313 research outputs found
Fabrication and analysis of vanadium oxides and vanadium oxide based magnetic hybrid structures
2021 Spring.Includes bibliographical references.Vanadium oxide films and vanadium oxide-based magnetic hybrid structures are fabricated using various techniques and studied optically, electrically, structurally, and magnetically for their potential applications into magnetic recording, room temperature refrigeration, and optical switches. The different types of behavior seen in the transitions of the vanadium oxide class of compounds can be altered and optimized according to desirable qualities for these applications. Several different techniques were used for the fabrication of vanadium oxide-ferromagnetic (FM) composites with the goal of causing magnetic coupling and the optimization of coupling between the vanadium oxide compound and the FM compound. The ball milling process was used as the primary step in formation of the composites, but was shown to be ineffective at causing coupling between the compounds if used alone. The addition of a sintering process was shown to successfully couple V2O3 and Ni, with an optimization of the process determined to be primarily dependent on temperature. Optimized composites showed up to 56% changes in coercivity at the transition temperature of the V2O3. VO2 based composites were unable to be coupled due to problems with the reduction and oxidation of the compounds involved, and a lack of diffusion. A sol-gel technique for the fabrication of VO2 layers was optimized for large transitional properties and refined for reproducibility. Magnetic hybrid structures formed from the sol-gel fabricated films were shown to have comparable properties to their sputtered counterparts. W doped films fabricated using the sol-gel technique, when compared to doping using a sputtering technique, were demonstrated to allow for larger control over the ideal doping range. Doping was shown to have negligible effect on the morphology of the films, but produced several W based impurities. Although doping produced expected shifts and decreases in the transitional electrical transport properties, there were also unexplained shifts in the absolute resistance for higher doping. Magnetic hybrid structures based on doped films still produced large changes in the magnetic properties of the FM layer, but these changes were shifted to lower temperatures and reduced. Transmission and reflection of VO2 films fabricated using different techniques were shown to have different qualitative and quantitative behaviors at different optical wavelengths of incidence. Most films were shown to have downward switching in both the transmission and reflection at the transition, however thinner films sometimes showed upward switching in the transmission. Downward bumps caused by interference were seen in the reflection at 980 nm, as well as at 635 nm for two other films. The model that was developed to try to reproduce this behavior is successful for 60% of the films, and able to reproduce all of the qualitative behaviors described. However the trends in the fitted refractive index do not help elucidate what physical mechanism is responsible for the differences seen between samples
On the universal structure of human lexical semantics
How universal is human conceptual structure? The way concepts are organized
in the human brain may reflect distinct features of cultural, historical, and
environmental background in addition to properties universal to human
cognition. Semantics, or meaning expressed through language, provides direct
access to the underlying conceptual structure, but meaning is notoriously
difficult to measure, let alone parameterize. Here we provide an empirical
measure of semantic proximity between concepts using cross-linguistic
dictionaries. Across languages carefully selected from a phylogenetically and
geographically stratified sample of genera, translations of words reveal cases
where a particular language uses a single polysemous word to express concepts
represented by distinct words in another. We use the frequency of polysemies
linking two concepts as a measure of their semantic proximity, and represent
the pattern of such linkages by a weighted network. This network is highly
uneven and fragmented: certain concepts are far more prone to polysemy than
others, and there emerge naturally interpretable clusters loosely connected to
each other. Statistical analysis shows such structural properties are
consistent across different language groups, largely independent of geography,
environment, and literacy. It is therefore possible to conclude the conceptual
structure connecting basic vocabulary studied is primarily due to universal
features of human cognition and language use.Comment: Press embargo in place until publicatio
Direct measurement of antiferromagnetic domain fluctuations
Measurements of magnetic noise emanating from ferromagnets due to domain
