14,015 research outputs found
Negative 4-Probe Conductances of Mesoscopic Superconducting Wires
We analyze the longitudinal 4-probe conductance of mesoscopic normal and
superconducting wires and predict that in the superconducting case, large
negative values can arise for both the weakly disordered and localized regimes.
This contrasts sharply with the behaviour of the longitudinal 4-probe
conductance of normal wires, which in the localized limit is always
exponentially small and positive.Comment: Latex, 3 figures available on request to [email protected]
(Simon Robinson
The Abundance Of Boron In Diffuse Interstellar Clouds
We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II lambda 1362 resonance line, with each detection confirmed by the presence of absorption from O I lambda 1355, Cu II lambda 1358, and Ga II lambda 1414 (when available) at the same velocity. Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis, yielding a combined sample that more than quadruples the number of sight lines with significant boron detections. Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper. The observations probe both high-and low-density diffuse environments, allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element. In the case of boron, the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium. The abundance of boron in warm, low-density gas is found to be B/H = (2.4 +/- 0.6) x 10(-10), which represents a depletion of 60% relative to the meteoritic boron abundance. Beyond the effects of depletion, our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of B-11, resulting from spallation reactions involving either cosmic rays or neutrinos. Future observations will help to disentangle the relative contributions from the two spallation channels for B-11 synthesis.Robert A. Welch Foundation F-634Space Telescope Science Institute HST-AR-11247.01-AAssociation of Universities for Research in Astronomy, Inc., under NASA NAS5-26555Astronom
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On evolution of relatively large combinational logic circuits
Evolvable hardware (EHW) (Yao and Higuchi, 1999) is a technique introduced to automatically design circuits where the circuit configuration is carried out by evolutionary algorithms. One of the main difficulties in using EHW to solve real-world problems is the scalability. Until now, several strategies have been proposed to avoid this problem, but none of them completely tackle the issue. In this paper three different methods for evolving the most complex circuits have been tested for their scalability. These methods are bi-directional incremental evolution (SO-BIE); generalised disjunction decomposition (GD-BIE) and evolutionary strategies (ES) with dynamic mutation rate. In order to achieve the generalised conclusions the chosen approaches were tested using multipliers, traditionally used in EHW, but also logic circuits taken from MCNC (Yang, 1991) benchmark library and randomly generated circuits. The analysis of the approaches demonstrated that PLA-based ES is capable of evolving logic circuits of up to 12 inputs. The use of SO-BIE allows the generation of fully functional circuits of 14 inputs and GD-BIE is estimated to be able to evolve circuits of 21 inputs
Boron Abundances in Diffuse Interstellar Clouds
We present a comprehensive survey of B abundances in diffuse interstellar
clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is
the result of a complete search of archival STIS data for the B II resonance
line at 1362 angstroms, with each detection confirmed by the presence of
absorption from other dominant ions at the same velocity. The data probe a
range of astrophysical environments including both high-density regions of
massive star formation as well as low-density paths through the Galactic halo,
allowing us to clearly define the trend of B depletion onto interstellar grains
as a function of gas density. Many extended sight lines exhibit complex
absorption profiles that trace both local gas and gas associated with either
the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher
B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of
the Sun, which may suggest that B production in the current epoch is dominated
by a secondary process. The average gas-phase B abundance in the warm diffuse
ISM is consistent with the abundances determined for a variety of Galactic disk
stars, but is depleted by 60 percent relative to the solar system value. Our
survey also reveals sight lines with enhanced B abundances that potentially
trace recent production of B-11 either by cosmic-ray or neutrino-induced
spallation. Such sight lines will be key to discerning the relative importance
of the two production routes for B-11 synthesis.Comment: To be published in the proceedings of the IAU Symposium 268, Light
Elements in the Universe, C. Charbonnel, M. Tosi, F. Primas & C. Chiappini,
ed
Electron spin manipulation and resonator readout in a double quantum dot nano-electromechanical system
Magnetically coupling a nano-mechanical resonator to a double quantum dot
