47,453 research outputs found
Large-deviation properties of the extended Moran model
The distributions of the times to the first common ancestor t_mrca is
numerically studied for an ecological population model, the extended Moran
model. This model has a fixed population size N. The number of descendants is
drawn from a beta distribution Beta(alpha, 2-alpha) for various choices of
alpha. This includes also the classical Moran model (alpha->0) as well as the
uniform distribution (alpha=1). Using a statistical mechanics-based
large-deviation approach, the distributions can be studied over an extended
range of the support, down to probabilities like 10^{-70}, which allowed us to
study the change of the tails of the distribution when varying the value of
alpha in [0,2]. We find exponential distributions p(t_mrca)~ delta^{t_mrca} in
all cases, with systematically varying values for the base delta. Only for the
cases alpha=0 and alpha=1, analytical results are known, i.e.,
delta=\exp(-2/N^2) and delta=2/3, respectively. We recover these values,
confirming the validity of our approach. Finally, we also study the
correlations between t_mrca and the number of descendants.Comment: 8 pages, 8 figure
Flexoelectric effect in finite samples
Static flexoelectric effect in a finite sample of a solid is addressed in
terms of phenomenological theory for the case of a thin plate subjected to
bending. It has been shown that despite an explicit asymmetry inherent to the
bulk constitutive electromechanical equations which take into account the
flexoelectric coupling, the electromechanical response for a finite sample is
"symmetric". "Symmetric" means that if a sensor and an actuator are made of a
flexoelectric element, performance of such devices can be characterized by the
same effective piezoelectric coefficient. This behavior is consistent with the
thermodynamic arguments offered earlier, being in conflict with the current
point of view on the matter in literature. This result was obtained using
standard mechanical boundary conditions valid for the case where the
polarization vanishes at the surface. It was shown that, for the case where
there is the polarization is nonzero at the surface, the aforementioned
symmetry of electromechanical response may be violated if standard mechanical
boundary conditions are used, leading to a conflict with the thermodynamic
arguments. It was argued that this conflict may be resolved when using modified
mechanical boundary conditions. It was also shown that the contribution of
surface piezoelectricity to the flexoelectric response of a finite sample is
expected to be comparable to that of the static bulk contribution (including
the material with high values of the dielectric constant) and to scale as the
bulk value of the dielectric constant (similar to the bulk contribution). This
finding implies that if the experimentally measured flexoelectric coefficient
scales as the dielectric constant of the material, this does not imply that the
measured flexoelectric response is controlled by the static bulk contribution
to the flexoelectric effect
Generalized synchronization: a modified system approach
The universal mechanism resulting in the generalized synchronization regime
arising in the chaotic oscillators with the dissipative coupling has been
described. The reasons of the generalized synchronization occurrence may be
clarified by means of a modified system approach. The main results are
illustrated by unidirectionally coupled Rossler systems, Rossler and Lorenz
systems and logistic maps.Comment: 4 pages, 2 figure
Critical behavior of the Random-Field Ising model at and beyond the Upper Critical Dimension
The disorder-driven phase transition of the RFIM is observed using exact
ground-state computer simulations for hyper cubic lattices in d=5,6,7
dimensions. Finite-size scaling analyses are used to calculate the critical
point and the critical exponents of the specific heat, magnetization,
susceptibility and of the correlation length. For dimensions d=6,7 which are
larger or equal to the assumed upper critical dimension, d_u=6, mean-field
behaviour is found, i.e. alpha=0, beta=1/2, gamma=1, nu=1/2. For the analysis
of the numerical data, it appears to be necessary to include recently proposed
corrections to scaling at and beyond the upper critical dimension.Comment: 8 pages and 13 figures; A consise summary of this work can be found
in the papercore database at http://www.papercore.org/Ahrens201
Material growth and characterization directed toward improving III-V heterojunction solar cells
In addition to the existing materials growth laboratory, the photolithographic facility and the device testing facility were completed. The majority of equipment for data acquisition, solar cell testing, materials growth and device characterization were received and are being put into operation. In the research part of the program, GaAs and GaA1As layers were grown reproducibly on GaAs substrates. These grown layers were characterized as to surface morphology, thickness and thickness uniformity. The liquid phase epitaxial growth process was used to fabricate p-n junctions in Ga(1-x)A1(x)As. Sequential deposition of two alloy layers was accomplished and detailed analysis of the effect of substrate quality and dopant on the GaA1As layer quality is presented. Finally, solar cell structures were formed by growing a thin p-GaA1As layer upon an epitaxial n-GaA1As layer. The energy gap corresponding to the long wavelength cutoff of the spectral response characteristic was 1.51-1.63 eV. Theoretical calculations of the spectral response were matched to the measured response
Field Equations and Equations of Motion in Post-Newtonian Approximation of the Projective Unified Field Theory
The equations of motion of gravitationally bound bodies are derived from
the field equations of Projective Unified Field Theory. The Newtonian and the
post-Newtonian approximations of the field equations and of the equations of
motion of this system of bodies are studied in detail. In analyzing some
experimental data we performed some numeric estimates of the ratio of the
inertial mass to the scalaric mass of matter.Comment: 17 page
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