67,509 research outputs found
Does International Trade Synchronize Business Cycles?
This paper studies the relationship between international trade and output fluctuations. The authors find evidence that the business cycles of countries that are more open to international trade are more likely to by synchronized with the business cycles of their major trading partners. A detailed study of the South Korean case shows that while business cycles are related to openness, the diversification of export destinations seems to weaken these links. The authors find no relationship between openness and output volatility.Coherence; Volatility; Business Cycles; Time Series
Inactivation of pathogens on food and contact surfaces using ozone as a biocidal agent
This study focuses on the inactivation of a range of food borne pathogens using ozone as a biocidal agent. Experiments were carried out using Campylobacter jejuni, E. coli and Salmonella enteritidis in which population size effects and different treatment temperatures were investigate
A Decidable Confluence Test for Cognitive Models in ACT-R
Computational cognitive modeling investigates human cognition by building
detailed computational models for cognitive processes. Adaptive Control of
Thought - Rational (ACT-R) is a rule-based cognitive architecture that offers a
widely employed framework to build such models. There is a sound and complete
embedding of ACT-R in Constraint Handling Rules (CHR). Therefore analysis
techniques from CHR can be used to reason about computational properties of
ACT-R models. For example, confluence is the property that a program yields the
same result for the same input regardless of the rules that are applied.
In ACT-R models, there are often cognitive processes that should always yield
the same result while others e.g. implement strategies to solve a problem that
could yield different results. In this paper, a decidable confluence criterion
for ACT-R is presented. It allows to identify ACT-R rules that are not
confluent. Thereby, the modeler can check if his model has the desired
behavior.
The sound and complete translation of ACT-R to CHR from prior work is used to
come up with a suitable invariant-based confluence criterion from the CHR
literature. Proper invariants for translated ACT-R models are identified and
proven to be decidable. The presented method coincides with confluence of the
original ACT-R models.Comment: To appear in Stefania Costantini, Enrico Franconi, William Van
Woensel, Roman Kontchakov, Fariba Sadri, and Dumitru Roman: "Proceedings of
RuleML+RR 2017". Springer LNC
The Raman Spectrum of Boron Trifluoride Gas
The Raman spectrum of BF3 was photographed using a purified preparation obtained from the thermal decomposition of C6H5N2BF4. Of the lines observed, that with the frequency 888 cm^—1 is certainly, and the band at 439–513 cm^—1 is probably due to BF3. The Raman frequencies and the infra-red results of Bailey et al. are assigned to the fundamental modes of vibrations
Spontaneous superconductivity and optical properties of high-Tc cuprates
We suggest that the high temperature superconductivity in cuprate compounds
may emerge due to interaction between copper-oxygen layers mediated by in-plane
plasmons. The strength of the interaction is determined by the c-axis geometry
and by the ab-plane optical properties. Without making reference to any
particular in-plane mechanism of superconductivity, we show that the interlayer
interaction favors spontaneous appearance of the superconductivity in the
layers. At a qualitative level the model describes correctly the dependence of
the transition temperature on the interlayer distance, and on the number of
adjacent layers in multilayered homologous compounds. Moreover, the model has a
potential to explain (i) a mismatch between the optimal doping levels for
critical temperature and superconducting density and (ii) a universal scaling
relation between the dc-conductivity, the superfluid density, and the
superconducting transition temperature.Comment: 4.4 pages, 2 figures; v2 matches the published version (clarifying
remarks and references are added
Bound pair states beyond the condensate for Fermi systems below T_c: the pseudogap as a necessary condition
As is known, the 1/q^2 theorem of Bogoliubov asserts that the mean density of
the fermion pair states with the total momentum q obeys the inequality n_q >
C/q^2 (q \to 0) in the case of the Fermi system taken at nonzero temperature
and in the superconducting state provided the interaction term of its
Hamiltonian is locally gauge invariant. With the principle of correlation
weakening it is proved in this paper that the reason for the mentioned singular
behaviour of n_q is the presence of the bound states of particle pairs with
nonzero total momenta. Thus, below the temperature of the superconducting phase
transition there always exist the bound states of the fermion couples beyond
the pair condensate. If the pseudogap observed in the normal phase of the
high-T_c superconductors is stipulated by the presence of the electron bound
pairs, then the derived result suggests, in a model-independent manner, that
the pseudogap survives below T_c.Comment: REVTeX, 8 pages, no figures, submitted to Phys. Rev.
