285 research outputs found
Contrasting patterns in the occurrence and biomass centers of gravity among fish and macroinvertebrates in a continental shelf ecosystem
The distribution of a group of fish and macroinvertebrates (n = 52) resident in the US Northeast Shelf large marine ecosystem were characterized with species distribution models (SDM), which in turn were used to estimate occurrence and biomass center of gravity (COG). The SDMs were fit using random forest machine learning and were informed with a range of physical and biological variables. The estimated probability of occurrence and biomass from the models provided the weightings to determine depth, distance to the coast, and along-shelf distance COG. The COGs of occupancy and biomass habitat tended to be separated by distances averaging 50 km, which approximates half of the minor axis of the subject ecosystem. During the study period (1978–2018), the biomass COG has tended to shift to further offshore positions whereas occupancy habitat has stayed at a regular spacing from the coastline. Both habitat types have shifted their along-shelf distances, indicating a general movement to higher latitude or to the Northeast for this ecosystem. However, biomass tended to occur at lower latitudes in the spring and higher latitude in the fall in a response to seasonal conditions. Distribution of habitat in relation to depth reveals a divergence in response with occupancy habitat shallowing over time and biomass habitat distributing in progressively deeper water. These results suggest that climate forced change in distribution will differentially affect occurrence and biomass of marine taxa, which will likely affect the organization of ecosystems and the manner in which human populations utilize marine resources.publishedVersio
The symmetry problem in NaV2O5
We discuss the symmetry of NaV2O5 in the high temperature phase on the basis
of optical conductivity data. Conclusive information cannot be obtained by
studying the optically allowed lattice vibrations. However, intensity and
polarization of the electronic excitations give a direct indication for a
broken-parity electronic ground-state. This is responsible for the detection of
charged bi-magnons in the optical spectrum.Comment: Revtex, 2 pages, 1 postscript picture embedded in the tex
Infrared study of spin-Peierls compound alpha'-NaV2O5
Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range
from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In
addition to phonon modes identification, for the a-polarised spectrum a broad
continuum absorption in the range of 1D magnetic excitation energies was found.
The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with
vanishing intensity in c-polarisation. Activation of new phonons due to the
lattice dimerisation were detected below 35K as well as pretransitional
structural fluctuations up to 65K.Comment: 3 pages, 2 figures, 1 table. Contributed paper for the SCES'98 (15-18
July 1998, Paris). To be published in Physica
Independent Component Analysis-motivated Approach to Classificatory Decomposition of Cortical Evoked Potentials
BACKGROUND: Independent Component Analysis (ICA) proves to be useful in the analysis of neural activity, as it allows for identification of distinct sources of activity. Applied to measurements registered in a controlled setting and under exposure to an external stimulus, it can facilitate analysis of the impact of the stimulus on those sources. The link between the stimulus and a given source can be verified by a classifier that is able to "predict" the condition a given signal was registered under, solely based on the components. However, the ICA's assumption about statistical independence of sources is often unrealistic and turns out to be insufficient to build an accurate classifier. Therefore, we propose to utilize a novel method, based on hybridization of ICA, multi-objective evolutionary algorithms (MOEA), and rough sets (RS), that attempts to improve the effectiveness of signal decomposition techniques by providing them with "classification-awareness." RESULTS: The preliminary results described here are very promising and further investigation of other MOEAs and/or RS-based classification accuracy measures should be pursued. Even a quick visual analysis of those results can provide an interesting insight into the problem of neural activity analysis. CONCLUSION: We present a methodology of classificatory decomposition of signals. One of the main advantages of our approach is the fact that rather than solely relying on often unrealistic assumptions about statistical independence of sources, components are generated in the light of a underlying classification problem itself
Coexistence of charge density wave and spin-Peierls orders in quarter-filled quasi-one dimensional correlated electron systems
Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compounds
consisting of coupled ladders and/or zig-zag chains are investigated. Hubbard
and t-J models including local Holstein and/or Peierls couplings to the lattice
are studied by numerical techniques. Next nearest neighbor hopping and magnetic
exchange, and short-range Coulomb interactions are also considered. We show
that, generically, these systems undergo instabilities towards the formation of
Charge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulated
structures. Moderate electron-electron and electron-lattice couplings can lead
