Second harmonic spectroscopy to optically detect valley polarization in 2D materials

Valley polarization (VP), an induced imbalance in the populations of a multi-valley electronic system, allows emission of second harmonic (SH) light even in centrosymmetric crystals such as graphene. Whereas in systems such as MoS$\mathrm{_2}$ or BN this adds to their intrinsic quadratic response, SH generation in a multi-valley inversion-symmetric crystal can provide a direct measure of valley polarization. By computing the nonlinear response and characterizing theoretically the respective SH as a function of polarization, temperature, electron density, and degree of VP, we demonstrate the possibility of disentangling and individually quantifying the intrinsic and valley contributions to the SH. A specific experimental setup is proposed to obtain direct quantitative information about the degree of VP and allow its remote mapping. This approach could prove useful for direct, contactless, real-space monitoring of valley injection and other applications of valley transport and valleytronics.Comment: Updating with published version, including typesetting corrections to eqs 3 and 4; 7 pages, 5 figure

Macroeconomic effects of fiscal consolidations in a DSGE model for the Euro Area: does composition matter?

We develop a new-Keynesian DSGE model with an extended fiscal policy block to assess the conditions for expansionary fiscal consolidations. In addition to several taxes, we consider public employment expenditures and government spending, which may have different degrees of productivity. We calibrate the model for the Euro Area and use it to simulate alternative fiscal consolidations with changes in the budget composition. Among the main conclusions we find that: (i) if conducted with a cut in weaklyproductive spending and a symmetric increase in highly-productive spending, fiscal consolidations have expansionary effects on investment and output; (ii) if consolidation is pursued through a pure reduction in weaklyproductive public employment, the effects on output decrease with the degree of labor market competition and turn out to be positive under perfect competition.fiscal policy, fiscal consolidation, new-Keynesian DSGE model

Nonlinear photocurrents in two-dimensional systems based on graphene and boron nitride

DC photoelectrical currents can be generated purely as a non-linear effect in uniform media lacking inversion symmetry without the need for a material junction or bias voltages to drive it, in what is termed photogalvanic effect. These currents are strongly dependent on the polarization state of the radiation, as well as on topological properties of the underlying Fermi surface such as its Berry curvature. In order to study the intrinsic photogalvanic response of gapped graphene (GG), biased bilayer graphene (BBG), and hexagonal boron nitride (hBN), we compute the non-linear current using a perturbative expansion of the density matrix. This allows a microscopic description of the quadratic response to an electromagnetic field in these materials, which we analyze as a function of temperature and electron density. We find that the intrinsic response is robust across these systems and allows for currents in the range of pA cm/W to nA cm/W. At the independent-particle level, the response of hBN-based structures is significant only in the ultra-violet due to their sizeable band-gap. However, when Coulomb interactions are accounted for by explicit solution of the Bethe-Salpeter equation, we find that the photoconductivity is strongly modified by transitions involving exciton levels in the gap region, whose spectral weight dominates in the overall frequency range. Biased bilayers and gapped monolayers of graphene have a strong photoconductivity in the visible and infrared window, allowing for photocurrent densities of several nA cm/W. We further show that the richer electronic dispersion of BBG at low energies and the ability to change its band-gap on demand allows a higher tunability of the photocurrent, including not only its magnitude but also, and significantly, its polarity.Comment: Updating with published version and respective references; 14 pages, 11 figure

A Monte Carlo Approach to Measure the Robustness of Boolean Networks

Emergence of robustness in biological networks is a paramount feature of evolving organisms, but a study of this property in vivo, for any level of representation such as Genetic, Metabolic, or Neuronal Networks, is a very hard challenge. In the case of Genetic Networks, mathematical models have been used in this context to provide insights on their robustness, but even in relatively simple formulations, such as Boolean Networks (BN), it might not be feasible to compute some measures for large system sizes. We describe in this work a Monte Carlo approach to calculate the size of the largest basin of attraction of a BN, which is intrinsically associated with its robustness, that can be used regardless the network size. We show the stability of our method through finite-size analysis and validate it with a full search on small networks.Comment: on 1st International Workshop on Robustness and Stability of Biological Systems and Computational Solutions (WRSBS

Enhanced Optical Dichroism of Graphene Nanoribbons

The optical conductivity of graphene nanoribbons is analytical and exactly derived. It is shown that the absence of translation invariance along the transverse direction allows considerable intra-band absorption in a narrow frequency window that varies with the ribbon width, and lies in the THz range domain for ribbons 10-100nm wide. In this spectral region the absorption anisotropy can be as high as two orders of magnitude, which renders the medium strongly dichroic, and allows for a very high degree of polarization (up to ~85) with just a single layer of graphene. The effect is resilient to level broadening of the ribbon spectrum potentially induced by disorder. Using a cavity for impedance enhancement, or a stack of few layer nanoribbons, these values can reach almost 100%. This opens a potential prospect of employing graphene ribbon structures as efficient polarizers in the far IR and THz frequencies.Comment: Revised version. 10 pages, 7 figure