motion were first carried out nearly 100 years ago and have underpinned much
science and technology. Antiferromagnets, which carry no net external magnetic
dipole moment, yet have a periodic arrangement of the electron spins extending
over macroscopic distances, should also display magnetic noise, but this must
be sampled at spatial wavelengths of order several interatomic spacings, rather
than the macroscopic scales characteristic of ferromagnets. Here we present the
first direct measurement of the fluctuations in the nanometre-scale spin-
(charge-) density wave superstructure associated with antiferromagnetism in
elemental Chromium. The technique used is X-ray Photon Correlation
Spectroscopy, where coherent x-ray diffraction produces a speckle pattern that
serves as a "fingerprint" of a particular magnetic domain configuration. The
temporal evolution of the patterns corresponds to domain walls advancing and
retreating over micron distances. While the domain wall motion is thermally
activated at temperatures above 100K, it is not so at lower temperatures, and
indeed has a rate which saturates at a finite value - consistent with quantum
fluctuations - on cooling below 40K. Our work is important because it provides
an important new measurement tool for antiferromagnetic domain engineering as
well as revealing a fundamental new fact about spin dynamics in the simplest
antiferromagnet.Comment: 19 pages, 4 figure
Finding minimal action sequences with a simple evaluation of actions
Animals are able to discover the minimal number of actions that achieves an outcome (the minimal action sequence). In most accounts of this, actions are associated with a measure of behavior that is higher for actions that lead to the outcome with a shorter action sequence, and learning mechanisms find the actions associated with the highest measure. In this sense, previous accounts focus on more than the simple binary signal of “was the outcome achieved?”; they focus on “how well was the outcome achieved?” However, such mechanisms may not govern all types of behavioral development. In particular, in the process of action discovery (Redgrave and Gurney, 2006), actions are reinforced if they simply lead to a salient outcome because biological reinforcement signals occur too quickly to evaluate the consequences of an action beyond an indication of the outcome’s occurrence. Thus, action discovery mechanisms focus on the simple evaluation of “was the outcome achieved?” and not “how well was the outcome achieved?” Notwithstanding this impoverishment of information, can the process of action discovery find the minimal action sequence? We address this question by implementing computational mechanisms, referred to in this paper as no-cost learning rules, in which each action that leads to the outcome is associated with the same measure of behavior. No-cost rules focus on “was the outcome achieved?” and are consistent with action discovery. No-cost rules discover the minimal action sequence in simulated tasks and execute it for a substantial amount of time. Extensive training, however, results in extraneous actions, suggesting that a separate process (which has been proposed in action discovery) must attenuate learning if no-cost rules participate in behavioral development. We describe how no-cost rules develop behavior, what happens when attenuation is disrupted, and relate the new mechanisms to wider computational and biological context
A study of localisation in dual-phase high-strength steels under dynamic loading using digital image correlation and FE analysis
Tensile tests were conducted on dual-phase high-strength steel in a
Split-Hopkinson Tension Bar at a strain-rate in the range of 150-600/s and in a
servo-hydraulic testing machine at a strain-rate between 10-3 and 100/s. A
novel specimen design was utilized for the Hopkinson bar tests of this sheet
material. Digital image correlation was used together with high-speed
photography to study strain localisation in the tensile specimens at high rates
of strain. By using digital image correlation, it is possible to obtain
in-plane displacement and strain fields during non-uniform deformation of the
gauge section, and accordingly the strains associated with diffuse and
localised necking may be determined. The full-field measurements in high
strain-rate tests reveal that strain localisation started even before the
maximum load was attained in the specimen. An elasto-viscoplastic constitutive
model is used to predict the observed stress-strain behaviour and strain
localisation for the dual-phase steel. Numerical simulations of dynamic tensile
tests were performed using the non-linear explicit FE code LS-DYNA. Simulations
were done with shell (plane stress) and brick elements. Good correlation
between experiments and numerical predictions was achieved, in terms of
engineering stress-strain behaviour, deformed geometry and strain fields.
