15,775 research outputs found
The Vector Curvaton
We analyze a massive vector field with a non-canonical kinetic term in the
action, minimally coupled to gravity, where the mass and kinetic function of
the vector field vary as functions of time during inflation. The vector field
is introduced following the same idea of a scalar curvaton, which must not
affect the inflationary dynamics since its energy density during inflation is
negligible compared to the total energy density in the Universe. Using this
hypothesis, the vector curvaton will be solely responsible for generating the
primordial curvature perturbation \zeta. We have found that the spectra of the
vector field perturbations are scale-invariant in superhorizon scales due to
the suitable choice of the time dependence of the kinetic function and the
effective mass during inflation. The preferred direction, generated by the
vector field, makes the spectrum of \zeta depend on the wavevector, i.e. there
exists statistical anisotropy in \zeta. This is discussed principally in the
case where the mass of the vector field increases with time during inflation,
where it is possible to find a heavy field (M >> H) at the end of inflation,
making the particle production be practically isotropic; thus, the longitudinal
and transverse spectra are nearly the same order which in turn causes that the
statistical anisotropy generated by the vector field is within the
observational bounds.Comment: LaTex file in Aipproc style, 6 pages, no figures. Prepared for the
conference proceedings of the IX Mexican School of the DGFM-SMF: Cosmology
for the XXIst Century. This work is entirely based on Refs. [23-26] and is
the result of Andres A. Navarro's MSc thesi
On the Credibility of the Irish Pound in the EMS
This paper assesses the degree of credibility of the Irish Pound in the European Monetary System between 1983 and 1997. Different credibility indicators proposed in the literature are used to measure agentsâ perceptions of the credibility of the ERM commitment in an attempt to distinguish between events stemming from problems in the ERM itself and those that appear to have been exclusive to Ireland.
Uncertain research country rankings. Should we continue producing uncertain rankings?
Citation based country rankings consistently categorize Japan as a developing
country, even in those from the most reputed institutions. This categorization
challenges the credibility of such rankings, considering Japan elevated
scientific standing. In most cases, these rankings use percentile indicators
and are accurate if country citations fit an ideal model of distribution, but
they can be misleading in cases of deviations. The ideal model implies a
lognormal citation distribution and a power law citation based double rank: in
the global and country lists. This report conducts a systematic examination of
deviations from the ideal model and their consequential impact on evaluations.
The study evaluates six selected countries across three scientifically relevant
topics and utilizes Leiden Ranking assessments of over 300 universities. The
findings reveal three types of deviations from the lognormal citation
distribution: i deviations in the extreme upper tail; ii inflated lower tails;
and iii deflated lower part of the distributions. These deviations stem from
structural differences among research systems that are prevalent and have the
potential to mislead evaluations across all research levels. Consequently,
reliable evaluations must consider these deviations. Otherwise, while some
countries and institutions will be correctly evaluated, failure to identify
deviations in each specific country or institution will render uncertain
evaluations. For reliable assessments, future research evaluations of countries
and institutions must identify deviations from the ideal model.Comment: 29 pages, 6 figures, 5 table
Countries pushing the boundaries of knowledge: the US dominance, China rise, and the EU stagnation
Knowing which countries contribute the most to pushing the boundaries of
knowledge in science and technology has social and political importance.
However, common citation metrics do not adequately measure this contribution.
This measure requires more stringent metrics appropriate for the highly
influential breakthrough papers that push the boundaries of knowledge, which
are very highly cited but very rare. Here I used the recently described Rk
index, specifically designed to address this issue. I applied this index to 25
countries and the EU across 10 key research topics, five technological and five
biomedical, studying domestic and international collaborative papers
independently. In technological topics, the Rk indices of domestic papers show
that overall, the USA, China, and the EU are leaders; other countries are
clearly behind. The USA is notably ahead of China, and the EU is far behind
China. The same approach to biomedical topics shows an overwhelming dominance
of the USA and that the EU is ahead of China. The analysis of internationally
collaborative papers further demonstrates the US dominance. These results
conflict with current country rankings based on less stringent indicators.Comment: 18 pages, 1 figure, 6 table
Numerical study of electrostatically-defined quantum dots in bilayer graphene
MĂ ster Oficial de CiĂšncia i Tecnologia QuĂ ntiques / Quantum Science and Technology, Facultat de FĂsica, Universitat de Barcelona. Curs: 2022-2023. Tutor: Iacopo TorreInteracting quantum many-body systems are so challenging to study that even simplified models, such as the Hubbard model, cannot be solved exactly.
