1,829 research outputs found
Thermodynamics in the vicinity of a relativistic quantum critical point in 2+1 dimensions
We study the thermodynamics of the relativistic quantum O() model in two
space dimensions. In the vicinity of the zero-temperature quantum critical
point (QCP), the pressure can be written in the scaling form
P(T)=P(0)+N(T^3/c^2)\calF_N(\Delta/T) where is the velocity of the
excitations at the QCP and is a characteristic zero-temperature energy
scale. Using both a large- approach to leading order and the nonperturbative
renormalization group, we compute the universal scaling function \calF_N. For
small values of () we find that \calF_N(x) is nonmonotonous
in the quantum critical regime () with a maximum near . The
large- approach -- if properly interpreted -- is a good approximation both
in the renormalized classical () and quantum disordered
() regimes, but fails to describe the nonmonotonous behavior of
\calF_N in the quantum critical regime. We discuss the renormalization-group
flows in the various regimes near the QCP and make the connection with the
quantum nonlinear sigma model in the renormalized classical regime. We compute
the Berezinskii-Kosterlitz-Thouless transition temperature in the quantum O(2)
model and find that in the vicinity of the QCP the universal ratio
\Tkt/\rho_s(0) is very close to , implying that the stiffness
\rho_s(\Tkt^-) at the transition is only slightly reduced with respect to the
zero-temperature stiffness . Finally, we briefly discuss the
experimental determination of the universal function \calF_2 from the
pressure of a Bose gas in an optical lattice near the
superfluid--Mott-insulator transition.Comment: v1) 16 pages, 10 figures. v2) Revised versio
Focal plane wavefront sensor achromatization : The multireference self-coherent camera
High contrast imaging and spectroscopy provide unique constraints for
exoplanet formation models as well as for planetary atmosphere models. But this
can be challenging because of the planet-to-star small angular separation and
high flux ratio. Recently, optimized instruments like SPHERE and GPI were
installed on 8m-class telescopes. These will probe young gazeous exoplanets at
large separations (~1au) but, because of uncalibrated aberrations that induce
speckles in the coronagraphic images, they are not able to detect older and
fainter planets. There are always aberrations that are slowly evolving in time.
They create quasi-static speckles that cannot be calibrated a posteriori with
sufficient accuracy. An active correction of these speckles is thus needed to
reach very high contrast levels (>1e7). This requires a focal plane wavefront
sensor. Our team proposed the SCC, the performance of which was demonstrated in
the laboratory. As for all focal plane wavefront sensors, these are sensitive
to chromatism and we propose an upgrade that mitigates the chromatism effects.
First, we recall the principle of the SCC and we explain its limitations in
polychromatic light. Then, we present and numerically study two upgrades to
mitigate chromatism effects: the optical path difference method and the
multireference self-coherent camera. Finally, we present laboratory tests of
the latter solution.
We demonstrate in the laboratory that the MRSCC camera can be used as a focal
plane wavefront sensor in polychromatic light using an 80 nm bandwidth at 640
nm. We reach a performance that is close to the chromatic limitations of our
bench: contrast of 4.5e-8 between 5 and 17 lambda/D.
The performance of the MRSCC is promising for future high-contrast imaging
instruments that aim to actively minimize the speckle intensity so as to detect
and spectrally characterize faint old or light gaseous planets.Comment: 14 pages, 20 figure
Dimensional crossover and metal-insulator transition in quasi-two-dimensional disordered conductors
We study the metal-insulator transition (MIT) in weakly coupled disordered
planes on the basis of a Non-Linear Sigma Model (NLM). Using two
different methods, a renormalization group (RG) approach and an auxiliary field
method, we calculate the crossover length between a 2D regime at small length
scales and a 3D regime at larger length scales. The 3D regime is described by
an anisotropic 3D NLM with renormalized coupling constants. We obtain
the critical value of the single particle interplane hopping which separates
the metallic and insulating phases. We also show that a strong parallel
magnetic field favors the localized phase and derive the phase diagram.Comment: 16 pages (RevTex), 4 poscript figure
Laboratory validation of the dual-zone phase mask coronagraph in broadband light at the high-contrast imaging THD-testbed
Specific high contrast imaging instruments are mandatory to characterize
circumstellar disks and exoplanets around nearby stars. Coronagraphs are
commonly used in these facilities to reject the diffracted light of an observed
star and enable the direct imaging and spectroscopy of its circumstellar
environment. One important property of the coronagraph is to be able to work in
broadband light.
Among several proposed coronagraphs, the dual-zone phase mask coronagraph is
a promising solution for starlight rejection in broadband light. In this paper,
we perform the first validation of this concept in laboratory.
First, we recall the principle of the dual-zone phase mask coronagraph. Then,
we describe the high-contrast imaging THD testbed, the manufacturing of the
components and the quality-control procedures. Finally, we study the
sensitivity of our coronagraph to low-order aberrations (inner working angle
and defocus) and estimate its contrast performance. Our experimental broadband
light results are compared with numerical simulations to check agreement with
the performance predictions.
