2,341 research outputs found
Temperature dependence of the charge carrier mobility in gated quasi-one-dimensional systems
The many-body Monte Carlo method is used to evaluate the frequency dependent
conductivity and the average mobility of a system of hopping charges,
electronic or ionic on a one-dimensional chain or channel of finite length. Two
cases are considered: the chain is connected to electrodes and in the other
case the chain is confined giving zero dc conduction. The concentration of
charge is varied using a gate electrode. At low temperatures and with the
presence of an injection barrier, the mobility is an oscillatory function of
density. This is due to the phenomenon of charge density pinning. Mobility
changes occur due to the co-operative pinning and unpinning of the
distribution. At high temperatures, we find that the electron-electron
interaction reduces the mobility monotonically with density, but perhaps not as
much as one might intuitively expect because the path summation favour the
in-phase contributions to the mobility, i.e. the sequential paths in which the
carriers have to wait for the one in front to exit and so on. The carrier
interactions produce a frequency dependent mobility which is of the same order
as the change in the dc mobility with density, i.e. it is a comparably weak
effect. However, when combined with an injection barrier or intrinsic disorder,
the interactions reduce the free volume and amplify disorder by making it
non-local and this can explain the too early onset of frequency dependence in
the conductivity of some high mobility quasi-one-dimensional organic materials.Comment: 9 pages, 8 figures, to be published in Physical Review
Facing the future : time for the EU to meet global challenges
There is a clear and growing need for the capacity to anticipate change to be embedded in policy. This is critical not only to be able to respond and adapt to new situations before they occur, but also to shape the future, building upon mutual understanding and common visions to be jointly pursued.
For policy responses to address all the pressing current global challenges, especially when these are seen separately from one another, is clearly a demanding task. Institutions face greater complexity and difficulty in providing solutions in due time. In particular, this is true when the policy focus extends beyond the challenges that societies face today, seeking to anticipate future challenges and transform them into opportunities.
This is the rationale for the report āFacing the future: time for the EU to meet global challengesā based on a study carried out in the course of 2009 by the Joint Research Centre, Institute for Prospective Technological Studies (JRC-IPTS) for the Bureau of European Policy Advisors (BEPA) of the European Commission.
The aim is to provide a comprehensive picture of the main trends ahead and possible future disruptive global challenges, and to examine how the EU could position itself to take an active role in shaping a response to them. The work described in this report brings a fresh perspective, by linking widely accepted quantified trends towards 2025 and beyond with expertsā and policy makersā opinions on the likely consequences of these trends and wild cards
Creativity and Autonomy in Swarm Intelligence Systems
This work introduces two swarm intelligence algorithms -- one mimicking the behaviour of one species of ants (\emph{Leptothorax acervorum}) foraging (a `Stochastic Diffusion Search', SDS) and the other algorithm mimicking the behaviour of birds flocking (a `Particle Swarm Optimiser', PSO) -- and outlines a novel integration strategy exploiting the local search properties of the PSO with global SDS behaviour. The resulting hybrid algorithm is used to sketch novel drawings of an input image, exploliting an artistic tension between the local behaviour of the `birds flocking' - as they seek to follow the input sketch - and the global behaviour of the `ants foraging' - as they seek to encourage the flock to explore novel regions of the canvas. The paper concludes by exploring the putative `creativity' of this hybrid swarm system in the philosophical light of the `rhizome' and Deleuze's well known `Orchid and Wasp' metaphor
The architectures of media power: editing, the newsroom, and urban public space
This paper considers the relation of the newsroom and the city as a lens into the more general relation of production spaces and mediated publics. Leading theoretically from Lee and LiPumaās (2002) notion of ācultures of circulationā, and drawing on an ethnography of the Toronto Star, the paper focuses on how media forms circulate and are enacted through particular practices and material settings. With its attention to the urban milieus and orientations of media organizations, this paper exhibits both affinities with but also differences to current interests in the urban architectures of media, which describe and theorize how media get ābuilt intoā the urban experience more generally. In looking at editing practices situated in the newsroom, an emphasis is placed on the phenomenological appearance of media forms both as objects for material assembly as well as more abstracted subjects of reflexivity, anticipation and purposiveness. Although this is explored with detailed attention to the settings of the newsroom and the city, the paper seeks to also provide insight into the more general question of how publicness is material shaped and sited
Measurement of the Surface Gravity of Boo
Direct angular size measurements of the G0IV subgiant Boo from the
Palomar Testbed Interferometer are presented, with limb-darkened angular size
of mas, which indicate a linear
radius of . A bolometric flux estimate of erg cms is computed, which indicates
an effective temperature of K and luminosity of for this object. Similar data are established for a check
star, HD 121860. The Boo results are compared to, and confirm, similar
parameters established by the {\it MOST} asteroseismology satellite. In
conjunction with the mass estimate from the {\it MOST} investigation, a surface
gravity of [cm s] is established for
Boo.Comment: To appear in March 1, 2007 ApJ v657 n
First Space-Based Microlens Parallax Measurement: Spitzer Observations of OGLE-2005-SMC-001
We combine Spitzer and ground-based observations to measure the microlens
parallax of OGLE-2005-SMC-001, the first such space-based determination since
S. Refsdal proposed the idea in 1966. The parallax measurement yields a
projected velocity \tilde v ~ 230 km/s, the typical value expected for halo
lenses, but an order of magnitude smaller than would be expected for lenses
lying in the Small Magellanic Cloud (SMC) itself. The lens is a weak (i.e.,
non-caustic-crossing) binary, which complicates the analysis considerably but
ultimately contributes additional constraints. Using a test proposed by Assef
et al. (2006), which makes use only of kinematic information about different
populations but does not make any assumptions about their respective mass
functions, we find that the likelihood ratio is L_halo/L_SMC = 20. Hence, halo
lenses are strongly favored but SMC lenses are not definitively ruled out.
Similar Spitzer observations of additional lenses toward the Magellanic Clouds
would clarify the nature of the lens population. The Space Interferometry
Mission could make even more constraining measurements.Comment: ApJ, in press. Text and figures are updated to match the journal
versio
Debris disks in main sequence binary systems
We observed 69 A3-F8 main sequence binary star systems using the Multiband
Imaging Photometer for Spitzer onboard the Spitzer Space Telescope. We find
emission significantly in excess of predicted photospheric flux levels for
9(+4/-3)% and 40(+7/-6)% of these systems at 24 and 70 microns, respectively.
Twenty two systems total have excess emission, including four systems that show
excess emission at both wavelengths. A very large fraction (nearly 60%) of
observed binary systems with small (<3 AU) separations have excess thermal
mission. We interpret the observed infrared excesses as thermal emission from
dust produced by collisions in planetesimal belts. The incidence of debris
disks around main sequence A3-F8 binaries is marginally higher than that for
single old AFGK stars. Whatever combination of nature (birth conditions of
binary systems) and nurture (interactions between the two stars) drives the
evolution of debris disks in binary systems, it is clear that planetesimal
formation is not inhibited to any great degree. We model these dust disks
through fitting the spectral energy distributions and derive typical dust
temperatures in the range 100--200 K and typical fractional luminosities around
10^-5, with both parameters similar to other Spitzer-discovered debris disks.
Our calculated dust temperatures suggest that about half the excesses we
observe are derived from circumbinary planetesimal belts and around one third
of the excesses clearly suggest circumstellar material. Three systems with
excesses have dust in dynamically unstable regions, and we discuss possible
scenarios for the origin of this short-lived dust.Comment: ApJ, in press. 57 pages, including 7 figures (one of which is in
color
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