2,977 research outputs found
Car-Parrinello Molecular Dynamics on excited state surfaces
This paper describes a method to do ab initio molecular dynamics in
electronically excited systems within the random phase approximation (RPA).
Using a dynamical variational treatment of the RPA frequency, which corresponds
to the electronic excitation energy of the system, we derive coupled equations
of motion for the RPA amplitudes, the single particle orbitals, and the nuclear
coordinates. These equations scale linearly with basis size and can be
implemented with only a single holonomic constraint. Test calculations on a
model two level system give exact agreement with analytical results.
Furthermore, we examined the computational efficiency of the method by modeling
the excited state dynamics of a one-dimensional polyene lattice. Our results
indicate that the present method offers a considerable decrease in
computational effort over a straight-forward configuration interaction
(singles) plus gradient calculation performed at each nuclear configuration
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Presynaptic facilitation at the crayfish neuromuscular junction: Role of calcium-activated potassium conductance
Membrane potential was recorded intracellularly near presynaptic
terminals of the excitor axon of the crayfish opener neuromuscular junction (NMJ),
while transmitter release was recorded postsynaptically. This study focused on the
effects of a presynaptic calcium-activated potassium conductance, gK(Ca), on the
transmitter release evoked by single and paired depolarizing current pulses.
Blocking gK(Ca) by adding tetraethylammonium ion (TEA; 5-20 mM) to a solution
containing tetrodotoxin and aminopyridines caused the relation between presynaptic
potential and transmitter release to steepen and shift to less depolarized
potentials. When two depolarizing current pulses were applied at 20-ms intervals
with gK(Ca) not blocked, the presynaptic voltage change to the second (test) pulse was
inversely related to the amplitude of the first (conditioning) pulse. This effect of the
conditioning prepulse on the response to the test pulse was eliminated by 20 mM
TEA and by solutions containing 0 mM Ca2+/1 mM EGTA, suggesting that the
reduction in the amplitude of the test pulse was due to activation of gK(Ca) by calcium
remaining from the conditioning pulse. In the absence of TEA, facilitation of
transmitter release evoked by a test pulse increased as the conditioning pulse grew
from -40 to -20 mV, but then decreased with further increase in the conditioning
depolarization. A similar nonmonotonic relationship between facilitation and the
amplitude of the conditioning depolarization was reported in previous studies using
extracellular recording, and interpreted as supporting an additional voltagedependent
step in the activation of transmitter release. We suggest that this result
was due instead to activation of a gK(Ca) by the conditioning depolarization, since
facilitation of transmitter release increased monotonically with the amplitude of the
conditioning depolarization, and the early time course of the decay of facilitation
was prolonged when gK(Ca) was blocked. The different time courses for decay of the
presynaptic potential (20 ms) and facilitation (> 50 ms) suggest either that residual free calcium does not account for facilitation at the crayfish NMJ or that the
transmitter release mechanism has a markedly higher affinity or stoichiometry for
internal free calcium than does g K(Ca). Finally, our data suggest that the calcium
channels responsible for transmitter release at the crayfish NMJ are not of the L, N,
or T type.This work was partially supported by NIAAA grant AA0776 to G. D. Bittner.Neuroscienc
Extremal transmission through a microwave photonic crystal and the observation of edge states in a rectangular Dirac billiard
This article presents experimental results on properties of waves propagating
in an unbounded and a bounded photonic crystal consisting of metallic cylinders
which are arranged in a triangular lattice. First, we present transmission
measurements of plane waves traversing a photonic crystal. The experiments are
performed in the vicinity of a Dirac point, i.e., an isolated conical
singularity of the photonic band structure. There, the transmission shows a
pseudodiffusive 1/L dependence, with being the thickness of the crystal, a
phenomenon also observed in graphene. Second, eigenmode intensity distributions
measured in a microwave analog of a relativistic Dirac billiard, a rectangular
microwave billiard that contains a photonic crystal, are discussed. Close to
the Dirac point states have been detected which are localized at the straight
edge of the photonic crystal corresponding to a zigzag edge in graphene
The Missing Link: Democratic Citizenship in Service Learning A Case Study of Undergraduate Course Offerings at a Large Urban University
The purpose of this study was to explore a discrete set of service-learning courses to determine. (1) were they of the type conducive to fostering democratic citizens. and (2) did the coordinating center that supported service-learning advocate it for democratic Citizenship. Sixteen university instructors and two administrative staff members from a coordinating center were interviewed, and documents describing the courses and coordinating center were reviewed. Drawing from the literature. a list of ten criteria for democratic citizenship was assembled, and two sets of questions-one for the instructors and another for the administrative staff-were devised to prompt the response of the participants. It was determined that: (1) the coordinating center exhibited nearly twice as many characteristics of democratic citizenship as did the instructors\u27 courses; (2) the coordinating center and the instructors had considerable room for improvement if democratic citizenship was a motivation and a goal for the students; (3) curricular interests were the primary reason for engaging in service learning; and (4) more support needs to be provided by the coordinating center if instructors are to gain confidence, and effectively develop service 1eaming for democratic citizenship
Application of a trace formula to the spectra of flat three-dimensional dielectric resonators
The length spectra of flat three-dimensional dielectric resonators of
circular shape were determined from a microwave experiment. They were compared
to a semiclassical trace formula obtained within a two-dimensional model based
on the effective index of refraction approximation and a good agreement was
found. It was necessary to take into account the dispersion of the effective
index of refraction for the two-dimensional approximation. Furthermore, small
deviations between the experimental length spectrum and the trace formula
prediction were attributed to the systematic error of the effective index of
refraction approximation. In summary, the methods developed in this article
enable the application of the trace formula for two-dimensional dielectric
resonators also to realistic, flat three-dimensional dielectric microcavities
and -lasers, allowing for the interpretation of their spectra in terms of
classical periodic orbits.Comment: 13 pages, 12 figures, 1 tabl
Experimental Observation of Localized Modes in a Dielectric Square Resonator
We investigated the frequency spectra and field distributions of a dielectric
square resonator in a microwave experiment. Since such systems cannot be
treated analytically, the experimental studies of their properties are
indispensable. The momentum representation of the measured field distributions
shows that all resonant modes are localized on specific classical tori of the
square billiard. Based on these observations a semiclassical model was
developed. It shows excellent agreement with all but a single class of measured
field distributions that will be treated separately.Comment: 6 pages, 5 figures, 1 tabl
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