22,113 research outputs found
The chronostratigraphy of Late Pleistocene glacial and periglacial aeolian activity in the Tuktoyaktuk Coastlands, NWT, Canada
Aeolian periglacial sand deposits are common in the Tuktoyaktuk Coastlands of Western Arctic Canada. Regionally extensive and thick aeolian sand-sheet deposits have been observed in two major stratigraphic settings: within a sand unit characterized by large aeolian dune deposits; and interbedded with glaciofluvial outwash from the Laurentide Ice Sheet (LIS). Small, localized sand sheets have also been observed along the tops of sandy bluffs, within sequences of drained thermokarst lakes deposits and as an involuted veneer above buried basal ice of the LIS.
On the basis of radiocarbon and optically stimulated luminescence (OSL) dates from preserved periglacial aeolian sand sheets and dunes a regional chronostratigraphy is presented which indicates that both extensive dunes and sand sheets accumulated mainly between ca 30 and 13 ka. A switch to dominantly sand-sheet aggradation at ca 14â13 ka, with sand sheets forming widely until ca 8 ka, is attributed to (a) surface armouring by glacial deposits associated with the advance of the LIS; and (b) amelioration of the climate from cold aridity. An absence of OSL dates between ca 8 and 1 ka suggests that sand sheets stabilized during much of the Holocene. Local sand-sheet aggradation during recent centuries has occurred near sandy bluffs and on the floors of drained thermokarst lakes. The OSL dates constrain the maximum extent of the LIS in the Tuktoyaktuk Coastlands to Marine Isotope Stage 2
Dimensional Evolution of Spin Correlations in the Magnetic Pyrochlore Yb2Ti2O7
The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional
(2D) magnetic correlations within a cubic lattice environment at low
temperatures, before entering an exotic disordered ground state below T=265mK.
We report neutron scattering measurements of the thermal evolution of the 2D
spin correlations in space and time. Short range three dimensional (3D) spin
correlations develop below 400 mK, accompanied by a suppression in the
quasi-elastic (QE) scattering below ~ 0.2 meV. These show a slowly fluctuating
ground state with spins correlated over short distances within a
kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated
kagome spin-stars at higher temperatures. Furthermore, low-temperature specific
heat results indicate a sample dependence to the putative transition
temperature that is bounded by 265mK, which we discuss in the context of recent
mean field theoretical analysis.Comment: 5 pages, 6 figure
Curve Shortening and the Rendezvous Problem for Mobile Autonomous Robots
If a smooth, closed, and embedded curve is deformed along its normal vector
field at a rate proportional to its curvature, it shrinks to a circular point.
This curve evolution is called Euclidean curve shortening and the result is
known as the Gage-Hamilton-Grayson Theorem. Motivated by the rendezvous problem
for mobile autonomous robots, we address the problem of creating a polygon
shortening flow. A linear scheme is proposed that exhibits several analogues to
Euclidean curve shortening: The polygon shrinks to an elliptical point, convex
polygons remain convex, and the perimeter of the polygon is monotonically
decreasing.Comment: 15 pages, 18 figure
Magnetoelastics of a spin liquid: X-ray diffraction studies of Tb2Ti2O7 in pulsed magnetic fields
We report high resolution single crystal x-ray diffraction measurements of
the frustrated pyrochlore magnet Tb2Ti2O7, collected using a novel low
temperature pulsed magnet system. This instrument allows characterization of
structural degrees of freedom to temperatures as low as 4.4 K, and in applied
magnetic fields as large as 30 Tesla. We show that Tb2Ti2O7 manifests
intriguing structural effects under the application of magnetic fields,
including strongly anisotropic giant magnetostriction, a restoration of perfect
pyrochlore symmetry in low magnetic fields, and ultimately a structural phase
transition in high magnetic fields. It is suggested that the magnetoelastic
coupling thus revealed plays a significant role in the spin liquid physics of
Tb2Ti2O7 at low temperatures.Comment: 4 pages, 4 figures, submitted for publicatio
Spin Waves in the Ferromagnetic Ground State of the Kagome Staircase System Co3V2O8
Inelastic neutron scattering measurements were performed on single crystal
Co3V2O8 wherein magnetic cobalt ions reside on distinct spine and cross-tie
sites within kagome staircase planes. This system displays a rich magnetic
phase diagram which culminates in a ferromagnetic ground state below Tc~6 K. We
have studied the low-lying magnetic excitations in this phase within the kagome
plane. Despite the complexity of the system at higher temperatures, linear
spin-wave theory describes most of the quantitative detail of the inelastic
neutron measurements. Our results show two spin-wave branches, the higher
energy of which displays finite spin-wave lifetimes well below Tc, and
negligible magnetic exchange coupling between Co moments on the spine sites.Comment: 4 pages and 4 figure
Spatio-temporal patterns of beaked whale echolocation signals in the North Pacific.
