9,236 research outputs found
Homo sapiens in the Americas. Overview of the earliest human expansion in the new world
Although it is widely recognised that America was the last continent to be populated by our species, researchers' views on various aspects of this process (e.g. the period in which it occurred, the area from which the colonizing populations came, the number of dispersal waves and the routes taken by these migrations) differ significantly. In this paper, we review both classical data and more recent findings from various research fields - including geology, paleoecology, archaeology, skeletal biology, and genetics - that may shed light on the dynamics of the colonization of the American continent, according to a critical reappraisal of the various hypotheses and models that have been advanced over time to explain this process
Numerical study of metastable states in Ising spin glasses
We study numerically the structure of metastable states in the
Sherrington-Kirkpatrick spin glass. We find that all non-paramagnetic
stationary points of the free energy are organized into pairs, consisting in a
minimum and a saddle of order one, which coalesce in the thermodynamic limit.
Within the annealed approximation, the entropic contribution of these states,
that is the complexity, is compatible with the supersymmetry-breaking
calculation performed in [A.J. Bray and M.A. Moore, J. Phys. C, 13 L469
(1980)]. This result indicates that the supersymmetry is spontaneously broken
in the Sherrington-Kirkpatrick model
Surface-acoustic-wave driven planar light-emitting device
Electroluminescence emission controlled by means of surface acoustic waves
(SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital
transducers for SAW generation were integrated onto pLEDs fabricated following
the scheme which we have recently developed. Current-voltage, light-voltage and
photoluminescence characteristics are presented at cryogenic temperatures. We
argue that this scheme represents a valuable building block for advanced
optoelectronic architectures
Analysis of Round Off Errors with Reversibility Test as a Dynamical Indicator
We compare the divergence of orbits and the reversibility error for discrete
time dynamical systems. These two quantities are used to explore the behavior
of the global error induced by round off in the computation of orbits. The
similarity of results found for any system we have analysed suggests the use of
the reversibility error, whose computation is straightforward since it does not
require the knowledge of the exact orbit, as a dynamical indicator. The
statistics of fluctuations induced by round off for an ensemble of initial
conditions has been compared with the results obtained in the case of random
perturbations. Significant differences are observed in the case of regular
orbits due to the correlations of round off error, whereas the results obtained
for the chaotic case are nearly the same. Both the reversibility error and the
orbit divergence computed for the same number of iterations on the whole phase
space provide an insight on the local dynamical properties with a detail
comparable with other dynamical indicators based on variational methods such as
the finite time maximum Lyapunov characteristic exponent, the mean exponential
growth factor of nearby orbits and the smaller alignment index. For 2D
symplectic maps the differentiation between regular and chaotic regions is well
full-filled. For 4D symplectic maps the structure of the resonance web as well
as the nearby weakly chaotic regions are accurately described.Comment: International Journal of Bifurcation and Chaos, 201
Методика и методология социолингвистических исследований в условиях билингвизма и диглоссии
Lithospheric-scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting, characterized by early activation of large boundary faults and basin subsidence, followed by localization of tectonic activity in internal faults at the rift axis. Integration of current and previous experiments shows that the evolution of deformation, in particular the amount of extension needed for the abandonment of boundary faults and migration of deformation to in-rift faults, is dependent on at least five boundary conditions: (i) thickness of brittle layers (including syn-rift sediments); (ii) thickness of ductile layers; (iii) extension rate; (iv) width of the weak zone localizing extension; and (v) rift obliquity with respect to the extension direction. An increase in the amount of extension corresponding to the inward migration of faulting (i.e., a longer phase of slip on boundary faults) is observed for (a) an increase in the thickness of both brittle and ductile crustal layers and syn-rift sediment accumulation, (b) a decrease in extension rate and width of the weak zone, and (c) a decrease in rift obliquity. A unified account of these correlations is presented, based on the hypothesis that fault migration occurs when boundary faults can no longer accommodate the imposed bulk extension, leading to time-space variations of internal strain and strain rate (and consequently stress) in the ductile layers which overcome the total resistance of brittle layers to thoroughgoing faulting
