1,771 research outputs found
On the temperature dependence of ballistic Coulomb drag in nanowires
We have investigated within the theory of Fermi liquid dependence of Coulomb
drag current in a passive quantum wire on the applied voltage across an
active wire and on the temperature for any values of . We assume
that the bottoms of the 1D minibands in both wires almost coincide with the
Fermi level. We come to conclusions that 1) within a certain temperature
interval the drag current can be a descending function of the temperature ;
2) the experimentally observed temperature dependence of the drag
current can be interpreted within the framework of Fermi liquid theory; 3) at
relatively high applied voltages the drag current as a function of the applied
voltage saturates; 4) the screening of the electron potential by metallic gate
electrodes can be of importance.Comment: 7 pages, 1 figur
The impact of heavy quark mass effects in the NNPDF global analysis
We discuss the implementation of the FONLL general-mass scheme for heavy
quarks in deep-inelastic scattering in the FastKernel framework, used in the
NNPDF series of global PDF analysis. We present the general features of FONLL
and benchmark the accuracy of its implementation in FastKernel comparing with
the Les Houches heavy quark benchmark tables. We then show preliminary results
of the NNPDF2.1 analysis, in which heavy quark mass effects are included
following the FONLL-A GM scheme.Comment: 5 pages, 3 figures; to appear in the proceedings of DIS 2010, Firenz
Progress in the Neural Network Determination of Polarized Parton Distributions
We review recent progress towards a determination of a set of polarized
parton distributions from a global set of deep-inelastic scattering data based
on the NNPDF methodology, in analogy with the unpolarized case. This method is
designed to provide a faithful and statistically sound representation of parton
distributions and their uncertainties. We show how the FastKernel method
provides a fast and accurate method for solving the polarized DGLAP equations.
We discuss the polarized PDF parametrizations and the physical constraints
which can be imposed. Preliminary results suggest that the uncertainty on
polarized PDFs, most notably the gluon, has been underestimated in previous
studies.Comment: 5 pages, 2 figures; to appear in the proceedings of DIS 2010, Firenz
Coulomb Drag in Coherent Mesoscopic Systems
We present a theory for Coulomb drag between two mesoscopic systems. Our
formalism expresses the drag in terms of scattering matrices and wave
functions, and its range of validity covers both ballistic and disordered
systems. The consequences can be worked out either by analytic means, such as
the random matrix theory, or by numerical simulations. We show that Coulomb
drag is sensitive to localized states, which usual transport measurements do
not probe. For chaotic 2D-systems we find a vanishing average drag, with a
nonzero variance. Disordered 1D-wires show a finite drag, with a large
variance, giving rise to a possible sign change of the induced current.Comment: 4 pages including 2 figures. Minor changes. Accepted for publication
in Phys. Rev. Let
Coulomb drag between ballistic one-dimensional electron systems
The presence of pronounced electronic correlations in one-dimensional systems
strongly enhances Coulomb coupling and is expected to result in distinctive
features in the Coulomb drag between them that are absent in the drag between
two-dimensional systems. We review recent Fermi and Luttinger liquid theories
of Coulomb drag between ballistic one-dimensional electron systems, and give a
brief summary of the experimental work reported so far on one-dimensional drag.
Both the Fermi liquid (FL) and the Luttinger liquid (LL) theory predict a
maximum of the drag resistance R_D when the one-dimensional subbands of the two
quantum wires are aligned and the Fermi wave vector k_F is small, and also an
exponential decay of R_D with increasing inter-wire separation, both features
confirmed by experimental observations. A crucial difference between the two
theoretical models emerges in the temperature dependence of the drag effect.
Whereas the FL theory predicts a linear temperature dependence, the LL theory
promises a rich and varied dependence on temperature depending on the relative
magnitudes of the energy and length scales of the systems. At higher
temperatures, the drag should show a power-law dependence on temperature, R_D
\~ T^x, experimentally confirmed in a narrow temperature range, where x is
determined by the Luttinger liquid parameters. The spin degree of freedom plays
an important role in the LL theory in predicting the features of the drag
effect and is crucial for the interpretation of experimental results.Comment: 25 pages, 14 figures, to appear in Semiconductor Science and
Technolog
Current drag in capacitevly coupled Luttinger constrictions
We study the current drag in the system of two electrostatically coupled
finite 1D electron channels. We present the perturbation theory results along
with the results for two non-perturbative regimes. It is shown that the drag
may become absolute, that is, the currents in the channels are equal in a
finite window of the bias voltages.Comment: 4 pages RevTeX, 3 postscript figure
Freezing transition of the vortex liquid in anisotropic superconductors
We study the solid-liquid transition of a model of pancake vortices in
laminar superconductors using a density functional theory of freezing. The
physical properties of the system along the melting line are discussed in
detail. We show that there is a very good agreement with experimental data in
the shape and position of the first order transition in the phase diagram and
in the magnitude and temperature dependence of the magnetic induction jump at
the transition. We analyze the validity of the Lindemann melting criterion and
the Hansen-Verlet freezing criterion. Both criteria are shown to be good to
predict the phase diagram in the region where a first order phase transition is
experimentally observed.Comment: 9 pages, 10 figure
Absence of gap for infinite half--integer spin ladders with an odd number of legs
A proof is presented for the absence of gap for spin ladders with an
odd number of legs, in the infinite leg length limit. This result is relevant
to the current discussion of coupled one--dimensional spin systems, a physical
realization of which are vanadyl pyrophosphate, (VO)PO, and
stoichiometric Sr Cu O (with ).Comment: REVTeX, 8 page
Stability and mechanical evaluation of bovine pericardium cross-linked with polyurethane prepolymer in aqueous medium
The present study investigates the potential use of non-catalyzed water-soluble blocked polyurethane prepolymer (PUP) as a bifunctional cross-linker for collagenous scaffolds. The effect of concentration (5, 10, 15 and 20%), time (4, 6, 12 and 24 h), medium volume (50, 100, 200 and 300%) and pH (7.4, 8.2, 9 and 10) over stability, microstructure and tensile mechanical behavior of acellular pericardial matrix was studied. The cross-linking index increased up to 81% while the denaturation temperature increased up to 12 °C after PUP crosslinking. PUP-treated scaffold resisted the collagenase degradation (0.167 ± 0.14 mmol/g of liberated amine groups vs. 598 ± 60 mmol/g for non-cross-linked matrix). The collagen fiber network was coated with PUP while viscoelastic properties were altered after cross-linking. The treatment of the pericardial scaffold with PUP allows (i) different densities of cross-linking depending of the process parameters and (ii) tensile properties similar to glutaraldehyde method
Energía gastada en el primer ciclo de histéresis como parámetro de selección de un biomaterial
Veinticuatro muestras de pericardio de ternera estabilizado, material similar al utilizado en la fabricación de velos de bioprótesis cardiacas, fueron sometidas a una prueba de esfuerzo a fatiga. Tras seis bloques de 100 ciclos a 1 MPa de presión máxima se rompieron 12 muestras, el 50%. La energía media gastada en este primer ciclo, en las muestras que sobrevivieron, fue de 0.16J, un valor inferior a la energía gastada en las muestra que terminaron rompiendo, 0.28J (p=0.005). Utilizando el cociente entre la energía disipada en el primer ciclo y el espesor medio de la muestra, con un valor de corte de 0.48J/mm para la selección de las mejores muestras, se obtiene un índice de validez del 87.5%, y un área bajo la curva ROC de 0.917. Este método no destructivo debe ayudar a los métodos ópticos en el reconocimiento y selección de las muestras más resistentes y en la obtención del material biológico más homogéne
- …