12,846 research outputs found
Analysis and application of ERTS-1 data for regional geological mapping
Combined visual and digital techniques of analysing ERTS-1 data for geologic information have been tried on selected areas in Pennsylvania. The major physiolographic and structural provinces show up well. Supervised mapping, following the imaged expression of known geologic features on ERTS band 5 enlargements (1:250,000) of parts of eastern Pennsylvania, delimited the Diabase Sills and the Precambrian rocks of the Reading Prong with remarkable accuracy. From unsupervised mapping, transgressive linear features are apparent in unexpected density, and exhibit strong control over river valley and stream channel directions. They are unaffected by bedrock type, age, or primary structural boundaries, which suggests they are either rejuvenated basement joint directions on different scales, or they are a recently impressed structure possibly associated with a drifting North American plate. With ground mapping and underflight data, 6 scales of linear features have been recognized
Spin Precession and Avalanches
In many magnetic materials, spin dynamics at short times are dominated by
precessional motion as damping is relatively small. In the limit of no damping
and no thermal noise, we show that for a large enough initial instability, an
avalanche can transition to an ergodic phase where the state is equivalent to
one at finite temperature, often above that for ferromagnetic ordering. This
dynamical nucleation phenomenon is analyzed theoretically. For small finite
damping the high temperature growth front becomes spread out over a large
region. The implications for real materials are discussed.Comment: 4 pages 2 figure
Radiation from low-momentum zoom-whirl orbits
We study zoom-whirl behaviour of equal mass, non-spinning black hole binaries
in full general relativity. The magnitude of the linear momentum of the initial
data is fixed to that of a quasi-circular orbit, and its direction is varied.
We find a global maximum in radiated energy for a configuration which completes
roughly one orbit. The radiated energy in this case exceeds the value of a
quasi-circular binary with the same momentum by 15%. The direction parameter
only requires minor tuning for the localization of the maximum. There is
non-trivial dependence of the energy radiated on eccentricity (several local
maxima and minima). Correlations with orbital dynamics shortly before merger
are discussed. While being strongly gauge dependent, these findings are
intuitive from a physical point of view and support basic ideas about the
efficiency of gravitational radiation from a binary system.Comment: 9 pages, 6 figures, Amaldi8 conference proceedings as publishe
Backreaction of Schwinger pair creation in massive QED
Particle-antiparticle pairs can be produced by background electric fields via
the Schwinger mechanism provided they are unconfined. If, as in QED in
(3+1)- these particles are massive, the particle production rate is
exponentially suppressed below a threshold field strength. Above this
threshold, the energy for pair creation must come from the electric field
itself which ought to eventually relax to the threshold strength. Calculating
this relaxation in a self-consistent manner, however, is difficult. Chu and
Vachaspati addressed this problem in the context of capacitor discharge in
massless QED [1] by utilizing bosonization in two-dimensions. When the bare
fermions are massless, the dual bosonized theory is free and capacitor
discharge can be analyzed exactly [1], however, special care is required in its
interpretation given that the theory exhibits confinement. In this paper we
reinterpret the findings of [1], where the capacitors Schwinger-discharge via
electrically neutral dipolar meson-production, and generalize this to the case
where the fermions have bare masses. Crucially, we note that when the initial
charge of the capacitor is large compared to the charge of the fermions, , the classical equation of motion for the bosonized model accurately
characterizes the dynamics of discharge. For massless QED, we find that the
discharge is suppressed below a critical plate separation that is commensurate
with the length scale associated with the meson dipole moment. For massive
QED, we find in addition, a mass threshold familiar from (3+1)-, and
show the electric field relaxes to a final steady state with a magnitude
proportional to the initial charge. We discuss the wider implications of our
findings and identify challenges in extending this treatment to higher
dimensions.Comment: 21 pages, 2 figures; several additional references and enhanced
discussion. Matches JHEP versio
Single-molecule stretching shows glycosylation sets tension in the hyaluronan-aggrecan bottlebrush
Large bottlebrush complexes formed from the polysaccharide hyaluronan (HA)
and the proteoglycan aggrecan contribute to cartilage compression resistance
and are necessary for healthy joint function. A variety of mechanical forces
act on these complexes in the cartilage extracellular matrix, motivating the
need for a quantitative description which links their structure and mechanical
response. Studies using electron microscopy have imaged the HA-aggrecan brush
but require adsorption to a surface, dramatically altering the complex from its
native conformation. We use magnetic tweezers force spectroscopy to measure
changes in extension and mechanical response of an HA chain as aggrecan
monomers bind and form a bottlebrush. This technique directly measures changes
undergone by a single complex with time and under varying solution conditions.