With the manufactured prototype and using a dark hole technique based on the
self-coherent camera, we obtain contrast levels down to between 5
and 17 in monochromatic light (640 nm). We also reach contrast
levels of between 7 and 17 in broadband
( nm, nm and %), which demonstrates the excellent chromatic performance of the dual-zone
phase mask coronagraph.
The performance reached by the dual-zone phase mask coronagraph is promising
for future high-contrast imaging instruments that aim at detecting and
spectrally characterizing old or light gaseous planets.Comment: 9 pages, 16 figure
Ordering dynamics of blue phases entails kinetic stabilization of amorphous networks
The cubic blue phases of liquid crystals are fascinating and technologically
promising examples of hierarchically structured soft materials, comprising
ordered networks of defect lines (disclinations) within a liquid crystalline
matrix. We present the first large-scale simulations of their domain growth,
starting from a blue phase nucleus within a supercooled isotropic or
cholesteric background. The nucleated phase is thermodynamically stable; one
expects its slow orderly growth, creating a bulk cubic. Instead, we find that
the strong propensity to form disclinations drives the rapid disorderly growth
of a metastable amorphous defect network. During this process the original
nucleus is destroyed; re-emergence of the stable phase may therefore require a
second nucleation step. Our findings suggest that blue phases exhibit
hierarchical behavior in their ordering dynamics, to match that in their
structure.Comment: 11 pages, 5 figures, 2 supplementary figures, 2 supplementary tables,
accepted by PNA
Large Deviations Analysis for Distributed Algorithms in an Ergodic Markovian Environment
We provide a large deviations analysis of deadlock phenomena occurring in
distributed systems sharing common resources. In our model transition
probabilities of resource allocation and deallocation are time and space
dependent. The process is driven by an ergodic Markov chain and is reflected on
the boundary of the d-dimensional cube. In the large resource limit, we prove
Freidlin-Wentzell estimates, we study the asymptotic of the deadlock time and
we show that the quasi-potential is a viscosity solution of a Hamilton-Jacobi
equation with a Neumann boundary condition. We give a complete analysis of the
colliding 2-stacks problem and show an example where the system has a stable
attractor which is a limit cycle
Decoupling with unitary approximate two-designs
Consider a bipartite system, of which one subsystem, A, undergoes a physical
evolution separated from the other subsystem, R. One may ask under which
conditions this evolution destroys all initial correlations between the
subsystems A and R, i.e. decouples the subsystems. A quantitative answer to
this question is provided by decoupling theorems, which have been developed
recently in the area of quantum information theory. This paper builds on
preceding work, which shows that decoupling is achieved if the evolution on A
consists of a typical unitary, chosen with respect to the Haar measure,
followed by a process that adds sufficient decoherence. Here, we prove a
generalized decoupling theorem for the case where the unitary is chosen from an
approximate two-design. A main implication of this result is that decoupling is
physical, in the sense that it occurs already for short sequences of random
two-body interactions, which can be modeled as efficient circuits. Our
decoupling result is independent of the dimension of the R system, which shows
that approximate 2-designs are appropriate for decoupling even if the dimension
of this system is large.Comment: Published versio
Large deviations for polling systems
Related INRIA Research report available at : http://hal.inria.fr/docs/00/07/27/62/PDF/RR-3892.pdfInternational audienceWe aim at presenting in short the technical report, which states a sample path large deviation principle for a resealed process n-1 Qnt, where Qt represents the joint number of clients at time t in a single server 1-limited polling system with Markovian routing. The main goal is to identify the rate function. A so-called empirical generator is introduced, which consists of Q t and of two empirical measures associated with S t the position of the server at time t. The analysis relies on a suitable change of measure and on a representation of fluid limits for polling systems. Finally, the rate function is solution of a meaningful convex program
Laser Ultrasonics: A Non Contacting NDT System
Ultrasound is today one of the commonest nondestructive methods in industry, in particular for the inspection of composites in the aerospace industry. Piezoelectric transducers are generally used to emit and receive the ultrasound, but have a number of disadvantages, both in use on a production line and for the inspection of complex parts
Spin-density-wave instabilities in the organic conductor (TMTSF)_2ClO_4: Role of anion ordering
We study the spin-density-wave instabilities in the quasi-one-dimensional
conductor (TMTSF)_2ClO_4. The orientational order of the anions ClO_4 doubles
the unit cell and leads to the presence of two electrnic bands at the Fermi
level. From the Ginzburg-Landau expansion of the free energy, we determine the
low-temperature phase diagram as a function of the strength of the Coulomb
potential due to the anions. Upon increasing the anion potential, we first find
a SDW phase corresponding to an interband pairing. This SDW phase is rapidly
supressed, the metallic phase being then stable down to zero temperature. The
SDW instability is restored when the anion potential becomes of the order of
the hopping amplitude. The metal-SDW transition corresponds to an intraband
pairing which leaves half of the Fermi surface metallic. At lower temperature,
a second transition, corresponding to the other intraband pairing, takes place
and opens a gap on the whole Fermi surface. We discuss the consequences of our
results for the experimental phase diagram of (TMTSF)_2ClO_4 at high magnetic
field.Comment: 13 pages, 10 figures, Version 2 with minor correction
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