confining two electrons can enable the manipulation of a single electron spin
and the readout of the resonator's natural frequency. When the Larmor frequency
matches the resonator frequency, the electron spin in one of the dots can be
selectively flipped by the magnetised resonator. By simultaneously measuring
the charge state of the two-electron double quantum dots, this transition can
be detected thus enabling the natural frequency of the mechanical resonator to
be determined.Comment: 7 pages, fixed typos, updated figures 4 and
(2,0) theory on circle fibrations
We consider (2,0) theory on a manifold M_6 that is a fibration of a spatial
S^1 over some five-dimensional base manifold M_5. Initially, we study the free
(2,0) tensor multiplet which can be described in terms of classical equations
of motion in six dimensions. Given a metric on M_6 the low energy effective
theory obtained through dimensional reduction on the circle is a Maxwell theory
on M_5. The parameters describing the local geometry of the fibration are
interpreted respectively as the metric on M_5, a non-dynamical U(1) gauge field
and the coupling strength of the resulting low energy Maxwell theory. We derive
the general form of the action of the Maxwell theory by integrating the reduced
equations of motion, and consider the symmetries of this theory originating
from the superconformal symmetry in six dimensions. Subsequently, we consider a
non-abelian generalization of the Maxwell theory on M_5. Completing the theory
with Yukawa and phi^4 terms, and suitably modifying the supersymmetry
transformations, we obtain a supersymmetric Yang-Mills theory which includes
terms related to the geometry of the fibration.Comment: 24 pages, v2 References added, typos correcte
Superconducting Proximity Effect and Universal Conductance Fluctuations
We examine universal conductance fluctuations (UCFs) in mesoscopic
normal-superconducting-normal (N-S-N) structures using a numerical solution of
the Bogoliubov - de Gennes equation. We discuss two cases depending on the
presence (``open'' structure) or absence (``closed'' structure) of
quasiparticle transmission. In contrast to N-S structures, where the onset of
superconductivity increases fluctuations, we find that UCFs are suppressed by
superconductivity for N-S-N structures. We demonstrate that the fluctuations in
``open'' and ``closed'' structures exhibit distinct responses to an applied
magnetic field and to an imposed phase variation of the superconducting order
parameter.Comment: (4 pages, 5 figures). Corrected typos in equations, added references,
changed Fig. 5 and its discussions. Phys. Rev. B, accepted for publicatio
Comparative Analysis of Nanoscale Ultrasound Contrast Agents
Current ultrasound contrast agents utilize microbubbles as a blood pooling agent, but the size inhibits access to small capillaries. The development of nanoscale ultrasound contrast agents can enter small capillaries of tissues and aid in the detection of diseased states. However, current nano-formulations are flushed from the body over a short period of time. We developed a nanoscale ultrasound contrast agent with increased circulation time to allow for better detection of diseased states in the microvasculature of the body. Characterization (zeta potential, size, echogenicity and stability) and pharmacokinetic analysis were conducted on three nanoscale formulations: 1) Liquid based Bovine Serum Albumin (BSA)- Perfluropentane (PFP), 2) Gas based Lipid -L61 Pluronic, and 3) Gas based L61-Pluronic with Polyethylene Glycol (PEG). Characterization was conducted using Dynamic Light Scattering (DLS) and Nanoparticle Tracking Analysis (NTA). Echogenicity testing was performed using ultrasound imaging (Vevo2100) in vitro with a tissue phantom. The concentration, frequency response, and bubble stability were evaluated. The BSA-PFP, L61, and PEG L61 formulations had mean sizes of approximately 300 nm, 250 nm, and 330 nm respectively (n = 6). Images taken on the high frequency ultrasound system revealed that the L61 nanobubbles had higher signal intensity. Stability of the different formulations was measured over 2 hours. Nanobubble intensities decreased overtime, except the BSA-PFP bubbles, most likely due to being liquid-derived. Future in vivo studies will be conducted for circulation times in an animal model. Utilization of these formulations could lead to targeting of proteins in the body through modified nanobubble formulations
Particle Production in Tachyon Condensation
We study particle production in the tachyon condensation process as described
by different effective actions for the tachyon. By making use of invariant
operators, we are able to obtain exact results for the density of produced
particles, which is shown to depend strongly on the specific action. In
particular, the rate of particle production remains finite only for one of the
actions considered, hence confirming results previously appeared in the
literature.Comment: LaTeX, 6 pages, 3 figure
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