413 research outputs found
Stabilité et renforcement des fronts de taille des tunnels : une approche analytique en contraintes-déformations
National audienceConstruction process of tunnels involve more and more frequently a full face excavation, with high faces. Therefore engineers have to analyse the face stability and to design reinforcement by longitudinal fibreglass bolts. The current methods are mainly based on stability analysis, but they scarcely allow an evaluation of the deformations, except when using numerical modelling, in which the consideration of reinforcements induces very heavy models, much difficult to use in engineering practice. This paper describes a new stress-strain approach, based on a spherical principle, and allowing the calculation of face deformations, including when bolts reinforcement is used. We present the general principles of the methods and its validation, then various parametric analysis, and finally some cases of practical use: evaluation of a safety factor, design of face reinforcement, effect of the confining pressure on the face deformations when using a TBM.Les tunnels sont de plus en plus réalisés en pleine section, ce qui conduit à des hauteurs du front de taille parfois très importantes. C'est pourquoi les ingénieurs sont fréquemment appelés à examiner la tenue du front, et à prévoir son renforcement par des boulons longitudinaux en fibre de verre. Les approches actuelles privilégient les analyses en stabilité, mais ne permettent guère une évaluation des déformations du front, sauf à utiliser des méthodes numériques, dans lesquels la prise en compte des renforcements conduit à des modèles très lourds et peu utilisables en pratique courante. Cet article présente une nouvelle approche analytique en contraintes-déformations, basée sur un principe de symétrie sphérique, et permettant d'évaluer les déformations d'extrusion du front, et ce même lorsque le front est renforcé par des boulons. Nous présentons en premier lieu les principes de la méthode ainsi que sa validation, puis différentes études paramétriques, ainsi que quelques cas possibles d'utilisation : évaluation d'un coefficient de sécurité, dimensionnement d'un renforcement par boulonnage, effet de la pression de confinement sur l'extrusion lors de l'utilisation de boucliers
Determination of the Fermion Pair Size in a Resonantly Interacting Superfluid
Fermionic superfluidity requires the formation of pairs. The actual size of
these fermion pairs varies by orders of magnitude from the femtometer scale in
neutron stars and nuclei to the micrometer range in conventional
superconductors. Many properties of the superfluid depend on the pair size
relative to the interparticle spacing. This is expressed in BCS-BEC crossover
theories, describing the crossover from a Bardeen-Cooper-Schrieffer (BCS) type
superfluid of loosely bound and large Cooper pairs to Bose-Einstein
condensation (BEC) of tightly bound molecules. Such a crossover superfluid has
been realized in ultracold atomic gases where high temperature superfluidity
has been observed. The microscopic properties of the fermion pairs can be
probed with radio-frequency (rf) spectroscopy. Previous work was difficult to
interpret due to strong and not well understood final state interactions. Here
we realize a new superfluid spin mixture where such interactions have
negligible influence and present fermion-pair dissociation spectra that reveal
the underlying pairing correlations. This allows us to determine the
spectroscopic pair size in the resonantly interacting gas to be 2.6(2)/kF (kF
is the Fermi wave number). The pairs are therefore smaller than the
interparticle spacing and the smallest pairs observed in fermionic superfluids.
