51 research outputs found
Ballistic resistivity in aluminum nanocontacts
One of the major industrial challenges is to profit from some fascinating
physical features present at the nanoscale. The production of dissipationless
nanoswitches (or nanocontacts) is one of such attractive applications.
Nevertheless, the lack of knowledge of the real efficiency of electronic
ballistic/non dissipative transport limits future innovations. For multi-valent
metallic nanosystems -where several transport channels per atom are involved-
the only experimental technique available for statistical transport
characterization is the conductance histogram. Unfortunately its interpretation
is difficult because transport and mechanical properties are intrinsically
interlaced. We perform a representative series of semiclassical molecular
dynamics simulations of aluminum nanocontact breakages, coupled to full quantum
conductance calculations, and put in evidence a linear relationship between the
conductance and the contact minimum cross-section for the geometrically favored
aluminum nanocontact configurations. Valid in a broad range of conductance
values, such relation allows the definition of a transport parameter for
nanomaterials, that represents the novel concept of ballistic resistivity
Intensity Distribution of Modes in Surface Corrugated Waveguides
Exact calculations of transmission and reflection coefficients in surface
randomly corrugated optical waveguides are presented. As the length of the
corrugated part of the waveguide increases, there is a strong preference to
forward coupling through the lowest mode. An oscillating behavior of the
enhanced backscattering as a function of the wavelength is predicted. Although
the transport is strongly non isotropic, the analysis of the probability
distributions of the transmitted waves confirms in this configuration
distributions predicted by Random Matrix Theory for volume disorder
Reflection and transmission of waves in surface-disordered waveguides
The reflection and transmission amplitudes of waves in disordered multimode
waveguides are studied by means of numerical simulations based on the invariant
embedding equations. In particular, we analyze the influence of surface-type
disorder on the behavior of the ensemble average and fluctuations of the
reflection and transmission coefficients, reflectance, transmittance, and
conductance. Our results show anomalous effects stemming from the combination
of mode dispersion and rough surface scattering: For a given waveguide length,
the larger the mode transverse momentum is, the more strongly is the mode
scattered. These effects manifest themselves in the mode selectivity of the
transmission coefficients, anomalous backscattering enhancement, and speckle
pattern both in reflection and transmission, reflectance and transmittance, and
also in the conductance and its universal fluctuations. It is shown that, in
contrast to volume impurities, surface scattering in quasi-one-dimensional
structures (waveguides) gives rise to the coexistence of the ballistic,
diffusive, and localized regimes within the same sample.Comment: LaTeX (REVTeX), 12 pages with 14 EPS figures (epsf macro), minor
change
Mesoscopic scattering in the half-plane: squeezing conductance through a small hole
We model the 2-probe conductance of a quantum point contact (QPC), in linear
response. If the QPC is highly non-adiabatic or near to scatterers in the open
reservoir regions, then the usual distinction between leads and reservoirs
breaks down and a technique based on scattering theory in the full
two-dimensional half-plane is more appropriate. Therefore we relate conductance
to the transmission cross section for incident plane waves. This is equivalent
to the usual Landauer formula using a radial partial-wave basis. We derive the
result that an arbitrarily small (tunneling) QPC can reach a p-wave channel
conductance of 2e^2/h when coupled to a suitable reflector. If two or more
resonances coincide the total conductance can even exceed this. This relates to
recent mesoscopic experiments in open geometries. We also discuss reciprocity
of conductance, and the possibility of its breakdown in a proposed QPC for atom
waves.Comment: 8 pages, 3 figures, REVTeX. Revised version (shortened), accepted for
publication in PR
Argument mining: A machine learning perspective
Argument mining has recently become a hot topic, attracting the interests of several and diverse research communities, ranging from artificial intelligence, to computational linguistics, natural language processing, social and philosophical sciences. In this paper, we attempt to describe the problems and challenges of argument mining from a machine learning angle. In particular, we advocate that machine learning techniques so far have been under-exploited, and that a more proper standardization of the problem, also with regards to the underlying argument model, could provide a crucial element to develop better systems
Force, charge, and conductance of an ideal metallic nanowire
The conducting and mechanical properties of a metallic nanowire formed at the
junction between two macroscopic metallic electrodes are investigated. Both
two- and three-dimensional wires with a W(ide)-N(arrow)-W(ide) geometry are
modelled in the free-electron approximation with hard-wall boundary conditions.
Tunneling and quantum-size effects are treated exactly using the scattering
matrix formalism. Oscillations of order E_F/lambda_F in the tensile force are
found when the wire is stretched to the breaking point, which are synchronized
with quantized jumps in the conductance. The force and conductance are shown to
be essentially independent of the width of the wide sections (electrodes). The
exact results are compared with an adiabatic approximation; the later is found
to overestimate the effects of tunneling, but still gives qualitatively
reasonable results for nanowires of length L>>lambda_F, even for this abrupt
geometry. In addition to the force and conductance, the net charge of the
nanowire is calculated and the effects of screening are included within linear
response theory. Mesoscopic charge fluctuations of order e are predicted which
are strongly correlated with the mesoscopic force fluctuations. The local
density of states at the Fermi energy exhibits nontrivial behavior which is
correlated with fine structure in the force and conductance, showing the
importance of treating the whole wire as a mesoscopic system rather than
treating only the narrow part.Comment: 23 pages, 8 figure
Intrinsic Factors Influencing the Infection by Helminth Parasites in Horses under an Oceanic Climate Area (NW Spain)
A coprological survey to determine the influence of some intrinsic factors (breed, age, and sex) on the infection by helminth parasites in equine livestock (n = 418) under an oceanic climate area (NW Spain) was conducted. Faecal samples were individually collected and analyzed by the coprological techniques. The main strongylid genera identified were Trichonema and Cyalocephalus spp (small strongyles) and Strongylus and Triodontophorus (large strongyles). The prevalence of gastrointestinal nematode was 89% (95% CI 86, 92) and 1% cestoda (0, 2). The percentage of horses with strongyloid parasites was 89% (86, 92), 11% (8, 14) for Parascaris, and 3% (1, 5) for Oxyuris. The highest prevalence for ascariosis was observed in the youngest horses (<3 years), for oxyurosis in the >10 years animals, and for strongylosis in the 3â10 years ones. Females were significantly more parasitized than males. A negative correlation between the age and the egg-excretion of ascarids and strongyles was recorded. The autochthonous and the English Pure Blood horses were the most parasitized. We concluded that the infections by helminths, especially the strongyloids, are significantly common in the region, so that greater importance should be given to this situation
PanorĂĄmicas para un corte de la vertiente surpirenaica en la zona de estructuras oblicuas de AĂnsa â Valle del Cinca
En la vertiente meridional de los Pirineos, la franja comprendida entre las latitudes 0Âș9â30ââO y 0Âș26â30ââE, que tiene su eje en torno al valle del
rĂo Cinca, tiene caracteres peculiares en su estructura: la direcciĂłn de los pliegues y cabalgamientos se aparta de la direcciĂłn pirenaica dominante..
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