1,655 research outputs found
A wide band gap metal-semiconductor-metal nanostructure made entirely from graphene
A blueprint for producing scalable digital graphene electronics has remained
elusive. Current methods to produce semiconducting-metallic graphene networks
all suffer from either stringent lithographic demands that prevent
reproducibility, process-induced disorder in the graphene, or scalability
issues. Using angle resolved photoemission, we have discovered a unique one
dimensional metallic-semiconducting-metallic junction made entirely from
graphene, and produced without chemical functionalization or finite size
patterning. The junction is produced by taking advantage of the inherent,
atomically ordered, substrate-graphene interaction when it is grown on SiC, in
this case when graphene is forced to grow over patterned SiC steps. This
scalable bottomup approach allows us to produce a semiconducting graphene strip
whose width is precisely defined within a few graphene lattice constants, a
level of precision entirely outside modern lithographic limits. The
architecture demonstrated in this work is so robust that variations in the
average electronic band structure of thousands of these patterned ribbons have
little variation over length scales tens of microns long. The semiconducting
graphene has a topologically defined few nanometer wide region with an energy
gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.
This work demonstrates how the graphene-substrate interaction can be used as a
powerful tool to scalably modify graphene's electronic structure and opens a
new direction in graphene electronics research.Comment: 11 pages, 7 figure
Evaluación del dispositivo Pavement Quality Indicator (PQI) en la determinación de la densidad in situ de mezclas fabricadas con emulsión bituminosa
The Pavement Quality Indicator (PQI) is a non-nuclear gauge used for the on-site density measurement of asphalt pavements without the need to extract core samples. Previous studies of hot asphalt mixes found that PQI density readings were very similar to laboratory density measurements of pavement cores. This paper describes the first stage of a research project whose objective is to analyze PQI density measurements of mixes manufactured with an asphalt emulsion binder. The PQI density variability for such mixes was verified and compared with the results obtained with other on-site methods for measuring pavement density.El equipo Pavement Quality Indicator es un dispositivo para la determinación de densidad in situ en pavimentos asfálticos sin extracción de testigos. Las experiencias con este equipo en mezclas bituminosas en caliente, recogidas en diferentes fuentes bibliográficas, muestran que las densidades medidas in situ con el PQI son muy similares a las obtenidas mediante la extracción de testigos. En este artÃculo se expone la primera etapa de un proyecto de investigación que tiene por objeto analizar los resultados de mediciones efectuadas con PQI en mezclas bituminosas donde se utiliza emulsión asfáltica como ligante. Se comprueba la variabilidad de la densidad obtenida con el equipo para este tipo de mezclas, y se comparan los resultados con otros métodos de medida de densidad in situ
Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment
Graphene stacked in a Bernal configuration (60 degrees relative rotations
between sheets) differs electronically from isolated graphene due to the broken
symmetry introduced by interlayer bonds forming between only one of the two
graphene unit cell atoms. A variety of experiments have shown that non-Bernal
rotations restore this broken symmetry; consequently, these stacking varieties
have been the subject of intensive theoretical interest. Most theories predict
substantial changes in the band structure ranging from the development of a Van
Hove singularity and an angle dependent electron localization that causes the
Fermi velocity to go to zero as the relative rotation angle between sheets goes
to zero. In this work we show by direct measurement that non-Bernal rotations
preserve the graphene symmetry with only a small perturbation due to weak
effective interlayer coupling. We detect neither a Van Hove singularity nor any
significant change in the Fermi velocity. These results suggest significant
problems in our current theoretical understanding of the origins of the band
structure of this material.Comment: 7 pages, 6 figures, submitted to PR
Mixing Effects in the Crystallization of Supercooled Quantum Binary Liquids
By means of Raman spectroscopy of liquid microjets we have investigated the
crystallization process of supercooled quantum liquid mixtures composed of
parahydrogen (pH) diluted with small amounts of up to 5\% of either neon or
orthodeuterium (oD), and of oD diluted with either Ne or pH. We
show that the introduction of Ne impurities affects the crystallization
kinetics in both the pH-Ne and oD-Ne mixtures in terms of a significant
reduction of the crystal growth rate, similarly to what found in our previous
work on supercooled pH-oD liquid mixtures [M. K\"uhnel et {\it al.},
Phys. Rev. B \textbf{89}, 180506(R) (2014)]. Our experimental results, in
combination with path-integral simulations of the supercooled liquid mixtures,
suggest in particular a correlation between the measured growth rates and the
ratio of the effective particle sizes originating from quantum delocalization
effects. We further show that the crystalline structure of the mixture is also
affected to a large extent by the presence of the Ne impurities, which likely
initiate the freezing process through the formation of Ne crystallites.Comment: 19 pages, 7 figures, submitted to J. Chem. Phy
Solidification of small para-H2 clusters at zero temperature
We have determined the ground-state energies of para-H clusters at zero
temperature using the diffusion Monte Carlo method. The liquid or solid
character of each cluster is investigated by restricting the phase through the
use of proper importance sampling. Our results show inhomogeneous
crystallization of clusters, with alternating behavior between liquid and solid
phases up to N=55. From there on, all clusters are solid. The ground-state
energies in the range N=13--75 are established and the stable phase of each
cluster is determined. In spite of the small differences observed between the
energy of liquid and solid clusters, the corresponding density profiles are
significantly different, feature that can help to solve ambiguities in the
determination of the specific phase of H clusters.Comment: 17 pages, accepted for publication in J. Phys. Chem.
