388 research outputs found
Time-dependent evolution of two coupled Luttinger liquids
We consider two disconnected Luttinger liquids which are coupled at
through chiral density-density interactions. Both for and the
system is exactly solvable by means of bosonization and this allows to evaluate
analytically the time-dependence of correlation functions. We find that in the
long-time limit the critical exponent governing the one-particle correlation
function differs from the exponent dictated by the equilibrium ground state of
the coupled system. We also discuss how this reflects on some physical
quantities which are accessible in real experiments.Comment: 6 pages, 1 eps fig, revised version accepted for publication in Phys.
Rev.
Influence of dimensionality on superconductivity in carbon nanotubes
We investigate the electronic instabilities in carbon nanotubes (CNs),
looking for the break-down of the one dimensional Luttinger liquid regime due
to the strong screening of the long-range part of the Coulomb repulsion. We
show that such a breakdown is realized both in ultra-small single wall CNs and
multi wall CNs, while a purely electronic mechanism could explain the
superconductivity (SC) observed recently in ultra-small (diameter ) single wall CNs () and entirely end-bonded multi-walled
ones (). We show that both the doping and the screening of
long-range part of the electron-electron repulsion, needed to allow the SC
phase, are related to the intrinsically 3D nature of the environment where the
CNs operate.Comment: 5 pages, 3 figures, PACS: 71.10.Pm,74.50.+r,71.20.Tx, to appear in J.
Phys. Cond. Ma
Surface ozone measurements at Taliarte, Gran Canaria (Canary Islands, Spain)
From December 1990 tropospheric ozone concentrations have been measured at Taliarte station (Gran Canaria, Canary Islands). Taliarte’s measurements are of great interest because of its proximity to Izaña BAPMoN observatory
(Tenerife, Canary Islands) and its relative distance to high-pollution regions. A comparative study with other North-Atlantic stations has been carried out. In order to
compare possible analogies and differences, a comparative study has been carried out. Studies involve seasonal cycle, diurnal oscillation and long-range transport. Ozone concentrations at Taliarte show high sensitivity to transport from higher latitudes. For diurnal variations, two different patterns have been found: “trade winds” and “marine breeze” situations
A generalized drift-diffusion model for rectifying Schottky contact simulation
We present a discussion on the modeling of Schottky barrier rectifying contacts (diodes) within the framework of partial-differential-equation-based physical simulations. We propose a physically consistent generalization of the drift-diffusion model to describe the boundary layer close to the Schottky barrier where thermionic emission leads to a non-Maxwellian carrier distribution, including a novel boundary condition at the contact. The modified drift-diffusion model is validated against Monte Carlo simulations of a GaAs device. The proposed model is in agreement with the Monte Carlo simulations not only in the current value but also in the spatial distributions of microscopic quantities like the electron velocity and concentratio
Comparative analysis of human and mouse expression data illuminates tissue-specific evolutionary patterns of miRNAs.
MicroRNAs (miRNAs) constitute an important class of gene regulators. While models have been proposed to explain their appearance and expansion, the validation of these models has been difficult due to the lack of comparative studies. Here, we analyze miRNA evolutionary patterns in two mammals, human and mouse, in relation to the age of miRNA families. In this comparative framework, we confirm some predictions of previously advanced models of miRNA evolution, e.g. that miRNAs arise more frequently de novo than by duplication, or that the number of protein-coding gene targeted by miRNAs decreases with evolutionary time. We also corroborate that miRNAs display an increase in expression level with evolutionary time, however we show that this relation is largely tissue-dependent, and especially low in embryonic or nervous tissues. We identify a bias of tag-sequencing techniques regarding the assessment of breadth of expression, leading us, contrary to predictions, to find more tissue-specific expression of older miRNAs. Together, our results refine the models used so far to depict the evolution of miRNA genes. They underline the role of tissue-specific selective forces on the evolution of miRNAs, as well as the potential co-evolution patterns between miRNAs and the protein-coding genes they target
Robust multiple-people tracking using color-based particle filters
Presentado al 3rd Iberian Conference on Pattern Recognition and Image Analysis (IbPRIA-2007) celebrado en Girona (Spain) del 6 al 8 de junio.Robust and accurate people tracking is a key task in many promising computer-vision applications. One must deal with non-rigid targets in open-world scenarios, whose shape and appearance evolve over time. Targets may interact, causing partial or complete occlusions. This paper improves tracking by means of particle filtering, where occlusions are handled considering the target's predicted trajectories. Model drift is tackled by careful updating, based on the history of likelihood measures. A colour-based likelihood, computed from histogram similarity, is used. Experiments are carried out using sequences from the CAVIAR database.This work has been supported by the Catalan Research Agency (AGAUR), by the Spanish Ministry of Education (MEC) under projects TIC2003-08865 and DPI-2004-5414, and by the EC grant IST-027110 under the HERMES project.Peer Reviewe
Deterministic and stochastic methods for gaze tracking in real-time
Psychological evidence demonstrates how eye gaze analysis is requested for human computer interaction endowed with emotion recognition capabilities. The existing proposals analyse eyelid and iris motion by using colour information and edge detectors, but eye movements are quite fast and difficult for precise and robust tracking. Instead, we propose to reduce the dimensionality of the image-data by using multi-Gaussian modelling and transition estimations by applying partial differences. The tracking system can handle illumination changes, low-image resolution and occlusions while estimating eyelid and iris movements as continuous variables. Therefore, this is an accurate and robust tracking system for eyelids and irises in 3D for standard image quality.Peer Reviewe
Assessing confidence in cased based reuse step
Case-Based Reasoning (CBR) is a learning approach that solves current situations by reusing previous solutions that are stored in a case base. In the CBR cycle the reuse step plays an important role into the problem solving process, since the solution for a new problem is based in the available solutions of the retrieved cases. In classification tasks a trivial reuse method is commonly used, which takes into account the most frequently solution proposed by the set of retrieved cases. We propose an alternative reuse process; we call confidence-reuse method, which make a qualitative assessment of the information retrieved. This approach is focused on measuring the solution accuracy, applying some confidence predictors based in a k-NN classifier with the aim of analyzing and evaluating the information offered by the retrieved cases.Peer Reviewe
Suppression of electron-electron repulsion and superconductivity in Ultra Small Carbon Nanotubes
Recently, ultra-small-diameter Single Wall Nano Tubes with diameter of have been produced and many unusual properties were observed, such as
superconductivity, leading to a transition temperature , much
larger than that observed in the bundles of larger diameter tubes.
By a comparison between two different approaches, we discuss the issue
whether a superconducting behavior in these carbon nanotubes can arise by a
purely electronic mechanism. The first approach is based on the Luttinger Model
while the second one, which emphasizes the role of the lattice and short range
interaction, is developed starting from the Hubbard Hamiltonian. By using the
latter model we predict a transition temperature of the same order of magnitude
as the measured one.Comment: 7 pages, 3 figures, to appear in J. Phys.-Cond. Ma
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