35,976 research outputs found
Quantum localized modes in capacitively coupled Josephson junctions
We consider the quantum dynamics of excitations in a system of two
capacitively coupled Josephson junctions. Quantum breather states are found in
the middle of the energy spectrum of the confined nonescaping states of the
system. They are characterized by a strong excitation of one junction. These
states perform slow tunneling motion from one junction to the other, while
keeping their coherent nature. The tunneling time sensitively depends on the
initial excitation energy. By using an external bias as a control parameter,
the tunneling time can be varied with respect to the escape time and the
experimentally limited coherence time. Thus one can control the flow of quantum
excitations between the two junctions.Comment: 5 pages, 3 figures. Improved version, title was slightly changed.
Accepted in Europhysics Letters (http://www.iop.org/EJ/journal/EPL
Explosive synchronization with partial degree-frequency correlation
Networks of Kuramoto oscillators with a positive correlation between the
oscillators frequencies and the degree of the their corresponding vertices
exhibits the so-called explosive synchronization behavior, which is now under
intensive investigation. Here, we study and report explosive synchronization in
a situation that has not yet been considered, namely when only a part,
typically small, of the vertices is subjected to a degree frequency
correlation. Our results show that in order to have explosive synchronization,
it suffices to have degree-frequency correlations only for the hubs, the
vertices with the highest degrees. Moreover, we show that a partial
degree-frequency correlation does not only promotes but also allows explosive
synchronization to happen in networks for which a full degree-frequency
correlation would not allow it. We perform exhaustive numerical experiments for
synthetic networks and also for the undirected and unweighted version of the
neural network of the worm Caenorhabditis elegans. The latter is an explicit
example where partial degree-frequency correlation leads to explosive
synchronization with hysteresis, in contrast with the fully correlated case,
for which no explosive synchronization is observed.Comment: 10 pages, 6 figures, final version to appear in PR
Optimal synchronization of Kuramoto oscillators: a dimensional reduction approach
A recently proposed dimensional reduction approach for studying
synchronization in the Kuramoto model is employed to build optimal network
topologies to favor or to suppress synchronization. The approach is based in
the introduction of a collective coordinate for the time evolution of the phase
locked oscillators, in the spirit of the Ott-Antonsen ansatz. We show that the
optimal synchronization of a Kuramoto network demands the maximization of the
quadratic function , where stands for the vector of
the natural frequencies of the oscillators, and for the network Laplacian
matrix. Many recently obtained numerical results can be re-obtained
analytically and in a simpler way from our maximization condition. A
computationally efficient {hill climb} rewiring algorithm is proposed to
generate networks with optimal synchronization properties. Our approach can be
easily adapted to the case of the Kuramoto models with both attractive and
repulsive interactions, and again many recent numerical results can be
rederived in a simpler and clearer analytical manner.Comment: 6 pages, 6 figures, final version to appear in PR
Quantum breathers in capacitively coupled Josephson junctions: Correlations, number conservation, and entanglement
We consider the classical and quantum dynamics of excitations in a system of
two capacitively coupled Josephson junctions. In the classical case the
equations of motion admit discrete breather solutions, which are time periodic
and localized predominantly on one of the junctions. In the quantum case
breather states are found in the central part of the energy spectrum of the
confined nonescaping states of the system. We perform a systematic analysis of
their tunneling frequency, site correlations, fluctuations of the number of
quanta, and entanglement. Quantum breather states show strong site correlation
of quanta and are characterized by a strong excitation of quanta on one
junction which perform slow coherent tunneling motion from one junction to the
other. They suppress fluctuations of the total number of excited quanta.
Quantum breather states are the least entangled states among the group of
eigenstates in the same range of the energy spectrum. We describe how quantum
breather excitations could be experimentally observed by employing the already
developed techniques for quantum information processing using Josephson
junctions.Comment: 10 pages, 9 figures. Improved version with further discussions.