to a coexistence of these three types of orders. In the ladder, a zig-zag
pattern is stabilized by the Holstein coupling and the nearest-neighbor Coulomb
repulsion. In the case of an isolated chain, bond-centered and site-centered
2k_F and 4k_F modulations are induced by the local Holstein coupling. In
addition, we show that, in contrast to the ladders, a small charge ordering in
the chains, strongly enhances the spin-Peierls instability. Our results are
applied to the NaV_2O_5 compound (trellis lattice) and various phases with
coexisting charge disproportionation and spin-Peierls order are proposed and
discussed in the context of recent experiments. The role of the long-range
Coulomb potential is also outlined.Comment: 10 pages, Revtex, 10 encapsulated figure
NaV_2O_5 as an Anisotropic t-J Ladder at Quarter Filling
Based on recent experimental evidences that the electronic charge degrees of
freedom plays an essential role in the spin-Peierls--like phase transition of
NaVO, we first make the mapping of low-energy electronic states of the
model for NaVO to the quarter-filled ladder with
anisotropic parameter values between legs and rungs, and then show that this
anisotropic ladder is in the Mott insulating state, of which
lowest-energy states can be modeled by the one-dimensional Heisenberg
antiferromagnet with the effective exchange interaction whose value
is consistent with experimental estimates. We furthermore examine the coupling
between the ladders as the trellis lattice model and show that the
nearest-neighbor Coulomb repulsion on the zigzag-chain bonds can lead to the
instability in the charge degrees of freedom of the ladders.Comment: 4 pages, 5 gif figures. Fig.3 corrected. Hardcopies of figures (or
the entire manuscript) can be obtained by e-mail request to
[email protected]
Superexchange in the quarter- filled two- leg ladder system NaV2O5
A theory of superexchange in the mixed valent layer compound NaV2O5 is
presented which provides a consistent description of exchange both in the
disordered and charge ordered state. Starting from results of band structure
calculations for NaV2O5 first an underlying electronic model for a ladder unit
in the Trellis lattice is formulated. By using the molecular orbital
representation for intra-rung electronic states a second-order perturbation
procedure is developed and an effective spin-chain model for a ladder is
derived. Variation of the resulting superexchange integral J is examined
numerically as the ladder system evolves from a charge disordered to the
extreme ('zig-zag') charge ordered state. It is found that the effective intra-
ladder superexchange is always antiferromagnetic.Comment: 18 pages Revtex, 7 Postscript figure
TYC 2675-663-1: A newly discovered W UMa system in an active state
The recently discovered eclipsing binary system TYC 2675-663-1 is a X-ray
source, and shows properties in the optical that are similar to the W UMa
systems, but are somewhat unusual compared to what is seen in other contact
binary systems. The goal of this work is to characterize its properties and
investigate its nature by means of detailed photometric and spectroscopic
observations. We have performed extensive V-band photometric measurements with
the INTEGRAL satellite along with ground-based multi-band photometric
observations, as well as high-resolution spectroscopic monitoring from which we
have measured the radial velocities of the components. These data have been
analysed to determine the stellar properties, including the absolute masses and
radii. Additional low-resolution spectroscopy was obtained to investigate
spectral features. From the measured eclipse timings we determine an orbital
period for the binary of P=0.4223576+-0.0000009 days. The light-curve and
spectroscopic analyses reveal the observations to be well represented by a
model of an overcontact system composed of main-sequence F5 and G7 stars
(temperature difference of nearly 1000 K), with the possible presence of a
third star. Low-resolution optical spectroscopy reveals a complex H alpha
emission, and other features that are not yet understood. The unusually large
mass ratio of q=0.81+-0.05 places it in the rare "H" (high mass ratio) subclass
of the W UMa systems, which are presumably on their way to coalescence.Comment: 12 pages in double column format. Accepted for publication in
Astronomy and Astrophysic
The yellow hypergiants HR 8752 and rho Cassiopeiae near the evolutionary border of instability
High-resolution near-ultraviolet spectra of the yellow hypergiants HR 8752
and rho Cassiopeiae indicate high effective temperatures placing both stars
near the T_eff border of the ``yellow evolutionary void''. At present, the
temperature of HR 8752 is higher than ever. For this star we found
Teff=7900+-200 K, whereas rho Cassiopeiae has Teff=7300+-200 K. Both, HR 8752
and rho Cassiopeiae have developed strong stellar winds with Vinf ~ 120 km/s
and Vinf ~ 100 km/s, respectively. For HR 8752 we estimate an upper limit for
the spherically symmetric mass-loss of 6.7X10^{-6}M_solar/yr. Over the past
decades two yellow hypergiants appear to have approached an evolutionary phase,
which has never been observed before. We present the first spectroscopic
evidence of the blueward motion of a cool super/hypergiant on the HR diagram.Comment: 13 pages including 3 figures. Accepted for publication in ApJ Letter
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