267 research outputs found
Casimir-Polder shifts on quantum levitation states
An ultracold atom above a horizontal mirror experiences quantum reflection
from the attractive Casimir-Polder interaction, which holds it against gravity
and leads to quantum levitation states. We analyze this system by using a
Liouville transformation of the Schr\"odinger equation and a Langer coordinate
adapted to problems with a classical turning point. Reflection on the
Casimir-Polder attractive well is replaced by reflection on a repulsive wall
and the problem is then viewed as an ultracold atom trapped inside a cavity
with gravity and Casimir-Polder potentials acting respectively as top and
bottom mirrors. We calculate numerically Casimir-Polder shifts of the energies
of the cavity resonances and propose a new approximate treatment which is
precise enough to discuss spectroscopy experiments aiming at tests of the weak
equivalence principle on antihydrogen. We also discuss the lifetimes by
calculating complex energies associated with cavity resonances.Comment: Accepted in PR
Quantum reflection of antihydrogen from a liquid helium film
We study the quantum reflection of ultracold antihydrogen atoms bouncing on
the surface of a liquid helium film. The Casimir-Polder potential and quantum
reflection are calculated for different thicknesses of the film supported by
different substrates. Antihydrogen can be protected from anni- hilation for as
long as 1.3s on a bulk of liquid 4He, and 1.7s for liquid 3He. These large
lifetimes open interesting perspectives for spectroscopic measurements of the
free fall acceleration of antihydrogen. Variation of the scattering length with
the thickness of a film of helium shows interferences which we interpret
through a Liouville transformation of the quantum reflection problem
Microstrain analysis of titanium aluminides
International audienceThe aeronautic and automotive industries have shown a renewed interest in TiAl based alloys. The main reasons for such an interest are their low density (~3,8g/cm3), a good stiffness and a high strength for temperatures up to 750°C. However, these alloys exhibit, in their polycrystalline form, a poor ductility at room temperature with widely scattered values. The aim of this study is therefore to characterise their mechanical behaviour with a multiscale methodology, coupling microstructure analysis and strain field measurements. This methodology employs orientation imaging microscopy as well as digital imaging correlation techniques with an intragranular step size of a few micrometers. Two chemical compositions (47 at. % Al and 48 at. % Al) and two processing routes (casting and powder metallurgy) are studied. Thus, four different types of final microstructures are considered, from fully lamellar Ti3Al (a2) + TiAl (g) microstructure to bimodal ones composed of two-phase (a2+g) lamellar grains and monolithic g grains. Firstly, the microstructure is characterised crystallographically and morphologically. This allows the identification of a representative volume element (RVE) inside the analysed volume. Then, uniaxial mechanical tests are performed for each microstructure, and the strain fields are analysed with a multiscale approach, which determines the spatial distribution of the strain field heterogeneity with respect to the different microstructures
Identification of crystalline behavior on macroscopic response and local strain field analysis: application to alpha zirconium alloys
The purpose of this paper is to present an identification method of the crystalline behavior of a material from a mechanical test performed on a polycrystalline sample. Because of the lack of knowledge about its crystalline behavior, this method is applied to a Zirconium alloy. This identification is based on a finite element modeling of the microstructure, and the results are compared to both the macroscopic and the microscopic experimental results. On the microscopic scale, the plastic strains are obtained using a micro-extensometry technique and the crystalline orientation using an EBSD technique. In order to validate the method, an identification is performed with only two free parameters: the evolutions of the macroscopic and microscopic errors appear to be regular and exhibit a well-defined minimum so that the parameters can be clearly identified
The Dynamics of Ecosystems, Biodiversity Management and Social Institutions at High Northern Latitudes
Ecosystems at high latitudes are highly dynamic, influenced
by a multitude of large-scale disturbances. Due to global
change processes these systems may be expected to be
particularly vulnerable, affecting the sustained production
of renewable wood resources and abundance of plants
and animals on which local cultures depend. In this paper,
we assess the implications of new understandings of high
northern latitude ecosystems and what must be done to
manage systems for resilience. We suggest that the focus
of land management should shift from recovery from local
disturbance to sustaining ecosystem functions in the face
of change and disruption. The role of biodiversity as insurance
for allowing a system to reorganize and develop during
the disturbance and reorganization phases needs to be
addressed in management and policy. We emphasize that
the current concepts of ecological reserves and protected
areas need to be reconsidered to developp dynamic tools
for sustainable management of ecosystems in face of
change. Characteristics of what may be considered as customary
reserves at high latitudes are often consistent with
a more dynamic view of reserves. We suggest new directions
for addressing biodiversity management in dynamic
landscapes at high latitudes, and provide empirical examples
of insights from unconventional perspectives that may
help improve the potential for sustainable management of
biodiversity and the generation of ecosystem services
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