10,060 research outputs found
Heterogeneities in amorphous systems under shear
The last decade has seen major progresses in studies of elementary mechanisms
of deformation in amorphous materials. Here, we start with a review of
physically-based theories of plasticity, going back to the identification of
"shear-transformations" as early as the 70's. We show how constructive
criticism of the theoretical models permits to formulate questions concerning
the role of structural disorder, mechanical noise, and long-ranged elastic
interactions. These questions provide the necessary context to understand what
has motivated recent numerical studies. We then summarize their results, show
why they had to focus on athermal systems, and point out the outstanding
questions.Comment: Chapter of "Dynamical Heterogeneities in glasses, colloids and
granular materials", Eds.: L. Berthier, G. Biroli, J-P Bouchaud, L.
Cipelletti and W. van Saarloos (Oxford University Press, to appear), more
info at http://w3.lcvn.univ-montp2.fr/~lucacip/DH_book.ht
Dark Energy and the Return of the Phoenix Universe
In cyclic universe models based on a single scalar field (e.g., the radion
determining the distance between branes in M-theory), virtually the entire
universe makes it through the ekpyrotic smoothing and flattening phase,
bounces, and enters a new epoch of expansion and cooling. This stable evolution
cannot occur, however, if scale-invariant curvature perturbations are produced
by the entropic mechanism because it requires two scalar fields (e.g., the
radion and the Calabi-Yau dilaton) evolving along an unstable classical
trajectory. In fact, we show here that an overwhelming fraction of the universe
fails to make it through the ekpyrotic phase; nevertheless, a sufficient volume
survives and cycling continues forever provided the dark energy phase of the
cycle lasts long enough, of order a trillion years. Two consequences are a new
role for dark energy and a global structure of the universe radically different
from that of eternal inflation.Comment: 5 pages, 3 figure
The spherical symmetry Black hole collapse in expanding universe
The spherical symmetry Black holes are considered in expanding background.
The singularity line and the marginally trapped tube surface behavior are
discussed. In particular, we address the conditions whether dynamical horizon
forms for these cosmological black holes. We also discuss about the
cosmological constant effect on these black hole and the redshift of the light
which comes from the marginally trapped tube surface.Comment: 7 pages, 3 figures. Accepted for publication in International Journal
of Modern Physics D (IJMPD). arXiv admin note: text overlap with
arXiv:gr-qc/0308033 and arXiv:gr-qc/030611
Driven activation versus thermal activation
Activated dynamics in a glassy system undergoing steady shear deformation is
studied by numerical simulations. Our results show that the external driving
force has a strong influence on the barrier crossing rate, even though the
reaction coordinate is only weakly coupled to the nonequilibrium system. This
"driven activation" can be quantified by introducing in the Arrhenius
expression an effective temperature, which is close to the one determined from
the fluctuation-dissipation relation. This conclusion is supported by
analytical results for a simplified model system.Comment: 5 pages, 3 figure
Big Bang Nucleosynthesis Constraints on the Self-Gravity of Pressure
Using big bang nucleosynthesis and present, high-precision measurements of
light element abundances, we constrain the self-gravity of radiation pressure
in the early universe. The self-gravity of pressure is strictly non-Newtonian,
and thus the constraints we set provide a direct test of this prediction of
general relativity and of the standard, Robertson-Walker-Friedmann cosmology.Comment: 5 pages, 1 figure. This paper was developed from an earlier version
which was posted as arXiv:0707.358
Enhancement of the Spin Accumulation at the Interface Between a Spin-Polarized Tunnel Junction and a Semiconductor
We report on spin injection experiments at a Co/AlO/GaAs interface
with electrical detection. The application of a transverse magnetic field
induces a large voltage drop at the interface as high as 1.2mV for a
current density of 0.34 nA.. This represents a dramatic increase of
the spin accumulation signal, well above the theoretical predictions for spin
injection through a ferromagnet/semiconductor interface. Such an enhancement is
consistent with a sequential tunneling process via localized states located in
the vicinity of the AlO/GaAs interface. For spin-polarized carriers
these states act as an accumulation layer where the spin lifetime is large. A
model taking into account the spin lifetime and the escape tunneling time for
carriers travelling back into the ferromagnetic contact reproduces accurately
the experimental results
High frequency GaAs nano-optomechanical disk resonator
Optomechanical coupling between a mechanical oscillator and light trapped in
a cavity increases when the coupling takes place in a reduced volume. Here we
demonstrate a GaAs semiconductor optomechanical disk system where both optical
and mechanical energy can be confined in a sub-micron scale interaction volume.
We observe giant optomechanical coupling rate up to 100 GHz/nm involving
picogram mass mechanical modes with frequency between 100 MHz and 1 GHz. The
mechanical modes are singled-out measuring their dispersion as a function of
disk geometry. Their Brownian motion is optically resolved with a sensitivity
of 10^(-17)m/sqrt(Hz) at room temperature and pressure, approaching the quantum
limit imprecision.Comment: 7 pages, 3 figure
Dark Matter Prediction from Canonical Quantum Gravity with Frame Fixing
We show how, in canonical quantum cosmology, the frame fixing induces a new
energy density contribution having features compatible with the (actual) cold
dark matter component of the Universe. First we quantize the closed
Friedmann-Robertson-Walker (FRW) model in a sinchronous reference and determine
the spectrum of the super-Hamiltonian in the presence of ultra-relativistic
matter and a perfect gas contribution. Then we include in this model small
inhomogeneous (spherical) perturbations in the spirit of the Lemaitre-Tolman
cosmology. The main issue of our analysis consists in outlining that, in the
classical limit, the non-zero eigenvalue of the super-Hamiltonian can make
account for the present value of the dark matter critical parameter.
Furthermore we obtain a direct correlation between the inhomogeneities in our
dark matter candidate and those one appearing in the ultra-relativistic matter.Comment: 5 pages, to appear on Modern Physics Letters
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