15,932 research outputs found
Quantum dissipative effects in moving mirrors: a functional approach
We use a functional approach to study various aspects of the quantum
effective dynamics of moving, planar, dispersive mirrors, coupled to scalar or
Dirac fields, in different numbers of dimensions. We first compute the
Euclidean effective action, and use it to derive the imaginary part of the
`in-out' effective action. We also obtain, for the case of the real scalar
field in 1+1 dimensions, the Schwinger-Keldysh effective action and a
semiclassical Langevin equation that describes the motion of the mirror
including noise and dissipative effects due to its coupling to the quantum
fields.Comment: References added. Version to appear in Phys. Rev.
Mathematical Model Identification of Self-Excited Systems Using Experimental Bifurcation Analysis Data
Self-excited vibrations can be found in many engineering applications such as flutter of aerofoils, stick-slip vibrations in drill strings, and wheel shimmy. These self-excited vibrations are generally unwanted since they can cause serious damage to the system. To avoid such phenomena, an accurate mathematical model of the system is crucial. Self-excited systems are typically modelled as dynamical systems with Hopf bifurcations. The identification of such non-linear dynamical system from data is much more challenging compared to linear systems. In this research, we propose two different mathematical model identification methods for self-excited systems that use experimental bifurcation analysis data. The first method considers an empirical mathematical model whose coefficients are identified to fit the measured bifurcation diagram. The second approach considers a fundamental Hopf normal form model and learns a data-driven coordinate transformation mapping the normal form state-space to physical coordinates. The approaches developed are applied to bifurcation data collected on a two degree-of-freedom flutter rig and the two methods show promising results. The advantages and disadvantages of the methods are discussed
Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures
Many plastic crystals, molecular solids with long-range, center-of-mass
crystalline order but dynamic disorder of the molecular orientations, are known
to exhibit exceptionally high ionic conductivity. This makes them promising
candidates for applications as solid-state electrolytes, e.g., in batteries.
Interestingly, it was found that the mixing of two different
plastic-crystalline materials can considerably enhance the ionic dc
conductivity, an important benchmark quantity for electrochemical applications.
An example is the admixture of different nitriles to succinonitrile, the latter
being one of the most prominent plastic-crystalline ionic conductors. However,
until now only few such mixtures were studied. In the present work, we
investigate succinonitrile mixed with malononitrile, adiponitrile, and
pimelonitrile, to which 1 mol% of Li ions were added. Using differential
scanning calorimetry and dielectric spectroscopy, we examine the phase behavior
and the dipolar and ionic dynamics of these systems. We especially address the
mixing-induced enhancement of the ionic conductivity and the coupling of the
translational ionic mobility to the molecular reorientational dynamics,
probably arising via a "revolving-door" mechanism.Comment: 9 pages, 7 figures; revised version as accepted for publication in J.
