991 research outputs found
Neptune at Summer Solstice: Zonal Mean Temperatures from Ground-Based Observations 2003-2007
Imaging and spectroscopy of Neptune's thermal infrared emission is used to
assess seasonal changes in Neptune's zonal mean temperatures between Voyager-2
observations (1989, heliocentric longitude Ls=236) and southern summer solstice
(2005, Ls=270). Our aim was to analyse imaging and spectroscopy from multiple
different sources using a single self-consistent radiative-transfer model to
assess the magnitude of seasonal variability. Globally-averaged stratospheric
temperatures measured from methane emission tend towards a quasi-isothermal
structure (158-164 K) above the 0.1-mbar level, and are found to be consistent
with spacecraft observations of AKARI. This remarkable consistency, despite
very different observing conditions, suggests that stratospheric temporal
variability, if present, is 5 K at 1 mbar and 3 K at 0.1 mbar during
this solstice period. Conversely, ethane emission is highly variable, with
abundance determinations varying by more than a factor of two. The retrieved
C2H6 abundances are extremely sensitive to the details of the T(p) derivation.
Stratospheric temperatures and ethane are found to be latitudinally uniform
away from the south pole (assuming a latitudinally-uniform distribution of
stratospheric methane). At low and midlatitudes, comparisons of synthetic
Voyager-era images with solstice-era observations suggest that tropospheric
zonal temperatures are unchanged since the Voyager 2 encounter, with cool
mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25-50
{\mu}m mapping of tropospheric temperatures and para-hydrogen disequilibrium
suggests a symmetric meridional circulation with cold air rising at
mid-latitudes (sub-equilibrium para-H2 conditions) and warm air sinking at the
equator and poles (super-equilibrium para-H2 conditions). The most significant
atmospheric changes are associated with the polar vortex (absent in 1989).Comment: 35 pages, 19 figures. Accepted for publication in Icaru
Observation of Pure Spin Transport in a Diamond Spin Wire
Spin transport electronics - spintronics - focuses on utilizing electron spin
as a state variable for quantum and classical information processing and
storage. Some insulating materials, such as diamond, offer defect centers whose
associated spins are well-isolated from their environment giving them long
coherence times; however, spin interactions are important for transport,
entanglement, and read-out. Here, we report direct measurement of pure spin
transport - free of any charge motion - within a nanoscale quasi 1D 'spin
wire', and find a spin diffusion length ~ 700 nm. We exploit the statistical
fluctuations of a small number of spins ( < 100 net spins) which are
in thermal equilibrium and have no imposed polarization gradient. The spin
transport proceeds by means of magnetic dipole interactions that induce
flip-flop transitions, a mechanism that can enable highly efficient, even
reversible, pure spin currents. To further study the dynamics within the spin
wire, we implement a magnetic resonance protocol that improves spatial
resolution and provides nanoscale spectroscopic information which confirms the
observed spin transport. This spectroscopic tool opens a potential route for
spatially encoding spin information in long-lived nuclear spin states. Our
measurements probe intrinsic spin dynamics at the nanometre scale, providing
detailed insight needed for practical devices which seek to control spin.Comment: 7 pages, 2 figures, under consideration at Nature Nanotechnolog
Logarithmic growth dynamics in software networks
In a recent paper, Krapivsky and Redner (Phys. Rev. E, 71 (2005) 036118)
proposed a new growing network model with new nodes being attached to a
randomly selected node, as well to all ancestors of the target node. The model
leads to a sparse graph with an average degree growing logarithmically with the
system size. Here we present compeling evidence for software networks being the
result of a similar class of growing dynamics. The predicted pattern of network
growth, as well as the stationary in- and out-degree distributions are
consistent with the model. Our results confirm the view of large-scale software
topology being generated through duplication-rewiring mechanisms. Implications
of these findings are outlined.Comment: 7 pages, 3 figures, published in Europhysics Letters (2005
Mid-Infrared Spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 2. Determination of the Mean Composition of the Upper Troposphere and Stratosphere
Mid-infrared spectral observations Uranus acquired with the Infrared
Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the
abundances of C2H2, C2H6, CH3C2H, C4H2, CO2, and tentatively CH3 on Uranus at
