A self-consistent quantum master equation approach to molecular transport

We propose a self-consistent generalized quantum master equation (GQME) to describe electron transport through molecular junctions. In a previous study [M.Esposito and M.Galperin. Phys. Rev. B 79, 205303 (2009)], we derived a time-nonlocal GQME to cure the lack of broadening effects in Redfield theory. To do so, the free evolution used in the Born-Markov approximation to close the Redfield equation was replaced by a standard Redfield evolution. In the present paper, we propose a backward Redfield evolution leading to a time-local GQME which allows for a self-consistent procedure of the GQME generator. This approach is approximate but properly reproduces the nonequilibrium steady state density matrix and the currents of an exactly solvable model. The approach is less accurate for higher moments such as the noise.Comment: 9 pages, 4 figure

Evidence for Quantum Interference in SAMs of Arylethynylene Thiolates in Tunneling Junctions with Eutectic Ga-In (EGaIn) Top-Contacts

This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga2O3 as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10-1 A/cm2 at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.

Interference-based molecular transistors

Molecular transistors have the potential for switching with lower gate voltages than conventional field-effect transistors. We have calculated the performance of a single-molecule device in which there is interference between electron transport through the highest occupied molecular orbital and the lowest unoccupied molecular orbital of a single molecule. Quantum interference results in a subthreshold slope that is independent of temperature. For realistic parameters the change in gate potential required for a change in source-drain current of two decades is 20 mV, which is a factor of six smaller than the theoretical limit for a metal-oxide-semiconductor field-effect transistor

Observation of Quantum Interference in Molecular Charge Transport

As the dimensions of a conductor approach the nano-scale, quantum effects will begin to dominate its behavior. This entails the exciting possibility of controlling the conductance of a device by direct manipulation of the electron wave function. Such control has been most clearly demonstrated in mesoscopic semiconductor structures at low temperatures. Indeed, the Aharanov-Bohm effect, conductance quantization and universal conductance fluctuations are direct manifestations of the electron wave nature. However, an extension of this concept to more practical emperatures has not been achieved so far. As molecules are nano-scale objects with typical energy level spacings (~eV) much larger than the thermal energy at 300 K (~25 meV), they are natural candidates to enable such a break-through. Fascinating phenomena including giant magnetoresistance, Kondo effects and conductance switching, have previously been demonstrated at the molecular level. Here, we report direct evidence for destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. Furthermore, we show that the degree of interference can be controlled by simple chemical modifications of the molecule. Not only does this provide the experimental demonstration of a new phenomenon in quantum charge transport, it also opens the road for a new type of molecular devices based on chemical or electrostatic control of quantum interference

Aide à l'évaluation des risques aux postes de travail des radiologues industriels

L'objectif de cet atelier du GT SFRP-COFREND a été de proposer une analyse des risques pour la réalisation future d'études de postes sur les tirs en milieu industriel (nucléaire et hors nucléaire). Le principe retenu a été de réaliser un découpage des différentes phases d'un chantier, en y associant à chaque étape les risques associés. Les examens et contrôles non destructifs (END et CND) sont des activités qui nécessitent une vigilance de tous les instants (Fig. 1) pour : s'assurer de la qualité du travail ; gérer les différents risques dont le risque d'exposition aux rayonnements ionisants ; lutter contre des baisses d'attention liées aux formes de fatigue, aux conditions de travail, aux difficultés d'accès et dans certains cas au travail nocturne, etc. ; gérer les pressions temporelles (audits, surveillances...) ; gérer la co-activité pour éviter d'exposer d'autres professionnels ; gérer le risque lié au milieu de travail (INB, présence de produits dangereux...). Le champ d'étude d'une analyse de risque couvre les métiers, les tâches et activités, les nuisances, et les risques... Le principe par la suite d'une étude de poste sera une approche descriptive et analytique de toutes les composantes de la réalité d'un travail donné, son organisation, son environnement, ses contraintes et moyens d'ajustement (Fig. 2). L'étude de poste s'appuiera sur des méthodes objectives d'observation, d'analyse de risque et de mesure (rapport du groupe de coordination étude de postes médecins du travail EDF). L'objectif de l'étude de poste est de lister l'ensemble des actions à mener pour garantir des conditions d'intervention et de travail acceptables afin de prévenir l'ensemble des risques

Le retour d'expérience des incidents et son exploitation

Établi dans le cadre du groupe de travail COFREND-SFRP, cet atelier avait pour principal objectif de recenser le retour d'expérience des incidents existant dans le domaine de la gammagraphie industrielle et, si possible, de l'améliorer. Les produits issus de l'atelier sont principalement destinés à enrichir le contenu des formations des opérateurs et des personnes compétentes en radioprotection des entreprises de gammagraphie industrielle. Différentes sources bibliographiques dans lesquelles des incidents/accidents sont recensés ont été analysées : les bases de données IRID et RELIR ont été particulièrement étudiées. Il a alors été identifié que le retour d'expérience existant n'était pas totalement représentatif des événements rencontrés en France aujourd'hui. De nouveaux types d'incidents (non maîtrise de la zone balisée, blocage du porte-source dans la gaine dû à la présence de corps étranger...) ont été identifiés à partir desquels de nouvelles fiches RELIR ont été établies. En parallèle, une synthèse des leçons issues des incidents recensés au cours de l'atelier a été élaborée

Large planar drift chambers

The authors describe 14 m/sup 2/ hexagonal planar drift chambers designed for the neutrino experiment of the CERN-Dortmund-Heidelberg- Saclay Collaboration. Details on mechanical construction, electronic read-out, results on efficiency and accuracy are presented. (6 refs)