888 research outputs found
Photoelectron spectra of anionic sodium clusters from time-dependent density-functional theory in real-time
We calculate the excitation energies of small neutral sodium clusters in the
framework of time-dependent density-functional theory. In the presented
calculations, we extract these energies from the power spectra of the dipole
and quadrupole signals that result from a real-time and real-space propagation.
For comparison with measured photoelectron spectra, we use the ionic
configurations of the corresponding single-charged anions. Our calculations
clearly improve on earlier results for photoelectron spectra obtained from
static Kohn-Sham eigenvalues
Violation of the `Zero-Force Theorem' in the time-dependent Krieger-Li-Iafrate approximation
We demonstrate that the time-dependent Krieger-Li-Iafrate approximation in
combination with the exchange-only functional violates the `Zero-Force
Theorem'. By analyzing the time-dependent dipole moment of Na5 and Na9+, we
furthermore show that this can lead to an unphysical self-excitation of the
system depending on the system properties and the excitation strength.
Analytical aspects, especially the connection between the `Zero-Force Theorem'
and the `Generalized-Translation Invariance' of the potential, are discussed.Comment: 5 pages, 4 figure
Dust rings and filaments around the isolated young star V1331 Cygni
We characterize the small and large scale environment of the young star V1331
Cygni with high resolution HST/WFPC2 and Digitized Sky Survey images. In
addition to a previously known outer dust ring (~30'' in diameter), the
HST/WFPC2 scattered light image reveals an inner dust ring for the first time.
This ring has a maximum radius of 6.5'' and is possibly related to a molecular
envelope. Large-scale optical images show that V1331 Cyg is located at the tip
of a long dust filament linking it to the dark cloud LDN 981. We discuss the
origin of the observed dust morphology and analyze the object's relation to its
parent dark cloud LDN 981. Finally, based on recent results from the
literature, we investigate the properties of V1331 Cyg and conclude that in its
current state the object does not show suffcient evidence to be characterized
as an FU Ori object.Comment: 15 pages ApJ preprint style including 3 figures, accepted for
publication in ApJ (Feb. 2007
Coupling a single atomic quantum bit to a high finesse optical cavity
The quadrupole S -- D optical transition of a single trapped
Ca ion, well suited for encoding a quantum bit of information, is
coherently coupled to the standing wave field of a high finesse cavity. The
coupling is verified by observing the ion's response to both spatial and
temporal variations of the intracavity field. We also achieve deterministic
coupling of the cavity mode to the ion's vibrational state by selectively
exciting vibrational state-changing transitions and by controlling the position
of the ion in the standing wave field with nanometer-precision
Physical signatures of discontinuities of the time-dependent exchange-correlation potential
The exact exchange-correlation (XC) potential in time-dependent
density-functional theory (TDDFT) is known to develop steps and discontinuities
upon change of the particle number in spatially confined regions or isolated
subsystems. We demonstrate that the self-interaction corrected adiabatic
local-density approximation for the XC potential has this property, using the
example of electron loss of a model quantum well system. We then study the
influence of the XC potential discontinuity in a real-time simulation of a
dissociation process of an asymmetric double quantum well system, and show that
it dramatically affects the population of the resulting isolated single quantum
wells. This indicates the importance of a proper account of the discontinuities
in TDDFT descriptions of ionization, dissociation or charge transfer processes.Comment: 17 pages, 6 figure
He I 10830 as a Probe of Winds in Accreting Young Stars
He I 10830 profiles acquired with Keck's NIRSPEC for 6 young low mass stars
with high disk accretion rates (AS 353A, DG Tau, DL Tau, DR Tau, HL Tau and SVS
13) provide new insight into accretion-driven winds. In 4 stars the profiles
have the signature of resonance scattering, and possess a deep and broad
blueshifted absorption that penetrates more than 50% into the 1 micron
continuum over a continuous range of velocities from near the stellar rest
velocity to the terminal velocity of the wind, unlike inner wind signatures
seen in other spectral features. This deep and broad absorption provides the
first observational tracer of the acceleration region of the inner wind and
suggests that this acceleration region is situated such that it occults a
significant portion of the stellar disk. The remaining 2 stars also have blue
absorption extending below the continuum although here the profiles are
dominated by emission, requiring an additional source of helium excitation
beyond resonant scattering. This is likely the same process that produces the
emission profiles seen at He I 5876
STATISTICAL ANALYSIS OF THE EFFECTS OF MIXING POTATO VARIETIES ON LATE BLIGHT
A field study in two regions of Peru was conducted to determine how host-diversity effects on potato late blight varied geographically. Foliar disease severity was evaluated separately for the potato varieties in mixtures as well as in the single-variety plots. The TAUDPC (truncated area under the disease progress curve) and RMR (relative mixture response) for each site were analyzed separately using SAS mixed effects model procedures. While there was little difference between the sites in the 1997-1998 season, host-diversity effects were generally greater near Huancayo than near Cajamarca in the 1998-1999 season. Estimates of host-diversity effects from studies in Oregon and Ecuador were also compared with results for Peru. Host-diversity effects for reduced disease were generally greater for sites where we predicted lower levels of outside inoculum
The Randomized Shortened Dental Arch Study: Tooth Loss
The evidence concerning the management of shortened dental arch (SDA) cases is sparse. This multi-center study was aimed at generating data on outcomes and survival rates for two common treatments, removable dental prostheses (RDP) for molar replacement or no replacement (SDA). The hypothesis was that the treatments lead to different incidences of tooth loss. We included 215 patients with complete molar loss in one jaw. Molars were either replaced by RDP or not replaced, according to the SDA concept. First tooth loss after treatment was the primary outcome measure. This event occurred in 13 patients in the RDP group and nine patients in the SDA group. The respective Kaplan-Meier survival rates at 38 months were 0.83 (95% CI: 0.74-0.91) in the RDP group and 0.86 (95% CI: 0.78-0.95) in the SDA group, the difference being non-significant
Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability
We present the extension of previous two-dimensional simulations of the
time-dependent evolution of non-relativistic outflows from the surface of
Keplerian accretion disks, to three dimensions. The accretion disk itself is
taken to provide a set of fixed boundary conditions for the problem. The 3-D
results are consistent with the theory of steady, axisymmetric, centrifugally
driven disk winds up to the Alfv\'en surface of the outflow. Beyond the
Alfv\'en surface however, the jet in 3-D becomes unstable to non-axisymmetric,
Kelvin-Helmholtz instabilities. We show that jets maintain their long-term
stability through a self-limiting process wherein the average Alfv\'enic Mach
number within the jet is maintained to order unity. This is accomplished in at
least two ways. First, poloidal magnetic field is concentrated along the
central axis of the jet forming a ``backbone'' in which the Alfv\'en speed is
sufficiently high to reduce the average jet Alfv\'enic Mach number to unity.
Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2)
reduce the efficiency with which the jet material is accelerated, and transfer
kinetic energy of the outflow into the stretched, poloidal field lines of the
distorted jet. This too has the effect of increasing the Alfv\'en speed, and
thus reducing the Alfv\'enic Mach number. The jet is able to survive the onset
of the more destructive m=1 mode in this way. Our simulations also show that
jets can acquire corkscrew, or wobbling types of geometries in this relatively
stable end-state, depending on the nature of the perturbations upon them.
Finally, we suggest that jets go into alternating periods of low and high
activity as the disappearance of unstable modes in the sub-Alfv\'enic regime
enables another cycle of acceleration to super-Alfv\'enic speeds.Comment: 57 pages, 22 figures, submitted to Ap
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