8,324 research outputs found
Frequency-Tunable Josephson Junction Resonator for Quantum Computing
We have fabricated and measured a high-Q Josephson junction resonator with a
tunable resonance frequency. A dc magnetic flux allows the resonance frequency
to be changed by over 10 %. Weak coupling to the environment allows a quality
factor of 7000 when on average less than one photon is stored in the
resonator. At large photon numbers, the nonlinearity of the Josephson junction
creates two stable oscillation states. This resonator can be used as a tool for
investigating the quality of Josephson junctions in qubits below the single
photon limit, and can be used as a microwave qubit readout at high photon
numbers.Comment: 3 pages, 5 figure
STS-1 operational flight profile. Volume 5: Descent, cycle 3
The trajectory data presented are to be used for orbiter systems and subsystems evalation, flight and mission control center software verification, flight techniques and timeline development, crew training, and evaluation of operational mission suitability. The entry profile is very similar to cycle 2, however, elevon and body flap temperature margins have increased and the elevon schedule was changed. The terminal area energy management (TAEM) profile was completely reshaped to conform with new angle of attack constraints and left hand turn around the heading alignment cylinder. Also, the entry/TAEM interface was adjusted to minimize guidance induced angle of attack transients across the interface. The approach and landing phase was reshaped for a 20 deg glideslope and reduced velocity at touchdown. The definition of the runway threshold was standardized for all landing sites. This results in a shift at Edwards Air Force Base in aim points and touchdown relative to the threshold of 1000 feet. The rollout remains essentially unchanged with the exception of the speedbrake, which is now deployed to 50 percent at touchdown
Rapid soft X-ray fluctuations in solar flares observed with the X-ray polychromator
Three flares observed by the Soft X-Ray Polychromator on the Solar Maximum Mission were studied. Flare light curves from the Flat Crystal Spectrometer and Bent Crystal Spectrometer were examined for rapid signal variations. Each flare was characterized by an initial fast (less than 1 min) burst, observed by the Hard X-Ray Burst Spectrometer (HXRBS), followed by softer gradual X-ray emission lasting several minutes. From an autocorrelation function analysis, evidence was found for quasi-periodic fluctuations with rise and decay times of 10 s in the Ca XIX and Fe XXV light curves. These variations were of small amplitude (less than 20%), often coincided with hard X-ray emissions, and were prominent during the onset of the gradual phase after the initial hard X-ray burst. It is speculated that these fluctuations were caused by repeated energy injections in a coronal loop that had already been heated and filled with dense plasma associated with the initial hard X-ray burst
Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter
The origin of both the diffuse high-latitude MeV gamma-ray emission and the
511 keV line flux from the Galactic bulge are uncertain. Previous studies have
invoked dark matter physics to independently explain these observations, though
as yet none has been able to explain both of these emissions within the
well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here
we use an unstable WIMP dark matter model to show that it is in fact possible
to simultaneously reconcile both of these observations, and in the process show
a remarkable coincidence: decaying dark matter with MeV mass splittings can
explain both observations if positrons and photons are produced with similar
branching fractions. We illustrate this idea with an unstable branon, which is
a standard WIMP dark matter candidate appearing in brane world models with
large extra dimensions. We show that because branons decay via three-body final
states, they are additionally unconstrained by searches for Galactic MeV
gamma-ray lines. As a result, such unstable long-lifetime dark matter particles
provide novel and distinct signatures that can be tested by future observations
of MeV gamma-rays.Comment: 19 pages, 4 figure
On the Nature of Trapped-Hole States in CdS Nanocrystals and the Mechanism of their Diffusion
Recent transient absorption experiments on CdS nanorods suggest that
photoexcited holes rapidly trap to the surface of these particles and then
undergo diffusion along the rod surface. In this paper, we present a
semiperiodic DFT model for the CdS nanocrystal surface, analyze it, and comment
on the nature of both the hole-trap states and the mechanism by which the holes
diffuse. Hole states near the top of the valence band form an energetic near
continuum with the bulk, and localize to the non-bonding sp orbitals on
surface sulfur atoms. After localization, the holes form nonadiabatic small
polarons that move between the sulfur orbitals on the surface of the particle
in a series of uncorrelated, incoherent, thermally-activated hops at room
temperature. The surface-trapped holes are deeply in the weak-electronic
