281 research outputs found
Microscopic origin of the conducting channels in metallic atomic-size contacts
We present a theoretical approach which allows to determine the number and
orbital character of the conducting channels in metallic atomic contacts. We
show how the conducting channels arise from the atomic orbitals having a
significant contribution to the bands around the Fermi level. Our theory
predicts that the number of conducting channels with non negligible
transmission is 3 for Al and 5 for Nb one-atom contacts, in agreement with
recent experiments. These results are shown to be robust with respect to
disorder. The experimental values of the channels transmissions lie within the
calculated distributions.Comment: 11 pages, 4 ps-figures. Submitted to Phys. Rev. Let
Evidence of spontaneous spin polarized transport in magnetic nanowires
The exploitation of the spin in charge-based systems is opening revolutionary
opportunities for device architecture. Surprisingly, room temperature
electrical transport through magnetic nanowires is still an unresolved issue.
Here, we show that ferromagnetic (Co) suspended atom chains spontaneously
display an electron transport of half a conductance quantum, as expected for a
fully polarized conduction channel. Similar behavior has been observed for Pd
(a quasi-magnetic 4d metal) and Pt (a non-magnetic 5d metal). These results
suggest that the nanowire low dimensionality reinforces or induces magnetic
behavior, lifting off spin degeneracy even at room temperature and zero
external magnetic field.Comment: 4 pages, 3 eps fig
High-bias stability of monatomic chains
For the metals Au, Pt and Ir it is possible to form freely suspended
monatomic chains between bulk electrodes. The atomic chains sustain very large
current densities, but finally fail at high bias. We investigate the breaking
mechanism, that involves current-induced heating of the atomic wires and
electromigration forces. We find good agreement of the observations for Au
based on models due to Todorov and coworkers. The high-bias breaking of atomic
chains for Pt can also be described by the models, although here the parameters
have not been obtained independently. In the limit of long chains the breaking
voltage decreases inversely proportional to the length.Comment: 7 pages, 5 figure
Whole body and hepatic insulin action in normal, starved, and diabetic rats
In normal (N), 3-days starved (S), and streptozotocin-treated (65 mg/kg) 3-days diabetic (D) rats we examined the in vivo dose-response relationship between plasma insulin levels vs. whole body glucose uptake (BGU) and inhibition of hepatic glucose production (HGP) in conscious rats, as determined with the four-step sequential hyperinsulinemic euglycemic clamp technique, combined with [3-3H]glucose infusion. Twelve-hour fasting (basal) HGP was 3.0 +/- 0.2, 2.1 +/- 0.2, and 5.4 +/- 0.5 mg/min in N, S, and D rats, respectively. Next, all rats were clamped at matched glycemia (6 mM). Lowering plasma glucose in D rats from +/- 20 to 6.0 mM did not increase plasma norepinephrine, epinephrine, glucagon, and corticosterone levels. For BGU, insulin sensitivity was increased (70 +/- 11 microU/ml) in S and unchanged (113 +/- 21 microU/ml) in D compared with N rats (105 +/- 10 microU/ml). Insulin responsiveness was unchanged (12.4 +/- 0.8 mg/min) in S and decreased (8.5 +/- 0.8 mg/min) in D compared with N rats (12.3 +/- 0.7 mg/min). For HGP, insulin sensitivity was unchanged (68 +/- 10 microU/ml) in S and decreased (157 +/- 21 microU/ml) in D compared with N rats (71 +/- 5 microU/ml). Insulin responsiveness was identical among N, S, and D rats (complete suppression of HGP). In summary, 1) insulin resistance in D rats is caused by hepatic insensitivity and by a reduction in BGU responsiveness. 2) S rats show normal hepatic insulin action, but insulin sensitivity for BGU is increased. Therefore, S and D rats both suffering from a comparable catabolic state (10-15% body wt loss in 3 days) show opposite effects on in vivo insulin action. This indicates that in vivo insulin resistance in D rats is not caused by the catabolic state per se
Quantum Suppression of the Rayleigh Instability in Nanowires
A linear stability analysis of metallic nanowires is performed in the
free-electron model using quantum chaos techniques. It is found that the
classical instability of a long wire under surface tension can be completely
suppressed by electronic shell effects, leading to stable cylindrical
configurations whose electrical conductance is a magic number 1, 3, 5, 6,...
times the quantum of conductance. Our results are quantitatively consistent
with recent experiments with alkali metal nanowires.Comment: 10 pages, 5 eps figures, updated and expanded, accepted for
publication in "Nonlinearity
Quantum Conductance in Semimetallic Bismuth Nanocontacts
Electronic transport properties of bismuth nanocontacts are analyzed by means
of a low temperature scanning tunneling microscope. The subquantum steps
observed in the conductance versus elongation curves give evidence of atomic
rearrangements in the contact. The underlying quantum nature of the conductance
reveals itself through peaks in the conductance histograms. The shape of the
conductance curves at 77 K is well described by a simple gliding mechanism for
the contact evolution during elongation. The strikingly different behaviour at
4 K suggests a charge carrier transition from light to heavy ones as the
contact cross section becomes sufficiently small.Comment: 5 pages including 4 figures. Accepted for publication in Phys. Rev.
Let
Theoretical study of Ga-based nanowires and the interaction of Ga with single-wall carbon nanotubes
Gallium displays physical properties which can make it a potential element to
produce metallic nanowires and high-conducting interconnects in
nanoelectronics. Using first-principles pseudopotential plane method we showed
that Ga can form stable metallic linear and zigzag monatomic chain structures.
The interaction between individual Ga atom and single-wall carbon nanotube
(SWNT) leads to a chemisorption bond involving charge transfer. Doping of SWNT
with Ga atom gives rise to donor states. Owing to a significant interaction
between individual Ga atom and SWNT, continuous Ga coverage of the tube can be
achieved. Ga nanowires produced by the coating of carbon nanotube templates are
found to be stable and high conducting.Comment: 8 pages, 8 figure
Enhanced Detection of Expanded Repeat mRNA Foci with Hybridization Chain Reaction
Transcribed nucleotide repeat expansions form detectable RNA foci in patient cells that contribute to disease pathogenesis. The most widely used method for detecting RNA foci, fluorescence in situ hybridization (FISH), is powerful but can suffer from issues related to signal above background. Here we developed a repeat-specific form of hybridization chain reaction (R-HCR) as an alternative method for detection of repeat RNA foci in two neurodegenerative disorders: C9orf72 associated ALS and frontotemporal dementia (C9 ALS/FTD) and Fragile X-associated tremor/ataxia syndrome. R-HCR to both G4C2 and CGG repeats exhibited comparable specificity but \u3e 40 × sensitivity compared to FISH, with better detection of both nuclear and cytoplasmic foci in human C9 ALS/FTD fibroblasts, patient iPSC derived neurons, and patient brain samples. Using R-HCR, we observed that integrated stress response (ISR) activation significantly increased the number of endogenous G4C2 repeat RNA foci and triggered their selective nuclear accumulation without evidence of stress granule co-localization in patient fibroblasts and patient derived neurons. These data suggest that R-HCR can be a useful tool for tracking the behavior of repeat expansion mRNA in C9 ALS/FTD and other repeat expansion disorders
A Current Induced Transition in atomic-sized contacts of metallic Alloys
We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating.Comment: 5 pages, 6 figure
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