However, mesh density plays a role in the localisation of deformation in
numerical simulations, particularly for the shell element analysis
Limits on Gravitational-Wave Emission from Selected Pulsars Using LIGO Data
We place direct upper limits on the amplitude of gravitational waves from 28 isolated radio pulsars by a coherent multidetector analysis of the data collected during the second science run of the LIGO interferometric detectors. These are the first direct upper limits for 26 of the 28 pulsars. We use coordinated radio observations for the first time to build radio-guided phase templates for the expected gravitational-wave signals. The unprecedented sensitivity of the detectors allows us to set strain upper limits as low as a few times 10^(-24). These strain limits translate into limits on the equatorial ellipticities of the pulsars, which are smaller than 10^(-5) for the four closest pulsars
Detector Description and Performance for the First Coincidence Observations between LIGO and GEO
For 17 days in August and September 2002, the LIGO and GEO interferometer
gravitational wave detectors were operated in coincidence to produce their
first data for scientific analysis. Although the detectors were still far from
their design sensitivity levels, the data can be used to place better upper
limits on the flux of gravitational waves incident on the earth than previous
direct measurements. This paper describes the instruments and the data in some
detail, as a companion to analysis papers based on the first data.Comment: 41 pages, 9 figures 17 Sept 03: author list amended, minor editorial
change
Upper limits on the strength of periodic gravitational waves from PSR J1939+2134
The first science run of the LIGO and GEO gravitational wave detectors
presented the opportunity to test methods of searching for gravitational waves
from known pulsars. Here we present new direct upper limits on the strength of
waves from the pulsar PSR J1939+2134 using two independent analysis methods,
one in the frequency domain using frequentist statistics and one in the time
domain using Bayesian inference. Both methods show that the strain amplitude at
Earth from this pulsar is less than a few times .Comment: 7 pages, 1 figure, to appear in the Proceedings of the 5th Edoardo
Amaldi Conference on Gravitational Waves, Tirrenia, Pisa, Italy, 6-11 July
200
Search for Gravitational Waves from Primordial Black Hole Binary Coalescences in the Galactic Halo
We use data from the second science run of the LIGO gravitational-wave
detectors to search for the gravitational waves from primordial black hole
(PBH) binary coalescence with component masses in the range 0.2--.
The analysis requires a signal to be found in the data from both LIGO
observatories, according to a set of coincidence criteria. No inspiral signals
were found. Assuming a spherical halo with core radius 5 kpc extending to 50
kpc containing non-spinning black holes with masses in the range 0.2--, we place an observational upper limit on the rate of PBH coalescence
of 63 per year per Milky Way halo (MWH) with 90% confidence.Comment: 7 pages, 4 figures, to be submitted to Phys. Rev.
Improving the sensitivity to gravitational-wave sources by modifying the input-output optics of advanced interferometers
We study frequency dependent (FD) input-output schemes for signal-recycling
interferometers, the baseline design of Advanced LIGO and the current
configuration of GEO 600. Complementary to a recent proposal by Harms et al. to
use FD input squeezing and ordinary homodyne detection, we explore a scheme
which uses ordinary squeezed vacuum, but FD readout. Both schemes, which are
sub-optimal among all possible input-output schemes, provide a global noise
suppression by the power squeeze factor, while being realizable by using
detuned Fabry-Perot cavities as input/output filters. At high frequencies, the
two schemes are shown to be equivalent, while at low frequencies our scheme
gives better performance than that of Harms et al., and is nearly fully
optimal. We then study the sensitivity improvement achievable by these schemes
in Advanced LIGO era (with 30-m filter cavities and current estimates of
filter-mirror losses and thermal noise), for neutron star binary inspirals, and
for narrowband GW sources such as low-mass X-ray binaries and known radio
pulsars. Optical losses are shown to be a major obstacle for the actual
implementation of these techniques in Advanced LIGO. On time scales of
third-generation interferometers, like EURO/LIGO-III (~2012), with
kilometer-scale filter cavities, a signal-recycling interferometer with the FD
readout scheme explored in this paper can have performances comparable to
existing proposals. [abridged]Comment: Figs. 9 and 12 corrected; Appendix added for narrowband data analysi
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