self-energy feedback by means of a decreasing quasi-particle weight can suppress superconducting tendencies more than anti-ferromagnetic correlations by which they are generated. As a supplement, we provide sample results for the self-energy in second-order perturbation theory and address some conceptual matters

    Pseudogap opening in the two-dimensional Hubbard model: A functional renormalization group analysis

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    Using the recently introduced multiloop extension of the functional renormalization group, we compute the frequency- and momentum-dependent self-energy of the two-dimensional Hubbard model at half filling and weak coupling. We show that, in the truncated-unity approach for the vertex, it is essential to adopt the Schwinger-Dyson form of the self-energy flow equation in order to capture the pseudogap opening. We provide an analytic understanding of the key role played by the flow scheme in correctly accounting for the impact of the antiferromagnetic fluctuations. For the resulting pseudogap, we present a detailed numerical analysis of its evolution with temperature, interaction strength, and loop order.Comment: 15 pages, 15 figures, version as publishe

    Pseudogap at hot spots in the two-dimensional Hubbard model at weak coupling

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    We analyze the interaction-induced renormalization of single-particle excitations in the two-dimensional Hubbard model at weak coupling using the Wick-ordered version of the functional renormalization group. The self energy is computed for real frequencies by integrating a flow equation with renormalized two-particle interactions. In the vicinity of hot spots, that is points where the Fermi surface intersects the umklapp surface, self energy effects beyond the usual quasi-particle renormalizations and damping occur near instabilities of the normal, metallic phase. Strongly enhanced renormalized interactions between particles at different hot spots generate a pronounced low-energy peak in the imaginary part of the self energy, leading to a pseudogap-like double-peak structure in the spectral function for single-particle excitations.Comment: 14 pages, 7 figure

    REPRESENTA√á√ēES SOCIAIS DO CRACK NA M√ćDIA

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    Crack cocaine has been a recurrent topic in newspaper articles and in recent political speeches. This researchaimed to identify how the local media represents the drug. Based on the Theory of Social Representations 76 articles publishedduring 2009 in the Correio Braziliense newspaper were analyzed. The content of the articles was analyzed with the softwareALCESTE, which generated six classes. Three social representations were highlighted: drug as a scourge of humanity,indistinct police actions against users and drug dealers and hospitalization as a solution of the problem. We observed that thesocial representations are consistent with a stigmatizing and repressive approach, which still consider drug users as criminalor sick persons, reinforcing the maintenance of their underground status and limiting the understanding of the phenomenon.O crack tem desafiado o sistema de sa√ļde, configurando-se em um dos alvos das pol√≠ticas de governo. O artigo tem como objetivo identificar as representa√ß√Ķes sociais veiculadas pela m√≠dia sobre o usu√°rio de crack. Com base na Teoria das Representa√ß√Ķes Sociais, foram analisadas as 76 reportagens do jornal Correio Braziliense publicadas em 2009 sobre o tema. Utilizou-se o software ALCESTE que gerou uma an√°lise lexical das 93 unidades de contexto inicial, agrupando-as em seis classes. A partir das classes, foram destacadas tr√™s representa√ß√Ķes sociais: a droga como flagelo da humanidade, as a√ß√Ķes indistintas da pol√≠cia contra usu√°rios ou traficantes e a interna√ß√£o do usu√°rio como solu√ß√£o do problema. Observou-se que as representa√ß√Ķes sociais se coadunam com a abordagem estigmatizante e repressiva do usu√°rio de drogas, a qual continua a considerar o usu√°rio como criminoso ou doente, refor√ßando a manuten√ß√£o de sua clandestinidade e limitando a compreens√£o do fen√īmeno

