293 research outputs found
Possible symmetries of the superconducting order parameter in a hexagonal ferromagnet
We study the order parameter symmetry in a hexagonal crystal with co-existing
superconductivity and ferromagnetism. An experimental example is provided by
carbon-based materials, such as graphite-sulfur composites, in which an
evidence of such co-existence has been recently discovered. The presence of a
non-zero magnetization in the normal phase brings about considerable changes in
the symmetry classification of superconducting states, compared to the
non-magnetic case.Comment: 4 pages, REVTe
Analysis of human performance as a measure of mental fatigue
In our day to day, we often experience a sense of being tired
due to mental or physical workload. Along with that, there is also a
feeling of degrading performance, even after the completion of simple
tasks. These mental states however, are often not felt consciously or are
ignored. This is an attitude that may result in human error, failure,
and may lead to potential health problems together with a decrease in
quality of life. States of acute mental fatigue may be detected with the
close monitoring of certain indicators, such as productivity, performance
and health indicators. In this paper, a model and prototype are proposed
to detect and monitor acute acute fatigue, based on non-invasive Humancomputer
Interaction (HCI). This approach will enable the development
of better working environments, with an impact on the quality of life and
the work produced.This work was developed in the context of the project CAMCoF - Contextaware
Multimodal Communication Framework funded by ERDF - European
Regional Development Fund through the COMPETE Programme (operational
programme for competitiveness) and by National Funds through the FCT - Funda
ção para a Ciência e a Tecnologia (Portuguese Foundation for Science and
Technology) within project FCOMP-01-0124-FEDER-028980
Self-assembly mechanism in colloids: perspectives from Statistical Physics
Motivated by recent experimental findings in chemical synthesis of colloidal
particles, we draw an analogy between self-assembly processes occurring in
biological systems (e.g. protein folding) and a new exciting possibility in the
field of material science. We consider a self-assembly process whose elementary
building blocks are decorated patchy colloids of various types, that
spontaneously drive the system toward a unique and predetermined targeted
macroscopic structure.
To this aim, we discuss a simple theoretical model -- the Kern-Frenkel model
-- describing a fluid of colloidal spherical particles with a pre-defined
number and distribution of solvophobic and solvophilic regions on their
surface. The solvophobic and solvophilic regions are described via a
short-range square-well and a hard-sphere potentials, respectively.
Integral equation and perturbation theories are presented to discuss
structural and thermodynamical properties, with particular emphasis on the
computation of the fluid-fluid (or gas-liquid) transition in the
temperature-density plane.
The model allows the description of both one and two attractive caps, as a
function of the fraction of covered attractive surface, thus interpolating
between a square-well and a hard-sphere fluid, upon changing the coverage.
By comparison with Monte Carlo simulations, we assess the pros and the cons
of both integral equation and perturbation theories in the present context of
patchy colloids, where the computational effort for numerical simulations is
rather demanding.Comment: 14 pages, 7 figures, Special issue for the SigmaPhi2011 conferenc
A tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr(0.52)Ti(0.48)O3
The perovskite-like ferroelectric system PbZr(1-x)Ti(x)O3 (PZT) has a nearly
vertical morphotropic phase boundary (MPB) around x=0.45-0.50. Recent
synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys.
Lett. 74, 2059 (1999)] have revealed a new monoclinic phase between the
previously-established tetragonal and rhombohedral regions. In the present work
we describe a Rietveld analysis of the detailed structure of the tetragonal and
monoclinic PZT phases on a sample with x= 0.48 for which the lattice parameters
are respectively: at= 4.044 A, ct= 4.138 A, at 325 K, and am= 5.721 A, bm=
5.708 A, cm= 4.138 A, beta= 90.496 deg., at 20K. In the tetragonal phase the
shifts of the atoms along the polar [001] direction are similar to those in
PbTiO3 but the refinement indicates that there are, in addition, local
disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis..
The monoclinic structure can be viewed as a condensation along one of the
directions of the local displacements present in the tetragonal phase. It
equally well corresponds to a freezing-out of the local displacements along one
of the directions recently reported by Corker et al.[J. Phys. Condens.
Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure
therefore provides a microscopic picture of the MPB region in which one of the
"locally" monoclinic phases in the "average" rhombohedral or tetragonal
structures freezes out, and thus represents a bridge between these two phases.Comment: REVTeX, 7 figures. Modifications after referee's suggestion: new
figure (figure 5), comments in 2nd para. (Sect.III) and in 2nd & 3rd para.
(Sect. IV-a), in the abstract: "...of ~0.2 A perpendicular to the polar
axis.
