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 $\pi^0$'s in the range 1 GeV/c $< p_T <$ 4 GeV/c have been measured by the PHENIX experiment in Au-Au collisions at $\sqrt{s}=130$ 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