coupling limit and, as a result, undergo slow diffusion.Comment: 4 figure
Diffuse continuum gamma rays from the Galaxy
A new study of the diffuse Galactic gamma-ray continuum radiation is
presented, using a cosmic-ray propagation model which includes nucleons,
antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of
the inverse Compton (IC) scattering includes the effect of anisotropic
scattering in the Galactic interstellar radiation field (ISRF) and a new
evaluation of the ISRF itself. Models based on locally measured electron and
nucleon spectra and synchrotron constraints are consistent with gamma-ray
measurements in the 30-500 MeV range, but outside this range excesses are
apparent. A harder nucleon spectrum is considered but fitting to gamma rays
causes it to violate limits from positrons and antiprotons. A harder
interstellar electron spectrum allows the gamma-ray spectrum to be fitted above
1 GeV as well, and this can be further improved when combined with a modified
nucleon spectrum which still respects the limits imposed by antiprotons and
positrons. A large electron/IC halo is proposed which reproduces well the
high-latitude variation of gamma-ray emission. The halo contribution of
Galactic emission to the high-latitude gamma-ray intensity is large, with
implications for the study of the diffuse extragalactic component and
signatures of dark matter. The constraints provided by the radio synchrotron
spectral index do not allow all of the <30 MeV gamma-ray emission to be
explained in terms of a steep electron spectrum unless this takes the form of a
sharp upturn below 200 MeV. This leads us to prefer a source population as the
origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal
(vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49
ps-figures, uses emulateapj.sty. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Giant Molecular Clouds in M33 - I. BIMA All Disk Survey
We present the first interferometric CO(J=1->0) map of the entire H-alpha
disk of M33. The 13" diameter synthesized beam corresponds to a linear
resolution of 50 pc, sufficient to distinguish individual giant molecular
clouds (GMCs). From these data we generated a catalog of 148 GMCs with an
expectation that no more than 15 of the sources are spurious. The catalog is
complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of
2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that
our survey detects ~50% of the CO flux, hence that the total molecular mass of
M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our
catalog are confined largely to the central region (R < 4 kpc). They show a
remarkable spatial and kinematic correlation with overdense HI filaments; the
geometry suggests that the formation of GMCs follows that of the filaments. The
GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper
than that found in the Milky Way and in the LMC. Combined with the total mass,
this steep function implies that the GMCs in M33 form with a characteristic
mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions,
implying that the GMCs have a short quiescent period. Our results suggest the
rapid assembly of molecular clouds from atomic gas, with prompt onset of
massive star formation.Comment: 19 pages, Accepted for Publication in the Astrophysical Journal
Supplemen
Density Matrix Renormalization Group Study of the Spin 1/2 Heisenberg Ladder with Antiferromagnetic Legs and Ferromagnetic Rungs
The ground state and low lying excitation of the spin 1/2 Heisenberg ladder
with antiferromagnetic leg () and ferromagnetic rung () interaction is studied by means of the density matrix renormalization
group method. It is found that the state remains in the Haldane phase even for
small suggesting the continuous transition to the gapless
phase at . The critical behavior for small is studied by
the finite size scaling analysis. The result is consistent with the recent
field theoretical prediction.Comment: 11 pages, revtex, figures upon reques
First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI
We present the first results using the Reuven Ramaty High-Energy Solar
Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated
with active regions, sunspots or flares (the quiet Sun). Using a newly
developed chopping technique (fan-beam modulation) during seven periods of
offpointing between June 2005 to October 2006, we obtained upper limits over
3-200 keV for the quietest times when the GOES12 1-8A flux fell below
Wm. These values are smaller than previous limits in the 17-120 keV
range and extend them to both lower and higher energies. The limit in 3-6 keV
is consistent with a coronal temperature MK. For quiet Sun periods
when the GOES12 1-8A background flux was between Wm and
Wm, the RHESSI 3-6 keV flux correlates to this as a power-law,
with an index of . The power-law correlation for microflares has
a steeper index of . We also discuss the possibility of
observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun
limits to estimate the axion-to-photon coupling constant for two different
axion emission scenarios.Comment: 4 pages, 3 figures, Accepted by ApJ letter
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