Optical Probe of Quantum Shot Noise Reduction at a Single-Atom Contact

Visible and infra-red light emitted at a Ag-Ag(111) junction has been investigated from tunneling to single atom contact conditions with a scanning tunneling microscope. The light intensity varies in a highly nonlinear fashion with the conductance of the junction and exhibits a minimum at conductances close to the conductance quantum. The data are interpreted in terms of current noise at optical frequencies, which is characteristic of partially open transport channels

Demokratie auf dem Rückzug? Zur Konfrontation zwischen amerikanischem Präsidenten und Kongress über einen Truppenabzug aus dem Irak

Im US-Präsidentschaftswahlkampf 2008 ist der Irakkrieg eines der wichtigsten Themen. Die Amerikaner sind allmählich kriegsmüde und die Mehrheit befürwortet einen Truppenrückzug aus dem Irak. Das brachten bereits der Ausgang der Kongresswahlen im Jahr 2006 sowie regelmäßige Meinungsumfragen zum Ausdruck. Es ist nun anzunehmen, dass in der ältesten Demokratie der Welt Volkes Wille direkten Einfluss auf die Politik haben müsste. Doch dies ist nur bedingt der Fall. Besonders Präsident George W. Bush hat wiederholt klargestellt, dass die Stimmung in der Bevölkerung für ihn in der Irakfrage nicht maßgebend sei. Für ihn zähle vielmehr, wie die Generäle vor Ort die aktuelle Sicherheitslage und die Fortschritte einschätzen. Dagegen versucht der seit Ende 2006 von Demokraten dominierte Kongress immer wieder, Rückzugspläne mit festen Fristen zu etablieren. Bislang ist sein Erfolg jedoch bescheiden. Vor allem die fehlende Unterstützung der Republikaner im Kongress und die Blockadepolitik des Präsidenten waren dafür verantwortlich. Hinter der Konfrontation steht eine tiefergehende Auseinandersetzung über die Kompetenzverteilung zwischen Präsident und Kongress bei Kriegsentscheidungen, die in der US-Verfassung in einer Grauzone bleibt. Stephanie Sohnius verknüpft vor diesem verfassungsrechtlichen Hintergrund die Aufarbeitung der aktuell wichtigsten USaußenpolitischen Debatte mit der Frage, ob sich die Demokratie in den USA in der Ära des „Krieges gegen den Terrorismus“ (weiterhin) auf dem Rückzug befindet

Fluorescent single-molecule STM probe

The plasmonic tip of a scanning tunnelling microscope (STM) is functionalized with a single fluorescent molecule and is scanned on a plasmonic substrate. The tunneling current flowing through the tip-molecule-substrate junction generates a narrow-line emission of light corresponding to the fluorescence of the negatively charged molecule suspended at the apex of the tip, i.e., the emission of the excited molecular anion (trion). The fluorescence of this molecular probe is recorded for tip-substrate nanocavities featuring different plasmonic resonances, for different tip-substrate distances and applied bias voltages, and on different substrates. We demonstrate that the width of the emission peak can be used as a probe of the trion-plasmon coupling strength and that the energy of the emitted photons is governed by the molecule interactions with its electrostatic environment. Additionally, we theoretically elucidate why the direct contact of the suspended molecule with the metallic tip does not totally quench the radiative emission of the molecule.Comment: 9 pages in the main manuscript, including 4 figures, 11 pages in the supporting info, including 5 figures and 1 tabl

Magnetism of CoPd self-organized alloy clusters on Au(111)

Magnetic properties of gold-encapsulated CoxPd1-x self-organized nano-clusters on Au(111) are analyzed by x-ray magnetic circular dichroism for x = 0.5, 0.7, and 1.0. The clusters are superparamagnetic with a blocking temperature decreasing with increasing Pd concentration, due to a reduction of the out-of-plane anisotropy strength. No magnetic moment is detected on Pd in these clusters, within the detection limit, contrary to thick CoPd films. Both reduction of anisotropy and vanishing Pd moment are attributed to strain. (C) 2013 AIP Publishing LLC

Charged pions from Ni on Ni collisions between 1 and 2 AGeV

Charged pions from Ni + Ni reactions at 1.05, 1.45 and 1.93 AGeV are measured with the FOPI detector. The mean $\pi^{\pm}$ multiplicities per mean number of participants increase with beam energy, in accordance with earlier studies of the Ar + KCl and La + La systems. The pion kinetic energy spectra have concave shape and are fitted by the superposition of two Boltzmann distributions with different temperatures. These apparent temperatures depend only weakly on bombarding energy. The pion angular distributions show a forward/backward enhancement at all energies, but not the $\Theta = 90^0$ enhancement which was observed in case of the Au + Au system. These features also determine the rapidity distributions which are therefore in disagreement with the hypothesis of one thermal source. The importance of the Coulomb interaction and of the pion rescattering by spectator matter in producing these phenomena is discussed

