3,044 research outputs found
EXTENSION BY CONSERVATION. SIKORSKI'S THEOREM
Constructive meaning is given to the assertion that every finite Boolean algebra is an injective object in the category of distributive lattices. To this end, we employ Scott's notion of entailment relation, in which context we describe Sikorski's extension theorem for finite Boolean algebras and turn it into a syntactical conservation result. As a by-product, we can facilitate proofs of related classical principles
Classical and quantum anisotropic Heisenberg antiferromagnets
We study classical and quantum Heisenberg antiferromagnets with exchange
anisotropy of XXZ-type and crystal field single-ion terms of quadratic and
cubic form in a field. The magnets display a variety of phases, including the
spin-flop (or, in the quantum case, spin-liquid) and biconical (corresponding,
in the quantum lattice gas description, to supersolid) phases. Applying
ground-state considerations, Monte Carlo and density matrix renormalization
group methods, the impact of quantum effects and lattice dimension is analysed.
Interesting critical and multicritical behaviour may occur at quantum and
thermal phase transitions.Comment: 13 pages, 14 figures, conferenc
Quantum Antiferromagnetism in Quasicrystals
The antiferromagnetic Heisenberg model is studied on a two-dimensional
bipartite quasiperiodic lattice. The distribution of local staggered magnetic
moments is determined on finite square approximants with up to 1393 sites,
using the Stochastic Series Expansion Quantum Monte Carlo method. A non-trivial
inhomogeneous ground state is found. For a given local coordination number, the
values of the magnetic moments are spread out, reflecting the fact that no two
sites in a quasicrystal are identical. A hierarchical structure in the values
of the moments is observed which arises from the self-similarity of the
quasiperiodic lattice. Furthermore, the computed spin structure factor shows
antiferromagnetic modulations that can be measured in neutron scattering and
nuclear magnetic resonance experiments.
This generic model is a first step towards understanding magnetic
quasicrystals such as the recently discovered Zn-Mg-Ho icosahedral structure.Comment: RevTex, 4 pages with 5 figure
An 8-cm ion thruster characterization
The performance of the Ion Auxiliary Propulsion System (IAPS) thruster was increased to thrust T = 32 mN, specific impulse I sub sp = 4062 s, and thrust-to-power ratio T/P = 33 mN/kW. This performance was obtained by increasing the discharge power, accelerating voltage, propellant flow rate, and chamber magnetic field. Adding a plenum and main vaporizer for propellant distribution was the only major change required in the thruster. The modified thruster characterization is presented. A cathode magnet assembly did not improve performance. A simplified power processing unit was designed and evaluated. This unit decreased the parts count of the IAPS power processing unit by a factor of ten
Dynamical structure factors and excitation modes of the bilayer Heisenberg model
Using quantum Monte Carlo simulations along with higher-order spin-wave
theory, bond-operator and strong-coupling expansions, we analyse the dynamical
spin structure factor of the spin-half Heisenberg model on the square-lattice
bilayer. We identify distinct contributions from the low-energy Goldstone modes
in the magnetically ordered phase and the gapped triplon modes in the quantum
disordered phase. In the antisymmetric (with respect to layer inversion)
channel, the dynamical spin structure factor exhibits a continuous evolution of
spectral features across the quantum phase transition, connecting the two types
of modes. Instead, in the symmetric channel we find a depletion of the spectral
weight when moving from the ordered to the disordered phase. While the
dynamical spin structure factor does not exhibit a well-defined distinct
contribution from the amplitude (or Higgs) mode in the ordered phase, we
identify an only marginally-damped amplitude mode in the dynamical singlet
structure factor, obtained from interlayer bond correlations, in the vicinity
of the quantum critical point. These findings provide quantitative information
in direct relation to possible neutron or light scattering experiments in a
fundamental two-dimensional quantum-critical spin system.Comment: 19 pages, 15 figure
Synchronization from Disordered Driving Forces in Arrays of Coupled Oscillators
The effects of disorder in external forces on the dynamical behavior of
coupled nonlinear oscillator networks are studied. When driven synchronously,
i.e., all driving forces have the same phase, the networks display chaotic
dynamics. We show that random phases in the driving forces result in regular,
periodic network behavior. Intermediate phase disorder can produce network
synchrony. Specifically, there is an optimal amount of phase disorder, which
can induce the highest level of synchrony. These results demonstrate that the
spatiotemporal structure of external influences can control chaos and lead to
synchronization in nonlinear systems.Comment: 4 pages, 4 figure
Twenty-One at TREC-8: using Language Technology for Information Retrieval
This paper describes the official runs of the Twenty-One group for TREC-8. The Twenty-One group participated in the Ad-hoc, CLIR, Adaptive Filtering and SDR tracks. The main focus of our experiments is the development and evaluation of retrieval methods that are motivated by natural language processing techniques. The following new techniques are introduced in this paper. In the Ad-Hoc and CLIR tasks we experimented with automatic sense disambiguation followed by query expansion or translation. We used a combination of thesaurial and corpus information for the disambiguation process. We continued research on CLIR techniques which exploit the target corpus for an implicit disambiguation, by importing the translation probabilities into the probabilistic term-weighting framework. In filtering we extended the use of language models for document ranking with a relevance feedback algorithm for query term reweightin
The generic mapping tools version 6
The Generic Mapping Tools (GMT) software is ubiquitous in the Earth and ocean sciences. As a cross-platform tool producing high-quality maps and figures, it is used by tens of thousands of scientists around the world. The basic syntax of GMT scripts has evolved very slowly since the 1990s, despite the fact that GMT is generally perceived to have a steep learning curve with many pitfalls for beginners and experienced users alike. Reducing these pitfalls means changing the interface, which would break compatibility with thousands of existing scripts. With the latest GMT version 6, we solve this conundrum by introducing a new "modern mode" to complement the interface used in previous versions, which GMT 6 now calls "classic mode." GMT 6 defaults to classic mode and thus is a recommended upgrade for all GMT 5 users. Nonetheless, new users should take advantage of modern mode to make shorter scripts, quickly access commonly used global data sets, and take full advantage of the new tools to draw subplots, place insets, and create animations.Funding Agency
National Science Foundation (NSF)
Appeared in article as
U.S. National Science Foundation
MSU Geological Sciences Endowmentinfo:eu-repo/semantics/publishedVersio
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