36,991 research outputs found
Agent-oriented Modeling for Collaborative Learning Environments: A Peer-to-Peer Helpdesk Case Study
In this paper, we present the analysis and modelling of Help&Learn, an agent-based peer-to-peer helpdesk system to support extra-class interactions among students and teachers. Help&Learn expands the student’s possibility of solving problems, getting involved in a cooperative learning experience that transcends the limits of classrooms. To model Help&Learn, we have used Agent-Object-Relationship Modeling Language (AORML), an UML extension for agent-oriented information systems modeling. The aim of this research is two-fold. On one hand, we aim at modeling the variety of roles and the complexity of their interactions and activities within the Help&Learn system. On the other hand, we aim at showing the expressive power and the modeling strengths of AORML
Thermal entanglement witness for materials with variable local spin lengths
We show that the thermal entanglement in a spin system using only magnetic
susceptibility measurements is restricted to the insulator materials. We
develop a generalization of the thermal entanglement witness that allows us to
get information about the system entanglement with variable local spin lengths
that can be used experimentally in conductor or insulator materials. As an
application, we study thermal entanglement for the half-filled Hubbard model
for linear, square and cubic clusters. We note that it is the itinerancy of
electrons that favors the entanglement. Our results suggest a weak dependence
between entanglement and external spin freedom degrees.Comment: 4 pages, 3 figure
Impact of Power Allocation and Antenna Directivity in the Capacity of a Multiuser Cognitive Ad Hoc Network
This paper studies the benefits that power control and antenna directivity can bring to the capacity of a multiuser cognitive radio network. The main objective is to optimize the secondary network sum rate under the capacity constraint of the primary network. Exploiting location awareness, antenna directivity, and the power control capability, the cognitive radio ad hoc network can broaden its coverage and improve capacity. Computer simulations show that by employing the proposed method the system performance is significantly enhanced compared to conventional fixed power allocation
Effects of geometric constraints on the nuclear multifragmentation process
We include in statistical model calculations the facts that in the nuclear
multifragmentation process the fragments are produced within a given volume and
have a finite size. The corrections associated with these constraints affect
the partition modes and, as a consequence, other observables in the process. In
particular, we find that the favored fragmenting modes strongly suppress the
collective flow energy, leading to much lower values compared to what is
obtained from unconstrained calculations. This leads, for a given total
excitation energy, to a nontrivial correlation between the breakup temperature
and the collective expansion velocity. In particular we find that, under some
conditions, the temperature of the fragmenting system may increase as a
function of this expansion velocity, contrary to what it might be expected.Comment: 16 pages, 5 figure
Statistical multifragmentation model with discretized energy and the generalized Fermi breakup. I. Formulation of the model
The Generalized Fermi Breakup recently demonstrated to be formally equivalent
to the Statistical Multifragmentation Model, if the contribution of excited
states are included in the state densities of the former, is implemented. Since
this treatment requires the application of the Statistical Multifragmentation
Model repeatedly on the hot fragments until they have decayed to their ground
states, it becomes extremely computational demanding, making its application to
the systems of interest extremely difficult. Based on exact recursion formulae
previously developed by Chase and Mekjian to calculate the statistical weights
very efficiently, we present an implementation which is efficient enough to
allow it to be applied to large systems at high excitation energies. Comparison
with the GEMINI++ sequential decay code shows that the predictions obtained
with our treatment are fairly similar to those obtained with this more
traditional model.Comment: 8 pages, 6 figure
Wannier-based calculation of the orbital magnetization in crystals
We present a first-principles scheme that allows the orbital magnetization of
a magnetic crystal to be evaluated accurately and efficiently even in the
presence of complex Fermi surfaces. Starting from an initial
electronic-structure calculation with a coarse ab initio k-point mesh,
maximally localized Wannier functions are constructed and used to interpolate
the necessary k-space quantities on a fine mesh, in parallel to a
previously-developed formalism for the anomalous Hall conductivity [X.Wang, J.
Yates, I. Souza, and D. Vanderbilt, Phys. Rev. B 74, 195118 (2006)]. We
formulate our new approach in a manifestly gauge-invariant manner, expressing
the orbital magnetization in terms of traces over matrices in Wannier space.
Since only a few (e.g., of the order of 20) Wannier functions are typically
needed to describe the occupied and partially occupied bands, these Wannier
matrices are small, which makes the interpolation itself very efficient. The
method has been used to calculate the orbital magnetization of bcc Fe, hcp Co,
and fcc Ni. Unlike an approximate calculation based on integrating orbital
currents inside atomic spheres, our results nicely reproduce the experimentally
measured ordering of the orbital magnetization in these three materials.Comment: 13 pages, 3 figures, 4 table
Analytical Multi-kinks in smooth potentials
In this work we present an approach which can be systematically used to
construct nonlinear systems possessing analytical multi-kink profile
configurations. In contrast with previous approaches to the problem, we are
able to do it by using field potentials which are considerably smoother than
the ones of Doubly Quadratic family of potentials. This is done without losing
the capacity of writing exact analytical solutions. The resulting field
configurations can be applied to the study of problems from condensed matter to
brane world scenarios
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