8,751 research outputs found
Efecto del tiempo y temperatura en el color generado por el proceso de tostado del café evaluado con la metodología de superficie de respuesta
En promedio el 95% de la producción nacional cafetalera es destinada a la exportación y el resto al consumo interno, el café es una importante fuente generadora de empleo y demandante de insumos, bienes y servicios, a lo largo de toda la cadena de valor (productores, comerciantes, acopiadores, transportistas, distribuidores, tostadores, comerciantes grandes y pequeños, administradores, entidades financieras, exportadores, técnicos y profe- sionales en todos los sectores mencionados, tostadurias, cafeterías, así como la agroindustria, entre otros). Siendo fuente de empleo para 123 mil familias que equivalen a 1 millón peruanos. Desde el año 1992 hasta el año 2001 se apreció un mayor incremento de las áreas y de los volúmenes de producción nacional (Abad, 2003). Con respecto al café tostado se pretende conocer los factores que determinan su procesamiento como el tiempo y temperatura que dependen del café a tostar por lo cual se intenta estudiar la temperatura y el tiempo apropiados para este fin evaluando el cambio de coloración de los granos tostados mediante el análisis de las imágenes por computadora. A este análisis de imágenes se le denomina visión artificial que es una herramienta eficaz en el desarrollo de diferentes tareas, ya que la imágenes digitales permiten la extracción de características cuantitativas y cualitativas de los objetos tales como la forma, tamaño y color (Sandoval, 2003).Este trabajo pretende servir de referencia a todo aquel que desee investigar acerca de la aplicación de los sistemas de visión por computadora en el control mediante color de alimentos y en especial de café tostado
Structural phase transition in IrTe: A combined study of optical spectroscopy and band structure calculations
IrPtTe is an interesting system showing competing phenomenon
between structural instability and superconductivity. Due to the large atomic
numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the
system which may lead to nonconventional superconductivity. We grew single
crystal samples of this system and investigated their electronic properties. In
particular, we performed optical spectroscopic measurements, in combination
with density function calculations, on the undoped compound IrTe in an
effort to elucidate the origin of the structural phase transition at 280 K. The
measurement revealed a dramatic reconstruction of band structure and a
significant reduction of conducting carriers below the phase transition. We
elaborate that the transition is not driven by the density wave type
instability but caused by the crystal field effect which further
splits/separates the energy levels of Te (p, p) and Te p bands.Comment: 16 pages, 5 figure
Rational sequences for the conductance in quantum wires from affine Toda field theories
We analyse the expression for the conductance of a quantum wire which is
decribed by an integrable quantum field theory. In the high temperature regime
we derive a simple formula for the filling fraction. This expression involves
only the inverse of a matrix which contains the information of the asymptotic
phases of the scattering matrix and the solutions of the constant thermodynamic
Bethe ansatz equations. Evaluating these expressions for minimal affine Toda
field theory we recover several sequences of rational numbers, which are
multiples of the famous Jain sequence for the filling fraction occurring in the
context of the fractional quantum Hall effect. For instance we obtain for -minimal affine Toda field theory. The matrices
involved have in general non-rational entries and are not part of previous
classification schemes based on integral lattices.Comment: 9 pages Latex, version to appear in Journal of Physics
On the origin of M81 group extended dust emission
Galactic cirrus emission at far-infrared wavelengths affects many extragalactic observations. Separating this emission from that associated with extragalactic objects is both important and difficult. In this paper we discuss a particular case, the M81 group, and the identification of diffuse structures prominent in the infrared, but also detected at optical wavelengths. The origin of these structures has previously been controversial, ranging from them being the result of a past interaction between M81 and M82 or due to more local Galactic emission. We show that over an order of a few arcmin scales, the far-infrared (Herschel 250 mu m) emission correlates spatially very well with a particular narrow-velocity (2-3 km s(-1)) component of the Galactic HI. We find no evidence that any of the far-infrared emission associated with these features actually originates in the M81 group. Thus we infer that the associated diffuse optical emission must be due to galactic light-back scattered off dust in our galaxy. Ultraviolet observations pick out young stellar associations around M81, but no detectable far-infrared emission. We consider in detail one of the Galactic cirrus features, finding that the far-infrared HI relation breaks down below arcmin scales and that at smaller scales there can be quite large dust-temperature variation
