27,274 research outputs found
General linewidth formula for steady-state multimode lasing in arbitrary cavities
A formula for the laser linewidth of arbitrary cavities in the multimode
non-linear regime is derived from a scattering analysis of the solutions to
semiclassical laser theory. The theory generalizes previous treatments of the
effects of gain and openness described by the Petermann factor. The linewidth
is expressed using quantities based on the non-linear scattering matrix, which
can be computed from steady-state ab initio laser theory; unlike previous
treatments, no passive cavity or phenomenological parameters are involved. We
find that low cavity quality factor, combined with significant dielectric
dispersion, can cause substantial deviations from the Schawlow-Townes-Petermann
theory.Comment: 5 pages, 2 figure
Three-dimensional flows in slowly-varying planar geometries
We consider laminar flow in channels constrained geometrically to remain
between two parallel planes; this geometry is typical of microchannels obtained
with a single step by current microfabrication techniques. For pressure-driven
Stokes flow in this geometry and assuming that the channel dimensions change
slowly in the streamwise direction, we show that the velocity component
perpendicular to the constraint plane cannot be zero unless the channel has
both constant curvature and constant cross-sectional width. This result implies
that it is, in principle, possible to design "planar mixers", i.e. passive
mixers for channels that are constrained to lie in a flat layer using only
streamwise variations of their in-plane dimensions. Numerical results are
presented for the case of a channel with sinusoidally varying width
Zero-energy states in graphene quantum dots and rings
We present exact analytical zero-energy solutions for a class of smooth
decaying potentials, showing that the full confinement of charge carriers in
electrostatic potentials in graphene quantum dots and rings is indeed possible
without recourse to magnetic fields. These exact solutions allow us to draw
conclusions on the general requirements for the potential to support fully
confined states, including a critical value of the potential strength and
spatial extent.Comment: 8 pages, 3 figures, references added, typos corrected, discussion
section expande
Transmission Oscillator Ultrasonic Spectrometer (TOUS): A new research instrument
TOUS is capable for measuring very small changes in acoustic attenuation and phase velocity. Its high sensitivity to small changes in ultrasonic absorption results in part from operation under marginal conditions. In spite of high sensitivity, TOUS system is relatively simple, inexpensive, and compact
Neural networks in geophysical applications
Neural networks are increasingly popular in geophysics.
Because they are universal approximators, these
tools can approximate any continuous function with an
arbitrary precision. Hence, they may yield important
contributions to finding solutions to a variety of geophysical applications.
However, knowledge of many methods and techniques
recently developed to increase the performance
and to facilitate the use of neural networks does not seem
to be widespread in the geophysical community. Therefore,
the power of these tools has not yet been explored to
their full extent. In this paper, techniques are described
for faster training, better overall performance, i.e., generalization,and the automatic estimation of network size
and architecture
Far Infrared Variability of Sagittarius A*: 25.5 Hours of Monitoring with
Variable emission from Sgr~A*, the luminous counterpart to the super-massive
black hole at the center of our Galaxy, arises from the innermost portions of
the accretion flow. Better characterization of the variability is important for
constraining models of the low-luminosity accretion mode powering Sgr~A*, and
could further our ability to use variable emission as a probe of the strong
gravitational potential in the vicinity of the
black hole. We use the \textit{Herschel}
Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr~A* at
wavelengths that are difficult or impossible to observe from the ground. We
find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal
structure that is highly correlated across these wavelengths. While the
variations correspond to 1% changes in the total intensity in the
\textit{Herschel} beam containing Sgr~A*, comparison to independent,
simultaneous observations at 0.85 mm strongly supports the reality of the
variations. The lowest point in the light curves, 0.5 Jy below the
time-averaged flux density, places a lower bound on the emission of Sgr~A* at
0.25 mm, the first such constraint on the THz portion of the SED. The
variability on few hour timescales in the SPIRE light curves is similar to that
seen in historical 1.3 mm data, where the longest time series is available, but
the distribution of variations in the sub-mm do not show a tail of
large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from
XMM-Newton shows no significant variation within our observing period, which
may explain the lack of very large variations if X-ray and submillimeter flares
are correlated.Comment: Accepted for publication in Ap
Quantifying the effect of interannual ocean variability on the attribution of extreme climate events to human influence
In recent years, the climate change research community has become highly
interested in describing the anthropogenic influence on extreme weather events,
commonly termed "event attribution." Limitations in the observational record
and in computational resources motivate the use of uncoupled,
atmosphere/land-only climate models with prescribed ocean conditions run over a
short period, leading up to and including an event of interest. In this
approach, large ensembles of high-resolution simulations can be generated under
factual observed conditions and counterfactual conditions that might have been
observed in the absence of human interference; these can be used to estimate
the change in probability of the given event due to anthropogenic influence.
However, using a prescribed ocean state ignores the possibility that estimates
of attributable risk might be a function of the ocean state. Thus, the
uncertainty in attributable risk is likely underestimated, implying an
over-confidence in anthropogenic influence.
In this work, we estimate the year-to-year variability in calculations of the
anthropogenic contribution to extreme weather based on large ensembles of
atmospheric model simulations. Our results both quantify the magnitude of
year-to-year variability and categorize the degree to which conclusions of
attributable risk are qualitatively affected. The methodology is illustrated by
exploring extreme temperature and precipitation events for the northwest coast
of South America and northern-central Siberia; we also provides results for
regions around the globe. While it remains preferable to perform a full
multi-year analysis, the results presented here can serve as an indication of
where and when attribution researchers should be concerned about the use of
atmosphere-only simulations
Advanced composite aileron for L-1011 transport aircraft, task 1
Structural design and maintainability criteria were established and used as a guideline for evaluating a variety of configurations and materials for each of the major subcomponents. From this array of subcomponent designs, several aileron assemblies were formulated and analyzed. The selected design is a multirib configuration with sheet skin covers mechanically fastened to channel section ribs and spars. Qualitative analysis of currently available composite material systems led to the selection of three candidate materials on which comparative structural tests were conducted to measure the effects of environment and impact damage on mechanical property retention. In addition, each system was evaluated for producibility characteristics. From these tests, Thornel 300/5208 unidirectional tape was selected for the front spar and covers, and Thornel 300 fabric/5208 was chosen for the ribs
Bohr-Sommerfeld quantization of spin Hamiltonians
The Bohr-Sommerfeld rule for a spin system is obtained, including the first
quantum corrections. The rule applies to both integer and half-integer spin,
and respects Kramers degeneracy for time-reversal invariant systems. It is
tested for various models, in particular the Lipkin-Meshkov-Glick model, and
found to agree very well with exact results.Comment: Revtex 4, no figures, 1 tabl
- …