18,649 research outputs found
Dynamical photo-induced electronic properties of molecular junctions
Nanoscale molecular-electronic devices and machines are emerging as promising
functional elements, naturally flexible and efficient, for next generation
technologies. A deeper understanding of carrier dynamics in molecular junctions
is expected to benefit many fields of nanoelectronics and power-devices. We
determine time-resolved charge current flowing at donor- acceptor interface in
molecular junctions connected to metallic electrodes by means of quantum
transport simulations. The current is induced by the interaction of the donor
with a Gaussian-shape femtosecond laser pulse. Effects of the molecular
internal coupling, metal- molecule tunneling and light-donor coupling on
photocurrent are discussed. We then examine the junction working through the
time-resolved donor density of states. Non-equilibrium reorganization of
hybridized molecular orbitals through the light-donor interaction gives rise to
two phenomena: the dynamical Rabi shift and the appearance of Floquet-like
states. Such insights into the dynamical photoelectronic structure of molecules
are of strong interest for ultrafast spectroscopy, and open avenues toward the
possibility of analyzing and controlling the internal properties of quantum
nanodevices with pump-push photocurrent spectroscopy
A survey of children's interests from grade IV through grade VIII in the fields of active recreation, entertainment, reading, school subjects, and vocations.
Thesis (Ed.M.)--Boston Universit
Detection and Mapping of Quantitative Trait Loci that Determine Responsiveness
Exposure to 70% N2O evokes a robust antinociceptive effect in C57BL/6 (B6) but not in DBA/2 (D2) inbred mice. This study was conducted to identify quantitative trait loci (QTL) in the mouse genome that might determine responsiveness to N2O. Offspring from the F2 generation bred from B6 and D2 progenitors exhibited a broad range of responsiveness to N2O antinociception as determined by the acetic acid-induced abdominal constriction test. QTL analysis was then used to dissect this continuous trait distribution into component loci, and to map them to broad chromosomal regions. To this end, 24 spleens were collected from each of the following four groups: male and female F2 mice responding to 70% N2O in oxygen with 100% response (high-responders); and male and female F2 mice responding with 0% response (low-responders). Genomic DNA was extracted from the spleens and genotyped with simple sequence length polymorphism MapPairs markers. Findings were combined with findings from the earlier QTL analysis from BXD recombinant inbred mice [Brain Res 725 (1996) 23]. Combined results revealed two significant QTL that influence responsiveness to nitrous oxide on proximal chromosome 2 and distal chromosome 5, and one suggestive QTL on midchromosome 18. The chromosome 2 QTL was evident only in males. A significant interaction was found between a locus on chromosome 6 and another on chromosome 13 with a substantial effect on N2O antinociception
Free energy of cluster formation and a new scaling relation for the nucleation rate
Recent very large molecular dynamics simulations of homogeneous nucleation
with Lennard-Jones atoms [Diemand et al. J. Chem. Phys. {\bf
139}, 074309 (2013)] allow us to accurately determine the formation free energy
of clusters over a wide range of cluster sizes. This is now possible because
such large simulations allow for very precise measurements of the cluster size
distribution in the steady state nucleation regime. The peaks of the free
energy curves give critical cluster sizes, which agree well with independent
estimates based on the nucleation theorem. Using these results, we derive an
analytical formula and a new scaling relation for nucleation rates: is scaled by , where the supersaturation ratio is ,
is the dimensionless surface energy, and is a dimensionless
nucleation rate. This relation can be derived using the free energy of cluster
formation at equilibrium which corresponds to the surface energy required to
form the vapor-liquid interface. At low temperatures (below the triple point),
we find that the surface energy divided by that of the classical nucleation
theory does not depend on temperature, which leads to the scaling relation and
implies a constant, positive Tolman length equal to half of the mean
inter-particle separation in the liquid phase.Comment: 7 figure
Automatic estimation of flux distributions of astrophysical source populations
In astrophysics a common goal is to infer the flux distribution of
populations of scientifically interesting objects such as pulsars or
supernovae. In practice, inference for the flux distribution is often conducted
using the cumulative distribution of the number of sources detected at a given
sensitivity. The resulting "-" relationship can be used to
compare and evaluate theoretical models for source populations and their
evolution. Under restrictive assumptions the relationship should be linear. In
practice, however, when simple theoretical models fail, it is common for
astrophysicists to use prespecified piecewise linear models. This paper
proposes a methodology for estimating both the number and locations of
"breakpoints" in astrophysical source populations that extends beyond existing
work in this field. An important component of the proposed methodology is a new
interwoven EM algorithm that computes parameter estimates. It is shown that in
simple settings such estimates are asymptotically consistent despite the
complex nature of the parameter space. Through simulation studies it is
demonstrated that the proposed methodology is capable of accurately detecting
structural breaks in a variety of parameter configurations. This paper
concludes with an application of our methodology to the Chandra Deep Field
North (CDFN) data set.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS750 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
How to Track Protists in Three Dimensions
We present an apparatus optimized for tracking swimming microorganisms in the
size range 10-1000 microns, in three dimensions (3D), far from surfaces, and
with negligible background convective fluid motion. CCD cameras attached to two
long working distance microscopes synchronously image the sample from two
perpendicular directions, with narrowband dark-field or bright-field
illumination chosen to avoid triggering a phototactic response. The images from
the two cameras can be combined to yield 3D tracks of the organism. Using
additional, highly directional broad-spectrum illumination with millisecond
timing control the phototactic trajectories in 3D of organisms ranging from
Chlamydomonas to Volvox can be studied in detail. Surface-mediated hydrodynamic
interactions can also be investigated without convective interference. Minimal
modifications to the apparatus allow for studies of chemotaxis and other taxes.Comment: 8 pages, 7 figure
The pre-shock gas of SN1006 from HST/ACS observations
We derive the pre-shock density and scale length along the line of sight for
the collisionless shock from a deep HST image that resolves the H alpha
filament in SN1006 and updated model calculations. The very deep ACS
high-resolution image of the Balmer line filament in the northwest (NW)
quadrant shows that 0.25 < n_0 < le$ 0.4 cm-3 and that the scale along the line
of sight is about 2 x 10^{18} cm, while bright features within the filament
correspond to ripples with radii of curvature less than 1/10 that size. The
derived densities are within the broad range of earlier density estimates, and
they agree well with the ionization time scale derived from the Chandra X-ray
spectrum of a region just behind the optical filament. This provides a test for
widely used models of the X-ray emission from SNR shocks. The scale and
amplitude of the ripples are consistent with expectations for a shock
propagating though interstellar gas with ~ 20% density fluctuations on parsec
scales as expected from studies of interstellar turbulence. One bulge in the
filament corresponds to a knot of ejecta overtaking the blast wave, however.
The interaction results from the rapid deceleration of the blast wave as it
encounters an interstellar cloud.Comment: 16 pages, 6 figures, to appear in Ap
Combined ultraviolet studies of astronomical sources
Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra
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