692 research outputs found
Safety Considerations in the Ground Environment
In the history of humankind, every great space adventure has begun on the ground. While this seems to be stating the obvious, mission and spacecraft designers who have overlooked this fact have paid a high price, either in loss or damage to the spacecraft pre-launch, or in mission failure or reduction. Spacecraft personnel may risk not only their flight hardware, but they may also risk their lives, their co-workers lives and even the general public by not heeding safety on the ground. Their eyes may be on the stars but their feet are on the ground! One additional comment: Although the design requirements are very different for human rated and nonhuman rated flight hardware, while on the ground that flight hardware (and its ground support equipment) doesn't care about what it is flying on. On the ground, additional requirements are often levied to protect the work force and general public. (Authors' Note: The source material for this chapter is primarily taken from the Kennedy Space Center Handbook (KHB) 1700.7/45 SW Handbook S-100 Space Shuttle Payload Ground Safety Handbook and the authors' personal experiences
Transport Anomalies and Marginal Fermi-Liquid Effects at a Quantum Critical Point
The conductivity and the tunneling density of states of disordered itinerant
electrons in the vicinity of a ferromagnetic transition at low temperature are
discussed. Critical fluctuations lead to nonanalytic frequency and temperature
dependences that are distinct from the usual long-time tail effects in a
disordered Fermi liquid. The crossover between these two types of behavior is
proposed as an experimental check of recent theories of the quantum
ferromagnetic critical behavior. In addition, the quasiparticle properties at
criticality are shown to be those of a marginal Fermi liquid.Comment: 4pp., REVTeX, no figs, final version as publishe
Influence of rare regions on magnetic quantum phase transitions
The effects of quenched disorder on the critical properties of itinerant
quantum magnets are considered. Particular attention is paid to locally ordered
rare regions that are formed in the presence of quenched disorder even when the
bulk system is still in the nonmagnetic phase. It is shown that these local
moments or instantons destroy the previously found critical fixed point in the
case of antiferromagnets. In the case of itinerant ferromagnets, the critical
behavior is unaffected by the rare regions due to an effective long-range
interaction between the order parameter fluctuations.Comment: 4 pp., REVTe
Spitzer Photometry of WISE-Selected Brown Dwarf and Hyper-Luminous Infrared Galaxy Candidates
We present Spitzer 3.6 and 4.5 m photometry and positions for a sample
of 1510 brown dwarf candidates identified by the WISE all-sky survey. Of these,
166 have been spectroscopically classified as objects with spectral types M(1),
L(7), T(146), and Y(12); Sixteen other objects are non-(sub)stellar in nature.
The remainder are most likely distant L and T dwarfs lacking spectroscopic
verification, other Y dwarf candidates still awaiting follow-up, and assorted
other objects whose Spitzer photometry reveals them to be background sources.
