34,162 research outputs found
Total destruction of invariant tori for the generalized Frenkel-Kontorova model
We consider generalized Frenkel-Kontorova models on higher dimensional
lattices. We show that the invariant tori which are parameterized by continuous
hull functions can be destroyed by small perturbations in the topology
with
Model anisotropic quantum Hall states
Model quantum Hall states including Laughlin, Moore-Read and Read-Rezayi
states are generalized into appropriate anisotropic form. The generalized
states are exact zero-energy eigenstates of corresponding anisotropic two- or
multi-body Hamiltonians, and explicitly illustrate the existence of geometric
degrees of in the fractional quantum Hall effect. These generalized model
quantum Hall states can provide a good description of the quantum Hall system
with anisotropic interactions. Some numeric results of these anisotropic
quantum Hall states are also presented.Comment: 10 pages, 5 figure
Renormalization of the Sigma-Omega model within the framework of U(1) gauge symmetry
It is shown that the Sigma-Omega model which is widely used in the study of
nuclear relativistic many-body problem can exactly be treated as an Abelian
massive gauge field theory. The quantization of this theory can perfectly be
performed by means of the general methods described in the quantum gauge field
theory. Especially, the local U(1) gauge symmetry of the theory leads to a
series of Ward-Takahashi identities satisfied by Green's functions and proper
vertices. These identities form an uniquely correct basis for the
renormalization of the theory. The renormalization is carried out in the
mass-dependent momentum space subtraction scheme and by the renormalization
group approach. With the aid of the renormalization boundary conditions, the
solutions to the renormalization group equations are given in definite
expressions without any ambiguity and renormalized S-matrix elememts are
exactly formulated in forms as given in a series of tree diagrams provided that
the physical parameters are replaced by the running ones. As an illustration of
the renormalization procedure, the one-loop renormalization is concretely
carried out and the results are given in rigorous forms which are suitable in
the whole energy region. The effect of the one-loop renormalization is examined
by the two-nucleon elastic scattering.Comment: 32 pages, 17 figure
Ammonia as a possible element in an energy infrastructure: catalysts for ammonia decomposition
The possible role of ammonia in a future energy infrastructure is discussed. The review is focused on the catalytic decomposition of ammonia as a key step. Other aspects, such as the catalytic removal of ammonia from gasification product gas or direct ammonia fuel cells, are highlighted as well. The more general question of the integration of ammonia in an infrastructure is also covered
Compositional redistribution during casting of Hg sub 0.8 Cd sub 0.2 Te alloys
A series of Hg(0.8)Cd(0.2)Te ingots was cast both vertically and horizontally under well-defined thermal conditions by using a two-zone furnace with isothermal heat-pipe liners. The main objective of the experiments was to establish correlations between casting parameters and compositional redistribution and to develop ground-based data for a proposed flight experiment of casting of Hg(1-x)Cd(x)Te alloys under reduced gravity conditions. The compositional variations along the axial and radial directions were determined by precision density measurements, infrared transmission spectra, and X-ray energy dispersion spectrometry. Comparison between the experimental results and a numerical simulation of the solidification process of Hg(0.8)Cd(0.2)Te is described
Exploiting Cognitive Structure for Adaptive Learning
Adaptive learning, also known as adaptive teaching, relies on learning path
recommendation, which sequentially recommends personalized learning items
(e.g., lectures, exercises) to satisfy the unique needs of each learner.
Although it is well known that modeling the cognitive structure including
knowledge level of learners and knowledge structure (e.g., the prerequisite
relations) of learning items is important for learning path recommendation,
existing methods for adaptive learning often separately focus on either
knowledge levels of learners or knowledge structure of learning items. To fully
exploit the multifaceted cognitive structure for learning path recommendation,
we propose a Cognitive Structure Enhanced framework for Adaptive Learning,
named CSEAL. By viewing path recommendation as a Markov Decision Process and
applying an actor-critic algorithm, CSEAL can sequentially identify the right
learning items to different learners. Specifically, we first utilize a
recurrent neural network to trace the evolving knowledge levels of learners at
each learning step. Then, we design a navigation algorithm on the knowledge
structure to ensure the logicality of learning paths, which reduces the search
space in the decision process. Finally, the actor-critic algorithm is used to
determine what to learn next and whose parameters are dynamically updated along
the learning path. Extensive experiments on real-world data demonstrate the
effectiveness and robustness of CSEAL.Comment: Accepted by KDD 2019 Research Track. In Proceedings of the 25th ACM
SIGKDD International Conference on Knowledge Discovery & Data Mining (KDD'19
Understanding the Clean Interface between Covalent Si and Ionic Al2O3
The atomic and electronic structures of the (001)-Si/(001)-gamma-Al2O3
heterointerface are investigated by first principles total energy calculations
combined with a newly developed "modified basin-hopping" method. It is found
that all interface Si atoms are fourfold coordinated due to the formation of
Si-O and unexpected covalent Si-Al bonds in the new abrupt interface model. And
the interface has perfect electronic properties in that the unpassivated
interface has a large LDA band gap and no gap levels. These results show that
it is possible to have clean semiconductor-oxide interfaces
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