8,337 research outputs found
Ultrafast electroabsorption dynamics in an InAs quantum dot saturable absorber at 1.3 mu m
The authors report a direct measurement of the absorption dynamics in an InAs p-i-n ridge waveguide quantum dot modulator. The carrier escape mechanisms are investigated via subpicosecond pump-probe measurements at room temperature, under reverse bias conditions. The optical pulses employed are degenerate in wavelength with the quantum dot ground state transition at 1.28 mu m. The absorption change recovers with characteristic times ranging from 62 ps (0 V) to similar to 700 fs (-10 V), showing a decrease of nearly two orders of magnitude. The authors show that at low applied fields, this recovery is attributed to thermionic emission while for higher applied fields, tunneling becomes the dominant mechanism. (c) 2006 American Institute of Physics.</p
Nonvolcanic tremor observed in the Mexican subduction zone
Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1â8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001â2007 when in 2001â2002 a large slow slip event (SSE) had occurred in the Guerrero-Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S-waves. More than 100 strong NVT bursts were recorded in the narrow band of ~40 Ă 150 km^2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001â2002 and 2006 SSE compared with that for the âquietâ period of 2003â2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes
S wave velocity structure below central Mexico using high-resolution surface wave tomography
Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB
Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline LaSrFeO: Magnetization, Mossbauer, and polarised neutron studies
The charge ordered LaSrFeO (LSFO) in bulk and
nanocrystalline forms are investigated using ac and dc magnetization,
M\"{o}ssbauer, and polarised neutron studies. A complex scenario of short range
charge and magnetic ordering is realized from the polarised neutron studies in
nanocrystalline specimen. This short range ordering does not involve any change
in spin state and modification in the charge disproportion between Fe
and Fe compared to bulk counterpart as evident in the M\"{o}ssbauer
results. The refinement of magnetic diffraction peaks provides magnetic moments
of Fe and Fe are about 3.15 and 1.57 for bulk, and
2.7 and 0.53 for nanocrystalline specimen, respectively. The
destabilization of charge ordering leads to magnetic phase separation, giving
rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K
attains a value as high as 4.4 kOe for average size 70 nm, which is zero
for the bulk counterpart. A strong frequency dependence of ac susceptibility
reveals cluster-glass like transition around 65 K, below which EB
appears. Overall results propose that finite size effect directs the complex
glassy magnetic behavior driven by unconventional short range charge and
magnetic ordering, and magnetic phase separation appears in nanocrystalline
LSFO.Comment: 10 pages, 9 figures. Fig. 1 available upon request or in
http://www.ffn.ub.es/oscar/Articles.html. Accepted in Phys. Rev.
Amplifying applied game development and uptake
The established (digital) leisure game industry is historically one dominated by large international hardware
vendors (e.g. Sony, Microsoft and Nintendo), major publishers and supported by a complex network of
development studios, distributors and retailers. New modes of digital distribution and development practice are
challenging this business model and the leisure games industry landscape is one experiencing rapid change. The
established (digital) leisure games industry, at least anecdotally, appears reluctant to participate actively in the
applied games sector (Stewart et al., 2013). There are a number of potential explanations as to why this may
indeed be the case including ; A concentration on large-scale consolidation of their (proprietary) platforms,
content, entertainment brand and credibility which arguably could be weakened by association with the
conflicting notion of purposefulness (in applied games) in market niches without clear business models or
quantifiable returns on investment.
In contrast, the applied games industry exhibits the characteristics of an emerging, immature industry namely:
weak interconnectedness, limited knowledge exchange, an absence of harmonising standards, limited
specialisations, limited division of labour and arguably insufficient evidence of the products efficacies (Stewart et
al., 2013; Garcia Sanchez, 2013) and could, arguably, be characterised as a dysfunctional market. To test these
assertions the Realising an Applied Gaming Ecosystem (RAGE) project will develop a number of self contained
gaming assets to be actively employed in the creation of a number of applied games to be implemented and
evaluated as regional pilots across a variety of European educational, training and vocational contexts.
RAGE is a European Commission Horizon 2020 project with twenty (pan European) partners from industry,
research and education with the aim of developing, transforming and enriching advanced technologies from the
leisure games industry into self-contained gaming assets (i.e. solutions showing economic value potential) that
could support a variety of stakeholders including teachers, students, and, significantly, game studios interested in
developing applied games. RAGE will provide these assets together with a large quantity of high-quality
knowledge resources through a self-sustainable Ecosystem, a social space that connects research, the gaming
industries, intermediaries, education providers, policy makers and end-users in order to stimulate the
development and application of applied games in educational, training and vocational contexts.
