Zero-bias molecular electronics: Exchange-correlation corrections to Landauer's formula

Standard first principles calculations of transport through single molecules miss exchange-correlation corrections to the Landauer formula. From Kubo response theory, both the Landauer formula and these corrections in the limit of zero bias are derived and calculations are presented.Comment: 4 pages, 3 figures, final version to appear in Phys. Rev. B, Rapid Communication

Control of beam propagation in optically written waveguides beyond the paraxial approximation

Beam propagation beyond the paraxial approximation is studied in an optically written waveguide structure. The waveguide structure that leads to diffractionless light propagation, is imprinted on a medium consisting of a five-level atomic vapor driven by an incoherent pump and two coherent spatially dependent control and plane-wave fields. We first study propagation in a single optically written waveguide, and find that the paraxial approximation does not provide an accurate description of the probe propagation. We then employ coherent control fields such that two parallel and one tilted Gaussian beams produce a branched waveguide structure. The tilted beam allows selective steering of the probe beam into different branches of the waveguide structure. The transmission of the probe beam for a particular branch can be improved by changing the width of the titled Gaussian control beam as well as the intensity of the spatially dependent incoherent pump field.Comment: 10 pages, 9 figure

Dynamic formation of Rydberg aggregates at off-resonant excitation

The dynamics of a cloud of ultra-cold two-level atoms is studied at off-resonant laser driving to a Rydberg state. We find that resonant excitation channels lead to strongly peaked spatial correlations associated with the buildup of asymmetric excitation structures. These aggregates can extend over the entire ensemble volume, but are in general not localized relative to the system boundaries. The characteristic distances between neighboring excitations depend on the laser detuning and on the interaction potential. These properties lead to characteristic features in the spatial excitation density, the Mandel $Q$ parameter, and the total number of excitations. As an application an implementation of the three-atom CSWAP or Fredkin gate with Rydberg atoms is discussed. The gate not only exploits the Rydberg blockade, but also utilizes the special features of an asymmetric geometric arrangement of the three atoms. We show that continuous-wave off-resonant laser driving is sufficient to create the required spatial arrangement of atoms out of a homogeneous cloud.Comment: 8 pages, 7 figure

Yoctosecond photon pulses from quark-gluon plasmas

Present ultra-fast laser optics is at the frontier between atto- and zeptosecond photon pulses, giving rise to unprecedented applications. We show that high-energetic photon pulses down to the yoctosecond timescale can be produced in heavy ion collisions. We focus on photons produced during the initial phase of the expanding quark-gluon plasma. We study how the time evolution and properties of the plasma may influence the duration and shape of the photon pulse. Prospects for achieving double peak structures suitable for pump-probe experiments at the yoctosecond timescale are discussed.Comment: 4 pages, 2 figures; final version as accepted by PR

Quantum Hall Transition in the Classical Limit

We study the quantum Hall transition using the density-density correlation function. We show that in the limit h->0 the electron density moves along the percolating trajectories, undergoing normal diffusion. The localization exponent coincides with its percolation value \nu=4/3. The framework provides a natural way to study the renormalization group flow from percolation to quantum Hall transition. We also confirm numerically that the critical conductivity of a classical limit of quantum Hall transition is \sigma_{xx} = \sqrt{3}/4.Comment: 8 pages, 4 figures; substantial changes include the critical conductivity calculatio

Ideology and the self-fulfilling prophecy in conservation and social science research

In this essay, I propose an analytical model, ‘zones of intermediality’, designed to research socio-cultural dynamics in foreign large-scale land projects. ‘Zones of intermediality’ refers to the ontological grids of (inter)national-local stakeholder encounters where diverse ideologies, discourses and practices of land use and valuation are mediated. The model was constructed to analyze conceptual similarities and differences between and within stakeholder groups in such land projects. Just as local ‘communities’ are composed of people with varied social realities, economies, political relations, knowledge, views and perceptions, so are other stakeholder groups. Researchers are not immune to such realities. The subjectivity and epistemologycal rooting of the researcher impact on what he or she sees in the field and what is eventually reported in research publications. Thus, the essay argues for a reflection on these processes in view of the fact that we ourselves mediate representations of ‘local’ people to academic and non-academic audiences. I hope that the ‘zones of intermediality’ model will be useful in facilitating such reflections. RÉSUMÉ Dans cet article, je propose de considérer un modèle analytique dénommé ‘zones d’intermédialité’ conçu pour faire progresser les outils de recherche des dynamiques socioculturelles associées avec des projets d’acquisition foncière de grande envergure en Afrique continentale et à Madagascar. Le modèle ‘zones d’intermédialité’ s’inscrit dans des grilles ontologiques de rencontres d’intervenants (inter)nationaux à locaux dans lesquelles divers idéologies, discours et pratiques ont une influence sur l’utilisation des terres et sur l’évaluation foncière. Le modèle a été conçu pour procéder à une analyse détaillée des différences et des similarités entre et au sein de tels projets d’acquisition foncière. Au même titre que les ‘communautés’ locales sont constituées de personnes avec des réalités sociales, économiques et politiques différentes, et que cette diversité a un effet sur leur opinion et leurs perceptions, convient-il de préciser que ces diverses réalités s’imposent également aux autres groupes d’intervenants et même aux chercheurs qui ne sont pas indifférents à de telles réalités. Les racines subjectives et épistémologiques du chercheur influencent ce qu’il observe sur le terrain et ce qu’il rapporte ultérieurement dans ses publications. C’est pour toutes ces raisons que j'invite à une réflexion sur ces procédés dans la mesure où nous sommes nous-mêmes amenés à influencer les représentations des gens locaux destinés à un public universitaire ou non. J’espère que le modèle ‘zones d’intermédialité’ facilitera de telles réflexions.

Density of states in graphene with vacancies: midgap power law and frozen multifractality

The density of states (DoS), $\varrho(E)$, of graphene is investigated numerically and within the self-consistent T-matrix approximation (SCTMA) in the presence of vacancies within the tight binding model. The focus is on compensated disorder, where the concentration of vacancies, $n_\text{A}$ and $n_\text{B}$, in both sub-lattices is the same. Formally, this model belongs to the chiral symmetry class BDI. The prediction of the non-linear sigma-model for this class is a Gade-type singularity $\varrho(E) \sim |E|^{-1}\exp(-|\log(E)|^{-1/x})$. Our numerical data is compatible with this result in a preasymptotic regime that gives way, however, at even lower energies to $\varrho(E)\sim E^{-1}|\log(E)|^{-\mathfrak{x}}$, $1\leq \mathfrak{x} < 2$. We take this finding as an evidence that similar to the case of dirty d-wave superconductors, also generic bipartite random hopping models may exhibit unconventional (strong-coupling) fixed points for certain kinds of randomly placed scatterers if these are strong enough. Our research suggests that graphene with (effective) vacancy disorder is a physical representative of such systems.Comment: References updated onl

An exploratory analysis of factors driving intermodal transportation usage

The purpose of this study is to investigate certain aspects of a transportation choice model proposed by Krapfel and Mentzer (1982) pertaining to the influence of shipper perceptions on the selection of a mode. Specifically, this study attempts to identify the impact that shipper perceptions of intermodal and over-the-road truck service, as well as other characteristics of the shipper, have on intermodal usage. The research findings support the notion that shipper perceptions affect modal usage and indicate areas in which intermodal providers should focus their attention to improve intermodal usage

Influence of Planting Date on the Growing Season of Three Crimson Clover Varieties

Last updated: 6/12/200