1,778 research outputs found
Development and fabrication of improved Schottky power diodes, phases I and II
Reproducible methods for the fabrication of silicon Schottky diodes were developed for the metals tungsten, aluminum, conventional platinum silicide and low temperature platinum silicide. Barrier heights and barrier lowering were measured permitting the accurate prediction of ideal forward and reverse diode performance. Processing procedures were developed which permit the fabrication of large area (approximately 1 sqcm) mesa-geometry power Schottky diodes with forward and reverse characteristics that approach theoretical values
Development and fabrication of improved Schottky power diodes
Reproducible methods for the fabrication of silicon Schottky diodes have been developed for tungsten, aluminum, conventional platinum silicide, and low temperature platinum silicide. Barrier heights and barrier lowering under reverse bias have been measured, permitting the accurate prediction of forward and reverse diode characteristics. Processing procedures have been developed that permit the fabrication of large area (about 1 sq cm) mesageometry power Schottky diodes with forward and reverse characteristics that approach theoretical values. A theoretical analysis of the operation of bridge rectifier circuits has been performed, which indicates the ranges of frequency and voltage for which Schottky rectifiers are preferred to p-n junctions. Power Schottky rectifiers have been fabricated and tested for voltage ratings up to 140 volts
Microarcsecond VLBI pulsar astrometry with PSRPI I. Two binary millisecond pulsars with white dwarf companions
Model-independent distance constraints to binary millisecond pulsars (MSPs)
are of great value to both the timing observations of the radio pulsars, and
multiwavelength observations of their companion stars. Very Long Baseline
Interferometry (VLBI) astrometry can be employed to provide these
model-independent distances with very high precision via the detection of
annual geometric parallax. Using the Very Long Baseline Array, we have observed
two binary millisecond pulsars, PSR J1022+1001 and J2145-0750, over a two-year
period and measured their distances to be 700 +14 -10 pc and 613 +16 -14 pc
respectively. We use the well-calibrated distance in conjunction with revised
analysis of optical photometry to tightly constrain the nature of their massive
(M ~ 0.85 Msun) white dwarf companions. Finally, we show that several
measurements of their parallax and proper motion of PSR J1022+1001 and PSR
J2145-0750 obtained by pulsar timing array projects are incorrect, differing
from the more precise VLBI values by up to 5 sigma. We investigate possible
causes for the discrepancy, and find that imperfect modeling of the solar wind
is a likely candidate for the timing model errors given the low ecliptic
latitude of these two pulsars.Comment: 14 pages, 9 figures, 6 tables; minor revisions in response to referee
comments to match version accepted by Ap
Semi- and Non-relativistic Limit of the Dirac Dynamics with External Fields
We show how to approximate Dirac dynamics for electronic initial states by
semi- and non-relativistic dynamics. To leading order, these are generated by
the semi- and non-relativistic Pauli hamiltonian where the kinetic energy is
related to and , respectively. Higher-order
corrections can in principle be computed to any order in the small parameter
v/c which is the ratio of typical speeds to the speed of light. Our results
imply the dynamics for electronic and positronic states decouple to any order
in v/c << 1.
To decide whether to get semi- or non-relativistic effective dynamics, one
needs to choose a scaling for the kinetic momentum operator. Then the effective
dynamics are derived using space-adiabatic perturbation theory by Panati et. al
with the novel input of a magnetic pseudodifferential calculus adapted to
either the semi- or non-relativistic scaling.Comment: 42 page
Microarcsecond VLBI pulsar astrometry with PSR II. parallax distances for 57 pulsars
We present the results of PSR, a large astrometric project targeting
radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric
database of 60 pulsars, we have obtained parallax-based distance measurements
for all but 3, with a parallax precision of typically 40 as and
approaching 10 as in the best cases. Our full sample doubles the number of
radio pulsars with a reliable (5) model-independent distance
constraint. Importantly, many of the newly measured pulsars are well outside
the solar neighbourhood, and so PSR brings a near-tenfold increase in the
number of pulsars with a reliable model-independent distance at kpc.
