2,053 research outputs found
Noise properties of nanoscale YBCO Josephson junctions
We present electric noise measurements of nanoscale biepitaxial YBa2Cu3O(7-x)
(YBCO) Josephson junctions fabricated by two different lithographic methods.
The first (conventional) technique defines the junctions directly by ion
milling etching through an amorphous carbon mask. The second (soft patterning)
method makes use of the phase competition between the superconducting YBCO
(Y123) and the insulating Y2BaCuO5 (Y211) phase at the grain boundary interface
on MgO (110) substrates. The voltage noise properties of the two methods are
compared in this study. For all junctions (having a thickness of 100 nm and
widths of 250-500 nm) we see a significant amount of individual charge traps.
We have extracted an approximate value for the effective area of the charge
traps from the noise data. From the noise measurements we infer that the soft
patterned junctions with a grain boundary (GB) interface manifesting a large
c-axis tunneling component have a uniform barrier and a SIS like behavior. The
noise properties of soft patterned junctions having a GB interface dominated by
transport parallel to the ab-planes are in accordance with a resonant tunneling
barrier model. The conventionally patterned junctions, instead, have suppressed
superconducting transport channels with an area much less than the nominal
junction area. These findings are important for the implementation of nanosized
Josephson junctions in quantum circuits.Comment: 11 Pages, 10 figure
Constraint damping for the Z4c formulation of general relativity
One possibility for avoiding constraint violation in numerical relativity
simulations adopting free-evolution schemes is to modify the continuum
evolution equations so that constraint violations are damped away. Gundlach et.
al. demonstrated that such a scheme damps low amplitude, high frequency
constraint violating modes exponentially for the Z4 formulation of General
Relativity. Here we analyze the effect of the damping scheme in numerical
applications on a conformal decomposition of Z4. After reproducing the
theoretically predicted damping rates of constraint violations in the linear
regime, we explore numerical solutions not covered by the theoretical analysis.
In particular we examine the effect of the damping scheme on low-frequency and
on high-amplitude perturbations of flat spacetime as well and on the long-term
dynamics of puncture and compact star initial data in the context of spherical
symmetry. We find that the damping scheme is effective provided that the
constraint violation is resolved on the numerical grid. On grid noise the
combination of artificial dissipation and damping helps to suppress constraint
violations. We find that care must be taken in choosing the damping parameter
in simulations of puncture black holes. Otherwise the damping scheme can cause
undesirable growth of the constraints, and even qualitatively incorrect
evolutions. In the numerical evolution of a compact static star we find that
the choice of the damping parameter is even more delicate, but may lead to a
small decrease of constraint violation. For a large range of values it results
in unphysical behavior.Comment: 13 pages, 24 figure
The Juridical Status of Privileged Combatants Under the Geneva Protocol of 1977 Concerning International Conflicts
Centralized control and coordination of the connections in a wireless network is not possible in practice. To keep the delay from measure-ment instants to actuating the decisions, distributed control is required. This paper focuses on the uplink (from mobiles to base stations) and dis-cusses distributing the decision of when and when not to transmit data (distributed scheduling) to the mobiles. The scheme, uplink transmission timing, utilizes mobile transmitter power control feedback from the base station receiver to determine whether the channel is favorable or not compared to the average channel condition. Thereby, the battery consumption and disturbing power to other connections are reduced. The algorithm can be described as a feedback control system. Some transient behaviors are analyzed using systems theory, and supported by wireless network simulations of a system with a WCDMA (Wideband Code Division Multiple Access) radio interface as in most 3G systems
Extragalactic gamma-ray signal from dark matter annihilation: an appraisal
We re-evaluate the extragalactic gamma-ray flux prediction from dark matter
annihilation in the approach of integrating over the nonlinear matter power
spectrum, extrapolated to the free-streaming scale. We provide an estimate of
the uncertainty based entirely on available N-body simulation results and
minimal theoretical assumptions. We illustrate how an improvement in the
simulation resolution, exemplified by the comparison between the Millennium and
Millennium II simulations, affects our estimate of the flux uncertainty and we
provide a "best guess" value for the flux multiplier, based on the assumption
of stable clustering for the dark matter perturbations described as a
collision-less fluid. We achieve results comparable to traditional Halo Model
calculations, but with a much simpler procedure and a more general approach, as
it relies only on one, directly measurable quantity. In addition we discuss the
extension of our calculation to include baryonic effects as modeled in
hydrodynamical cosmological simulations and other possible sources of
uncertainty that would in turn affect indirect dark matter signals. Upper limit
on the integrated power spectrum from supernovae lensing magnification are also
derived and compared with theoretical expectations.Comment: 20 pages, 9 figures, 1 table. Updated to match the published version.
