Spin-dependent electronic hybridization in a rope of carbon nanotubes

We demonstrate single electron addition to different strands of a carbon nanotube rope. Anticrossings of anomalous conductance peaks occur in quantum transport measurements through the parallel quantum dots forming on the individual strands. We determine the magnitude and the sign of the hybridization as well as the Coulomb interaction between the carbon nanotube quantum dots, finding that the bonding states dominate the transport. In a magnetic field the hybridization is shown to be selectively suppressed due to spin effects.Comment: 4 pages, 4 figure

Selected-fit versus induced-fit protein binding: Kinetic differences and mutational analysis

The binding of a ligand molecule to a protein is often accompanied by conformational changes of the protein. A central question is whether the ligand induces the conformational change (induced-fit), or rather selects and stabilizes a complementary conformation from a pre-existing equilibrium of ground and excited states of the protein (selected-fit). We consider here the binding kinetics in a simple four-state model of ligand-protein binding. In this model, the protein has two conformations, which can both bind the ligand. The first conformation is the ground state of the protein when the ligand is off, and the second conformation is the ground state when the ligand is bound. The induced-fit mechanism corresponds to ligand binding in the unbound ground state, and the selected-fit mechanism to ligand binding in the excited state. We find a simple, characteristic difference between the on- and off-rates in the two mechanisms if the conformational relaxation into the ground states is fast. In the case of selected-fit binding, the on-rate depends on the conformational equilibrium constant, while the off-rate is independent. In the case of induced-fit binding, in contrast, the off-rate depends on the conformational equilibrium, while the on-rate is independent. Whether a protein binds a ligand via selected-fit or induced-fit thus may be revealed by mutations far from the protein's binding pocket, or other "perturbations" that only affect the conformational equilibrium. In the case of selected-fit, such mutations will only change the on-rate, and in the case of induced-fit, only the off-rate.Comment: 6 pages, 3 figures; to appear in "Proteins: Structure, Function, and Bioinformatics

Saturation spectroscopy in optically thick three-level gas media

High-contrast transmission spectra arising from saturated absorption in optically thick Doppler-broadened three-level media are analyzed within the formalism of Maxwell-Bloch-type equations. It is found that although in the strong pumping engine propagation-induced line narrowing and light-shift elimination occur, spectral features lying within the natural linewidth cannot be resolved

Surface band structure of Bi1−xSbx\text{Bi}_{1-x}\text{Sb}_{x}(111)

Theoretical and experimental studies agree that $\text{Bi}_{1-x}\text{Sb}_{x}$ ($0.07 \leq x \leq 0.21$) to be a three-dimensional topological insulator. However, there is still a debate on the corresponding $\text{Bi}_{1-x}\text{Sb}_{x}$(111) surface band structure. While three spin polarized bands have been claimed experimentally, theoretically, only two surface bands appear, with the third band being attributed to surface imperfections. Here, we address this controversy using angle-resolved photoemission spectroscopy (ARPES) on $\text{Bi}_{1-x}\text{Sb}_{x}$ films. To minimize surface imperfections, we have optimized the sample growth recipe. We have measured the evolution of the surface band structure of $\text{Bi}_{1-x}\text{Sb}_{x}$ with $x$ increasing gradually from $x = 0$ to $x = 0.6$. Our ARPES data show better agreement with the theoretical calculations, where the system is topologically non-trivial with two surface bands

