Disordered two-dimensional superconductors: roles of temperature and interaction strength

We have considered the half-filled disordered attractive Hubbard model on a square lattice, in which the on-site attraction is switched off on a fraction $f$ of sites, while keeping a finite $U$ on the remaining ones. Through Quantum Monte Carlo (QMC) simulations for several values of $f$ and $U$, and for system sizes ranging from $8\times 8$ to $16\times 16$, we have calculated the configurational averages of the equal-time pair structure factor $P_s$, and, for a more restricted set of variables, the helicity modulus, $\rho_s$, as functions of temperature. Two finite-size scaling {\it ansatze} for $P_s$ have been used, one for zero-temperature and the other for finite temperatures. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, $f_c$, which increases with $U$, at least up to U=4 (in units of the hopping energy). Also, the normalized zero-temperature gap as a function of $f$ shows a maximum near $f\sim 0.07$, for $2\lesssim U\lesssim 6$. Analyses of the helicity modulus and of the pair structure factor led to the determination of the critical temperature as a function of $f$, for $U=3,$ 4 and 6: they also show maxima near $f\sim 0.07$, with the highest $T_c$ increasing with $U$ in this range. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to "push" electrons towards attractive sites, the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe

Changes in postural sway behavior across the life span

The present study aimed to investigate human balance control by assessing postural sway on three groups representing three stages of life (6-12, 19-40 and 65-74 years old). There were 14 individuals in each group and they were tested during upright bipedal stance with either eyes open or closed. Focus was given to multiple sway indices representing multi-dimensional features of postural sway in quiet stance and included: the center of pressure area, amplitude, root mean square (RMS), velocity, jerkiness, and sample entropy. Results confirmed that children and seniors swayed more (p\u3c.004), faster (p\u3c.001) and their body sway was shakier (p\u3c.001) than young adults. Seniors also presented faster (p\u3c.006) and shakier (p\u3c.001) sway than children and a more unpredictable pattern of body sway in time (p\u3c.002) than children and young adults. In addition, children presented a more random anterior-posterior sway (p\u3c.034) and a more regular medio-lateral sway (p\u3c.043) than young adults, and a higher synchronization between anterior-posterior and medio-lateral body sway (p\u3c.012) than young adults and seniors. We also observed that postural control of children and young adults becomes relatively more challenged in experimental situations when eyes were closed for most postural indices. In conclusion, this study suggests that multi-dimension posturography is sensitive to detect subtle age-related changes in the postural behavior and each stage of life may have their own signature patterns of postural behavior. Therefore, we expect that quantifications of this nature may be used to assess not only postural instability and fall risk but also to aid the testing of the efficacy of balance interventional protocols

Densidade de estocagem e manejo alimentar na criação de Acarás-bandeira.

O comércio de peixes ornamentais é considerado um dos setores mais lucrativos da piscicultura brasileira e vem se expandindo rapidamente com o aumento na demanda mundial, sendo o acará-bandeira (Pterophyllum scalare) uma das espécies nativas de água doce de maior procura no mercado nacional.bitstream/item/122979/1/Densidade-de-estocagem-COT145.pd

Scattering by linear defects in graphene: a tight-binding approach

We develop an analytical scattering formalism for computing the transmittance through periodic defect lines within the tight-binding model of graphene. We first illustrate the method with a relatively simple case, the pentagon-only defect line. Afterwards, more complex defect lines are treated, namely the zz(558) and the zz(5757) ones. The formalism developed, only uses simple tight-binding concepts, reducing the problem to matrix manipulations which can be easily worked out by any computational algebraic calculator.JNBR was supported by Fundacao para a Ciencia e a Tecnologia (FCT) through Grant No. SFRH/BD/44456/2008. NMRP was supported by Fundos FEDER through the Programa Operacional Factores de Competitividade-COMPETE and by FCT under project no. PEst-C/FIS/UI0607/2011. NMRP acknowledges both the hospitality and the funding from the Graphene Research Centre at the National University of Singapore, where this work was completed

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

Phenomenological study of the electronic transport coefficients of graphene

Using a semi-classical approach and input from experiments on the conductivity of graphene, we determine the electronic density dependence of the electronic transport coefficients -- conductivity, thermal conductivity and thermopower -- of doped graphene. Also the electronic density dependence of the optical conductivity is obtained. Finally we show that the classical Hall effect (low field) in graphene has the same form as for the independent electron case, characterized by a parabolic dispersion, as long as the relaxation time is proportional to the momentum.Comment: 4 pages, 1 figur