4,175 research outputs found
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
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, âzones of intermedialityâ, designed to research socio-cultural dynamics in foreign large-scale land projects. âZones of intermedialityâ 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 âcommunitiesâ 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 âlocalâ people to academic and non-academic audiences. I hope that the âzones of intermedialityâ model will be useful in facilitating such reflections. RĂSUMĂ Dans cet article, je propose de considĂ©rer un modĂšle analytique dĂ©nommĂ© âzones dâintermĂ©dialitĂ©â conçu pour faire progresser les outils de recherche des dynamiques socioculturelles associĂ©es avec des projets dâacquisition fonciĂšre de grande envergure en Afrique continentale et Ă Madagascar. Le modĂšle âzones dâintermĂ©dialitĂ©â sâinscrit dans des grilles ontologiques de rencontres dâintervenants (inter)nationaux Ă locaux dans lesquelles divers idĂ©ologies, discours et pratiques ont une influence sur lâutilisation des terres et sur lâĂ©valuation fonciĂšre. Le modĂšle a Ă©tĂ© conçu pour procĂ©der Ă une analyse dĂ©taillĂ©e des diffĂ©rences et des similaritĂ©s entre et au sein de tels projets dâacquisition fonciĂšre. Au mĂȘme titre que les âcommunautĂ©sâ locales sont constituĂ©es de personnes avec des rĂ©alitĂ©s sociales, Ă©conomiques et politiques diffĂ©rentes, et que cette diversitĂ© a un effet sur leur opinion et leurs perceptions, convient-il de prĂ©ciser que ces diverses rĂ©alitĂ©s sâimposent Ă©galement aux autres groupes dâintervenants et mĂȘme aux chercheurs qui ne sont pas indiffĂ©rents Ă de telles rĂ©alitĂ©s. Les racines subjectives et Ă©pistĂ©mologiques du chercheur influencent ce quâil observe sur le terrain et ce quâil rapporte ultĂ©rieurement dans ses publications. Câest pour toutes ces raisons que j'invite Ă une rĂ©flexion sur ces procĂ©dĂ©s dans la mesure oĂč nous sommes nous-mĂȘmes amenĂ©s Ă influencer les reprĂ©sentations des gens locaux destinĂ©s Ă un public universitaire ou non. JâespĂšre que le modĂšle âzones dâintermĂ©dialitĂ©â facilitera de telles rĂ©flexions.
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
Density of states in graphene with vacancies: midgap power law and frozen multifractality
The density of states (DoS), , 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, and
, 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 . Our numerical data is compatible with this
result in a preasymptotic regime that gives way, however, at even lower
energies to , . 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
Influence of Planting Date on the Growing Season of Three Crimson Clover Varieties
Last updated: 6/12/200
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