1,173 research outputs found
Polychromatic solitons in a quadratic medium
We introduce the simplest model to describe parametric interactions in a
quadratically nonlinear optical medium with the fundamental harmonic containing
two components with (slightly) different carrier frequencies [which is a direct
analog of wavelength-division multiplexed (WDM) models, well known in media
with cubic nonlinearity]. The model takes a closed form with three different
second-harmonic components, and it is formulated in the spatial domain. We
demonstrate that the model supports both polychromatic solitons (PCSs), with
all the components present in them, and two types of mutually orthogonal simple
solitons, both types being stable in a broad parametric region. An essential
peculiarity of PCS is that its power is much smaller than that of a simple
(usual) soliton (taken at the same values of control parameters), which may be
an advantage for experimental generation of PCSs. Collisions between the
orthogonal simple solitons are simulated in detail, leading to the conclusion
that the collisions are strongly inelastic, converting the simple solitons into
polychromatic ones, and generating one or two additional PCSs. A collision
velocity at which the inelastic effects are strongest is identified, and it is
demonstrated that the collision may be used as a basis to design a simple
all-optical XOR logic gate.Comment: 9 pages, 8 figures, accepted to Phys. Rev.
Small anisotropy of the lower critical field and -wave two-gap feature in single crystal LiFeAs
The in- and out-of-plane lower critical fields and magnetic penetration
depths for LiFeAs were examined. The anisotropy ratio is
smaller than the expected theoretical value, and increased slightly with
increasing temperature from 0.6 to . This small degree of anisotropy
was numerically confirmed by considering electron correlation effect. The
temperature dependence of the penetration depths followed a power
law() below 0.3, with 3.5 for both and
. Based on theoretical studies of iron-based superconductors, these
results suggest that the superconductivity of LiFeAs can be represented by an
extended -wave due to weak impurity scattering effect. And the
magnitudes of the two gaps were also evaluted by fitting the superfluid density
for both the in- and out-of-plane to the two-gap model. The estimated values
for the two gaps are consistent with the results of angle resolved
photoemission spectroscopy and specific heat experiments.Comment: 10 pages, 5 figure
Health monitoring in composite structures using piezoceramic sensors and fiber optic sensors
Abstract: Health monitoring is a major concern not only in the design and manufacturing but also in service stages for composite laminated structures. Excessive loads or low velocity impact can cause matrix cracks and delaminations that may severely degrade the load carrying capability of the composite laminated structures. To develop the health monitoring techniques providing on-line diagnostics of smart composite structures can be helpful in keeping the composite structures sound during their service. In this presentation, we discuss the signal processing techniques and some applications for health monitoring of composite structures using piezoceramic sensors and fiber optic sensors
The incommensurate charge-density-wave instability in the extended three-band Hubbard model
The infinite-U three-band Hubbard model is considered in order to describe
the CuO_2 planes of the high temperature superconducting cuprates. The charge
instabilities are investigated when the model is extended with a
nearest-neighbor repulsion between holes on copper d and oxygen p orbitals and
in the presence of a long-range Coulombic repulsion. It is found that a
first-order valence instability line ending with a critical point is present
like in the previously investigated model without long-range forces. However,
the dominant critical instability is the formation of incommensurate
charge-density-waves, which always occur before the valence-instability
critical point is reached. An effective singular attraction arises in the
proximity of the charge-density wave instability, accounting for both a strong
pairing mechanism and for the anomalous normal state properties.Comment: 15 pages in RevteX. Figures available from M. Grill
HST/WFPC2 snapshot imaging of symbiotic stars
The results of a HST/WFPC2 snapshot imaging survey of selected symbiotic
stars in 1999/2000 are presented. Seven sources - HD 149427 (PC 11), PU Vul, RT
Ser, He2-104 (Southern Crab), V1329 Cyg (HBV 475), V417 Cen, AS 201 - were
observed in filters F218W (ultraviolet continuum), F502N ([OIII]lambda
4959,5007) and F656N (Halpha); an eighth source, RS Oph, was observed in F437N
([OIII]lambda 4363), F502N and F656N. The presence of extended emission was
detected in He2-104, V1329 Cyg and possibly HD 149427. In He2-104 we detected
the [OIII] and Halpha counterparts to the inner lobes found in [NII] by Corradi
et al. For V1329 Cyg, comparison with previously published HST/FOC results
indicates expanding ejecta which may be associated with an ejection event in
1982 (+/-2 years) at a velocity of 260 +/- 50 km/s in the plane of the sky and
at an assumed distance of 3.4kpc. We also present previously unpublished radio
images of HD 149427, which we have obtained from the archives of the Australia
Telescope Compact Array and which reveal the presence of extended emission at a
similar orientation to that of the possible optical extension. Finally we also
include HST/WFPC2 GO observations of AG Peg and detect possible extended
emission in the F218W filter.Comment: 8 pages, accepted for publication in MNRA
Spin and Charge Structure Factor of the 2-d Hubbard Model
The spin and charge structure factors are calculated for the Hubbard model on
the square lattice near half-filling using a spin-rotation invariant six-slave
boson representation. The charge structure factor shows a broad maximum at the
zone corner and is found to decrease monotonically with increasing interaction
strength and electron density and increasing temperature. The spin structure
factor develops with increasing interaction two incommensurate peaks at the
zone boundary and along the zone diagonal. Comparison with results of Quantum
Monte Carlo and variational calculations is carried out and the agreement is
found to be good. The limitations of an RPA-type approach are pointed out.Comment: 18 pages, revtex, 13 postscript figures, submitted to Phys. Rev.
A Cognitive Model of an Epistemic Community: Mapping the Dynamics of Shallow Lake Ecosystems
We used fuzzy cognitive mapping (FCM) to develop a generic shallow lake
ecosystem model by augmenting the individual cognitive maps drawn by 8
scientists working in the area of shallow lake ecology. We calculated graph
theoretical indices of the individual cognitive maps and the collective
cognitive map produced by augmentation. The graph theoretical indices revealed
internal cycles showing non-linear dynamics in the shallow lake ecosystem. The
ecological processes were organized democratically without a top-down
hierarchical structure. The steady state condition of the generic model was a
characteristic turbid shallow lake ecosystem since there were no dynamic
environmental changes that could cause shifts between a turbid and a clearwater
state, and the generic model indicated that only a dynamic disturbance regime
could maintain the clearwater state. The model developed herein captured the
empirical behavior of shallow lakes, and contained the basic model of the
Alternative Stable States Theory. In addition, our model expanded the basic
model by quantifying the relative effects of connections and by extending it.
In our expanded model we ran 4 simulations: harvesting submerged plants,
nutrient reduction, fish removal without nutrient reduction, and
biomanipulation. Only biomanipulation, which included fish removal and nutrient
reduction, had the potential to shift the turbid state into clearwater state.
The structure and relationships in the generic model as well as the outcomes of
the management simulations were supported by actual field studies in shallow
lake ecosystems. Thus, fuzzy cognitive mapping methodology enabled us to
understand the complex structure of shallow lake ecosystems as a whole and
obtain a valid generic model based on tacit knowledge of experts in the field.Comment: 24 pages, 5 Figure
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