Silicon intercalation into the graphene-SiC interface
In this work we use LEEM, XPEEM and XPS to study how the excess Si at the
graphene-vacuum interface reorders itself at high temperatures. We show that
silicon deposited at room temperature onto multilayer graphene films grown on
the SiC(000[`1]) rapidly diffuses to the graphene-SiC interface when heated to
temperatures above 1020. In a sequence of depositions, we have been able to
intercalate ~ 6 ML of Si into the graphene-SiC interface.Comment: 6 pages, 8 figures, submitted to PR
Vacuum Polarization and Dynamical Chiral Symmetry Breaking: Phase Diagram of QED with Four-Fermion Contact Interaction
We study chiral symmetry breaking for fundamental charged fermions coupled
electromagnetically to photons with the inclusion of four-fermion contact
self-interaction term. We employ multiplicatively renormalizable models for the
photon dressing function and the electron-photon vertex which minimally ensures
mass anomalous dimension = 1. Vacuum polarization screens the interaction
strength. Consequently, the pattern of dynamical mass generation for fermions
is characterized by a critical number of massless fermion flavors above which
chiral symmetry is restored. This effect is in diametrical opposition to the
existence of criticality for the minimum interaction strength necessary to
break chiral symmetry dynamically. The presence of virtual fermions dictates
the nature of phase transition. Miransky scaling laws for the electromagnetic
interaction strength and the four-fermion coupling, observed for quenched QED,
are replaced by a mean-field power law behavior corresponding to a second order
phase transition. These results are derived analytically by employing the
bifurcation analysis, and are later confirmed numerically by solving the
original non-linearized gap equation. A three dimensional critical surface is
drawn to clearly depict the interplay of the relative strengths of interactions
and number of flavors to separate the two phases. We also compute the
beta-function and observe that it has ultraviolet fixed point. The power law
part of the momentum dependence, describing the mass function, reproduces the
quenched limit trivially. We also comment on the continuum limit and the
triviality of QED.Comment: 9 pages, 10 figure
Determinación de neofitadieno en grasa subcutánea de cerdo Ibérico con diferentes sistemas de alimentación
This study was carried out in order to determine the relationship between the content of a branched hydrocarbon (neophytadiene) in the subcutaneous fat and the feeding system of Iberian pigs during the fattening period previous to slaughter. Thirty-six batches of Iberian pigs, belonging to the four categories of feeding systems described in the Spanish legislation (Bellota, Recebo, Campo and Cebo) were studied. These categories differ in their degree of utilization of the natural resources (acorn and pasture) of the extensive rearing system or in their level of replacement by concentrated feed. SPE combined with GC-MS techniques were carried out to isolate and to identify hydrocarbons. Recebo and Bellota batches showed the highest levels of neophytadiene content, followed by Campo batches, and finally, batches of the Cebo category. The results showed a great variability between batches from the same feeding category, mainly from Bellota, Recebo and Campo categories, which makes the classification of the animals in these three categories difficult. However, the animals from the Cebo category were correctly classified when reared in intensive conditions.El objetivo del trabajo se centró en determinar el contenido de un hidrocarburo ramificado (neofitadieno) en la grasa subcutánea del cerdo ibérico, y su relación con el tipo de alimentación recibida durante la etapa de cebo previa al sacrificio (en la fase final de cebo). Para ello, se estudiaron treinta y seis lotes de cerdos Ibéricos, pertenecientes a las cuatro categorÃas de calidad en base a la alimentación de los animales establecidas en la Norma de Calidad (Bellota, Recebo, Campo y Cebo). Estas categorÃas se diferencian en el grado de aprovechamiento de los recursos naturales de la dehesa (bellotas y hierba) y en el nivel de sustitución de los mismos por piensos concentrados. Para el aislamiento e identificación de los hidrocarburos se utilizaron técnicas de SPE combinadas con GC-MS. Los lotes de Recebo y Bellota presentaron los mayores niveles de neofitadieno, seguidos por los lotes de Campo, y por último los lotes de Cebo. Se observó una gran variabilidad en los niveles de neofitadieno entre los lotes pertenecientes a una misma categorÃa de alimentación, sobre todo en los lotes de Bellota, Recebo y Campo, por lo que es difÃcil una correcta clasificación de los animales en las tres categorÃas en base al contenido en neofitadieno. Sin embargo, la clasificación dentro de la categorÃa de cebo se hace de forma correcta cuando los cerdos son producidos en condiciones intensivas
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