34,226 research outputs found
Fixed-domain asymptotic properties of tapered maximum likelihood estimators
When the spatial sample size is extremely large, which occurs in many
environmental and ecological studies, operations on the large covariance matrix
are a numerical challenge. Covariance tapering is a technique to alleviate the
numerical challenges. Under the assumption that data are collected along a line
in a bounded region, we investigate how the tapering affects the asymptotic
efficiency of the maximum likelihood estimator (MLE) for the microergodic
parameter in the Mat\'ern covariance function by establishing the fixed-domain
asymptotic distribution of the exact MLE and that of the tapered MLE. Our
results imply that, under some conditions on the taper, the tapered MLE is
asymptotically as efficient as the true MLE for the microergodic parameter in
the Mat\'ern model.Comment: Published in at http://dx.doi.org/10.1214/08-AOS676 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum
In magnetic materials, skyrmions are nanoscale regions where the orientation
of electron spin changes in a vortex-type manner. Here we show that spin-orbit
coupling in a focused vector beam results in a skyrmion-like photonic spin
distribution of the excited waveguided fields. While diffraction limits the
spatial size of intensity distributions, the direction of the field, defining
photonic spin, is not subject to this limitation. We demonstrate that the
skyrmion spin structure varies on the deep-subwavelength scales down to 1/60 of
light wavelength, which corresponds to about 10 nanometre lengthscale. The
application of photonic skyrmions may range from high-resolution imaging and
precision metrology to quantum technologies and data storage where the spin
structure of the field, not its intensity, can be applied to achieve
deep-subwavelength optical patterns
Tuning Strain in Flexible Graphene Nanoelectromechanical Resonators
The structural flexibility of low dimensional nanomaterials offers unique
opportunities for studying the impact of strain on their physical properties
and for developing innovative devices utilizing strain engineering. A key
towards such goals is a device platform which allows the independent tuning and
reliable calibration of the strain. Here we report the fabrication and
characterization of graphene nanoelectromechanical resonators(GNEMRs) on
flexible substrates. Combining substrate bending and electrostatic gating, we
achieve the independent tuning of the strain and sagging in graphene and
explore the nonlinear dynamics over a wide parameter space. Analytical and
numerical studies of a continuum mechanics model, including the competing
higher order nonlinear terms, reveal a comprehensive nonlinear dynamics phase
diagram, which quantitatively explains the complex behaviors of GNEMRs
Kinetic simulations of X-B and O-X-B mode conversion
We have performed fully-kinetic simulations of X-B and O-X-B mode conversion
in one and two dimensional setups using the PIC code EPOCH. We have recovered
the linear dispersion relation for electron Bernstein waves by employing
relatively low amplitude incoming waves. The setups presented here can be used
to study non-linear regimes of X-B and O-X-B mode conversion.Comment: 4 pages, 3 figure
Fabrication and Electric Field Dependent Transport Measurements of Mesoscopic Graphite Devices
We have developed a unique micromechanical method to extract extremely thin
graphite samples. Graphite crystallites with thicknesses ranging from 10 - 100
nm and lateral size 2 m are extracted from bulk. Mesoscopic
graphite devices are fabricated from these samples for electric field dependent
conductance measurements. Strong conductance modulation as a function of gate
voltage is observed in the thinner crystallite devices. The temperature
dependent resistivity measurements show more boundary scattering contribution
in the thinner graphite samples.Comment: 3 pages, 3 figures included, submitted to Appl. Phys. Let
Anomalous Hall effect in NiPt thin films
We study Hall effect in sputtered NixPt1-x thin films with different Ni
concentrations. Temperature, magnetic field and angular dependencies are
analyzed and the phase diagram of NiPt thin films is obtained. It is found that
films with sub-critical Ni concentration exhibit cluster-glass behavior at low
temperatures with a perpendicular magnetic anisotropy below the freezing
temperature. Films with over-critical Ni concentration are ferromagnetic with
parallel anisotropy. At the critical concentration the state of the film is
strongly frustrated. Such films demonstrate canted magnetization with the easy
axis rotating as a function of temperature. The magnetism appears via
consecutive paramagnetic - cluster glass - ferromagnetic transitions, rather
than a single second-order phase transition. But most remarkably, the
extraordinary Hall effect changes sign at the critical concentration. We
suggest that this is associated with a reconstruction of the electronic
structure of the alloy at the normal metal - ferromagnet quantum phase
transition.Comment: 12 pages, 17 figure
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