Diffusion probabiliste dans les réseaux dynamiques

National audienceLa diffusion probabiliste est une des techniques les plus populaires pour diffuser de l'information dans les réseaux à grande échelle. Cette technique est appréciée pour sa simplicité, sa robustesse et son efficacité. Dans le cas du protocole \push, chaque nœud informé choisit à chaque étape un de ses voisins aléatoirement de manière uniforme, et lui transmet l'information. Ce protocole est connu pour permettre la diffusion en $O(\log n)$ étapes, avec forte probabilité, dans plusieurs familles de réseaux \emph{statiques} de $n$ nœuds. De plus, il a été montré empiriquement que le protocole \push\/ offre de très bonnes performances en pratique. En particulier, il se montre robuste aux évolutions dynamiques de la structure réseau. Dans cet article, nous analysons le protocole \push\/ dans le cas de réseaux \emph{dynamiques}. Nous considérons le modèle des graphes à évolution arête-markovienne, qui permet de capturer une forme de dépendance temporelle entre la structure du réseau au temps $t$ et celle au temps $t+1$. Plus précisément, une arête inexistante apparaît avec probabilité $p$, tandis qu'une arête existante disparaît avec probabilité $q$. Ayant pour objectif de coller avec des traces réelles, nous concentrons principalement notre étude sur le cas $p=\Omega(\frac{1}{n})$ et $q$ constant. Nous prouvons que, dans ce cas réaliste, le protocole \push\/ permet de diffuser l'information en $O(\log n)$ étapes, avec forte probabilité. Cette borne reste valide même lorsque, avec forte probabilité, le réseau est déconnecté à chaque étape (typiquement, lorsque $p\ll \frac{\log n}{n}$). Ce résultat démontre ainsi formellement la robustesse du protocole \push\/ dans le cadre d'évolution temporelle de la structure du réseau. La version complète de cet article, en cours de soumission, est disponible sur arXiv (voir~\cite{CCDFPS13} qui contient un sur-ensemble des résultats présentés ici)

Spatial Distribution of Nucleosynthesis Products in Cassiopeia A: Comparison Between Observations and 3D Explosion Models

We examine observed heavy element abundances in the Cassiopeia A supernova remnant as a constraint on the nature of the Cas A supernova. We compare bulk abundances from 1D and 3D explosion models and spatial distribution of elements in 3D models with those derived from X-ray observations. We also examine the cospatial production of 26Al with other species. We find that the most reliable indicator of the presence of 26Al in unmixed ejecta is a very low S/Si ratio (~0.05). Production of N in O/S/Si-rich regions is also indicative. The biologically important element P is produced at its highest abundance in the same regions. Proxies should be detectable in supernova ejecta with high spatial resolution multiwavelength observations.Comment: To appear in the Conference Proceedings for the "10th Symposium on Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island, Michigan, US

Studies on Age-Related Changes in Equine Cheek Teeth Angulation and Dental Drift

BACKGROUND: Cheek teeth (second through fourth premolars and first through third molars) diastema is a common and painful equine disorder caused by the absence of effective tight interdental contact between these teeth. Limited objective information is available on the angulation of equine cheek teeth that control dental drift or on mesial or distal equine cheek teeth drift that should normally prevent this disorder. OBJECTIVES: To measure the angulation of the mesial and distal cheek teeth in horses of different ages, quantify age-related cheek teeth mesial and distal dental drift, and measure the cheek teeth row length in horses of different ages. STUDY DESIGN: Retrospective review of computed tomographic images of equine heads. METHODS: Case details and CT images from clinical equine cases that had undergone standing CT head examination were collated. Three sets of measurements were acquired from each head. “Head size” calculated as the distance between the caudal aspect of the orbit and the caudal aspect of the naso-incisive notch was used to standardize measurements in different sized heads. The length of the cheek teeth rows measured from the mesial aspect of the Triadan 06 occlusal surface to the distal aspect of the Triadan 11 occlusal surface. The rostro-caudal (antero-posterior) position and angulation of the mandibular and maxillary Triadan 06 and 11 teeth were measured in relation to reference lines drawn on CT images. RESULTS: Significant mesial drift occurred in the maxillary and mandibular Triadan 11s. Despite their distal angulation, the upper and lower Triadan 06s also drifted mesially. The mean angulation of Triadan 06 and 11 mandibular teeth (17.8 and 26.2°, respectively) was almost double that of maxillary teeth (9.2 and 13.3°, respectively) with both Triadan 11s having greater angulation than the 06s. Cheek teeth angulation only significantly decreased in the mandibular 06s. Cheek teeth arcade lengths decreased with age, but these decreases were not significant. MAIN LIMITATIONS: Limitations include the relatively small sample size. CONCLUSIONS: In the population of horses used for this study, age related mesial drift occurred in both Triadan 06 and 11s, and the angulation of these teeth did not decrease with age in most arcades