7,709 research outputs found
Three-dimensional Dirac oscillator in a thermal bath
The thermal properties of the three-dimensional Dirac oscillator are
considered. The canonical partition function is determined, and the
high-temperature limit is assessed. The degeneracy of energy levels and their
physical implications on the main thermodynamic functions are analyzed,
revealing that these functions assume values greater than the one-dimensional
case. So that at high temperatures, the limit value of the specific heat is
three times bigger.Comment: 9 pages, 4 figures. Text improved, references added. Revised to match
accepted version in Europhysics Letters
Electronic transport through bilayer graphene flakes
We investigate the electronic transport properties of a bilayer graphene
flake contacted by two monolayer nanoribbons. Such a finite-size bilayer flake
can be built by overlapping two semiinfinite ribbons or by depositing a
monolayer flake onto an infinite nanoribbon. These two structures have a
complementary behavior, that we study and analyze by means of a tight-binding
method and a continuum Dirac model. We have found that for certain energy
ranges and geometries, the conductance of these systems oscillates markedly
between zero and the maximum value of the conductance, allowing for the design
of electromechanical switches. Our understanding of the electronic transmission
through bilayer flakes may provide a way to measure the interlayer hopping in
bilayer graphene.Comment: 11 pages, 8 figure
Treating some solid state problems with the Dirac equation
The ambiguity involved in the definition of effective-mass Hamiltonians for
nonrelativistic models is resolved using the Dirac equation. The multistep
approximation is extended for relativistic cases allowing the treatment of
arbitrary potential and effective-mass profiles without ordering problems. On
the other hand, if the Schrodinger equation is supposed to be used, our
relativistic approach demonstrate that both results are coincidents if the
BenDaniel and Duke prescription for the kinetic-energy operator is implemented.
Applications for semiconductor heterostructures are discussed.Comment: 06 pages, 5 figure
Direct observation of melting in a 2-D superconducting vortex lattice
Topological defects such as dislocations and disclinations are predicted to
determine the twodimensional (2-D) melting transition. In 2-D superconducting
vortex lattices, macroscopic measurements evidence melting close to the
transition to the normal state. However, the direct observation at the scale of
individual vortices of the melting sequence has never been performed. Here we
provide step by step imaging through scanning tunneling spectroscopy of a 2-D
system of vortices up to the melting transition in a focused-ion-beam
nanodeposited W-based superconducting thin film. We show directly the
transition into an isotropic liquid below the superconducting critical
temperature. Before that, we find a hexatic phase, characterized by the
appearance of free dislocations, and a smectic-like phase, possibly originated
through partial disclination unbinding. These results represent a significant
step in the understanding of melting of 2-D systems, with impact across several
research fields, such as liquid crystal molecules, or lipids in membranes.Comment: Submitted to Nature Physic
Fenologia reprodutiva de Couratari stellata A. C. Smith.
Anais do CONGRESSO NACIONAL SOBRE ESSÊNCIAS NATIVAS, 2., 1992, São Paulo, SP
Identificação de carotenoides em polpa de goiaba microencapsulada utilizando cromatografia líquida de alta eficiência.
Título: goaiba [i.e. goiaba]
Biotechnologies and bioinspired materials for the construction industry : an overview
Published online: 16 Oct 2013Looking back to less than three centuries of industrialization, responsible for alarming levels of pollution and consumption
of non-renewable resources that has led to the exhaustion of the earth’s capacity, the humankind only now begins to grasp
the overwhelming potential of natural systems. During almost 40 million centuries, Nature has developed materials and
processes with optimal performance which are totally biodegradable. Analysis of bioinspired materials requires the
knowledge of both biological and engineering principles which are being a part of a large research area termed
biotechnology. This hot area is one of the six strategic Key Enabling Technologies that will be funded under the EU
Framework Programme Horizon 2020. This paper reviews the current knowledge on the potential of this emerging field,
particularly in the development of materials and technologies for the construction industry. It covers the use of bacteria for
enhancing concrete durability and for soil stabilization. It also covers bioinspired tough composite materials, bioinspired
adhesives and coatings, and self-cleaning materials. Incorporation of biology basics in the civil engineering curriculum
would ease the communication between biologists and civil engineers, helping to foster research on biotechnologies and
bioinspired materials for the construction industry
Rotor Unbalance Kind and Severity Identification by Current Signature Analysis with Adaptative Update to Multiclass Machine Learning Algorithms
The health of a rotating electric machine can be evaluated by monitoring electrical and mechanical parameters. As more information is available, it easier can become the diagnosis of the machine operational condition. We built a laboratory test bench to study rotor unbalance issues according to ISO standards. Using the electric stator current harmonic analysis, this paper presents a comparison study among Support-Vector Machines, Decision Tree classifies, and One-vs-One strategy to identify rotor unbalance kind and severity problem – a nonlinear multiclass task. Moreover, we propose a methodology to update the classifier for dealing better with changes produced by environmental variations and natural machinery usage. The adaptative update means to update the training data set with an amount of recent data, saving the entire original historical data. It is relevant for engineering maintenance. Our results show that the current signature analysis is appropriate to identify the type and severity of the rotor unbalance problem. Moreover, we show that machine learning techniques can be effective for an industrial application
Superconducting density of states at the border of an amorphous thin film grown by focused-ion-beam
We present very low temperature Scanning Tunneling Microscopy and
Spectroscopy (STM/S) measurements of a W based amorphous thin film grown with
focused-ion-beam. In particular, we address the superconducting properties
close to the border, where the thickness of the superconducting film decreases,
and the Au substrate emerges. When approaching the Au substrate, the
superconducting tunneling conductance strongly increases around the Fermi
level, and the quasiparticle peaks do not significantly change its position.
Under magnetic fields, the vortex lattice is observed, with vortices positioned
very close to the Au substrate.Comment: To appear in Journal of Physics: Conference serie
Adaptabilidade e estabilidade de híbridos de milho nos Estados do Piauí e Maranhão na safra 2005/2006.
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