422 research outputs found
Current-sensitive single-gun color cathode ray tube
Nonlinear phosphors for production of current sensitive single gun color cathode ray tube
Bosonic sector of the two-dimensional Hubbard model studied within a two-pole approximation
The charge and spin dynamics of the two-dimensional Hubbard model in the
paramagnetic phase is first studied by means of the two-pole approximation
within the framework of the Composite Operator Method. The fully
self-consistent scheme requires: no decoupling, the fulfillment of both Pauli
principle and hydrodynamics constraints, the simultaneous solution of fermionic
and bosonic sectors and a very rich momentum dependence of the response
functions. The temperature and momentum dependencies, as well as the dependency
on the Coulomb repulsion strength and the filling, of the calculated charge and
spin susceptibilities and correlation functions are in very good agreement with
the numerical calculations present in the literature
Time scale competition leading to fragmentation and recombination transitions in the coevolution of network and states
We study the co-evolution of network structure and node states in a model of
multiple state interacting agents. The system displays two transitions, network
recombination and fragmentation, governed by time scales that emerge from the
dynamics. The recombination transition separates a frozen configuration,
composed by disconnected network components whose agents share the same state,
from an active configuration, with a fraction of links that are continuously
being rewired. The nature of this transition is explained analytically as the
maximum of a characteristic time. The fragmentation transition, that appears
between two absorbing frozen phases, is an anomalous order-disorder transition,
governed by a crossover between the time scales that control the structure and
state dynamics.Comment: 5 pages, 5 figures, figures 2 and 4 changed, tile changed, to be
published in PR
A Study of the Antiferromagnetic Phase in the Hubbard Model by means of the Composite Operator Method
We have investigated the antiferromagnetic phase of the 2D, the 3D and the
extended Hubbard models on a bipartite cubic lattice by means of the Composite
Operator Method within a two-pole approximation. This approach yields a fully
self-consistent treatment of the antiferromagnetic state that respects the
symmetry properties of both the model and the algebra. The complete phase
diagram, as regards the antiferromagnetic and the paramagnetic phases, has been
drawn. We firstly reported, within a pole approximation, three kinds of
transitions at half-filling: Mott-Hubbard, Mott-Heisenberg and Heisenberg. We
have also found a metal-insulator transition, driven by doping, within the
antiferromagnetic phase. This latter is restricted to a very small region near
half filling and has, in contrast to what has been found by similar approaches,
a finite critical Coulomb interaction as lower bound at half filling. Finally,
it is worth noting that our antiferromagnetic gap has two independent
components: one due to the antiferromagnetic correlations and another coming
from the Mott-Hubbard mechanism.Comment: 20 pages, 37 figures, RevTeX, submitted to Phys. Rev.
High-order correlation effects in the two-dimensional Hubbard model
The electronic states of the two-dimensional Hubbard model are investigated
by means of a 4-pole approximation within the Composite Operator Method. In
addition to the conventional Hubbard operators, we consider other two operators
which come from the hierarchy of the equations of motion and carry information
regarding nearest-neighbor spin and charge configurations. By means of this
operatorial basis, we can study the physics related to the energy scale of
J=4t^2/U in addition to the one of U. Present results show relevant physical
features, well beyond those previously obtained by means of a 2-pole
approximation, such as a four-band structure with shadow bands and a
quasi-particle peak at the Fermi level. The Fermi level stays pinned to the
band flatness located at (pi,0)-point within a wide range of hole-doping (0 <=
delta <= 0.15). A comprehensive analysis of double occupancy, internal energy,
specific heat and entropy features have been also performed. All reported
results are in excellent agreement with the data of numerical simulations.Comment: 13 pages, 8 figure
Jet fuel property changes and their effect on producibility and cost in the U.S., Canada, and Europe
The effects of changes in properties and blending stocks on the refinery output and cost of jet fuel in the U.S., Canada, and Europe were determined. Computerized refinery models that minimize production costs and incorporated a 1981 cost structure and supply/demand projections to the year 2010 were used. Except in the West U.S., no changes in jet fuel properties were required to meet all projected demands, even allowing for deteriorating crude qualities and changes in competing product demand. In the West U.S., property changes or the use of cracked blendstocks were projected to be required after 1990 to meet expected demand. Generally, relaxation of aromatics and freezing point, or the use of cracked stocks produced similar results, i.e., jet fuel output could be increased by up to a factor of three or its production cost lowered by up to $10/cu m. High quality hydrocracked stocks are now used on a limited basis to produce jet fuel. The conversion of U.S. and NATO military forces from wide-cut to kerosene-based jet fuel is addressed. This conversion resulted in increased costs of several hundred million dollars annually. These costs can be reduced by relaxing kerosene jet fuel properties, using cracked stocks and/or considering the greater volumetric energy content of kerosene jet fuel
The internationalisation of the Spanish SME sector
As part of a wider research program, we analysed the theoretical framework and the recent developments of the process of internationalisation (transnationalisation) of the small- and medium-sized enterprises in Spain. The paper highlights the main trends and barriers of this internationalisation process. Methodology included document analyses, interviews, and the analyses of statistical databases
Tracking spin and charge with spectroscopy in spin-polarised 1D systems
We calculate the spectral function of a one-dimensional strongly interacting
chain of fermions, where the response can be well understood in terms of spinon
and holon excitations. Upon increasing the spin imbalance between the spin
species, we observe the single-electron response of the fully polarised system
to emanate from the holon peak while the spinon response vanishes. For
experimental setups that probe one-dimensional properties, we propose this
method as an additional generic tool to aid the identification of spectral
structures, e.g. in ARPES measurements. We show that this applies even to
trapped systems having cold atomic gas experiments in mind.Comment: 5 pages, 4 figure
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
Behavioral Modernity and the Cultural Transmission of Structured Information: The Semantic Axelrod Model
Cultural transmission models are coming to the fore in explaining increases
in the Paleolithic toolkit richness and diversity. During the later
Paleolithic, technologies increase not only in terms of diversity but also in
their complexity and interdependence. As Mesoudi and O'Brien (2008) have shown,
selection broadly favors social learning of information that is hierarchical
and structured, and multiple studies have demonstrated that teaching within a
social learning environment can increase fitness. We believe that teaching also
provides the scaffolding for transmission of more complex cultural traits.
Here, we introduce an extension of the Axelrod (1997} model of cultural
differentiation in which traits have prerequisite relationships, and where
social learning is dependent upon the ordering of those prerequisites. We
examine the resulting structure of cultural repertoires as learning
environments range from largely unstructured imitation, to structured teaching
of necessary prerequisites, and we find that in combination with individual
learning and innovation, high probabilities of teaching prerequisites leads to
richer cultural repertoires. Our results point to ways in which we can build
more comprehensive explanations of the archaeological record of the Paleolithic
as well as other cases of technological change.Comment: 24 pages, 7 figures. Submitted to "Learning Strategies and Cultural
Evolution during the Paleolithic", edited by Kenichi Aoki and Alex Mesoudi,
and presented at the 79th Annual Meeting of the Society for American
Archaeology, Austin TX. Revised 5/14/1
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