We present a catalog of Spitzer photometry for all astrophysical sources
identified in these fields and use this catalog to identify 7 fainter (4.5
m 17.0 mag) brown dwarf candidates, which are possibly wide-field
companions to the original WISE sources. To test this hypothesis, we use a
sample of 919 Spitzer observations around WISE-selected high-redshift
hyper-luminous infrared galaxy (HyLIRG) candidates. For this control sample we
find another 6 brown dwarf candidates, suggesting that the 7 companion
candidates are not physically associated. In fact, only one of these 7 Spitzer
brown dwarf candidates has a photometric distance estimate consistent with
being a companion to the WISE brown dwarf candidate. Other than this there is
no evidence for any widely separated ( 20 AU) ultra-cool binaries. As an
adjunct to this paper, we make available a source catalog of 7.33
objects detected in all of these Spitzer follow-up fields for use
by the astronomical community. The complete catalog includes the Spitzer 3.6
and 4.5 m photometry, along with positionally matched and
photometry from USNO-B; , , and photometry from 2MASS; and ,
, , and photometry from the WISE all-sky catalog
Statistical Transfer Matrix Study of the Multileg Ising Ladders and Tubes
Finite temperature properties of symmetric multileg Ising ladders and
tubes are investigated using the statistical transfer matrix method. The
temperature dependences of the specific heat and entropy are calculated. In the
case of tubes, it is found that the ground state entropy shows an even-odd
oscillation with respect to the number of legs. The same type of oscillation is
also found in the ground state energy. On the contrary, these oscillations do
not take place in ladders. From the temperature-dependence of the specific
heat, it is found that the lowest excitation energy is 4J for even-leg ladders
while it is 2J otherwise, The physical origin of these behaviors is discussed
based on the structure of excitations.Comment: 6 pages, 9 figure
Nonanalytic behavior of the spin susceptibility in clean Fermi systems
The wavevector and temperature dependent static spin susceptibility,
\chi_s(Q,T), of clean interacting Fermi systems is considered in dimensions
1\leq d \leq 3. We show that at zero temperature \chi_s is a nonanalytic
function of |Q|, with the leading nonanalyticity being |Q|^{d-1} for 1<d<3, and
Q^2\ln|Q| for d=3. For the homogeneous spin susceptibility we find a
nonanalytic temperature dependence T^{d-1} for 1<d<3. We give qualitative
mode-mode coupling arguments to that effect, and corroborate these arguments by
a perturbative calculation to second order in the electron-electron interaction
amplitude. The implications of this, in particular for itinerant
ferromagnetism, are discussed. We also point out the relation between our
findings and established perturbative results for 1-d systems, as well as for
the temperature dependence of \chi_s(Q=0) in d=3.Comment: 12pp., REVTeX, 5 eps figures, final version as publishe
Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs
We present the first mid-infrared spectra of brown dwarfs, together with
observations of a low-mass star. Our targets are the M3.5 dwarf GJ 1001A, the
L8 dwarf DENIS-P J0255-4700, and the T1/T6 binary system epsilon Indi Ba/Bb. As
expected, the mid-infrared spectral morphology of these objects changes rapidly
with spectral class due to the changes in atmospheric chemistry resulting from
their differing effective temperatures and atmospheric structures. By taking
advantage of the unprecedented sensitivity of the Infrared Spectrograph on the
Spitzer Space Telescope we have detected the 7.8 micron methane and 10 micron
ammonia bands for the first time in brown dwarf spectra.Comment: 4 pages, 2 figure
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Anderson-Mott transition as a quantum glass problem
We combine a recent mapping of the Anderson-Mott metal-insulator transition
on a random-field problem with scaling concepts for random-field magnets to
argue that disordered electrons near an Anderson-Mott transition show
glass-like behavior. We first discuss attempts to interpret experimental
results in terms of a conventional scaling picture, and argue that some of the
difficulties encountered point towards a glassy nature of the electrons. We
then develop a general scaling theory for a quantum glass, and discuss critical
properties of both thermodynamic and transport variables in terms of it. Our
most important conclusions are that for a correct interpretation of experiments
one must distinguish between self-averaging and non-self averaging observables,
and that dynamical or temperature scaling is not of power-law type but rather
activated, i.e. given by a generalized Vogel-Fulcher law. Recent mutually
contradicting experimental results on Si:P are discussed in the light of this,
and new experiments are proposed to test the predictions of our quantum glass
scaling theory.Comment: 25pp, REVTeX, 5 ps figs, final version as publishe
Differences between regular and random order of updates in damage spreading simulations
We investigate the spreading of damage in the three-dimensional Ising model
by means of large-scale Monte-Carlo simulations. Within the Glauber dynamics we
use different rules for the order in which the sites are updated. We find that
the stationary damage values and the spreading temperature are different for
different update order. In particular, random update order leads to larger
damage and a lower spreading temperature than regular order. Consequently,
damage spreading in the Ising model is non-universal not only with respect to
different update algorithms (e.g. Glauber vs. heat-bath dynamics) as already
known, but even with respect to the order of sites.Comment: final version as published, 4 pages REVTeX, 2 eps figures include
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