This finding highlights the importance of small fermion pairs for superfluidity
at high critical temperatures. We have also identified transitions from fermion
pairs into bound molecular states and into many-body bound states in the case
of strong final state interactions.Comment: 8 pages, 7 figures; Figures updated; New Figures added; Updated
discussion of fit function
From Cavity Electromechanics to Cavity Optomechanics
We present an overview of experimental work to embed high-Q mesoscopic
mechanical oscillators in microwave and optical cavities. Based upon recent
progress, the prospect for a broad field of "cavity quantum mechanics" is very
real. These systems introduce mesoscopic mechanical oscillators as a new
quantum resource and also inherently couple their motion to photons throughout
the electromagnetic spectrum.Comment: 8 pages, 6 figures, ICAP proceedings submissio
Broad Feshbach resonance in the 6Li-40K mixture
We study the widths of interspecies Feshbach resonances in a mixture of the
fermionic quantum gases 6Li and 40K. We develop a model to calculate the width
and position of all available Feshbach resonances for a system. Using the model
we select the optimal resonance to study the 6Li/40K mixture. Experimentally,
we obtain the asymmetric Fano lineshape of the interspecies elastic cross
section by measuring the distillation rate of 6Li atoms from a potassium-rich
6Li/40K mixture as a function of magnetic field. This provides us with the
first experimental determination of the width of a resonance in this mixture,
Delta B=1.5(5) G. Our results offer good perspectives for the observation of
universal crossover physics using this mass-imbalanced fermionic mixture.Comment: 4 pages, 2 figure
Tensile strained membranes for cavity optomechanics
We investigate the optomechanical properties of tensile-strained ternary
InGaP nanomembranes grown on GaAs. This material system combines the benefits
of highly strained membranes based on stoichiometric silicon nitride, with the
unique properties of thin-film semiconductor single crystals, as previously
demonstrated with suspended GaAs. Here we employ lattice mismatch in epitaxial
growth to impart an intrinsic tensile strain to a monocrystalline thin film
(approximately 30 nm thick). These structures exhibit mechanical quality
factors of 2*10^6 or beyond at room temperature and 17 K for eigenfrequencies
up to 1 MHz, yielding Q*f products of 2*10^12 Hz for a tensile stress of ~170
MPa. Incorporating such membranes in a high finesse Fabry-Perot cavity, we
extract an upper limit to the total optical loss (including both absorption and
scatter) of 40 ppm at 1064 nm and room temperature. Further reductions of the
In content of this alloy will enable tensile stress levels of 1 GPa, with the
potential for a significant increase in the Q*f product, assuming no
deterioration in the mechanical loss at this composition and strain level. This
materials system is a promising candidate for the integration of strained
semiconductor membrane structures with low-loss semiconductor mirrors and for
realizing stacks of membranes for enhanced optomechanical coupling.Comment: 10 pages, 3 figure
Production of cold molecules via magnetically tunable Feshbach resonances
Magnetically tunable Feshbach resonances were employed to associate cold
diatomic molecules in a series of experiments involving both atomic Bose as
well as two spin component Fermi gases. This review illustrates theoretical
concepts of both the particular nature of the highly excited Feshbach molecules
produced and the techniques for their association from unbound atom pairs.
Coupled channels theory provides the rigorous formulation of the microscopic
physics of Feshbach resonances in cold gases. Concepts of dressed versus bare
energy states, universal properties of Feshbach molecules, as well as the
classification in terms of entrance- and closed-channel dominated resonances
are introduced on the basis of practical two-channel approaches. Their
significance is illustrated for several experimental observations, such as
binding energies and lifetimes with respect to collisional relaxation.
Molecular association and dissociation are discussed in the context of
techniques involving linear magnetic field sweeps in cold Bose and Fermi gases
as well as pulse sequences leading to Ramsey-type interference fringes. Their
descriptions in terms of Landau-Zener, two-level mean field as well as beyond
mean field approaches are reviewed in detail, including the associated ranges
of validity.Comment: 50 pages, 26 figures, to be published in Reviews of Modern Physics,
final version with updated reference
Formation of a vortex lattice in a rotating BCS Fermi gas
We investigate theoretically the formation of a vortex lattice in a
superfluid two-spin component Fermi gas in a rotating harmonic trap, in a
BCS-type regime of condensed non-bosonic pairs. Our analytical solution of the
superfluid hydrodynamic equations, both for the 2D BCS equation of state and
for the 3D unitary quantum gas, predicts that the vortex free gas is subject to
a dynamic instability for fast enough rotation. With a numerical solution of
the full time dependent BCS equations in a 2D model, we confirm the existence
of this dynamic instability and we show that it leads to the formation of a
regular pattern of quantum vortices in the gas.Comment: 14 page
Superfluidity and binary-correlations within clusters of fermions
We propose a method for simulating the behaviour of small clusters of
particles that explicitly accounts for all mean-field and binary-correlation
effects. Our approach leads to a set of variational equations that can be used
to study both the dynamics and thermodynamics of these clusters. As an
illustration of this method, we explore the BCS-BEC crossover in the simple
model of four fermions, interacting with finite-range potentials, in a harmonic
potential. We find, in the crossover regime, that the particles prefer to
occupy two distinct pair states as opposed to the one assumed by BCS theory
On Quartet Superfluidity of Fermionic Atomic Gas
Possibility of a quartet superfluidity in fermionic systems is studied as a
new aspect of atomic gas at ultra low temperatures. The four-fold degeneracy of
hyperfine state and moderate coupling is indispensable for the quartet
superfluidity to occur. Possible superconductivity with quartet condensation in
electron systems is discussed.Comment: 7 pages, 1 figure. J. Phys. Soc. Jpn. vol.74 (2005) No.7, in press;
Note added for related previous works; some typographic errors revise
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