Upon addition of aggrecan, we find a large swelling effect manifests when the
HA chain is under very low external tension (i.e. stretching forces less than
~1 pN). We use models of force-extension behavior to show that repulsion
between the aggrecans induces an internal tension in the HA chain. Through
reference to theories of bottlebrush polymer behavior, we demonstrate that the
experimental values of internal tension are consistent with a polydisperse
aggrecan population, likely caused by varying degrees of glycosylation. By
enzymatically deglycosylating aggrecan, we show that aggrecan glycosylation is
the structural feature which causes HA stiffening. We then construct a simple
stochastic binding model to show that variable glycosylation leads to a wide
distribution of internal tensions in HA, causing variations in the mechanics at
much longer length-scales. Our results provide a mechanistic picture of how
flexibility and size of HA and aggrecan lead to the brush architecture and
mechanical properties of this important component of cartilage
Paleoproterozoic sterol biosynthesis and the rise of oxygen
Natural products preserved in the geological record can function as ‘molecular fossils’, providing insight into organisms and physiologies that existed in the deep past. One important group of molecular fossils is the steroidal hydrocarbons (steranes), which are the diagenetic remains of sterol lipids. Complex sterols with modified side chains are unique to eukaryotes, although simpler sterols can also be synthesized by a few bacteria. Sterol biosynthesis is an oxygen-intensive process; thus, the presence of complex steranes in ancient rocks not only signals the presence of eukaryotes, but also aerobic metabolic processes. In 1999, steranes were reported in 2.7 billion year (Gyr)-old rocks from the Pilbara Craton in Australia, suggesting a long delay between photosynthetic oxygen production and its accumulation in the atmosphere (also known as the Great Oxidation Event) 2.45–2.32 Gyr ago. However, the recent reappraisal and rejection of these steranes as contaminants pushes the oldest reported steranes forward to around 1.64 Gyr ago (ref. 6). Here we use a molecular clock approach to improve constraints on the evolution of sterol biosynthesis. We infer that stem eukaryotes shared functionally modern sterol biosynthesis genes with bacteria via horizontal gene transfer. Comparing multiple molecular clock analyses, we find that the maximum marginal probability for the divergence time of bacterial and eukaryal sterol biosynthesis genes is around 2.31 Gyr ago, concurrent with the most recent geochemical evidence for the Great Oxidation Event. Our results therefore indicate that simple sterol biosynthesis existed well before the diversification of living eukaryotes, substantially predating the oldest detected sterane biomarkers (approximately 1.64 Gyr ago), and furthermore, that the evolutionary history of sterol biosynthesis is tied to the first widespread availability of molecular oxygen in the ocean–atmosphere system
Opposite Thermodynamic Arrows of Time
A model in which two weakly coupled systems maintain opposite running
thermodynamic arrows of time is exhibited. Each experiences its own retarded
electromagnetic interaction and can be seen by the other. The possibility of
opposite-arrow systems at stellar distances is explored and a relation to dark
matter suggested.Comment: To appear in Phys. Rev. Let
Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4
Crystal structure, specific heat, thermal expansion, magnetic susceptibility
and electrical resistivity studies of the heavy fermion system
CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state
by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic
Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features)
towards a magnetically ordered, effectively twofold degenerate ground state in
CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ \chi ~
-ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents
the first system where a substitution-driven quantum phase transition is
connected not only with changes of the relative strength of Kondo effect and
RKKY interaction, but also with a reduction of the effective crystal field
ground state degeneracy.Comment: 15 pages, 9 figure
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