Theory for Gossamer and Resonating Valence Bond Superconductivity
We use an effective Hamiltonian for two-dimensional Hubbard model including
an antiferromagnetic spin-spin coupling term to study recently proposed
gossamer superconductivity. We formulate a renormalized mean field theory to
approximately take into account the strong correlation effect in the partially
projected Gutzwiller wavefucntions. At the half filled, there is a first order
phase transition to separate a Mott insulator at large Coulomb repulsion U from
a gossamer superconductor at small U. Away from the half filled,the Mott
insulator is evolved into an resonating valence bond state, which is
adiabatically connected to the gossamer superconductor.Comment: 10 pages, 13 figure
Phase diagrams of correlated electrons: systematic corrections to the mean field theory
Perturbative corrections to the mean field theory for particle-hole
instabilities of interacting electron systems are computed within a scheme
which is equivalent to the recently developed variational approach to the
Kohn-Luttinger superconductivity. This enables an unbiased comparison of
particle-particle and particle-hole instabilities within the same approximation
scheme. A spin-rotation invariant formulation for the particle-hole
instabilities in the triplet channel is developed. The method is applied to the
phase diagram of the t-t' Hubbard model on the square lattice. At the Van Hove
density, antiferromagnetic and d-wave Pomeranchuk phases are found to be stable
close to half filling. However, the latter phase is confined to an extremely
narrow interval of densities and away from the singular filling, d-wave
superconducting instability dominates
Search for Ferromagnetism in doped semiconductors in the absence of transition metal ions
In contrast to semiconductors doped with transition metal magnetic elements,
which become ferromagnetic at temperatures below ~ 100K, semiconductors doped
with non-magnetic ions (e.g. silicon doped with phosphorous) have not shown
evidence of ferromagnetism down to millikelvin temperatures. This is despite
the fact that for low densities the system is expected to be well modeled by
the Hubbard model, which is predicted to have a ferromagnetic ground state at
T=0 on 2- or 3-dimensional bipartite lattices in the limit of strong
correlation near half-filling. We examine the impurity band formed by
hydrogenic centers in semiconductors at low densities, and show that it is
described by a generalized Hubbard model which has, in addition to strong
electron-electron interaction and disorder, an intrinsic electron-hole
asymmetry. With the help of mean field methods as well as exact diagonalization
of clusters around half filling, we can establish the existence of a
ferromagnetic ground state, at least on the nanoscale, which is more robust
than that found in the standard Hubbard model. This ferromagnetism is most
clearly seen in a regime inaccessible to bulk systems, but attainable in
quantum dots and 2D heterostructures. We present extensive numerical results
for small systems that demonstrate the occurrence of high-spin ground states in
both periodic and positionally disordered 2D systems. We consider how
properties of real doped semiconductors, such as positional disorder and
electron-hole asymmetry, affect the ground state spin of small 2D systems. We
also discuss the relationship between this work and diluted magnetic
semiconductors, such as Ga_(1-x)Mn_(x)As, which though disordered, show
ferromagnetism at relatively high temperatures.Comment: 47 page
Monotonicity and logarithmic convexity relating to the volume of the unit ball
Let stand for the volume of the unit ball in for
. In the present paper, we prove that the sequence
is logarithmically convex and that the sequence
is strictly
decreasing for . In addition, some monotonic and concave properties of
several functions relating to are extended and generalized.Comment: 12 page
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