Disorder Induced Localized States in Graphene

We consider the electronic structure near vacancies in the half-filled honeycomb lattice. It is shown that vacancies induce the formation of localized states. When particle-hole symmetry is broken, localized states become resonances close to the Fermi level. We also study the problem of a finite density of vacancies, obtaining the electronic density of states, and discussing the issue of electronic localization in these systems. Our results also have relevance for the problem of disorder in d-wave superconductors.Comment: Replaced with published version. 4 pages, 4 figures. Fig. 1 was revise

Hydrogen generation by borohydrides: critical issues for portable applications

High volumetric and gravimetric efficiency are key to potential hydrogen energy carriers. Sodium borohydride emerges as such potentiality and a storage capacity well within DOE targets for 2015. Limitations exist due to the fact that hydrolysis is restricted by available water and due to the lack of low cost re-usable catalysts. An extensive amount of work has been done in our laboratories on Ni and Ru based catalysts, including synthesis and characterization and solutions have been found for durability, stability and reutilization under operating conditions in small volume batch reactors. Results showed that the Langmuir-Hinshelwood model described fairly well the reaction kinetics for all tested temperatures up to 60ºC and up to reactant exhaustion. In this work, issues such as self-hydrolysis, stability of solutions for storage, water management, some aspects of the catalyzed hydrolysis as well as gas conditioning are studied in order to associate a storage solution with sodium borohydride to a low power air breathing cathode PEM fuel cell

Modelling the SFRC flexural behavior using a stress-strain relationship

No presente trabalho foi desenvolvida uma estratégia numérica de forma a avaliar a possibilidade de ser definida uma lei tensão-extensão capaz de simular o comportamento fendilhado do betão de custo competitivo reforçado com fibras de aço (BRFA) concebido no âmbito de um projecto de investigação. Essa estratégia englobou dois modelos de complexidade distinta. O primeiro, designado por modelo de secção, foi utilizado para, com recurso a análise inversa e utilizando respostas força-flecha registadas em ensaios experimentais, se determinarem os parâmetros que definem uma lei trilinear tensão-extensão capaz de reproduzir as respostas experimentais com o rigor desejado. De seguida, esta lei foi utilizada para definir o modo I de fractura da lei constitutiva da fenda, no âmbito de um código computacional de análise não linear material de estruturas submetidas a estado plano de tensão, suportado no método dos elementos finitos, onde o processo de fendilhação é simulado por um modelo de fendilhação distribuída. Da aplicação dos dois modelos constatou-se que o modelo de secção não permite definir a lei tensão-extensão pós pico (lei de amolecimento) com o rigor necessário. Enquadrado numa análise inversa, o modelo baseado no MEF foi de seguida utilizado para se determinar a lei de amolecimento do BRFA, tendo-se constatado que essa lei não pode ser explicitada unicamente em função do conceito de resistência equivalente, tal como sugere o RILEM TC 162-TDF.To assess the post-cracking stress-strain relationship for a developed cost competitive steel fibre reinforced concrete (SFRC), a numerical strategy involving two numerical models of distinct complexity was carried out. Inserted into an inverse analysis framework, a cross-sectional-layer-model (CSLM) was used to evaluate the post-cracking stress-strain relationship able of reproducing, with the desired accuracy, the force-deflection relationship recorded in three-point bending tests carried out according to the RILEM TC 162-TDF recommendations. This relationship was used as the strain softening law of the fracture mode I crack constitutive law of a smeared crack model based on the strain decomposition concept, under the framework of the finite element method (FEM). Applying this model it was verified that the CSLM couldn't define a strain softening law able of simulating, with the necessary accuracy, the fracture mode I of the SFRC. Using an inverse analysis and the FEM model, the softening law for the SFRC analysed was evaluated. From the values obtained for the definition of this law it was concluded that this law cannot be exclusively dependent on the concept of equivalent flexural tensile strength, as RILEM TC 162-TDF recommends

On-demand hydrogen generation by hydrolysis of sodium borohydride in batch reactors: effect of the buffer pressure

A study was undertaken in order to investigate the potential of hydrogen generation by hydrolysis of sodium borohydride in batch reactors, operating at moderate pressures, in the presence of a reused nickel-ruthenium based catalyst, to feed on-demand a proton exchange membrane fuel cell. The effect of the buffer pressure is explored and hydrogen generation rates are evaluated by changing catalyst amount, operating pressure and successive refuelin