The authors identify barriers (real and perceived) and opportunities facing stakeholders in engaging, exploring
new emergent business models ,developing, establishing and sustaining an applied gaming eco system in Europe
Normalization factors for magnetic relaxation of small particle systems in non-zero magnetic field
We critically discuss relaxation experiments in magnetic systems that can be
characterized in terms of an energy barrier distribution, showing that proper
normalization of the relaxation data is needed whenever curves corresponding to
different temperatures are to be compared. We show how these normalization
factors can be obtained from experimental data by using the
scaling method without making any assumptions about the nature of the energy
barrier distribution. The validity of the procedure is tested using a
ferrofluid of Fe_3O_4 particles.Comment: 5 pages, 6 eps figures added in April 22, to be published in Phys.
Rev. B 55 (1 April 1997
Magnetic Field scaling of Relaxation curves in Small Particle Systems
We study the effects of the magnetic field on the relaxation of the
magnetization of small monodomain non-interacting particles with random
orientations and distribution of anisotropy constants. Starting from a master
equation, we build up an expression for the time dependence of the
magnetization which takes into account thermal activation only over barriers
separating energy minima, which, in our model, can be computed exactly from
analytical expressions. Numerical calculations of the relaxation curves for
different distribution widths, and under different magnetic fields H and
temperatures T, have been performed. We show how a \svar scaling of the
curves, at different T and for a given H, can be carried out after proper
normalization of the data to the equilibrium magnetization. The resulting
master curves are shown to be closely related to what we call effective energy
barrier distributions, which, in our model, can be computed exactly from
analytical expressions. The concept of effective distribution serves us as a
basis for finding a scaling variable to scale relaxation curves at different H
and a given T, thus showing that the field dependence of energy barriers can be
also extracted from relaxation measurements.Comment: 12 pages, 9 figures, submitted to Phys. Rev.
An Information-Theoretic Characterization of the Optimal Gradient Sensing Response of Cells
Many cellular systems rely on the ability to interpret spatial heterogeneities in chemoattractant concentration to direct cell migration. The accuracy of this process is limited by stochastic fluctuations in the concentration of the external signal and in the internal signaling components. Here we use information theory to determine the optimal scheme to detect the location of an external chemoattractant source in the presence of noise. We compute the minimum amount of mutual information needed between the chemoattractant gradient and the internal signal to achieve a prespecified chemotactic accuracy. We show that more accurate chemotaxis requires greater mutual information. We also demonstrate that a priori information can improve chemotaxis efficiency. We compare the optimal signaling schemes with existing experimental measurements and models of eukaryotic gradient sensing. Remarkably, there is good quantitative agreement between the optimal response when no a priori assumption is made about the location of the existing source, and the observed experimental response of unpolarized Dictyostelium discoideum cells. In contrast, the measured response of polarized D. discoideum cells matches closely the optimal scheme, assuming prior knowledge of the external gradientâfor example, through prolonged chemotaxis in a given direction. Our results demonstrate that different observed classes of responses in cells (polarized and unpolarized) are optimal under varying information assumptions
Ge(113) reconstruction stabilized by subsurface interstitials: An x-ray diffraction structure analysis
The three-dimensional atomic coordinates of the Ge(113)-(3Ă1) surface have been determined by analyzing in-plane and out-of-plane x-ray intensity data. Besides dimer and adatom motifs, which reduce the number of dangling bonds, a random distribution of subsurface interstitials has been identified. Subsurface interstitials relieve elastic stress and lower the energy of the electronic system. Together with the delicate balance between the energy gain due to reduction of dangling bonds and the energy costs due to induced strain this determines the nature of the (3Ă1) reconstruction of Ge(113)
Self-T-Dual Brane Cosmology and the Cosmological Constant Problem
We consider a codimension-one brane embedded in a gravity-dilaton bulk
action, whose symmetries are compatible with T-duality along the space-like
directions parallel to the brane, and the bulk time-like direction. The
equations of motions in the string frame allow for a smooth background obtained
by the union of two symmetric patches of AdS space. The Poincar\'{e} invariance
of the solution appears to hold independently of the value of the brane vacuum
energy, through a self-tuning property of the dilaton ground state. Moreover,
the effective cosmology displays a bounce, at which the scale factor does not
shrink to zero. Finally, by exploiting the T-duality symmetry, we show how to
construct an ever-expanding Universe, along the lines of the Pre-Big Bang
scenario.Comment: Minor corrections, comments & references added. Accepted for
publicatio
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