2,251 research outputs found
Hierarchical Parallelisation of Functional Renormalisation Group Calculations -- hp-fRG
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
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
Fermionic renormalization group flow into phases with broken discrete symmetry: charge-density wave mean-field model
We generalize the application of the functional renormalization group (fRG)
method for the fermionic flow into the symmetry-broken phase to finite
temperatures. We apply the scheme to the case of a broken discrete symmetry:
the charge-density wave (CDW) mean-field model at half filling. We show how an
arbitrarily small initial CDW order parameter starts to grow at the CDW
instability and how it flows to the correct final value, suppressing the
divergence of the effective interaction in the fRG flow. The effective
interaction peaks at the instability and saturates at low energy scales or
temperatures. The relation to the mean-field treatment, differences compared to
the flow for a broken continuous symmetry, and the prospects of the new method
are discussed.Comment: 10 pages, 7 figures. V2: minor corrections, journal references and
DOI adde
Pseudogap opening in the two-dimensional Hubbard model: A functional renormalization group analysis
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
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
Interaction flow method for many-fermion systems
We propose an interaction flow scheme that sums up the perturbation expansion
of many-particle systems by successively increasing the interaction strength.
It combines the unbiasedness of renormalization group methods with the
simplicity of straight-forward perturbation theory. Applying the scheme to
fermions in one dimension and to the two-dimensional Hubbard model we find that
at one-loop level and low temperatures there is ample agreement with previous
one-loop renormalization group approaches. We furthermore present results for
the momentum-dependence of spin, charge and pairing interactions in the
two-dimensional Hubbard model.Comment: 14 pages, 14 figure
Pair fluctuation induced pseudogap in the normal phase of the two-dimensional attractive Hubbard model at weak coupling
One-particle spectral properties in the normal phase of the two-dimensional
attractive Hubbard model are investigated in the weak coupling regime using the
non-selfconsistent T-matrix approximation. The corresponding equations are
evaluated numerically directly on the real frequency axis. For temperatures
sufficiently close to the superconducting transition temperature a pseudogap in
the one-particle spectral function is observed, which can be assigned to the
increasing importance of pair fluctuations.Comment: 22 pages, 13 figure
REPRESENTAÇÕES SOCIAIS DO CRACK NA MÍDIA
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 saúde, configurando-se em um dos alvos das políticas de governo. O artigo tem como objetivo identificar as representações sociais veiculadas pela mídia sobre o usuário de crack. Com base na Teoria das Representações Sociais, foram analisadas as 76 reportagens do jornal Correio Braziliense publicadas em 2009 sobre o tema. Utilizou-se o software ALCESTE que gerou uma análise lexical das 93 unidades de contexto inicial, agrupando-as em seis classes. A partir das classes, foram destacadas três representações sociais: a droga como flagelo da humanidade, as ações indistintas da polícia contra usuários ou traficantes e a internação do usuário como solução do problema. Observou-se que as representações sociais se coadunam com a abordagem estigmatizante e repressiva do usuário de drogas, a qual continua a considerar o usuário como criminoso ou doente, reforçando a manutenção de sua clandestinidade e limitando a compreensão do fenômeno
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