Chem. Phy
Cultural selection drives the evolution of human communication systems
Human communication systems evolve culturally, but the evolutionary mechanisms that drive this evolution are not well understood. Against a baseline that communication variants spread in a population following neutral evolutionary dynamics (also known as drift models), we tested the role of two cultural selection models: coordination- and content-biased. We constructed a parametrized mixed probabilistic model of the spread of communicative variants in four 8-person laboratory micro-societies engaged in a simple communication game. We found that selectionist models, working in combination, explain the majority of the empirical data. The best-fitting parameter setting includes an egocentric bias and a content bias, suggesting that participants retained their own previously used communicative variants unless they encountered a superior (content-biased) variant, in which case it was adopted. This novel pattern of results suggests that (i) a theory of the cultural evolution of human communication systems must integrate selectionist models and (ii) human communication systems are functionally adaptive complex systems
Semiclassical Gravity Theory and Quantum Fluctuations
We discuss the limits of validity of the semiclassical theory of gravity in
which a classical metric is coupled to the expectation value of the stress
tensor. It is argued that this theory is a good approximation only when the
fluctuations in the stress tensor are small. We calculate a dimensionless
measure of these fluctuations for a scalar field on a flat background in
particular cases, including squeezed states and the Casimir vacuum state. It is
found that the fluctuations are small for states which are close to a coherent
state, which describes classical behavior, but tend to be large otherwise. We
find in all cases studied that the energy density fluctuations are large
whenever the local energy density is negative. This is taken to mean that the
gravitational field of a system with negative energy density, such as the
Casimir vacuum, is not described by a fixed classical metric but is undergoing
large metric fluctuations. We propose an operational scheme by which one can
describe a fluctuating gravitational field in terms of the statistical behavior
of test particles. For this purpose we obtain an equation of the form of the
Langevin equation used to describe Brownian motion.Comment: In REVTEX. 20pp + 4 figures(not included, available upon request)
TUTP-93-
Quantum Mechanics of Damped Systems II. Damping and Parabolic Potential Barrier
We investigate the resonant states for the parabolic potential barrier known
also as inverted or reversed oscillator. They correspond to the poles of
meromorphic continuation of the resolvent operator to the complex energy plane.
As a byproduct we establish an interesting relation between parabolic cylinder
functions (representing energy eigenfunctions of our system) and a class of
Gel'fand distributions used in our recent paper.Comment: 14 page
Confidential genetic testing and electronic health records: A survey of current practices among Huntington disease testing centers
BACKGROUND: Clinical care teams providing presymptomatic genetic testing often employ advanced confidentiality practices for documentation and result storage. However, patient requests for increased confidentiality may be in conflict with the legal obligations of medical providers to document patient care activities in the electronic health record (EHR). Huntington disease presents a representative case study for investigating the ways centers currently balance the requirements of EHRs with the privacy demands of patients seeking presymptomatic genetic testing.
METHODS: We surveyed 23 HD centers (53% response rate) regarding their use of the EHR for presymptomatic HD testing.
RESULTS: Our survey revealed that clinical care teams and laboratories have each developed their own practices, which are cumbersome and often include EHR avoidance. We found that a majority of HD care teams record appointments in the EHR (91%), often using vague notes. Approximately half of the care teams (52%) keep presymptomatic results of out of the EHR.
CONCLUSION: As genetic knowledge grows, linking more genes to late-onset conditions, institutions will benefit from having professional recommendations to guide development of policies for EHR documentation of presymptomatic genetic results. Policies must be sensitive to the ethical differences and patient demands for presymptomatic genetic testing compared to those undergoing confirmatory genetic testing
Quantum Electrodynamics near a Huttner-Barnett dielectric
We build up a consistent theory of quantum electrodynamics in the presence of
macroscopic polarizable media. We use the Huttner-Barnett model of a dispersive
and absorbing dielectric medium and formulate the theory in terms of
interacting quantum fields. We integrate out the damped polaritons by using
diagrammatic techniques and find an exact expression for the displacement field
(photon) propagator in the presence of a dispersive and absorbing dielectric
half-space. This opens a new route to traceable perturbative calculations of
the same kind as in free-space quantum electrodynamics. As a worked-through
example we consider the interaction of a neutral atom with a dispersive and
absorbing dielectric half-space. For that we use the multipolar coupling
of the atomic dipole moment to the
electromagnetic displacement field. We apply the newly developed formalism to
calculate the one-loop correction to the atomic electron propagator and find
the energy-level shift and changes in the spontaneous decay rates for a neutral
atom close to an absorptive dielectric mirror.Comment: 25 pages, 4 figure
Large-area CCD imagers for spacecraft applications
Backside illuminated CCD imagers with 100 x 160 resolution elements have been fabricated using double level metal technology. Detailed study of the optical performance of such arrays has been performed between 24 C and -40 C using data rates from 10 kHz to 1 MHz. A 400 x 400 array is presently being fabricated
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