    Social representation of crack cocaine in Brasilia's media press

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    O crack tem sido tema recorrente em mat√©rias jornal√≠sticas e discursos pol√≠ticos. O objetivo do estudo foi identificar como a m√≠dia local representa a droga. Baseadas na Teoria das Representa√ß√Ķes Sociais foram analisadas 76 reportagens de 2009 do jornal Correio Braziliense, utilizando-se o software ALCESTE que gerou seis classes. Destacaram-se tr√™s representa√ß√Ķes sociais: droga como flagelo da humanidade, a√ß√Ķes policiais indistintas contra usu√°rios ou traficantes e interna√ß√£o do usu√°rio como solu√ß√£o do problema. As representa√ß√Ķes sociais encontradas alinham-se com a abordagem estigmatizante e repressiva ao usu√°rio de drogas que ainda o considera ora como criminoso, ora como doente e perpetuam sua clandestinidade e limitam a compreens√£o do fen√īmeno.Crack cocaine has been a recurrent topic in newspaper articles and in recent political speeches. This research aimed to identify how the local media represents the drug. Based on the Theory of Social Representations 76 articles published during 2009 in the Correio Braziliense newspaper were analyzed. The content of the articles was analyzed with the software ALCESTE, which generated six classes. Three social representations were highlighted: drug as a scourge of humanity, indistinct police actions against users and drug dealers and hospitalization as a solution of the problem. We observed that the social representations are consistent with a stigmatizing and repressive approach, which still consider drug users as criminal or sick persons, reinforcing the maintenance of their underground status and limiting the understanding of the phenomenon

    Speeding up Energy System Models - a Best Practice Guide

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    Background Energy system models (ESM) are widely used in research and industry to analyze todays and future energy systems and potential pathways for the European energy transition. Current studies address future policy design, analysis of technology pathways and of future energy systems. To address these questions and support the transformation of today‚Äôs energy systems, ESM have to increase in complexity to provide valuable quantitative insights for policy makers and industry. Especially when dealing with uncertainty and in integrating large shares of renewable energies, ESM require a detailed implementation of the underlying electricity system. The increased complexity of the models makes the application of ESM more and more difficult, as the models are limited by the available computational power of today‚Äôs decentralized workstations. Severe simplifications of the models are common strategies to solve problems in a reasonable amount of time ‚Äď naturally significantly influencing the validity of results and reliability of the models in general. Solutions for Energy-System Modelling Within BEAM-ME a consortium of researchers from different research fields (system analysis, mathematics, operations research and informatics) develop new strategies to increase the computational performance of energy system models and to transform energy system models for usage on high performance computing clusters. Within the project, an ESM will be applied on two of Germany‚Äôs fastest supercomputers. To further demonstrate the general application of named techniques on ESM, a model experiment is implemented as part of the project. Within this experiment up to six energy system models will jointly develop, implement and benchmark speed-up methods. Finally, continually collecting all experiences from the project and the experiment, identified efficient strategies will be documented and general standards for increasing computational performance and for applying ESM to high performance computing will be documented in a best-practice guide

    Scaling study of the pion electroproduction cross sections and the pion form factor

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    The 1^{1}H(e,e‚Ä≤ŌÄ+e,e^\prime \pi^+)n cross section was measured for a range of four-momentum transfer up to Q2Q^2=3.91 GeV2^2 at values of the invariant mass, WW, above the resonance region. The Q2Q^2-dependence of the longitudinal component is consistent with the Q2Q^2-scaling prediction for hard exclusive processes. This suggests that perturbative QCD concepts are applicable at rather low values of Q2Q^2. Pion form factor results, while consistent with the Q2Q^2-scaling prediction, are inconsistent in magnitude with perturbative QCD calculations. The extraction of Generalized Parton Distributions from hard exclusive processes assumes the dominance of the longitudinal term. However, transverse contributions to the cross section are still significant at Q2Q^2=3.91 GeV2^2.Comment: 6 pages, 3 figure

    Nuclear transparency and effective kaon-nucleon cross section from the A(e, e'K+) reaction

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    We have determined the transparency of the nuclear medium to kaons from A(e,e‚Ä≤K+)A(e,e^{'} K^{+}) measurements on 12^{12}C, 63^{63}Cu, and 197^{197}Au targets. The measurements were performed at the Jefferson Laboratory and span a range in four-momentum-transfer squared Q2^2=1.1 -- 3.0 GeV2^2. The nuclear transparency was defined as the ratio of measured kaon electroproduction cross sections with respect to deuterium, (ŌÉA/ŌÉD\sigma^{A}/\sigma^{D}). We further extracted the atomic number (AA) dependence of the transparency as parametrized by T=(A/2)őĪ‚ąí1T= (A/2)^{\alpha-1} and, within a simple model assumption, the in-medium effective kaon-nucleon cross sections. The effective cross sections extracted from the electroproduction data are found to be smaller than the free cross sections determined from kaon-nucleon scattering experiments, and the parameter őĪ\alpha was found to be significantly larger than those obtained from kaon-nucleus scattering. We have included similar comparisons between pion- and proton-nucleon effective cross sections as determined from electron scattering experiments, and pion-nucleus and proton-nucleus scattering data.Comment: 7 pages, 5 figure
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