985 research outputs found
Similarity-Based Models of Word Cooccurrence Probabilities
In many applications of natural language processing (NLP) it is necessary to
determine the likelihood of a given word combination. For example, a speech
recognizer may need to determine which of the two word combinations ``eat a
peach'' and ``eat a beach'' is more likely. Statistical NLP methods determine
the likelihood of a word combination from its frequency in a training corpus.
However, the nature of language is such that many word combinations are
infrequent and do not occur in any given corpus. In this work we propose a
method for estimating the probability of such previously unseen word
combinations using available information on ``most similar'' words.
We describe probabilistic word association models based on distributional
word similarity, and apply them to two tasks, language modeling and pseudo-word
disambiguation. In the language modeling task, a similarity-based model is used
to improve probability estimates for unseen bigrams in a back-off language
model. The similarity-based method yields a 20% perplexity improvement in the
prediction of unseen bigrams and statistically significant reductions in
speech-recognition error.
We also compare four similarity-based estimation methods against back-off and
maximum-likelihood estimation methods on a pseudo-word sense disambiguation
task in which we controlled for both unigram and bigram frequency to avoid
giving too much weight to easy-to-disambiguate high-frequency configurations.
The similarity-based methods perform up to 40% better on this particular task.Comment: 26 pages, 5 figure
Effects of oil spillage on Soil and Surface Water in Odoro Ikot Ukanafun Area Akwa Ibom State
A hydrogeochemical survey of water and soil sample in Odoro Ikot and its environment was carried out using Atomic Absorption Spectrometry (AAS) techniques. It was done to assess the impact of oil spillage on soil and surface water in the area. The soil samples were taken from the top to a depth of 30cm at the interval depth of 5cm each. Results obtained from these analyses were compared with World Health Organization (WHO) Department of Petroleum Resources (DPR)and Federal Environmental Protection Agency (FEPA) standards. The following parameters were measured and analyzed, temperature, pH, electrical conductivity, oil/grease, copper (Cu), cadmium (Cd) lead (Pb), Zinc (Zn), chromium (Cr), Iron (Fe), phosphorus, (Pox) and sulphate (S04). The results of the analysis show that the surface water samples exceeds the WHO, FEPA and DPR acceptable standards. Apart from the water pH which was almost moderately acidic and oil/grease value in the study area, other parameters also exceeds the recommended standards, therefore there is need for remediation or clean up exercise to be carried out in the study area. The comparison with the soil sample analytical results show that the crude oil has polluted the soil and rendered it unsuitable for agricultural purpose since the ecology has been damaged and the soil degraded. ©JASEMKeywords: Tozicity, heavy metals, soil and water, WHO standard, Odoro Ikot Ukanafun, Akwa Ibom State Nigeria
Kinetic simulations of ladder climbing by electron plasma waves
The energy of plasma waves can be moved up and down the spectrum using
chirped modulations of plasma parameters, which can be driven by external
fields. Depending on whether the wave spectrum is discrete (bounded plasma) or
continuous (boundless plasma), this phenomenon is called ladder climbing (LC)
or autoresonant acceleration of plasmons. It was first proposed by Barth
\textit{et al.} [PRL \textbf{115}, 075001 (2015)] based on a linear fluid
model. In this paper, LC of electron plasma waves is investigated using fully
nonlinear Vlasov-Poisson simulations of collisionless bounded plasma. It is
shown that, in agreement with the basic theory, plasmons survive substantial
transformations of the spectrum and are destroyed only when their wave numbers
become large enough to trigger Landau damping. Since nonlinear effects decrease
the damping rate, LC is even more efficient when practiced on structures like
quasiperiodic Bernstein-Greene-Kruskal (BGK) waves rather than on Langmuir
waves \textit{per~se}
Unusual interplay between copper-spin and vortex dynamics in slightly overdoped La{1.83}Sr{0.17}CuO{4}
Our inelastic neutron scattering experiments of the spin excitations in the
slightly overdoped La{1.83}Sr{0.17}CuO{4} compound show that, under the
application of a magnetic field of 5 Tesla, the low-temperature susceptibility
undergoes a weight redistribution centered at the spin-gap energy. Furthermore,
by comparing the temperature dependence of the neutron data with
ac-susceptibility and magnetization measurements, we conclude that the filling
in of the spin gap tracks the irreversibility/melting temperature rather than
Tc2, which indicates an unusual interplay between the magnetic vortices and the
spin excitations even in the slightly overdoped regime of high-temperature
superconductors.Comment: 7 pages, including 5 figure
Ni-impurity effects on the superconducting gap of LaSrCuO studied from the magnetic field and temperature dependence of the electronic specific heat
The magnetic field and temperature dependence of the electronic specific heat
have been systematically investigated in (LSCNO) in order to study Ni-impurity
effects on the superconducting (SC) gap. In LSCNO with =0.15 and =0.015,
the value of () at =0 K, , is
enhanced under the magnetic field applied along the -axis. The
increment of , , follows the Volovik relation
=, characteristic of the SC gap with line nodes,
with prefactor similar to that of a pure sample. The vs.
curve under =0 shows a d-wave-like SC anomaly with an abrupt increase at
and -linear dependence at , although the
-value in the vs. curve increases with increasing
Ni concentrations. Interestingly, as the SC part of , , decreases in LSCNO, is
reduced in proportion to the decrease of . These findings can
be explained phenomenologically by a simple model in which Ni impurities bring
about strong pair breaking at the edges of the coherent nodal part of the Fermi
surface but in the vicinity of the nodes of the SC gap. The reduction of the SC
condensation energy in LSCNO, evaluated from at
{0.3em}\raisebox{0.4ex}{} {-0.75em}\raisebox{-.7ex}{} {0.3em}, is also understood by the same model.Comment: 7 pages, 6 figures, accepted in Phys. Rev.
STM/STS Study on 4a X 4a Electronic Charge Order and Inhomogeneous Pairing Gap in Superconducting Bi2Sr2CaCu2O8+d
We performed STM/STS measurements on underdoped Bi2212 crystals with doping
levels p ~ 0.11, ~ 0.13 and ~ 0.14 to examine the nature of the nondispersive
4a X 4a charge order in the superconducting state at T << Tc. The charge order
appears conspicuously within the pairing gap, and low doping tends to favor the
charge order. We point out the possibility that the 4a X 4a charge order will
be dynamical in itself, and pinned down over regions with effective pinning
centers. The pinned 4a X 4a charge order is closely related to the spatially
inhomogeneous pairing gap structure, which has often been reported in STS
measurements on high-Tc cuprates.Comment: 12 pages, 16 figures, to be published in Phys. Rev.
Causality effects on accelerating light pulses
We study accelerating and decelerating shape-preserving temporal Airy wave-packets propagating in dispersive media. We explore the effects of causality, and find that, whereas decelerating pulses can asymptotically reach zero group velocity, pulses that accelerate towards infinite group velocity inevitably break up, after a specific critical point. The trajectories and the features of causal pulses are analyzed, along with the requirements for the existence of the critical point and experimental schemes for its observation. Finally, we show that causality imposes similar effects on accelerating pulses in the presence of local Kerr-like nonlinearities
- …