Crystal Field Triplets: A New Route to Non-Fermi Liquid Physics
A model for crystal field triplet ground states on rare earth or actinide
ions with dipolar and quadrupolar couplings to conduction electrons is studied
for the first time with renormalization group methods. The quadrupolar coupling
leads to a new nontrivial, non-Fermi liquid fixed point, which survives in an
intermediate valence Anderson model. The calculated magnetic susceptibility
displays one parameter scaling, going as ()
at intermediate temperatures, reminiscent of the non-Fermi liquid alloy
UCu_{5-x}Pd_x.Comment: 4 pages, 3 figures, REVTe
Quantum phase transitions
In recent years, quantum phase transitions have attracted the interest of
both theorists and experimentalists in condensed matter physics. These
transitions, which are accessed at zero temperature by variation of a
non-thermal control parameter, can influence the behavior of electronic systems
over a wide range of the phase diagram. Quantum phase transitions occur as a
result of competing ground state phases. The cuprate superconductors which can
be tuned from a Mott insulating to a d-wave superconducting phase by carrier
doping are a paradigmatic example. This review introduces important concepts of
phase transitions and discusses the interplay of quantum and classical
fluctuations near criticality. The main part of the article is devoted to bulk
quantum phase transitions in condensed matter systems. Several classes of
transitions will be briefly reviewed, pointing out, e.g., conceptual
differences between ordering transitions in metallic and insulating systems. An
interesting separate class of transitions are boundary phase transitions where
only degrees of freedom of a subsystem become critical; this will be
illustrated in a few examples. The article is aimed on bridging the gap between
high-level theoretical presentations and research papers specialized in certain
classes of materials. It will give an overview over a variety of different
quantum transitions, critically discuss open theoretical questions, and
frequently make contact with recent experiments in condensed matter physics.Comment: 50 pages, 7 figs; (v2) final version as publishe
Gemini Observations of Disks and Jets in Young Stellar Objects and in Active Galaxies
We present first results from the Near-infrared Integral Field Spectrograph
(NIFS) located at Gemini North. For the active galaxies Cygnus A and Perseus A
we observe rotationally-supported accretion disks and adduce the existence of
massive central black holes and estimate their masses. In Cygnus A we also see
remarkable high-excitation ionization cones dominated by photoionization from
the central engine. In the T-Tauri stars HV Tau C and DG Tau we see
highly-collimated bipolar outflows in the [Fe II] 1.644 micron line, surrounded
by a slower molecular bipolar outflow seen in the H_2 lines, in accordance with
the model advocated by Pyo et al. (2002).Comment: Invited paper presented at the 5th Stromlo Symposium. 9 pages, 7
figures. Accepted for publication in Astrophysics & Space Scienc
Magnetic-field dependence of the critical currents in a periodic coplanar array of narrow superconducting strip
We calculate the magnetic-field dependence of the critical current due to
both geometrical edge barriers and bulk pinning in a periodic coplanar array of
narrow superconducting strips. We find that in zero or low applied magnetic
fields the critical current can be considerably enhanced by the edge barriers,
but in modest applied magnetic fields the critical current reduces to that due
to bulk pinning alone.Comment: 23 pages, 7 figure
Systematic first-principles study of impurity hybridization in NiAl
We have performed a systematic first-principles computational study of the
effects of impurity atoms (boron, carbon, nitrogen, oxygen, silicon, phosporus,
and sulfur) on the orbital hybridization and bonding properties in the
intermetallic alloy NiAl using a full-potential linear muffin-tin orbital
method. The matrix elements in momentum space were used to calculate real-space
properties: onsite parameters, partial densities of states, and local charges.
In impurity atoms that are empirically known to be embrittler (N and O) we
found that the 2s orbital is bound to the impurity and therefore does not
participate in the covalent bonding. In contrast, the corresponding 2s orbital
is found to be delocalized in the cohesion enhancers (B and C). Each of these
impurity atoms is found to acquire a net negative local charge in NiAl
irrespective of whether they sit in the Ni or Al site. The embrittler therefore
reduces the total number of electrons available for covalent bonding by
removing some of the electrons from the neighboring Ni or Al atoms and
localizing them at the impurity site. We show that these correlations also hold
for silicon, phosporus, and sulfur.Comment: Revtex, 8 pages, 7 eps figures, to appear in Phys. Rev.
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