Sistema modular para superfícies ajardinadas-produzido com geopolímeros e aglomerado negro de cortiça

O aglomerado negro de cortiça expandida foi fornecido pela ISOCOR/SOFALCA.A integração de superfícies ajardinadas em edifícios, como coberturas e paredes verdes, pode constituir uma estratégia passiva de poupança de energia, entre outros benefícios, como se discute neste trabalho. Em primeiro lugar é apresentada uma síntese dos principais benefícios da integração das superfícies ajardinadas em edifícios. Em seguida, são analisadas as principais características das coberturas e paredes verdes. Por fim, é apresentada uma descrição detalhada da concepção do sistema modular GEOGREEN e as suas características inovadoras relevantes.Fundação Portuguesa para a Ciência e Tecnologia, FCT

``X-Ray Edge'' Singularities in Nanotubes and Quantum Wires with Multiple Subbands

Band theory predicts an inverse square root van Hove singularity in the tunneling density of states at the minimum energy of an unoccupied subband in a one-dimensional quantum wire. With interactions, an orthogonality catastrophe analogous to the x-ray edge effect for core levels in a metal strongly reduces this singularity by a power B of the energy above threshold, with B approximately 0.3 for typical carbon nanotubes. Despite the anomalous tunneling characteristic, good quasiparticles corresponding to the unoccupied subband states do exist.Comment: 4 page

Fluorographynes: Stability, Structural and Electronic Properties

The presence in the graphyne sheets of a variable amount of sp2/sp1 atoms, which can be transformed into sp3-like atoms by covalent binding with one or two fluorine atoms, respectively, allows one to assume the formation of fulorinated graphynes (fluorographynes) with variable F/C stoichiometry. Here, employing DFT band structure calculations, we examine a series of fluorographynes, and the trends in their stability, structural and electronic properties have been discussed as depending on their stoichiometry: from C2F3 (F/C= 1.5) to C4F7 (F/C= 1.75).Comment: 13 pages, 3 table

Evolution of the social network of scientific collaborations

The co-authorship network of scientists represents a prototype of complex evolving networks. By mapping the electronic database containing all relevant journals in mathematics and neuro-science for an eight-year period (1991-98), we infer the dynamic and the structural mechanisms that govern the evolution and topology of this complex system. First, empirical measurements allow us to uncover the topological measures that characterize the network at a given moment, as well as the time evolution of these quantities. The results indicate that the network is scale-free, and that the network evolution is governed by preferential attachment, affecting both internal and external links. However, in contrast with most model predictions the average degree increases in time, and the node separation decreases. Second, we propose a simple model that captures the network's time evolution. Third, numerical simulations are used to uncover the behavior of quantities that could not be predicted analytically.Comment: 14 pages, 15 figure

Kondo lattice model with a direct exchange interaction between localized moments

We study the Kondo lattice model with a direct antiferromagnetic exchange interaction between localized moments. Ferromagnetically long-range ordered state coexisting with the Kondo screening shows a continuous quantum phase transition to the Kondo singlet state. We obtain the value of the critical point where the magnetizations of the localized moments and the conduction electrons vanish. The magnetization curves yield a universal critical exponent independent of the filling factors and the strength of the interaction between localized moments. It is shown that the direct exchange interaction between localized moments introduces another phase transition from an antiferromagnetic ordering to a ferromagnetic ordering for small Kondo exchange interaction. We also explain the local minimum of the Kondo temperature in recent experiments.Comment: 6 pages, 5 figures, final versio

Superconductivity in graphene stacks: from the bilayer to graphite

We study the superconducting phase transition, both in a graphene bilayer and in graphite. For that purpose we derive the mean-field effective potential for a stack of graphene layers presenting hopping between adjacent sheets. For describing superconductivity, we assume there is an on-site attractive interaction between electrons and determine the superconducting critical temperature as a function of the chemical potential. This displays a dome-shaped curve, in agreement with previous results for two-dimensional Dirac fermions. We show that the hopping between adjacent layers increases the critical temperature for small values of the chemical potential. Finally, we consider a minimal model for graphite and show that the transition temperature is higher than that for the graphene bilayer for small values of chemical potential. This might explain why intrinsic superconductivity is observed in graphite

Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet

We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic insulator, and coupled to it via a weak antiferromagnetic spin exchange interaction. We argue that this model may qualitatively capture the physics of a single charge stripe in the cuprates on length- and time scales shorter than those set by its fluctuation dynamics. Using a local mean-field approach we identify the low-energy effective theory that describes the electronic spin sector of the stripe as that of a sine-Gordon model. We determine its phases via a perturbative renormalization group analysis. For realistic values of the model parameters we obtain a phase characterized by enhanced spin density and composite charge density wave correlations, coexisting with subleading triplet and composite singlet pairing correlations. This result is shown to be independent of the spatial orientation of the stripe on the square lattice. Slow transverse fluctuations of the stripes tend to suppress the density correlations, thus promoting the pairing instabilities. The largest amplitudes for the composite instabilities appear when the stripe forms an antiphase domain wall in the antiferromagnet. For twisted spin alignments the amplitudes decrease and leave room for a new type of composite pairing correlation, breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure

Exciton swapping in a twisted graphene bilayer as a solid-state realization of a two-brane model

It is shown that exciton swapping between two graphene sheets may occur under specific conditions. A magnetically tunable optical filter is described to demonstrate this new effect. Mathematically, it is shown that two turbostratic graphene layers can be described as a "noncommutative" two-sheeted (2+1)-spacetime thanks to a formalism previously introduced for the study of braneworlds in high energy physics. The Hamiltonian of the model contains a coupling term connecting the two layers which is similar to the coupling existing between two braneworlds at a quantum level. In the present case, this term is related to a K-K' intervalley coupling. In addition, the experimental observation of this effect could be a way to assess the relevance of some theoretical concepts of the braneworld hypothesis.Comment: 15 pages, 3 figures, final version published in European Physical Journal

Partially filled stripes in the two dimensional Hubbard model: statics and dynamics

The internal structure of stripes in the two dimensional Hubbard model is studied by going beyond the Hartree-Fock approximation. Partially filled stripes, consistent with experimental observations, are stabilized by quantum fluctuations, included through the Configuration Interaction method. Hopping of short regions of the stripes in the transverse direction is comparable to the bare hopping element. The integrated value of $n_{\bf \vec{k}}$ compares well with experimental results.Comment: 4 page

Libxc: a library of exchange and correlation functionals for density functional theory

The central quantity of density functional theory is the so-called exchange-correlation functional. This quantity encompasses all non-trivial many-body effects of the ground-state and has to be approximated in any practical application of the theory. For the past 50 years, hundreds of such approximations have appeared, with many successfully persisting in the electronic structure community and literature. Here, we present a library that contains routines to evaluate many of these functionals (around 180) and their derivatives.Comment: 15 page