Acoustic charge transport in n-i-n three terminal device
We present an unconventional approach to realize acoustic charge transport
devices that takes advantage from an original input region geometry in place of
standard Ohmic input contacts. Our scheme is based on a n-i-n lateral junction
as electron injector, an etched intrinsic channel, a standard Ohmic output
contact and a pair of in-plane gates. We show that surface acoustic waves are
able to pick up electrons from a current flowing through the n-i-n junction and
steer them toward the output contact. Acoustic charge transport was studied as
a function of the injector current and bias, the SAW power and at various
temperatures. The possibility to modulate the acoustoelectric current by means
of lateral in-plane gates is also discussed. The main advantage of our approach
relies on the possibility to drive the n-i-n injector by means of both voltage
or current sources, thus allowing to sample and process voltage and current
signals as well.Comment: 9 pages, 3 figures. Submitted to Applied Physics Letter
Generation of different Bell states within the SPDC phase-matching bandwidth
We study the frequency-angular lineshape for a phase-matched nonlinear
process producing entangled states and show that there is a continuous variety
of maximally-entangled states generated for different mismatch values within
the natural bandwidth. Detailed considerations are made for two specific
methods of polarization entanglement preparation, based on type-II spontaneous
parametric down-conversion (SPDC) and on SPDC in two subsequent type-I crystals
producing orthogonally polarized photon pairs. It turns out that different Bell
states are produced at the center of the SPDC line and on its slopes,
corresponding to about half-maximum intensity level. These Bell states can be
filtered out by either frequency selection or angular selection, or both. Our
theoretical calculations are confirmed by a series of experiments, performed
for the two above-mentioned schemes of producing polarization-entangled photon
pairs and with two kinds of measurements: frequency-selective and
angular-selective.Comment: submitted for publicatio
On the stationary points of the TAP free energy
In the context of the p-spin spherical model, we introduce a method for the
computation of the number of stationary points of any nature (minima, saddles,
etc.) of the TAP free energy. In doing this we clarify the ambiguities related
to the approximations usually adopted in the standard calculations of the
number of states in mean field spin glass models.Comment: 11 pages, 1 Postscript figure, plain Te
From rods to helices: evidence of a screw-like nematic phase
Evidence of a special chiral nematic phase is provided using numerical
simulation and Onsager theory for systems of hard helical particles. This phase
appears at the high density end of the nematic phase, when helices are well
aligned, and is characterized by the C symmetry axes of the helices
spiraling around the nematic director with periodicity equal to the particle
pitch. This coupling between translational and rotational degrees of freedom
allows a more efficient packing and hence an increase of translational entropy.
Suitable order parameters and correlation functions are introduced to identify
this screw-like phase, whose main features are then studied as a function of
radius and pitch of the helical particles. Our study highlights the physical
mechanism underlying a similar ordering observed in colloidal helical flagella
[E. Barry et al. \textit{Phys. Rev. Lett.} \textbf{96}, 018305 (2006)] and
raises the question of whether it could be observed in other helical particle
systems, such as DNA, at sufficiently high densities.Comment: List of authors correcte
Deformation patterns in a second-gradient lattice annular plate composed of "Spira mirabilis" fibers
In this paper, we aim to explore the mechanical potentialities of a material made of an orthogonal net of fibers arranged in logarithmic spirals. Therefore, an annular plate described with a second-gradient model is envisaged to evaluate the behavior of such material in a nonlinear elastic regime when large displacements and deformations occur. Several mechanical tests are performed numerically under the finite element method approximation obtained directly with a weak formulation based on the elastic energy that it is assumed to be predictive for this kind of network system of fibers. Plots reporting the mechanical characteristics in all the considered tests are provided to illustrate the overall mechanical behavior of the evaluated system
- …