For this reason, it is interesting to engineer controllable quantum systems, called quantum simulators, that can emulate the behavior of these models.
This makes quantum simulators a promising platform for studying the Hubbard model. These can be implemented, for example, using interacting arrays of quantum dots realized in semiconducting materials.
The capability to tune the bands in bilayer graphene with patterned gate electrodes provides an innovative platform to study such a model, as it is the first time to explore the Hubbard model with quantum dots in a twodimensional material. Moreover, this platform opens a wide range of possibilities to study the different parameters of the model.
In this work, we study theoretically and numerically realistic models of electrostatically defined quantum dots in bilayer graphene. We can calculate the proposed deviceâs potential and band-gap landscape induced in bilayer
graphene by solving the Poisson equation. The result is then fed to a lowenergy model to calculate the bound states of the quantum dots. This allows calculating the parameters of the corresponding Hubbard model, including tunneling amplitudes and on-site interactions. Our results can be directly used to design quantum-simulation devices based on quantum dots that are realized electrostatically in bilayer graphene
Technological research in the EU is less efficient than the world average. EU research policy risks Europeans' future
We have studied the efficiency of research in the EU by a percentile-based
citation approach that analyzes the distribution of country papers among the
world papers. Going up in the citation scale, the frequency of papers from
efficient countries increases while the frequency from inefficient countries
decreases. In the percentile-based approach, this trend, which is permanent at
any citation level, is measured by the ep index that equals the Ptop 1%/Ptop
10% ratio. By using the ep index we demonstrate that EU research on
fast-evolving technological topics is less efficient than the world average and
that the EU is far from being able to compete with the most advanced countries.
The ep index also shows that the USA is well ahead of the EU in both fast- and
slow-evolving technologies, which suggests that the advantage of the USA over
the EU in innovation is due to low research efficiency in the EU. In accord
with some previous studies, our results show that the European Commission's
ongoing claims about the excellence of EU research are based on a wrong
diagnosis. The EU must focus its research policy on the improvement of its
inefficient research. Otherwise, the future of Europeans is at risk.Comment: 30 pages, 3 figures, 7 tables, in one single file. Version accepted
in Journal of Informetric
Common bibliometric approaches fail to assess correctly the number of important scientific advances for most countries and institutions
Although not explicitly declared, most research rankings of countries and
institutions are supposed to reveal their contribution to the advancement of
knowledge. However, such advances are based on very highly cited publications
with very low frequency, which can only very exceptionally be counted with
statistical reliability. Percentile indicators enable calculations of the
probability or frequency of such rare publications using counts of much more
frequent publications; the general rule is that rankings based on the number of
top 10% or 1% cited publications (Ptop 10%, Ptop 1%) will also be valid for the
rare publications that push the boundaries of knowledge. Japan and its
universities are exceptions, as their frequent Nobel Prizes contradicts their
low Ptop 10% and Ptop 1%. We explain that this occurs because, in single
research fields, the singularity of percentile indicators holds only for
research groups that are homogeneous in their aims and efficiency. Correct
calculations for ranking countries and institutions should add the results of
their homogeneous groups, instead of considering all publications as a single
set. Although based on Japan, our findings have a general character. Common
predictions of scientific advances based on Ptop 10% might be severalfold lower
than correct calculations.Comment: 30 pages, tables and figures embedded in a single pdf fil
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