Using our sample along with previously published results, we show that even the
most recent models of the Galactic electron density distribution model contain
significant shortcomings, particularly at high Galactic latitudes. When
comparing our results to pulsar timing, two of the four millisecond pulsars in
our sample exhibit significant discrepancies in the estimates of proper motion
obtained by at least one pulsar timing array. With additional VLBI observations
to improve the absolute positional accuracy of our reference sources and an
expansion of the number of millisecond pulsars, we will be able to extend the
comparison of proper motion discrepancies to a larger sample of pulsar
reference positions, which will provide a much more sensitive test of the
applicability of the solar system ephemerides used for pulsar timing. Finally,
we use our large sample to estimate the typical accuracy attainable for
differential astrometry with the VLBA when observing pulsars, showing that for
sufficiently bright targets observed 8 times over 18 months, a parallax
uncertainty of 4 as per arcminute of separation between the pulsar and
calibrator can be expected.Comment: updated to version accepted by ApJ: 30 pages, 20 figures, 9 table
PSR J1856+0245: Arecibo Discovery of a Young, Energetic Pulsar Coincident with the TeV Gamma-ray Source HESS J1857+026
We present the discovery of the Vela-like radio pulsar J1856+0245 in the
Arecibo PALFA survey. PSR J1856+0245 has a spin period of 81ms, a
characteristic age of 21kyr, and a spin-down luminosity Edot = 4.6 x 10^36
ergs/s. It is positionally coincident with the TeV gamma-ray source HESS
J1857+026, which has no other known counterparts. Young, energetic pulsars
create wind nebulae, and more than a dozen pulsar wind nebulae have been
associated with very-high-energy (100GeV-100TeV) gamma-ray sources discovered
with the HESS telescope. The gamma-ray emission seen from HESS J1857+026 is
potentially produced by a pulsar wind nebula powered by PSR J1856+0245; faint
X-ray emission detected by ASCA at the pulsar's position supports this
hypothesis. The inferred gamma-ray efficiency is epsilon_gamma = L_gamma/Edot =
3.1% (1-10TeV, for a distance of 9kpc), comparable to that observed in similar
associations.Comment: 13 pages, 1 figure, accepted for publication in The Astrophysical
Journal Letter
Measurements of the Crab Pulsar's Giant Radio Pulse Amplitude Power-Law Index Using Low-Frequency Arecibo and Green Bank Telescope Observations
We report two low-frequency measurements of the power-law index for the
amplitudes of giant radio pulses from the Crab pulsar. The two observations
were taken with the Arecibo and Green Bank radio telescopes at center
frequencies of 327 MHz and 350 MHz, respectively. We find best-fit values for
the differential power-law index (where and
is pulse amplitude) of and from the Arecibo and
Green Bank data sets, respectively. Both values are broadly consistent with
other values previously measured for the Crab pulsar at low radio frequencies.
These reported values may be useful in future giant pulse studies of the Crab
pulsar.Comment: 7 pages with 1 table and 1 figure. Accepted by The Astrophysical
Journa
The structure of the exact effective action and the quark confinement in MSSM QCD
An expression for the exact (nonperturbative) effective action of =1
supersymmetric gauge theories is proposed, supposing, that all particles except
for the gauge bosons are massive. Analysis of its form shows, that instanton
effects in the supersymmetric theories can lead to the quark confinement. The
typical scale of confinement in MSSM QCD, calculated from the first principles,
is in agreement with the experimental data. The proposed explanation is quite
different from the dual Higgs mechanism.Comment: Final version to appear in Sov.J.Nucl.Phys. Some insignificant errors
and misprints are correcte
Competition of Spin-Fluctuations and Phonons in Superconductivity of ZrZn2
It has been long suspected that spin fluctuations in the weak itinerant
ferromagnet ZrZn2 may lead to a triplet superconductivity in this material.
Here we point out another possibility, a spatially inhomogeneous singlet
superconducting state (a Fulde-Ferrell-Larkin-Ovchinnikov state). We report
detailed electronic structure calculations, as well as calculations of the zone
center phonons and their coupling with electrons. We find that the exchange
splitting is nonuniform and may allow for gap formation at some parts of the
Fermi surface. We also find that there is substantial coupling of Zr rattling
modes with electrons, which can, in principle, provide the necessary pairing in
the s-channel.Comment: 4 pages, embedded color postscript figures. JPEG versions available
from the author
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