27,918 research outputs found
Spontaneous spatial fractal pattern formation in absorptive systems
We predict, for the first time to our knowledge, that purely-absorptive nonlinearity can support spontaneous spatial fractal pattern formation. A passive optical ring cavity with a thin slice of saturable absorber is analyzed. Linear stability analysis yields threshold curves for Turing (static) instabilities with features proposed as characteristics of potential fractal pattern formation. Numerical simulations of the fully-nonlinear dynamics, with both one and two transverse dimensions, confirm theoretical predictions
Revisiting the Cooling Flow Problem in Galaxies, Groups, and Clusters of Galaxies
We present a study of 107 galaxies, groups, and clusters spanning ~3 orders
of magnitude in mass, ~5 orders of magnitude in central galaxy star formation
rate (SFR), ~4 orders of magnitude in the classical cooling rate (dM/dt) of the
intracluster medium (ICM), and ~5 orders of magnitude in the central black hole
accretion rate. For each system in this sample, we measure dM/dt using archival
Chandra X-ray data and acquire the SFR and systematic uncertainty in the SFR by
combining over 330 estimates from dozens of literature sources. With these
data, we estimate the efficiency with which the ICM cools and forms stars,
finding e_cool = SFR/(dM/dt) = 1.4 +/- 0.4% for systems with dM/dt > 30
Msun/yr. For these systems, we measure a slope in the SFR-dM/dt relation
greater than unity, suggesting that the systems with the strongest cool cores
are also cooling more efficiently. We propose that this may be related to, on
average, higher black hole accretion rates in the strongest cool cores, which
could influence the total amount (saturating near the Eddington rate) and
dominant mode (mechanical vs radiative) of feedback. For systems with dM/dt <
30 Msun/yr, we find that the SFR and dM/dt are uncorrelated, and show that this
is consistent with star formation being fueled at a low (but dominant) level by
recycled ISM gas in these systems. We find an intrinsic log-normal scatter in
SFR at fixed dM/dt of 0.52 +/- 0.06 dex, suggesting that cooling is tightly
self-regulated over very long timescales, but can vary dramatically on short
timescales. There is weak evidence that this scatter may be related to the
feedback mechanism, with the scatter being minimized (~0.4 dex) in systems for
which the mechanical feedback power is within a factor of two of the cooling
luminosity.Comment: 16 pages, 10 figures, 6 tables. Submitted to ApJ. Comments welcome
Precise study of asymptotic physics with subradiant ultracold molecules
Weakly bound molecules have physical properties without atomic analogues,
even as the bond length approaches dissociation. In particular, the internal
symmetries of homonuclear diatomic molecules result in formation of two-body
superradiant and subradiant excited states. While superradiance has been
demonstrated in a variety of systems, subradiance is more elusive due to the
inherently weak interaction with the environment. Here we characterize the
properties of deeply subradiant molecular states with intrinsic quality factors
exceeding via precise optical spectroscopy with the longest
molecule-light coherent interaction times to date. We find that two competing
effects limit the lifetimes of the subradiant molecules, with different
asymptotic behaviors. The first is radiative decay via weak magnetic-dipole and
electric-quadrupole interactions. We prove that its rate increases
quadratically with the bond length, confirming quantum mechanical predictions.
The second is nonradiative decay through weak gyroscopic predissociation, with
a rate proportional to the vibrational mode spacing and sensitive to
short-range physics. This work bridges the gap between atomic and molecular
metrology based on lattice-clock techniques, yielding new understanding of
long-range interatomic interactions and placing ultracold molecules at the
forefront of precision measurements.Comment: 12 pages, 6 figure
High-precision spectroscopy of ultracold molecules in an optical lattice
The study of ultracold molecules tightly trapped in an optical lattice can
expand the frontier of precision measurement and spectroscopy, and provide a
deeper insight into molecular and fundamental physics. Here we create, probe,
and image microkelvin Sr molecules in a lattice, and demonstrate
precise measurements of molecular parameters as well as coherent control of
molecular quantum states using optical fields. We discuss the sensitivity of
the system to dimensional effects, a new bound-to-continuum spectroscopy
technique for highly accurate binding energy measurements, and prospects for
new physics with this rich experimental system.Comment: 12 pages, 4 figure
Statistical Mechanics and Lorentz Violation
The theory of statistical mechanics is studied in the presence of
Lorentz-violating background fields. The analysis is performed using the
Standard-Model Extension (SME) together with a Jaynesian formulation of
statistical inference. Conventional laws of thermodynamics are obtained in the
presence of a perturbed hamiltonian that contains the Lorentz violating terms.
As an example, properties of the nonrelativistic ideal gas are calculated in
detail. To lowest order in Lorentz violation, the scalar thermodynamic
variables are only corrected by a rotationally invariant combination of
parameters that mimics a (frame dependent) effective mass. Spin couplings can
induce a temperature independent polarization in the classical gas that is not
present in the conventional case. Precision measurements in the residual
expectation values of the magnetic moment of Fermi gases in the limit of high
temperature may provide interesting limits on these parameters.Comment: 7 pages, revte
Optical Production of Stable Ultracold Sr Molecules
We have produced large samples of ultracold Sr molecules in the
electronic ground state in an optical lattice. The molecules are bound by 0.05
cm and are stable for several milliseconds. The fast, all-optical method
of molecule creation via intercombination line photoassociation relies on a
near-unity Franck-Condon factor. The detection uses a weakly bound vibrational
level corresponding to a very large dimer. This is the first of two steps
needed to create Sr in the absolute ground quantum state. Lattice-trapped
Sr is of interest to frequency metrology and ultracold chemistry.Comment: 5 pages, 3 figure
Agriculture, meteorology and water quality in Ireland: a regional evaluation of pressures and pathways of nutrient loss to water
peer-reviewedThe main environmental impact of Irish agriculture on surface and ground water quality is the potential transfer of nutrients to water. Soil water dynamics mediate the transport of nutrients to water, and these dynamics in turn depend on agro-meteorological conditions, which show large variations between regions, seasons and years. In this paper we quantify and map the spatio-temporal variability of agro-meteorological factors that control nutrient pressures and pathways of nutrient loss. Subsequently, we evaluate their impact on the water quality of Irish rivers. For nitrogen, pressure and pathways factors coincide in eastern and southern areas, which is reflected in higher nitrate levels of the rivers in these regions. For phosphorus, pathway factors are most pronounced in north-western parts of the country. In south-eastern parts, high pressure factors result in reduced biological water quality. These regional differences require that farm practices be customised to reflect the local risk of nutrient loss to water. Where pathways for phosphorus loss are present almost year-round—as is the case in most of the north-western part of the country—build-up of pressures should be prevented, or ameliorated where already high. In south-eastern areas, spatio-temporal coincidence of nutrient pressures and pathways should be prevented, which poses challenges to grassland management
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