11,397 research outputs found
Displaced path integral formulation for the momentum distribution of quantum particles
The proton momentum distribution, accessible by deep inelastic neutron
scattering, is a very sensitive probe of the potential of mean force
experienced by the protons in hydrogen-bonded systems. In this work we
introduce a novel estimator for the end to end distribution of the Feynman
paths, i.e. the Fourier transform of the momentum distribution. In this
formulation, free particle and environmental contributions factorize. Moreover,
the environmental contribution has a natural analogy to a free energy surface
in statistical mechanics, facilitating the interpretation of experiments. The
new formulation is not only conceptually but also computationally advantageous.
We illustrate the method with applications to an empirical water model,
ab-initio ice, and one dimensional model systems
Boosting infrared energy transfer in 3D nanoporous gold antennas
The applications of plasmonics to energy transfer from free-space radiation to molecules are currently limited to the visible region of the electromagnetic spectrum due to the intrinsic optical properties of bulk noble metals that support strong electromagnetic field confinement only close to their plasma frequency in the visible/ultraviolet range. In this work, we show that nanoporous gold can be exploited as a plasmonic material for the mid-infrared region to obtain strong electromagnetic field confinement, co-localized with target molecules into the nanopores and resonant with their vibrational frequency. The effective optical response of the nanoporous metal enables the penetration of optical fields deep into the nanopores, where molecules can be loaded thus achieving a more efficient lightâmatter coupling if compared to bulk gold. In order to realize plasmonic resonators made of nanoporous gold, we develop a nanofabrication method based on polymeric templates for metal deposition and we obtain antenna arrays resonating at mid-infrared wavelengths selected by design. We then coat the antennas with a thin (3 nm) silica layer acting as the target dielectric layer for optical energy transfer. We study the strength of the lightâmatter coupling at the vibrational absorption frequency of silica at 1240 cmâ1 through the analysis of the experimental Fano lineshape that is benchmarked against identical structures made of bulk gold. The boost in the optical energy transfer from free-space mid-infrared radiation to molecular vibrations in nanoporous 3D nanoantenna arrays can open new application routes for plasmon-enhanced physicalâchemical reactions
Uncovering Spiral Structure in Flocculent Galaxies
We present K'(2.1 micron) observations of four nearby flocculent spirals,
which clearly show low-level spiral structure and suggest that kiloparsec-scale
spiral structure is more prevalent in flocculent spirals than previously
supposed. In particular, the prototypical flocculent spiral NGC 5055 is shown
to have regular, two-arm spiral structure to a radius of 4 kpc in the near
infrared, with an arm-interarm contrast of 1.3. The spiral structure in all
four galaxies is weaker than that in grand design galaxies. Taken in unbarred
galaxies with no large, nearby companions, these data are consistent with the
modal theory of spiral density waves, which maintains that density waves are
intrinsic to the disk. As an alternative, mechanisms for driving spiral
structure with non-axisymmetric perturbers are also discussed. These
observations highlight the importance of near infrared imaging for exploring
the range of physical environments in which large-scale dynamical processes,
such as density waves, are important.Comment: 12 pages AASTeX; 3 compressed PS figures can be retrieved from
ftp://ftp.astro.umd.edu/pub/michele as file thornley.tar (1.6Mbytes).
Accepted to Ap.J. Letters.(Figures now also available here, and from
ftp://ftp.astro.umd.edu/pub/michele , in GIF format.
Race/ethnicity and gender differences in drug use and abuse among college students
This study examines race/ethnicity and gender differences in drug use and abuse for substances other than alcohol among undergraduate college students. A probability-based sample of 4,580 undergraduate students at a Midwestern research university completed a cross-sectional Web-based questionnaire that included demographic information and several substance use measures. Male students were generally more likely to report drug use and abuse than female students. Hispanic and White students were more likely to report drug use and abuse than Asian and African American students prior to coming to college and during college. The findings of the present study reveal several important racial/ethnic differences in drug use and abuse that need to be considered when developing collegiate drug prevention and intervention efforts.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2377408/https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2377408/Accepted manuscrip
Non-gaussian effects in the cage dynamics of polymers
The correlation between the fast cage dynamics and structural relaxation is
investigated in a model polymer system. It is shown that the cage vibration
amplitude, as expressed by the Debye-Waller factor, and the relaxation time
collapse on a single universal curve with a simple analytic form
when the temperature, the density, the chain length and the monomer-monomer
interaction potential are changed. For the physical states with the same
coincidence of the mean-square displacement, the intermediate
scattering function and the non-Gaussian parameter is observed in a wide time
window spanning from the ballistic regime to the onset of the Rouse dynamics
driven by the chain connectivity. The role of the non-Gaussian effects is
discussed.Comment: 8 pages, 5 figure
Fractional and noncommutative spacetimes
We establish a mapping between fractional and noncommutative spacetimes in
configuration space. Depending on the scale at which the relation is
considered, there arise two possibilities. For a fractional spacetime with
log-oscillatory measure, the effective measure near the fundamental scale
determining the log-period coincides with the non-rotation-invariant but
cyclicity-preserving measure of \kappa-Minkowski. At scales larger than the
log-period, the fractional measure is averaged and becomes a power-law with
real exponent. This can be also regarded as the cyclicity-inducing measure in a
noncommutative spacetime defined by a certain nonlinear algebra of the
coordinates, which interpolates between \kappa-Minkowski and canonical
spacetime. These results are based upon a braiding formula valid for any
nonlinear algebra which can be mapped onto the Heisenberg algebra.Comment: 15 pages. v2: typos correcte
Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study
We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in
the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The
sample includes 14 galaxies covering a wide range of physical properties:
morphology, total absolute magnitude, integrated color. We take advantage of
the latter characteristic of the sample to check existing empirical
calibrations of absolute SBF magnitudes both in the I- and V-passbands.
Additionally, by comparing our SBF and color data with the Teramo-SPoT simple
stellar population models, and other recent sets of population synthesis
models, we discuss the feasibility of stellar population studies based on
fluctuation magnitudes analysis. The main result of this study is that
multiband optical SBF data and integrated colors can be used to significantly
constrain the chemical composition of the dominant stellar system in the
galaxy, but not the age in the case of systems older than 3 Gyr.
SBF color gradients are also detected and analyzed. These SBF gradient data,
together with other available data, point to the existence of mass dependent
metallicity gradients in galaxies, with the more massive objects showing a
non--negligible SBF versus color gradient. The comparison with models suggests
that such gradients imply more metal rich stellar populations in the galaxies'
inner regions with respect to the outer ones.Comment: ApJ Accepte
Infrared dust emission in the outer disk of M51
We examine faint infrared emission features detected in Spitzer Space
Telescope images of M51, which are associated with atomic hydrogen in the outer
disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The
infrared colors of these features are consistent with the colors of dust
associated with star formation in the bright disk. However, the star formation
efficiency (as a ratio of star formation rate to neutral gas mass) implied in
the outer disk is lower than that in the bright disk of M51 by an order of
magnitude, assuming a similar relationship between infrared emission and star
formation rate in the inner and outer disks.Comment: 13 pages in manuscript form, 2 figures; download PDF of manuscript
with original-resolution Figure 1 at
http://www.eg.bucknell.edu/physics/thornley/thornleym51.pd
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