520 research outputs found
Cryogenic Sapphire Oscillator using a low-vibration design pulse-tube cryocooler: First results
A Cryogenic Sapphire Oscillator has been implemented at 11.2 GHz using a
low-vibration design pulse-tube cryocooler. Compared with a state-of-the-art
liquid helium cooled CSO in the same laboratory, the square root Allan variance
of their combined fractional frequency instability is for integration times s, dominated by
white frequency noise. The minimum for the two
oscillators was reached at s. Assuming equal contributions from
both CSOs, the single oscillator phase noise at 1 Hz offset from the carrier.Comment: 5 pages, 5 figures, accepted in IEEE Trans on Ultrasonics,
Ferroelectrics and Frequency Contro
On the global evolution of self-gravitating matter. Nonlinear interactions in Gowdy symmetry
We are interested in the evolution of a compressible fluid under its
self-generated gravitational field. Assuming here Gowdy symmetry, we
investigate the algebraic structure of the Euler equations satisfied by the
mass density and velocity field. We exhibit several interaction functionals
that provide us with a uniform control on weak solutions in suitable Sobolev
norms or in bounded variation. These functionals allow us to study the local
regularity and nonlinear stability properties of weakly regular fluid flows
governed by the Euler-Gowdy system. In particular for the Gowdy equations, we
prove that a spurious matter field arises under weak convergence, and we
establish the nonlinear stability of weak solutions.Comment: 37 pages. v2: fix typos to match version published in 201
Well-posedness theory for geometry-compatible hyperbolic conservation laws on manifolds
Motivated by many applications (geophysical flows, general relativity), we
attempt to set the foundations for a study of entropy solutions to nonlinear
hyperbolic conservation laws posed on a (Riemannian or Lorentzian) manifold.
The flux of the conservation laws is viewed as a vector-field on the manifold
and depends on the unknown function as a parameter. We introduce notions of
entropy solutions in the class of bounded measurable functions and in the class
of measure-valued mappings. We establish the well-posedness theory for
conservation laws on a manifold, by generalizing both Kruzkov's and DiPerna's
theories originally developed in the Euclidian setting. The class of {\sl
geometry-compatible} (as we call it) conservation laws is singled out as an
important case of interest, which leads to robust estimates independent
of the geometry of the manifold. On the other hand, general conservation laws
solely enjoy the contraction property and leads to a unique contractive
semi-group of entropy solutions. Our framework allows us to construct entropy
solutions on a manifold via the vanishing diffusion method or the finite volume
method.Comment: 30 pages. This is Part 1 of a serie
Cyclic spacetimes through singularity scattering maps. The laws of quiescent bounces
For spacetimes containing singularity hypersurfaces we propose a general
notion of junction conditions based on a prescribed singularity scattering map,
as we call it, and we introduce the notion of a cyclic spacetime (also called a
multiverse) consisting of spacetime domains bounded by spacelike or timelike
singularity hypersurfaces, across which our scattering map is applied. A local
existence theory is established here while, in a companion paper, we construct
plane-symmetric cyclic spacetimes. We study the singularity data space
consisting of the suitably rescaled metric, extrinsic curvature, and matter
fields which can be prescribed on each side of the singularity, and for the
class of so-called quiescent singularities we establish restrictions that a
singularity scattering map must satisfy. We obtain a full characterization of
all scattering maps that are covariant and ultralocal, in a sense we define
and, in particular, we distinguish between, on the one hand, three laws of
bouncing cosmology of universal nature and, on the other hand, model-dependent
junction conditions. The theory proposed in this paper applies to spacelike and
timelike hypersurfaces and without symmetry restriction, and encompasses
bouncing-cosmology scenarios, both in string theory and in loop quantum
cosmology, and puts strong restrictions on their possible explicit
realizations.Comment: 56 pages. Construction of plane symmetric cyclic spacetimes moved to
a separate pape
Universal scattering laws for bouncing cosmology
Bouncing cosmologies can arise from various gravity theories. We model them
through a singularity scattering map, as we call it, relating large scale
geometries before and after the bounce. By classifying all suitably local maps
we uncover universal laws (scaling of Kasner exponents, canonical
transformation of matter). We study the singularity scattering map for Bianchi
I bounces in string theory, loop quantum cosmology and modified matter models:
our classification then determines how general spatial inhomogeneities and
anisotropies (without BKL oscillations) are transmitted through bounces.Comment: 5 page
Detrapping and retrapping of free carriers in nominally pure single crystal GaP, GaAs and 4H-SiC semiconductors under light illumination at cryogenic temperatures
We report on extremely sensitive measurements of changes in the microwave
properties of high purity non-intentionally-doped single-crystal semiconductor
samples of gallium phosphide, gallium arsenide and 4H-silicon carbide when
illuminated with light of different wavelengths at cryogenic temperatures.
Whispering gallery modes were excited in the semiconductors whilst they were
cooled on the coldfinger of a single-stage cryocooler and their frequencies and
Q-factors measured under light and dark conditions. With these materials, the
whispering gallery mode technique is able to resolve changes of a few parts per
million in the permittivity and the microwave losses as compared with those
measured in darkness. A phenomenological model is proposed to explain the
observed changes, which result not from direct valence to conduction band
transitions but from detrapping and retrapping of carriers from impurity/defect
sites with ionization energies that lay in the semiconductor band gap.
Detrapping and retrapping relaxation times have been evaluated from comparison
with measured data.Comment: 7 pages, 6 figure
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