148 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
Rising serum values of beta-subunit human chorionic gonadotrophin (hCG) in patients with progressive vulvar carcinomas.
Elevated serum levels of the beta-subunit of human chorionic gonadotrophin (hCG) were measured in 50% of patients with locoregional recurrences or progressive vulvar carcinoma (n = 14). At diagnosis of vulvar cancer, however, the incidence of elevated serum levels was low (5%) in 104 patients. The rising serum levels during progression of disease indicate that the synthesis of the beta-subunit hCG can be increased in vulvar carcinoma
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
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
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
Electron-Transport Properties of Na Nanowires under Applied Bias Voltages
We present first-principles calculations on electron transport through Na
nanowires at finite bias voltages. The nanowire exhibits a nonlinear
current-voltage characteristic and negative differential conductance. The
latter is explained by the drastic suppression of the transmission peaks which
is attributed to the electron transportability of the negatively biased plinth
attached to the end of the nanowire. In addition, the finding that a voltage
drop preferentially occurs on the negatively biased side of the nanowire is
discussed in relation to the electronic structure and conduction.Comment: 4 pages, 6 figure
Crossover from Electronic to Atomic Shell Structure in Alkali Metal Nanowires
After making a cold weld by pressing two clean metal surfaces together, upon
gradually separating the two pieces a metallic nanowire is formed, which
progressively thins down to a single atom before contact is lost. In previous
experiments [1,2] we have observed that the stability of such nanowires is
influenced by electronic shell filling effects, in analogy to shell effects in
metal clusters [3]. For sodium and potassium at larger diameters there is a
crossover to crystalline wires with shell-closings corresponding to the
completion of additional atomic layers. This observation completes the analogy
between shell effects observed for clusters and nanowires.Comment: 4 page
Towards unified understanding of conductance of stretched monatomic contacts
When monatomic contacts are stretched, their conductance behaves in
qualitatively different ways depending on their constituent atomic elements.
Under a single assumption of resonance formation, we show that various
conductance behavior can be understood in a unified way in terms of the
response of the resonance to stretching. This analysis clarifies the crucial
roles played by the number of valence electrons, charge neutrality, and orbital
shapes.Comment: 2 figure
A quantum point contact for neutral atoms
We show that the conductance of neutral atoms through a tightly confining
waveguide constriction is quantized in units of lambda_dB^2/pi, where lambda_dB
is the de Broglie wavelength of the incident atoms. Such a constriction forms
the atom analogue of an electron quantum point contact and is an example of
quantum transport of neutral atoms in an aperiodic system. We present a
practical constriction geometry that can be realized using a microfabricated
magnetic waveguide, and discuss how a pair of such constrictions can be used to
study the quantum statistics of weakly interacting gases in small traps.Comment: 5 pages with 3 figures. To appear in Phys. Rev. Let
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