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 “Facing the future: time for the EU to meet global challenges” 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 experts’ and policy makers’ 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 LiPuma’s (2002) notion of ‘cultures of circulation’, 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 ‘built into’ 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 η\eta Boo

Direct angular size measurements of the G0IV subgiant $\eta$ Boo from the Palomar Testbed Interferometer are presented, with limb-darkened angular size of $\theta_{LD}= 2.1894^{+0.0055}_{-0.0140}$ mas, which indicate a linear radius of $R=2.672 \pm 0.028 R_\odot$. A bolometric flux estimate of $F_{BOL} = 22.1 \pm 0.28\times 10^{-7}$ erg cm$^{-2}$s$^{-1}$ is computed, which indicates an effective temperature of $T_{EFF}=6100 \pm 28$ K and luminosity of $L = 8.89 \pm 0.16 L_\odot$ for this object. Similar data are established for a check star, HD 121860. The $\eta$ 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 $\log g=3.817 \pm 0.016$ [cm s$^{-2}$] is established for $\eta$ 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