At least ten species of beaked whales inhabit the North Pacific, but little is known about their abundance, ecology, and behavior, as they are elusive and difficult to distinguish visually at sea. Six of these species produce known species-specific frequency modulated (FM) echolocation pulses: Baird's, Blainville's, Cuvier's, Deraniyagala's, Longman's, and Stejneger's beaked whales. Additionally, one described FM pulse (BWC) from Cross Seamount, Hawai'i, and three unknown FM pulse types (BW40, BW43, BW70) have been identified from almost 11 cumulative years of autonomous recordings at 24 sites throughout the North Pacific. Most sites had a dominant FM pulse type with other types being either absent or limited. There was not a strong seasonal influence on the occurrence of these signals at any site, but longer time series may reveal smaller, consistent fluctuations. Only the species producing BWC signals, detected throughout the Pacific Islands region, consistently showed a diel cycle with nocturnal foraging. By comparing stranding and sighting information with acoustic findings, we hypothesize that BWC signals are produced by ginkgo-toothed beaked whales. BW43 signal encounters were restricted to Southern California and may be produced by Perrin's beaked whale, known only from Californian waters. BW70 signals were detected in the southern Gulf of California, which is prime habitat for Pygmy beaked whales. Hubb's beaked whale may have produced the BW40 signals encountered off central and southern California; however, these signals were also recorded off Pearl and Hermes Reef and Wake Atoll, which are well south of their known range
Quaternion Convolutional Neural Networks for End-to-End Automatic Speech Recognition
Recently, the connectionist temporal classification (CTC) model coupled with
recurrent (RNN) or convolutional neural networks (CNN), made it easier to train
speech recognition systems in an end-to-end fashion. However in real-valued
models, time frame components such as mel-filter-bank energies and the cepstral
coefficients obtained from them, together with their first and second order
derivatives, are processed as individual elements, while a natural alternative
is to process such components as composed entities. We propose to group such
elements in the form of quaternions and to process these quaternions using the
established quaternion algebra. Quaternion numbers and quaternion neural
networks have shown their efficiency to process multidimensional inputs as
entities, to encode internal dependencies, and to solve many tasks with less
learning parameters than real-valued models. This paper proposes to integrate
multiple feature views in quaternion-valued convolutional neural network
(QCNN), to be used for sequence-to-sequence mapping with the CTC model.
Promising results are reported using simple QCNNs in phoneme recognition
experiments with the TIMIT corpus. More precisely, QCNNs obtain a lower phoneme
error rate (PER) with less learning parameters than a competing model based on
real-valued CNNs.Comment: Accepted at INTERSPEECH 201
Mokopuna rising: Developing a best practice for early intervention in whanau violence
In a very difficult meeting, an extremely unpleasant man was concerned about the possible loss of his power and the continuation of his plans and dreams for the future. And I said, yeah well, you should be worried, because according to your philosophy, when youâre dead, youâre dead. But when Iâm dead, my mokopuna will be rising. (Personal Communication, Rob Cooper, 2007)
âMokopuna Risingâ is about claiming space for Maori to define best practice for reducing and avoiding whanau violence1 It is research in progress towards a PhD, being carried out with the Ngati Hine Health Trust, in Te Taitokerau (Northland)
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