92,778 research outputs found
Structured Prediction of Sequences and Trees using Infinite Contexts
Linguistic structures exhibit a rich array of global phenomena, however
commonly used Markov models are unable to adequately describe these phenomena
due to their strong locality assumptions. We propose a novel hierarchical model
for structured prediction over sequences and trees which exploits global
context by conditioning each generation decision on an unbounded context of
prior decisions. This builds on the success of Markov models but without
imposing a fixed bound in order to better represent global phenomena. To
facilitate learning of this large and unbounded model, we use a hierarchical
Pitman-Yor process prior which provides a recursive form of smoothing. We
propose prediction algorithms based on A* and Markov Chain Monte Carlo
sampling. Empirical results demonstrate the potential of our model compared to
baseline finite-context Markov models on part-of-speech tagging and syntactic
parsing
An instability mechanism for particulate pipe flow
We present linear stability analysis for a simple model of particle-laden
pipe flow. The model consists of a continuum approximation for the particles
two-way coupled to the fluid velocity field via Stokes drag (Saffman 1962). We
extend previous analysis in a channel (Klinkenberg et al. 2011) to allow for
the initial distribution of particles to be inhomogeneous and in particular
consider the effect of allowing the particles to be preferentially located
around one radius in accordance with experimental observations. This simple
modification of the problem is enough to alter the stability properties of the
flow, and in particular can lead to a linear instability at experimentally
realistic parameters. The results are compared to the experimental work of
Matas et al. (2004a) and are shown to be consistent with the reported flow
regimes.Comment: 15 pages, 11 figure
Averaged null energy condition in Loop Quantum Cosmology
Wormhole and time machine are very interesting objects in general relativity.
However, they need exotic matters which are impossible in classical level to
support them. But if we introduce the quantum effects of gravity into the
stress-energy tensor, these peculiar objects can be constructed
self-consistently. Fortunately, loop quantum cosmology (LQC) has the potential
to serve as a bridge connecting the classical theory and quantum gravity.
Therefore it provides a simple way for the study of quantum effect in the
semiclassical case. As is well known, loop quantum cosmology is very successful
to deal with the behavior of early universe. In the early stage, if taken the
quantum effect into consideration, inflation is natural because of the
violation of every kind of local energy conditions. Similar to the inflationary
universe, the violation of the averaged null energy condition is the necessary
condition for the traversable wormholes. In this paper, we investigate the
averaged null energy condition in LQC in the framework of effective
Hamiltonian, and find out that LQC do violate the averaged null energy
condition in the massless scalar field coupled model.Comment: 5 page
Optical interface states protected by synthetic Weyl points
Weyl fermions have not been found in nature as elementary particles, but they
emerge as nodal points in the band structure of electronic and classical wave
crystals. Novel phenomena such as Fermi arcs and chiral anomaly have fueled the
interest in these topological points which are frequently perceived as
monopoles in momentum space. Here we report the experimental observation of
generalized optical Weyl points inside the parameter space of a photonic
crystal with a specially designed four-layer unit cell. The reflection at the
surface of a truncated photonic crystal exhibits phase vortexes due to the
synthetic Weyl points, which in turn guarantees the existence of interface
states between photonic crystals and any reflecting substrates. The reflection
phase vortexes have been confirmed for the first time in our experiments which
serve as an experimental signature of the generalized Weyl points. The
existence of these interface states is protected by the topological properties
of the Weyl points and the trajectories of these states in the parameter space
resembles those of Weyl semimetal "Fermi arcs surface states" in momentum
space. Tracing the origin of interface states to the topological character of
the parameter space paves the way for a rational design of strongly localized
states with enhanced local field.Comment: 36 pages, 9 figures. arXiv admin note: text overlap with
arXiv:1610.0434
Failure mechanisms of graphene under tension
Recent experiments established pure graphene as the strongest material known
to mankind, further invigorating the question of how graphene fails. Using
density functional theory, we reveal the mechanisms of mechanical failure of
pure graphene under a generic state of tension. One failure mechanism is a
novel soft-mode phonon instability of the -mode, whereby the graphene
sheet undergoes a phase transition and is driven towards isolated benzene rings
resulting in a reduction of strength. The other is the usual elastic
instability corresponding to a maximum in the stress-strain curve. Our results
indicate that finite wave vector soft modes can be the key factor in limiting
the strength of monolayer materials
Towards offering more useful data reliably to mobile cloudfrom wireless sensor network
The integration of ubiquitous wireless sensor network (WSN) and powerful mobile cloud computing (MCC) is a research topic that is attracting growing interest in both academia and industry. In this new paradigm, WSN provides data to the cloud, and mobile users request data from the cloud. To support applications involving WSN-MCC integration, which need to reliably offer data that are more useful to the mobile users from WSN to cloud, this paper first identifies the critical issues that affect the usefulness of sensory data and the reliability of WSN, then proposes a novel WSN-MCC integration scheme named TPSS, which consists of two main parts: 1) TPSDT (Time and Priority based Selective Data Transmission) for WSN gateway to selectively transmit sensory data that are more useful to the cloud, considering the time and priority features of the data requested by the mobile user; 2) PSS (Priority-based Sleep Scheduling) algorithm for WSN to save energy consumption so that it can gather and transmit data in a more reliable way. Analytical and experimental results demonstrate the effectiveness of TPSS in improving usefulness of sensory data and reliability of WSN for WSN-MCC integration
Phonon-phason coupling in icosahedral quasicrystals
From relaxation simulations of decoration-based quasicrystal structure models
using microscopically based interatomic pair potentials, we have calculated the
(usually neglected) phonon-phason coupling constant. Its sign is opposite for
the two alloys studied, i-AlMn and i-(Al,Cu)Li; a dimensionless measure of its
magnitude relative to the phonon and phason elastic constants is of order 1/10,
suggesting its effects are small but detectable. We also give a criterion for
when phonon-phason effects are noticeable in diffuse tails of Bragg peaks.Comment: 7 pages, LaTeX, uses Europhys Lett macros (included
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