New material and figures added, conclusions unchange
Soft nanostructuring of YBCO Josephson Junctions by phase separation
We have developed a new method to fabricate biepitaxial YBa2Cu3O(7-x) (YBCO)
Josephson junctions at the nanoscale, allowing junctions widths down to 100 nm
and simultaneously avoiding the typical damage in grain boundary interfaces due
to conventional patterning procedures. By using the competition between the
superconducting YBCO and the insulating Y2BaCuO5 phases during film growth, we
formed nanometer sized grain boundary junctions in the insulating Y2BaCuO5
matrix as confirmed by high resolution transmission electron microscopy.
Electrical transport measurements give clear indications that we are close to
probing the intrinsic properties of the grain boundaries.Comment: 16 pages, 6 figure
Constraint preserving boundary conditions for the Z4c formulation of general relativity
We discuss high order absorbing constraint preserving boundary conditions for
the Z4c formulation of general relativity coupled to the moving puncture family
of gauges. We are primarily concerned with the constraint preservation and
absorption properties of these conditions. In the frozen coefficient
approximation, with an appropriate first order pseudo-differential reduction,
we show that the constraint subsystem is boundary stable on a four dimensional
compact manifold. We analyze the remainder of the initial boundary value
problem for a spherical reduction of the Z4c formulation with a particular
choice of the puncture gauge. Numerical evidence for the efficacy of the
conditions is presented in spherical symmetry.Comment: 18 pages, 8 figure
The R Coronae Borealis stars - carbon abundances from forbidden carbon lines
Spectra of several R Coronae Borealis (RCB) stars at maximum light were
examined for the [C I] 9850 A and 8727 A absorption lines. The 9850 A line is
variously blended with a Fe II and CN lines but positive identifications of the
[C I] line are made for R CrB and SU Tau. The 8727 A line is detected in the
spectrum of the five stars observed in this wavelength region. Carbon
abundances are derived from the [C I] lines using the model atmospheres and
atmospheric parameters used by Asplund et al. (2000).
Although the observed strength of a C I line is constant from cool to hot RCB
stars, the strength is weaker than predicted by an amount equivalent to a
factor of four reduction of a line's gf-value. Asplund et al. dubbed this 'the
carbon problem' and discussed possible solutions.
The [C I] 9850 A line seen clearly in R CrB and SU Tau confirms the magnitude
of the carbon problem revealed by the C I lines. The [C I] 8727 A line measured
in five stars shows an enhanced carbon problem. The gf-value required to fit
the observed [C I] 8727 A line is a factor of 15 less than the well-determined
theoretical gf-value. We suggest that the carbon problem for all lines may be
alleviated to some extent by a chromospheric-like temperature rise in these
stars. The rise far exceeds that predicted by our non-LTE calculations, and
requires a substantial deposition of mechanical energy.Comment: 11 pages (embedded 5 figures and 3 tables), accepted for publication
in MNRA
Neutral Pion Distributions in PHENIX at RHIC
Transverse momentum spectra for identified 's in the range 1 GeV/c 4 GeV/c have been measured by the PHENIX experiment in Au-Au collisions
at GeV. The spectra from peripheral nuclear collisions are
consistent with the simple expectation of scaling the spectra from p+p
collisions by the average number of nucleon-nucleon binary collisions. The
spectra from central collisions and the ratio of central/peripheral spectra are
significantly suppressed when compared to point-like scaling.Comment: 6 pages, 3 figure
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