Excitonic effects in the optical conductivity of gated graphene

We study the effect of electron-electron interactions in the optical conductivity of graphene under applied bias and derive a generalization of Elliot's formula, commonly used for semiconductors, for the optical intensity. We show that {\it excitonic resonances} are responsible for several features of the experimentally measured mid-infrared response of graphene such as the increase of the conductivity beyond the "universal" value above the Fermi blocked regime, the broadening of the absorption at the threshold, and the decrease of the optical conductivity at higher frequencies. Our results are also in agreement with {\it ab initio} calculations in the neutral regime.Comment: New version with discussion improve

Optical Properties of Strained Graphene

The optical conductivity of graphene strained uniaxially is studied within the Kubo-Greenwood formalism. Focusing on inter-band absorption, we analyze and quantify the breakdown of universal transparency in the visible region of the spectrum, and analytically characterize the transparency as a function of strain and polarization. Measuring transmittance as a function of incident polarization directly reflects the magnitude and direction of strain. Moreover, direction-dependent selection rules permit identification of the lattice orientation by monitoring the van-Hove transitions. These photoelastic effects in graphene can be explored towards atomically thin, broadband optical elements

Distortion of the perfect lattice structure in bilayer graphene

We consider the instability of bilayer graphene with respect to a distorted configuration in the same spirit as the model introduced by Su, Schrieffer and Heeger. By computing the total energy of a distorted bilayer, we conclude that the ground state of the system favors a finite distortion. We explore how the equilibrium configuration changes with carrier density and an applied potential difference between the two layers

On the relation between Transversal and Longitudinal Scaling in Cities

Given that a group of cities follows a scaling law connecting urban population with socio-economic or infrastructural metrics (transversal scaling), should we expect that each city would follow the same behavior over time (longitudinal scaling)? This assumption has important policy implications, although rigorous empirical tests have been so far hindered by the lack of suitable data. Here, we advance the debate by looking into the temporal evolution of the scaling laws for 5507 municipalities in Brazil. We focus on the relationship between population size and two urban variables, GDP and water network length, analyzing the time evolution of the system of cities as well as their individual trajectory. We find that longitudinal (individual) scaling exponents are city-specific, but they are distributed around an average value that approaches to the transversal scaling exponent when the data are decomposed to eliminate external factors, and when we only consider cities with a sufficiently large growth rate. Such results give support to the idea that the longitudinal dynamics is a micro-scaling version of the transversal dynamics of the entire urban system. Finally, we propose a mathematical framework that connects the microscopic level to global behavior, and, in all analyzed cases, we find good agreement between theoretical prediction and empirical evidence

Rejuvenescimento do cajueiro por meio de podas e substituição de copas.

Este trabalho teve como objetivo informar aos produtores de caju sobre as tecnologias para melhoria da cajucultura, tais como, rejuvenescimento de cajueiros antigos, substituição de copas por meio da eliminação seletiva de plantas...bitstream/item/80682/1/circular-44.PD

Síntese de nanopartículas de prata para aplicação na sanitização de embalagens.

bitstream/CNPDIA-2009-09/11896/1/CT99_2008.pd

Edge phonons in black phosphorus

Exfoliated black phosphorus has recently emerged as a new two-dimensional crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have potentially important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and the behavior of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.Comment: 15 pages, 4 figure

Induction of IL-17A Precedes Development of Airway Hyperresponsiveness during Diet-Induced Obesity and Correlates with Complement Factor D

Obesity is a risk factor for the development of asthma. Obese mice exhibit innate airway hyperresponsiveness (AHR), a characteristic feature of asthma, and IL-17A is required for development of AHR in obese mice. The purpose of this study was to examine the temporal association between the onset of AHR and changes in IL-17A during the development of obesity by high-fat feeding in mice. At weaning, C57BL/6J mice were placed either on mouse chow or on a high-fat diet (HFD) and examined 9, 12, 15, 18, or 24 weeks later. Airway responsiveness to aerosolized methacholine (assessed via the forced oscillation technique) was greater in mice fed HFD versus chow for 24 weeks but not at earlier time points. Bronchoalveolar lavage and serum IL-17A were not affected by either the type or duration of diet, but increased pulmonary IL17a mRNA abundance was observed in HFD versus chow fed mice after both 18 and 24 weeks. Flow cytometry also confirmed an increase in IL-17A+ γδ T cells and IL-17A+ CD4+ T (Th17) cells in lungs of HFD versus chow fed mice. Pulmonary expression of Cfd (complement factor D, adipsin), a gene whose expression can be reduced by IL-17A, decreased after both 18 and 24 weeks in HFD versus chow fed mice. Furthermore, pulmonary Cfd mRNA abundance correlated with elevations in pulmonary Il17a mRNA expression and with AHR. Serum levels of TNFα, MIP-1α, and MIP-1β, and classical markers of systemic inflammation of obesity were significantly greater in HFD than chow fed mice after 24 weeks, but not earlier. In conclusion, our data indicate that pulmonary rather than systemic IL-17A is important for obesity-related AHR and suggest that changes in pulmonary Cfd expression contribute to these effects of IL-17A. Further, the observation that increases in Il17a preceded the development of AHR by several weeks suggests that IL-17A interacts with other factors to promote AHR. The observation that the onset of the systemic inflammation of obesity coincided temporally with the development of AHR suggest that systemic inflammation may be one of these factors