Accepted in Physical Review
Discontinuous Almost Automorphic Functions and Almost Automorphic Solutions of Differential Equations with Piecewise Constant Argument
In this article we introduce a class of discontinuous almost automorphic
functions which appears naturally in the study of almost automorphic solutions
of differential equations with piecewise constant argument. Their fundamental
properties are used to prove the almost automorphicity of bounded solutions of
a system of differential equations with piecewise constant argument. Due to the
strong discrete character of these equations, the existence of a unique
discrete almost automorphic solution of a non-autonomous almost automorphic
difference system is obtained, for which conditions of exponential dichotomy
and discrete Bi-almost automorphicity are fundamental
Flux emergence in a magnetized convection zone
We study the influence of a dynamo magnetic field on the buoyant rise and
emergence of twisted magnetic flux-ropes, and their influence on the global
external magnetic field. We ran 3D MHD numerical simulations using the ASH code
and analysed the dynamical evolution of such buoyant flux-ropes from the bottom
of the convection zone until the post-emergence phases. The global nature of
this model represents very crudely and inaccurately the local dynamics of the
buoyant rise, but allows to study the influence of global effects such as
self-consistently generated differential rotation, meridional circulation and
Coriolis forces. Although motivated by the solar context, this model cannot be
thought of as a realistic model of the rise of magnetic structures and their
emergence in the Sun where the local dynamics are completely different. The
properties of initial phases of the buoyant rise in good agreement with
previous studies. However, the effects of the interaction of the background
dynamo field become increase as the flux-ropes evolve. During the buoyant rise
across the CZ, the flux-rope's magnetic field strength and scales as ,
with . An increase of velocity, density and current precedes flux
emergence at all longitudes. The geometry, latitude and relative orientation of
the flux-ropes with respect to the background magnetic field influences the
rise speeds, zonal flow amplitudes (which develop within the flux-ropes) and
the corresponding surface signatures. This influences the morphology, duration
and amplitude of the associated surface shearing and Poynting flux. The emerged
flux influences the system's global polarity, leading in some cases to a
polarity reversal while inhibiting background dynamo from doing so in some
others. The emerged magnetic flux is slowly advected poleward, while being
diffused and assimilated by the background dynamo field.Comment: Accepted for publication in Ap
Modelling a layer for real-time management of interactions in web based distance learning
In the last few years, the University of Aveiro, Portugal, has been offering several distance learning courses
over the Web, using e-learning platforms.
Experience showed that different editions of a same course, using the same contents and structure, and having
similar target learners, had different success rates. What would be the reason for that?
A hypothesis was considered: The level of success could be directly related with the remote follow-up of the
learners’ participation in the courses; the best results usually occur when the follow-up is closer.
The existing e-learning platforms offer and the standardization works being developed by organizations and
consortiums like IMS (IMS Global Learning Consortium, Inc), ADL SCORM (Advanced Distributed Learning
Sherable Content Object Reference Model), IEEE LTSC LOM (Institute of Electrical and Electronic Engineers
Learning Technologies Standard Committee Learning Object Metadata), ARIADNE (ARIADNE Foundation
for the European Knowledge Pool), AICC CMI (Aviation Industry CBT Committee Computer Managed
Instruction), etc, don’t cover the course monitorization concerns mentioned. Those projects were focused on
aspects like contents and its delivery in the context of the execution of the courses’ activities. This is even
true in the SCORM project that doesn’t include any reference to the management of the e-learning processes.
Recently, in the context of the IMS Global Consortium, a new project designated IMS LD (Learning Design) is
under development, providing a framework for the description of learning units under a three level model. In
the most recently defined level, the C level, some functionalities related to notifications were proposed,
expressing similar concerns to the ones that triggered our research. However, the extent at which IMS LD
takes the functionalities is, from our point of view, not complete.
This article describes a proposal of a reference model and functionalities towards a specification of a layer for
real-time management of user interactions on LMSs, and its possible integration with the ADL SCORM
standard proposal. The paper includes a discussion of the management metadata model for the LMS
sub-system and how the integration of the management module under SCORM may be achieved
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