2,324 research outputs found
Impurity effects on Fabry-Perot physics of ballistic carbon nanotubes
We present a theoretical model accounting for the anomalous Fabry-Perot
pattern observed in the ballistic conductance of a single-wall carbon
nanotubes. Using the scattering field theory, it is shown that the presence of
a limited number of impurities along the nanotube can be identified by a
measurement of the conductance and their position determined. Impurities can be
made active or silent depending on the interaction with the substrate via the
back-gate. The conceptual steps for designing a bio-molecules detector are
briefly discussed.Comment: 4 pages, 4 figure
Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series
Recently we have observed in compounds of the 4,n-alkyloxybenzoic acid
series, with the homologous index n ranging from 6 to 9, a texture transition
in the nematic range which subdivides the nematic phase in two sub-phases
displaying different textures in polarised light analysis. To investigate a
persistence of texture transitions in nematic phases, we prepared binary
mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-,8-,9-,12-,
16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the
temperature ranges of both smectic and nematic phases. A nematic temperature
range of 75 C is observed. In the nematic phase, in spite of the microscopic
disorder introduced by mixing two components, the polarised light optics
analysis of the liquid crystal cells reveals a texture transition. In the case
of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of
compounds with monomers of rather different lengths, the texture transition
temperature is not homogeneous in the cell, probably due to a local variation
in the relative concentrations of compounds.Comment: 13 pages, 9 figure
The CHORUS experiment: a status report
sample of 2622 fully analyzed 1 events, has been presented, confirming that the CHORUS proposal sensitivity for Run I \mbox{()} is reachable
Labor market regimes and monetary policy
In this paper we propose straightforward extensions of multi-union, monopolistic competition models appearing in the recent literature on the macroeconomic effects of monetary policy. We extend these models from the Stackelberg equilibrium to the Nash equilibrium under variations in labor market regime in order to evaluate propositions about non-neutrality of monetary policy.policy game monetary policy neutrality trade union monopolistic competition
Effective Lagrangian for Heavy and Light Mesons: Semileptonic Decays
We introduce an effective lagrangian including negative and positive parity
heavy mesons containing a heavy quark, light pseudoscalars, and light vector
resonances, with their allowed interactions, using heavy quark spin-flavour
symmetry, chiral symmetry, and the hidden symmetry approach for light vector
resonances. On the basis of such a lagrangian, by considering the allowed weak
currents and by including the contributions from the nearest unitarity poles we
calculate the form factors for semileptonic decays of and mesons into
light pseudoscalars and light vector resonances. The available data, together
with some additional assumptions, allow for a set of predictions in the
different semileptonic channels, which can be compared with those following
{}from different approaches. A discussion of non-dominant terms in our
approach, which attempts at including a rather complete dynamics, will however
have to wait till more abundant data become available.Comment: LaTeX (style article), 19 pages, UGVA-DPT 1992/11-790, BARI-TH/92-12
A local field emission study of partially aligned carbon-nanotubes by AFM probe
We report on the application of Atomic Force Microscopy (AFM) for studying
the Field Emission (FE) properties of a dense array of long and vertically
quasi-aligned multi-walled carbon nanotubes grown by catalytic Chemical Vapor
Deposition on a silicon substrate. The use of nanometric probes enables local
field emission measurements allowing investigation of effects non detectable
with a conventional parallel plate setup, where the emission current is
averaged on a large sample area. The micrometric inter-electrode distance let
achieve high electric fields with a modest voltage source. Those features
allowed us to characterize field emission for macroscopic electric fields up to
250 V/m and attain current densities larger than 10 A/cm. FE
behaviour is analyzed in the framework of the Fowler-Nordheim theory. A field
enhancement factor 40-50 and a turn-on field 15 V/m at an inter-electrode distance of 1 m are estimated.
Current saturation observed at high voltages in the I-V characteristics is
explained in terms of a series resistance of the order of M. Additional
effects as electrical conditioning, CNT degradation, response to laser
irradiation and time stability are investigated and discussed
Field emission from single multi-wall carbon nanotubes
Electron field emission characteristics of individual multiwalled carbon
nanotubes have been investigated by a piezoelectric nanomanipulation system
operating inside a scanning electron microscopy chamber. The experimental setup
ensures a high control capability on the geometric parameters of the field
emission system (CNT length, diameter and anode-cathode distance). For several
multiwalled carbon nanotubes, reproducible and quite stable emission current
behaviour has been obtained with a dependence on the applied voltage well
described by a series resistance modified Fowler-Nordheim model. A turn-on
field of about 30 V/um and a field enhancement factor of around 100 at a
cathode-anode distance of the order of 1 um have been evaluated. Finally, the
effect of selective electron beam irradiation on the nanotube field emission
capabilities has been extensively investigated.Comment: 16 pages, 5 figure
Local probing of the field emission stability of vertically aligned multiwalled carbon nanotubes
Metallic cantilever in high vacuum atomic force microscope has been used as
anode for field emission experiments from densely packed vertically aligned
multi-walled carbon nanotubes. The high spatial resolution provided by the
scanning probe technique allowed precise setting of the tip-sample distance in
the submicron region. The dimension of the probe (curvature radius below 50nm)
allowed to measure current contribution from sample areas smaller than 1um^2.
The study of long-term stability evidenced that on these small areas the field
emission current remains stable (within 10% fluctuations) several hours (at
least up to 72 hours) at current intensities between 10-5A and 10-8A.
Improvement of the current stability has been observed after performing
long-time Joule heating conditioning to completely remove possible adsorbates
on the nanotubes.Comment: 15 pages, 7 figure
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