HST hot-Jupiter transmission spectral survey: Evidence for aerosols and lack of TiO in the atmosphere of WASP-12b
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present Hubble Space Telescope (HST) optical transmission spectra of the transiting hot-Jupiter WASP-12b, taken with the Space Telescope Imaging Spectrograph instrument. The resulting spectra cover the range 2900–10 300 Å which we combined with archival Wide Field Camera 3 spectra and Spitzer photometry to cover the full optical to infrared wavelength regions. With high spatial resolution, we are able to resolve WASP-12A's stellar companion in both our images and spectra, revealing that the companion is in fact a close binary M0V pair, with the three stars forming a triple-star configuration. We derive refined physical parameters of the WASP-12 system, including the orbital ephemeris, finding the exoplanet's density is ∼20 per cent lower than previously estimated. From the transmission spectra, we are able to decisively rule out prominent absorption by TiO in the exoplanet's atmosphere, as there are no signs of the molecule's characteristic broad features nor individual bandheads. Strong pressure-broadened Na and K absorption signatures are also excluded, as are significant metal-hydride features. We compare our combined broad-band spectrum to a wide variety of existing aerosol-free atmospheric models, though none are satisfactory fits. However, we do find that the full transmission spectrum can be described by models which include significant opacity from aerosols: including Rayleigh scattering, Mie scattering, tholin haze and settling dust profiles. The transmission spectrum follows an effective extinction cross-section with a power law of index α, with the slope of the transmission spectrum constraining the quantity αT = −3528 ± 660 K, where T is the atmospheric temperature. Rayleigh scattering (α = −4) is among the best-fitting models, though requires low terminator temperatures near 900 K. Sub-micron size aerosol particles can provide equally good fits to the entire transmission spectrum for a wide range of temperatures, and we explore corundum as a plausible dust aerosol. The presence of atmospheric aerosols also helps to explain the modestly bright albedo implied by Spitzer observations, as well as the near blackbody nature of the emission spectrum. Ti-bearing condensates on the cooler night-side is the most natural explanation for the overall lack of TiO signatures in WASP-12b, indicating the day/night cold trap is an important effect for very hot Jupiters. These findings indicate that aerosols can play a significant atmospheric role for the entire wide range of hot-Jupiter atmospheres, potentially affecting their overall spectrum and energy balance.NASA, through grants under the HST-GO-12473 programme from the STScISTFC (Science & Technology Facilities Council)French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 ‘Exo-Atmos
Multiband tight-binding theory of disordered ABC semiconductor quantum dots: Application to the optical properties of alloyed CdZnSe nanocrystals
Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a
broad range of applications, as their spectrum and thus their excitation gap
can be tailored by variation of their size. Additionally, nanocrystals of the
type ABC can be realized by alloying of two pure compound semiconductor
materials AC and BC, which allows for a continuous tuning of their absorption
and emission spectrum with the concentration x. We use the single-particle
energies and wave functions calculated from a multiband sp^3 empirical
tight-binding model in combination with the configuration interaction scheme to
calculate the optical properties of CdZnSe nanocrystals with a spherical shape.
In contrast to common mean-field approaches like the virtual crystal
approximation (VCA), we treat the disorder on a microscopic level by taking
into account a finite number of realizations for each size and concentration.
We then compare the results for the optical properties with recent experimental
data and calculate the optical bowing coefficient for further sizes
Turnover of passerine birds on islands in the Aegean Sea (Greece)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73442/1/j.1365-2699.2007.01695.x.pd
Biogenic nano-magnetite and nano-zero valent iron treatment of alkaline Cr(VI) leachate and chromite ore processing residue
Highly reactive nano-scale biogenic magnetite (BnM), synthesized by the Fe(III)-reducing bacterium Geobacter sulfurreducens, was tested for the potential to remediate alkaline Cr(VI) contaminated waters associated with chromite ore processing residue (COPR). The performance of this biomaterial, targeting aqueous Cr(VI) removal, was compared to a synthetic alternative, nano-scale zero valent iron (nZVI). Samples of highly contaminated alkaline groundwater and COPR solid waste were obtained from a contaminated site in Glasgow, UK. During batch reactivity tests, Cr(VI) removal from groundwater was inhibited by ~25% (BnM) and ~50% (nZVI) when compared to the treatment of less chemically complex model pH 12 Cr(VI) solutions. In both the model Cr(VI) solutions and contaminated groundwater experiments the surface of the nanoparticles became passivated, preventing complete coupling of their available electrons to Cr(VI) reduction. To investigate this process, the surfaces of the reacted samples were analyzed by TEM-EDX, XAS and XPS, confirming Cr(VI) reduction to the less soluble Cr(III) on the nanoparticle surface. In groundwater reacted samples the presence of Ca, Si and S was also noted on the surface of the nanoparticles, and is likely responsible for earlier onset of passivation. Treatment of the solid COPR material in contact with water, by addition of increasing weight % of the nanoparticles, resulted in a decrease in aqueous Cr(VI) concentrations to below detection limits, via the addition of ≥5% w/w BnM or ≥1% w/w nZVI. XANES analysis of the Cr K edge, showed that the % Cr(VI) in the COPR dropped from 26% to a minimum of 4-7% by the addition of 5% w/w BnM or 2% w/w nZVI, with higher additions unable to reduce the remaining Cr(VI). The treated materials exhibited minimal re-mobilization of soluble Cr(VI) by re-equilibration with atmospheric oxygen, with the bulk of the Cr remaining in the solid fraction. Both nanoparticles exhibited a considerable capacity for the remediation of COPR related Cr(VI) contamination, with the synthetic nZVI demonstrating greater reactivity than the BnM. However, the biosynthesized BnM was also capable of significant Cr(VI) reduction and demonstrated a greater efficiency for the coupling of its electrons towards Cr(VI) reduction than the nZVI
Why ‘Good Governance’ Fails: Lessons from Regional Economic Development in Colombia
By critically reviewing different strands of literature on institutional change and development, this essay argues that, in order to fully understand subnational economic development, we need to move away from ‘good governance' explanations in which geography‐specific analyses of power structures and elite interests are largely absent. Using findings for Colombia and insights from economic geography and heterodox political economy theories, this essay gives theoretical and conceptual guidelines and approximations for future studies on regional economic development. The contribution provides a place‐based discussion of how the historically evolved distribution of power balances, context‐specific elite interests, and the interaction between place‐bound actors and place‐less dynamics affect subnational institutional arrangements shaping policies and development outcomes. The conclusions drawn are not limited to Colombia and will prove beneficial to researchers studying regional economic development in subnational contexts elsewhere in the world
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