4,992 research outputs found
Dual Mode Hydrazine Propulsion System
Dual mode hydrazine propulsion system for attitude control and maneuvering of interplanetary spacecraf
Breaking of the overall permutation symmetry in nonlinear optical susceptibilities of one-dimensional periodic dimerized Huckel model
Based on infinite one-dimensional single-electron periodic models of
trans-polyacetylene, we show analytically that the overall permutation symmetry
of nonlinear optical susceptibilities is, albeit preserved in the molecular
systems with only bound states, no longer generally held for the periodic
systems. The overall permutation symmetry breakdown provides a fairly natural
explanation to the widely observed large deviations of Kleinman symmetry for
periodic systems in off-resonant regions. Physical conditions to experimentally
test the overall permutation symmetry break are discussed.Comment: 7 pages, 1 figur
Nonequilibrium Steady State of Photoexcited Correlated Electrons in the Presence of Dissipation
We present a framework to determine nonequilibrium steady states in strongly
correlated electron systems in the presence of dissipation. This is
demonstrated for a correlated electron (Falicov-Kimball) model attached to a
heat bath and irradiated by an intense pump light, for which an exact solution
is obtained with the Floquet method combined with the nonequilibrium dynamical
mean-field theory. On top of a Drude-like peak indicative of photometallization
as observed in recent pump-probe experiments, new nonequilibrium phenomena are
predicted to emerge, where the optical conductivity exhibits dip and kink
structures around the frequency of the pump light, a midgap absorption arising
from photoinduced Floquet subbands, and a negative attenuation (gain) due to a
population inversion.Comment: 5 pages, 3 figures, published versio
Hawking radiation from "phase horizons" in laser filaments?
Belgiorno et al have reported on experiments aiming at the detection of (the
analogue of) Hawking radiation using laser filaments [F. Belgiorno et al, Phys.
Rev. Lett. 105, 203901 (2010)]. They sent intense focused Bessel pulses into a
non-linear dielectric medium in order to change its refractive index via the
Kerr effect and saw creation of photons orthogonal to the direction of travel
of the pluses. Since the refractive index change in the pulse generated a
"phase horizon" (where the phase velocity of these photons equals the pulse
speed), they concluded that they observed the analogue of Hawking radiation. We
study this scenario in a model with a phase horizon and a phase velocity very
similar to that of their experiment and find that the effective metric does not
quite correspond to a black hole. The photons created in this model are not due
to the analogue of black hole evaporation but have more similarities to
cosmological particle creation. Nevertheless, even this effect cannot explain
the observations -- unless the pulse has significant small scale structure in
both the longitudinal and transverse dimensions.Comment: 13 pages RevTeX, 2 figure
Using Wave-Packet Interferometry to Monitor the External Vibrational Control of Electronic Excitation Transfer
We investigate the control of electronic energy transfer in molecular dimers
through the preparation of specific vibrational coherences prior to electronic
excitation, and its observation by nonlinear wave-packet interferometry.
Laser-driven coherent nuclear motion can affect the instantaneous resonance
between site-excited electronic states and thereby influence short-time
electronic excitation transfer (EET). We first illustrate this control
mechanism with calculations on a dimer whose constituent monomers undergo
harmonic vibrations. We then consider the use of nonlinear wave-packet
interferometry (nl-WPI) experiments to monitor the nuclear dynamics
accompanying EET in general dimer complexes following impulsive vibrational
excitation by a sub-resonant control pulse (or control pulse sequence). In
measurements of this kind, two pairs of polarized phase-related femtosecond
pulses following the control pulse generate superpositions of coherent nuclear
wave packets in optically accessible electronic states. Interference
contributions to the time- and frequency-integrated fluorescence signal due to
overlaps among the superposed wave packets provide amplitude-level information
on the nuclear and electronic dynamics. We derive the basic expression for a
control-pulse-dependent nl-WPI signal. The electronic transition moments of the
constituent monomers are assumed to have a fixed relative orientation, while
the overall orientation of the complex is distributed isotropically. We include
the limiting case of coincident arrival by pulses within each phase-related
pair in which control-influenced nl-WPI reduces to a fluorescence-detected
pump-probe difference experiment. Numerical calculations of pump-probe signals
based on these theoretical expressions are presented in the following paper
The living aortic valve: From molecules to function.
The aortic valve lies in a unique hemodynamic environment, one characterized by a range of stresses (shear stress, bending forces, loading forces and strain) that vary in intensity and direction throughout the cardiac cycle. Yet, despite its changing environment, the aortic valve opens and closes over 100,000 times a day and, in the majority of human beings, will function normally over a lifespan of 70-90 years. Until relatively recently heart valves were considered passive structures that play no active role in the functioning of a valve, or in the maintenance of its integrity and durability. However, through clinical experience and basic research the aortic valve can now be characterized as a living, dynamic organ with the capacity to adapt to its complex mechanical and biomechanical environment through active and passive communication between its constituent parts. The clinical relevance of a living valve substitute in patients requiring aortic valve replacement has been confirmed. This highlights the importance of using tissue engineering to develop heart valve substitutes containing living cells which have the ability to assume the complex functioning of the native valve
Hierarchy of integrable Hamiltonians describing of nonlinear n-wave interaction
In the paper we construct an hierarchy of integrable Hamiltonian systems
which describe the variation of n-wave envelopes in nonlinear dielectric
medium. The exact solutions for some special Hamiltonians are given in terms of
elliptic functions of the first kind.Comment: 17 page
Hydrazine electrolysis for spacecraft propulsion. Analysis and test of a dual-mode gas and liquid system Final report
Dual-mode hydrazine electrolysis for spacecraft propulsion syste
Piracy in a Contested Periphery: Incorporation and the Emergence of the Modern World-System in the Colonial Atlantic Frontier
This article uses world-systems analysis to examine the role that pirates and privateers played in the competition between European core states in the Atlantic and Caribbean frontier during the seventeenth and eighteenth centuries. Piracy was an integral part of core-periphery interaction, as a force that nations could use against one another in the form of privateers, and as a reaction against increasing constraints on freedom of action by those same states, thus forming a semiperiphery. Although modern portrayals of pirates and privateers paint a distinct line between the two groups, historical records indicate that their actual status was rather fluid, with particular people moving back and forth between the two. As a result, the individuals were on a margin between legality and treason, often crossing from one to the other. In this study we discuss how pirates and privateers fit into the margins of society in the seventeenth and eighteenth centuries, also known as the Golden Age of Piracy, specifically using the example of Edward Teach, aka Blackbeard. The present analysis can contribute to our understanding not only of piracy, but also of the structure of peripheries and semiperipheries that in some ways reflect resistance to incorporation
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