34,156 research outputs found
Sulfate Burial Constraints on the Phanerozoic Sulfur Cycle
The sulfur cycle influences the respiration of sedimentary organic matter, the oxidation state of the atmosphere and oceans, and the composition of seawater. However, the factors governing the major sulfur fluxes between seawater and sedimentary reservoirs remain incompletely understood. Using macrostratigraphic data, we quantified sulfate evaporite burial fluxes through Phanerozoic time. Approximately half of the modern riverine sulfate flux comes from weathering of recently deposited evaporites. Rates of sulfate burial are unsteady and linked to changes in the area of marine environments suitable for evaporite formation and preservation. By contrast, rates of pyrite burial and weathering are higher, less variable, and largely balanced, highlighting a greater role of the sulfur cycle in regulating atmospheric oxygen
Exponential decay in a spin bath
We show that the coherence of an electron spin interacting with a bath of
nuclear spins can exhibit a well-defined purely exponential decay for special
(`narrowed') bath initial conditions in the presence of a strong applied
magnetic field. This is in contrast to the typical case, where spin-bath
dynamics have been investigated in the non-Markovian limit, giving
super-exponential or power-law decay of correlation functions. We calculate the
relevant decoherence time T_2 explicitly for free-induction decay and find a
simple expression with dependence on bath polarization, magnetic field, the
shape of the electron wave function, dimensionality, total nuclear spin I, and
isotopic concentration for experimentally relevant heteronuclear spin systems.Comment: 4+ pages, 3 figures; v2: 9 pages, 3 figures (added four appendices
with extensive technical details, version to appear in Phys. Rev. B
Speed Limits in General Relativity
Some standard results on the initial value problem of general relativity in
matter are reviewed. These results are applied first to show that in a well
defined sense, finite perturbations in the gravitational field travel no faster
than light, and second to show that it is impossible to construct a warp drive
as considered by Alcubierre (1994) in the absence of exotic matter.Comment: 7 pages; AMS-LaTeX; accepted for publication by Classical and Quantum
Gravit
Climate change and the selective signature of the Late Ordovician mass extinction
Selectivity patterns provide insights into the causes of ancient extinction events. The Late Ordovician mass extinction was related to Gondwanan glaciation; however, it is still unclear whether elevated extinction rates were attributable to record failure, habitat loss, or climatic cooling. We examined Middle Ordovician-Early Silurian North American fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per-taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk. Genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the Katian stage were particularly hard hit. The duration of the subsequent sampling gaps had little effect on extinction risk, suggesting that this extinction pulse cannot be entirely attributed to rock record failure; rather, it was caused, in part, by habitat loss. Extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled, a macroecological trait linked to thermal tolerance. A model trained on the relationship between 16 explanatory variables and extinction patterns during the early Katian interval substantially underestimates the extinction of exclusively tropical taxa during the late Katian interval. These results indicate that glacioeustatic sea-level fall and tropical ocean cooling played important roles in the first pulse of the Late Ordovician mass extinction in Laurentia
Perturbations of Spatially Closed Bianchi III Spacetimes
Motivated by the recent interest in dynamical properties of topologically
nontrivial spacetimes, we study linear perturbations of spatially closed
Bianchi III vacuum spacetimes, whose spatial topology is the direct product of
a higher genus surface and the circle. We first develop necessary mode
functions, vectors, and tensors, and then perform separations of (perturbation)
variables. The perturbation equations decouple in a way that is similar to but
a generalization of those of the Regge--Wheeler spherically symmetric case. We
further achieve a decoupling of each set of perturbation equations into
gauge-dependent and independent parts, by which we obtain wave equations for
the gauge-invariant variables. We then discuss choices of gauge and stability
properties. Details of the compactification of Bianchi III manifolds and
spacetimes are presented in an appendix. In the other appendices we study
scalar field and electromagnetic equations on the same background to compare
asymptotic properties.Comment: 61 pages, 1 figure, final version with minor corrections, to appear
in Class. Quant. Gravi
Small-Angle Excess Scattering: Glassy Freezing or Local Orientational Ordering?
We present Monte Carlo simulations of a dense polymer melt which shows
glass-transition-like slowing-down upon cooling, as well as a build up of
nematic order. At small wave vectors q this model system shows excess
scattering similar to that recently reported for light-scattering experiments
on some polymeric and molecular glass-forming liquids. For our model system we
can provide clear evidence that this excess scattering is due to the onset of
short-range nematic order and not directly related to the glass transition.Comment: 3 Pages of Latex + 4 Figure
Thermoelectric properties of Zn_5Sb_4In_(2-δ)(δ=0.15)
The polymorphic intermetallic compound Zn_5Sb_4In_(2−δ) (δ = 0.15(3)) shows promising thermoelectric properties at low temperatures, approaching a figure of merit ZT of 0.3 at 300 K. However, thermopower and electrical resistivity changes discontinuously at around 220 K. Measurement of the specific heat locates the previously unknown temperature of the order-disorder phase transition at around 180 K. Investigation of the charge carrier concentration and mobility by Hall measurements and infrared reflection spectroscopy indicate a mixed conduction behavior and the activation of charge carriers at temperatures above 220 K. Zn_5Sb_4In_(2−δ) has a low thermal stability, and at temperatures above 470 K samples decompose into a mixture of Zn, InSb, and Zn_4Sb_3
Free-induction decay and envelope modulations in a narrowed nuclear spin bath
We evaluate free-induction decay for the transverse components of a localized
electron spin coupled to a bath of nuclear spins via the Fermi contact
hyperfine interaction. Our perturbative treatment is valid for special
(narrowed) bath initial conditions and when the Zeeman energy of the electron
exceeds the total hyperfine coupling constant : . Using one unified
and systematic method, we recover previous results reported at short and long
times using different techniques. We find a new and unexpected modulation of
the free-induction-decay envelope, which is present even for a purely isotropic
hyperfine interaction without spin echoes and for a single nuclear species. We
give sub-leading corrections to the decoherence rate, and show that, in
general, the decoherence rate has a non-monotonic dependence on electron Zeeman
splitting, leading to a pronounced maximum. These results illustrate the
limitations of methods that make use of leading-order effective Hamiltonians
and re-exponentiation of short-time expansions for a strongly-interacting
system with non-Markovian (history-dependent) dynamics.Comment: 13 pages, 9 figure
Synthese von Kohlenstoff-14-markierten Terphenylen. EUR 2527. = Synthesis of C14-labeled terphenyl. EUR 2527.
Instrumentation design study for testing a hypersonic ramjet engine on the x-15 a-2. volume 3- conceptual design of measurement systems
Instrumentation for testing hypersonic ramjet engine on X-15A-2 aircraf
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