the time of the 2007 equinox. For vertically uniform eddy diffusion
coefficients in the range 2200-2600 cm2 s-1, photochemical models that
reproduce the observed methane emission also predict C2H6 profiles that compare
well with emission in the 11.6-12.5 micron wavelength region, where the nu9
band of C2H6 is prominent. Our nominal model with a uniform eddy diffusion
coefficient Kzz = 2430 cm2 sec-1 and a CH4 tropopause mole fraction of 1.6x10-5
provides a good fit to other hydrocarbon emission features, such as those of
C2H2 and C4H2, but the model profile for CH3C2H must be scaled by a factor of
0.43, suggesting that improvements are needed in the chemical reaction
mechanism for C3Hx species. The nominal model is consistent with a CH3D/CH4
ratio of 3.0+-0.2x10-4. From the best-fit scaling of these photochemical-model
profiles, we derive column abundances above the 10-mbar level of 4.5+01.1/-0.8
x 10+19 molecule-cm-2 for CH4, 6.2 +- 1.0 x 10+16 molecule-cm-2 for C2H2 (with
a value 24% higher from a different longitudinal sampling), 3.1 +- 0.3 x 10+16
molecule-cm-2 for C2H6, 8.6 +- 2.6 x 10+13 molecule-cm-2 for CH3C2H, 1.8 +- 0.3
x 10+13 molecule-cm-2 for C4H2, and 1.7 +- 0.4 x 10+13 molecule-cm-2 for CO2 on
Uranus. Our results have implications with respect to the influx rate of
exogenic oxygen species and the production rate of stratospheric hazes on
Uranus, as well as the C4H2 vapor pressure over C4H2 ice at low temperatures
Nonlinear Meissner Effect in CuO Superconductors
Recent theories of the NMR in the CuO superconductors are based on a
spin-singlet order parameter. Since this state has nodal lines on
the Fermi surface, nonlinear effects associated with low-energy quasiparticles
become important, particularly at low temperatures. We show that the
field-dependence of the supercurrent, below the nucleation field for vortices,
can be used to locate the positions of the nodal lines of an unconventional gap
in momentum space, and hence test the proposed state.Comment: 5 pages (RevTex), 1 figure (postscript file incl.
Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots
We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR)
field , found in a square lattice of circular Permalloy dots when the
interdot distance gets comparable to the dot diameter . The minimum
, along the lattice axes,
differ by 50 Oe at = 1.1. This anisotropy, not expected in
uniformly magnetized dots, is explained by a non-uniform magnetization
\bm(\br) in a dot in response to dipolar forces in the patterned magnetic
structure. It is well described by an iterative solution of a continuous
variational procedure.Comment: 4 pages, 3 figures, revtex, details of analytic calculation and new
references are adde
Force-detected nuclear magnetic resonance: Recent advances and future challenges
We review recent efforts to detect small numbers of nuclear spins using
magnetic resonance force microscopy. Magnetic resonance force microscopy (MRFM)
is a scanning probe technique that relies on the mechanical measurement of the
weak magnetic force between a microscopic magnet and the magnetic moments in a
sample. Spurred by the recent progress in fabricating ultrasensitive force
detectors, MRFM has rapidly improved its capability over the last decade. Today
it boasts a spin sensitivity that surpasses conventional, inductive nuclear
magnetic resonance detectors by about eight orders of magnitude. In this review
we touch on the origins of this technique and focus on its recent application
to nanoscale nuclear spin ensembles, in particular on the imaging of nanoscale
objects with a three-dimensional (3D) spatial resolution better than 10 nm. We
consider the experimental advances driving this work and highlight the
underlying physical principles and limitations of the method. Finally, we
discuss the challenges that must be met in order to advance the technique
towards single nuclear spin sensitivity -- and perhaps -- to 3D microscopy of
molecules with atomic resolution.Comment: 15 pages & 11 figure
Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets Executive Summary of a Report of the ExoPlanet Task Force Astronomy and Astrophysics Advisory Committee Washington, DC June 23, 2008
[No abstract
Williams Syndrome and Neonatal Cardiac Surgery for Congenital Single Ventricle
Williams syndrome (WS) is an arteriopathic derangement associated with supravalvular aortic stenosis and branch pulmonary stenosis. We describe double-outlet right ventricle with mitral atresia and aortic arch hypoplasia in an infant with WS. This case demonstrates the difficulty in managing patients with WS with complex cardiac defects. To our knowledge, this is the first reported single-ventricle physiology in a patient with WS. (Level of Difficulty: Advanced.)
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