Caracterização da dinâmica da complexação do receptor tipo Toll 4 humano a MD-2

Os receptores tipo Toll desempenham um importante papel na resposta imune inata, reconhecendo tanto padrões moleculares associados a patógenos (PAMPs), quanto padrões moleculares associados a danos (DAMPs), liberados sob condições de injúria ou estresse celular. O receptor tipo Toll 4 humano (hTLR4), associado ao seu co-receptor, o fator de diferenciação mielóide 2 (MD-2), forma um heterodímero caracterizado como responsável pelo reconhecimento de lipopolissacarídeos bacterianos (LPS), derivados de bactérias Gram-negativas. Nestes casos, sabe-se que o MD-2 reconhece LPS e promove a dimerização do complexo hTLR4 - MD-2 - LPS, promovendo a sinalização intracelular. Já foi reportada a ausência da associação hTLR4 a MD-2, no reconhecimento de outros ligantes por hTLR4, e, nesses casos, pouco é conhecido a respeito das mudanças estruturais e conformacionais sofridas por este receptor. No presente estudo, empregando a técnica de simulação por dinâmica molecular, foram exploradas as propriedades dinâmicas do complexo de reconhecimento de LPS, hTLR4 - MD-2, bem como investigou-se as implicações da presença do co-receptor para a biologia estrutural do hTLR4. Os resultados mostram que o receptor apresenta um movimento do tipo pinça, o qual leva a um estado final mais aberto da estrutura em forma de ferradura. Ademais, a estabilidade desta estrutura parece ser influenciada pela presença do co-receptor, MD-2.Toll-like receptors (TLRs) play an important role in innate immunity recognizing pathogen-associated molecular patterns (PAMPs), as also damageassociated molecular patterns (DAMPs), released after cellular injury or stress. Human Toll-Like Receptor 4 (hTLR4) and its co-receptor, myeloid differentiation factor 2 (MD-2), as a heterodimer, is a well-known complex of Gram-negative bacteria lipopolysaccharide (LPS) recognition. In this process, MD-2 recognizes LPS and promotes the dimerization of the complex hTLR4 - MD-2 - LPS, initiating an intracellular immune signaling. Moreover, it has been reported that hTLR4 can also act in the absence of MD-2, in the case of other ligands recognition, and, in these cases, little is known about the structural and conformational changes that hTLR4 structure underwent. In the current study, employing molecular dynamics simulations, we had explored the dynamical properties of the hTLR4 - MD-2 complex and investigated the implications of the co-receptor complexation to the structural biology of hTR4. We observed that the receptor showed a tweezers-like movement, leading to a more oppened final state of its horseshoe-shaped structure. Additionally, the stability of this structure seems to be influenced by the presence of the co-receptor, MD-2

Electroluminescence of monolayer WS2_2 in a scanning tunneling microscope: the effect of bias polarity on the spectral and angular distribution of the emitted light

Inelastic electron tunneling in a scanning tunneling microscope (STM) is used to generate excitons in monolayer tungsten disulfide (WS$_2$). Excitonic electroluminescence is measured both at positive and negative sample bias. Using optical spectroscopy and Fourier-space optical microscopy, we show that the bias polarity of the tunnel junction determines the spectral and angular distribution of the emitted light. At positive sample bias, only emission from excitonic species featuring an in-plane transition dipole moment is detected. Based on the spectral distribution of the emitted light, we infer that the dominant contribution is from charged excitons, i.e., trions. At negative sample bias, additional contributions from lower-energy excitonic species are evidenced in the emission spectra and the angular distribution of the emitted light reveals a mixed character of in-plane and out-of-plane transition dipole moments

Tip-Induced and Electrical Control of the Photoluminescence Yield of Monolayer WS₂

The photoluminescence (PL) of monolayer tungsten disulfide (WS2) is locally and electrically controlled using the nonplasmonic tip and tunneling current of a scanning tunneling microscope (STM). The spatial and spectral distribution of the emitted light is determined using an optical microscope. When the STM tip is engaged, short-range PL quenching due to near-field electromagnetic effects is present, independent of the sign and value of the bias voltage applied to the tip–sample tunneling junction. In addition, a bias-voltage-dependent long-range PL quenching is measured when the sample is positively biased. We explain these observations by considering the native n-doping of monolayer WS2 and the charge carrier density gradients induced by electron tunneling in micrometer-scale areas around the tip position. The combination of wide-field PL microscopy and charge carrier injection using an STM opens up new ways to explore the interplay between excitons and charge carriers in two-dimensional semiconductors