30,147 research outputs found
Transport Processes in Metal-Insulator Granular Layers
Tunnel transport processes are considered in a square lattice of metallic
nanogranules embedded into insulating host to model tunnel conduction in real
metal/insulator granular layers. Based on a simple model with three possible
charging states (, or 0) of a granule and three kinetic processes
(creation or recombination of a pair, and charge transfer) between
neighbor granules, the mean-field kinetic theory is developed. It describes the
interplay between charging energy and temperature and between the applied
electric field and the Coulomb fields by the non-compensated charge density.
The resulting charge and current distributions are found to be essentially
different in the free area (FA), between the metallic contacts, or in the
contact areas (CA), beneath those contacts. Thus, the steady state dc transport
is only compatible with zero charge density and ohmic resistivity in FA, but
charge accumulation and non-ohmic behavior are \emph{necessary} for conduction
over CA. The approximate analytic solutions are obtained for characteristic
regimes (low or high charge density) of such conduction. The comparison is done
with the measurement data on tunnel transport in related experimental systems.Comment: 10 pages, 11 figures, 1 reference corrected, acknowlegments adde
Shot Noise in Magnetic Tunnel Junctions: Evidence for Sequential Tunneling
We report the experimental observation of sub-Poissonian shot noise in single
magnetic tunnel junctions, indicating the importance of tunneling via impurity
levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in
conductance, the Fano factor (normalized shot noise) depends on the magnetic
configuration being enhanced for antiparallel alignment of the ferromagnetic
electrodes. We propose a model of sequential tunneling through nonmagnetic and
paramagnetic impurity levels inside the tunnel barrier to qualitatively explain
the observations.Comment: 5 pages, 5 figure
Soft Spheres Make More Mesophases
We use both mean-field methods and numerical simulation to study the phase
diagram of classical particles interacting with a hard-core and repulsive, soft
shoulder. Despite the purely repulsive interaction, this system displays a
remarkable array of aggregate phases arising from the competition between the
hard-core and shoulder length scales. In the limit of large shoulder width to
core size, we argue that this phase diagram has a number of universal features,
and classify the set of repulsive shoulders that lead to aggregation at high
density. Surprisingly, the phase sequence and aggregate size adjusts so as to
keep almost constant inter-aggregate separation.Comment: 4 pages, 2 included figure
Jamming non-local quantum correlations
We present a possible scheme to tamper with non-local quantum correlations in
a way that is consistent with relativistic causality, but goes beyond quantum
mechanics. A non-local ``jamming" mechanism, operating within a certain
space-time window, would not violate relativistic causality and would not lead
to contradictory causal loops. The results presented in this Letter do not
depend on any model of how quantum correlations arise and apply to any jamming
mechanism.Comment: 10 pp, LaTe
Quantum capacitor with discrete charge-anticharge: spectrum and forces
The quantum capacitor with discrete charge is modeled by a Hamiltonian
containing an inductive intrinsic term (tunnel effect between plates). The
spectrum is obtained using a double Hilbert space. Fluctuations in the
charge-anticharge pairs (zero total charge) give rise to an elementary
attraction which is compared to the Casimir force. In this case, the
field-fluctuations force could be also interpreted as charge-fluctuations
force
First experimental test of Bell inequalities performed using a non-maximally entangled state
We report on the realisation of a new test of Bell inequalities using the
superposition of type I parametric down conversion produced in two different
non-linear crystals pumped by the same laser, but with different polarisation.
The produced state is non-maximally entangled. We discuss the advantages and
the possible developments of this configuration
Correspondences and Quantum Description of Aharonov-Bohm and Aharonov-Casher Effects
We establish systematic consolidation of the Aharonov-Bohm and
Aharonov-Casher effects including their scalar counterparts. Their formal
correspondences in acquiring topological phases are revealed on the basis of
the gauge symmetry in non-simply connected spaces and the adiabatic condition
for the state of magnetic dipoles. In addition, investigation of basic two-body
interactions between an electric charge and a magnetic dipole clarifies their
appropriate relative motions and discloses physical interrelations between the
effects. Based on the two-body interaction, we also construct an exact
microscopic description of the Aharonov-Bohm effect, where all the elements are
treated on equal footing, i.e., magnetic dipoles are described
quantum-mechanically and electromagnetic fields are quantized. This microscopic
analysis not only confirms the conventional (semiclassical) results and the
topological nature but also allows one to explore the fluctuation effects due
to the precession of the magnetic dipoles with the adiabatic condition relaxed
Small-scale CMB Temperature and Polarization Anisotropies due to Patchy Reionization
We study contributions from inhomogeneous (patchy) reionization to arcminute
scale () cosmic microwave background (CMB) anisotropies.
We show that inhomogeneities in the ionization fraction, rather than in the
mean density, dominate both the temperature and the polarization power spectra.
Depending on the ionization history and the clustering bias of the ionizing
sources, we find that rms temperature fluctuations range from 2 K to 8
K and the corresponding values for polarization are over two orders of
magnitude smaller. Reionization can significantly bias cosmological parameter
estimates and degrade gravitational lensing potential reconstruction from
temperature maps but not from polarization maps. We demonstrate that a simple
modeling of the reionization temperature power spectrum may be sufficient to
remove the parameter bias. The high- temperature power spectrum will
contain some limited information about the sources of reionization.Comment: 11 pages, 8 figures. Minor changes to match version accepted by Ap
Aluminization of Scintillating Fibers for the Luminosity Detector of ATLAS
We report on tests on 25 cm long squared, 5mmx5mm SCF-3HF optical scintillating fibers, which have been aluminized by the magnetron sputtering technique in Lisbon during 2005. Two groups of fibers were used: 9 isolated fibers (Set I) and 20 stacked inside an acrylic piece (Set II). Fibers in Set I were used to measure the attenuation length of the fibers and were processed in Lisbon but not aluminized. Fibers in Set II were polished in a milling machine with a diamond blade and aluminium coated at one end. The other end was polished at CERN. From this set 10 were cut and polished at 90º with the axis of the fiber and the other 10 the end to be aluminized has been cut at 45º. The reflectivity of the aluminium mirror has been measured in a dedicated test bench. Preliminary results give a value of reflectivity of 51% for the fibers with a 90º cut
Signatures of magnetic activity in the seismic data of solar-type stars observed by Kepler
In the Sun, the frequencies of the acoustic modes are observed to vary in
phase with the magnetic activity level. These frequency variations are expected
to be common in solar-type stars and contain information about the
activity-related changes that take place in their interiors. The unprecedented
duration of Kepler photometric time-series provides a unique opportunity to
detect and characterize stellar magnetic cycles through asteroseismology. In
this work, we analyze a sample of 87 solar-type stars, measuring their temporal
frequency shifts over segments of length 90 days. For each segment, the
individual frequencies are obtained through a Bayesian peak-bagging tool. The
mean frequency shifts are then computed and compared with: 1) those obtained
from a cross-correlation method; 2) the variation in the mode heights; 3) a
photometric activity proxy; and 4) the characteristic timescale of the
granulation. For each star and 90-d sub-series, we provide mean frequency
shifts, mode heights, and characteristic timescales of the granulation.
Interestingly, more than 60% of the stars show evidence for (quasi-)periodic
variations in the frequency shifts. In the majority of the cases, these
variations are accompanied by variations in other activity proxies. About 20%
of the stars show mode frequencies and heights varying approximately in phase,
in opposition to what is observed for the Sun.Comment: Accepted for publication in ApJS, 19(+86) pages, 11(+89) figures,
2(+87) table
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