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

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

C58 on Au(111): a scanning tunneling microscopy study

C58 fullerenes were adsorbed onto room temperature Au(111) surface by low-energy (~6 eV) cluster ion beam deposition under ultrahigh vacuum conditions. The topographic and electronic properties of the deposits were monitored by means of scanning tunnelling microscopy (STM at 4.2 K). Topographic images reveal that at low coverages fullerene cages are pinned by point dislocation defects on the herringbone reconstructed gold terraces (as well as by step edges). At intermediate coverages, pinned monomers, act as nucleation centres for the formation of oligomeric C58 chains and 2D islands. At the largest coverages studied, the surface becomes covered by 3D interlinked C58 cages. STM topographic images of pinned single adsorbates are essentially featureless. The corresponding local densities of states are consistent with strong cage-substrate interactions. Topographic images of [C58]n oligomers show a stripe-like intensity pattern oriented perpendicular to the axis connecting the cage centers. This striped pattern becomes even more pronounced in maps of the local density of states. As supported by density functional theory, DFT calculations, and also by analogous STM images previously obtained for C60 polymers (M. Nakaya et al., J. Nanosci. Nanotechnol. 11, 2829 (2011)), we conclude that these striped orbital patterns are a fingerprint of covalent intercage bonds. For thick C58 films we have derived a band gap of 1.2 eV from scanning tunnelling spectroscopy data, STS, confirming that the outermost C58 layer behaves as a wide band semiconductor