466 research outputs found
Packing and Hausdorff measures of stable trees
In this paper we discuss Hausdorff and packing measures of random continuous
trees called stable trees. Stable trees form a specific class of L\'evy trees
(introduced by Le Gall and Le Jan in 1998) that contains Aldous's continuum
random tree (1991) which corresponds to the Brownian case. We provide results
for the whole stable trees and for their level sets that are the sets of points
situated at a given distance from the root. We first show that there is no
exact packing measure for levels sets. We also prove that non-Brownian stable
trees and their level sets have no exact Hausdorff measure with regularly
varying gauge function, which continues previous results from a joint work with
J-F Le Gall (2006).Comment: 40 page
Rigidity and intermediate phases in glasses driven by speciation
The rigid to floppy transitions and the associated intermediate phase in
glasses are studied in the case where the local structure is not fully
determined from the macroscopic concentration. The approach uses size
increasing cluster approximations and constraint counting algorithms. It is
shown that the location and the width of the intermediate phase and the
corresponding structural, mechanical and energetical properties of the network
depend crucially on the way local structures are selected at a given
concentration. The broadening of the intermediate phase is obtained for
networks combining a large amount of flexible local structural units and a high
rate of medium range order.Comment: 4 pages, 4 figure
Irreversible magnetization switching using surface acoustic waves
An analytical and numerical approach is developped to pinpoint the optimal
experimental conditions to irreversibly switch magnetization using surface
acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and
two switching mechanisms are considered. In precessional switching, a small
in-plane field initially tilts the magnetization and the passage of the SAW
modifies the magnetic anisotropy parameters through inverse magneto-striction,
which triggers precession, and eventually reversal. Using the micromagnetic
parameters of a fully characterized layer of the magnetic semiconductor
(Ga,Mn)(As,P), we then show that there is a large window of accessible
experimental conditions (SAW amplitude/wave-vector, field
amplitude/orientation) allowing irreversible switching. As this is a resonant
process, the influence of the detuning of the SAW frequency to the magnetic
system's eigenfrequency is also explored. Finally, another - non-resonant -
switching mechanism is briefly contemplated, and found to be applicable to
(Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small
perpendicular field is applied opposite the initial magnetization and the
passage of the SAW lowers the domain nucleation barrier.Comment: 11 pages, 4 figure
Extending Elliptic Curve Chabauty to higher genus curves
We give a generalization of the method of "Elliptic Curve Chabauty" to higher
genus curves and their Jacobians. This method can sometimes be used in
conjunction with covering techniques and a modified version of the Mordell-Weil
sieve to provide a complete solution to the problem of determining the set of
rational points of an algebraic curve .Comment: 24 page
Combining leak--resistant arithmetic for elliptic curves defined over \F_p and RNS representation
In this paper we combine the residue number system (RNS) representation
and the leak-resistant arithmetic on elliptic curves. These two techniques are relevant for implementation of elliptic curve
cryptography on embedded devices.\\
% since they have leak-resistance properties.
It is well known that the RNS multiplication is very efficient whereas the reduction step is costly. Hence, we optimize formulae for basic operations arising in leak-resistant arithmetic on elliptic curves (unified addition, Montgomery ladder) in order to minimize the number of modular reductions. We also improve the complexity of the RNS modular reduction step. As a result, we show how to obtain a competitive secured implementation.\\
Finally, %we recall the main advantages of the RNS representation,
%especially in hardware and for embedded devices, and
we show that,
contrary to other approaches, ours takes optimally the advantage of a dedicated parallel
architecture
Universal behavior of internal friction in glasses below T : anharmonicity vs relaxation
Comparison of the internal friction at hypersonic frequencies between a few K
and the glass transition temperature Tg for various glasses brings out general
features. At low temperature, internal friction is only weakly dependent on the
material. At high temperature but still below Tg the internal friction for
strong glasses shows a T-independent plateau in a very wide domain of
temperature; in contrast, for fragile glass, a nearly linear variation of
internal friction with T is observed. Anharmonicity appears dominant over
thermally activated relaxational processes at high temperature.Comment: accepted in Physical Review
Effect of binder on performance of intumescent coatings
This study investigates the role of the polymeric binder on the properties and performance of an intumescent coating. Waterborne resins of different types (vinylic, acrylic, and styrene-acrylic) were incorporated in an intumescent paint formulation, and characterized extensively in terms of thermal degradation behavior, intumescence thickness, and thermal insulation. Thermal microscopy images of charred foam development provided further information on the particular performance of each type of coating upon heating. The best foam expansion and heat protection results were obtained with the vinyl binders. Rheological measurements showed a complex evolution of the viscoelastic characteristics of the materials with temperature. As an example, the vinyl binders unexpectedly hardened significantly after thermal degradation. The values of storage moduli obtained at the onset of foam blowing (melamine decomposition) were used to explain different intumescence expansion behaviors.Funding for this work was provided by FCT-Fundacao para a Ciencia e Tecnologia (Project PTDC/EQU-EQU/65300/2006), and by FEDER/QREN (project RHED) in the - framework of Programa Operacional Factor de Competitividade-COMPETE. Joana Pimenta thanks FCT for PhD Grant SFRH/BDE/33431/2008
- …