2,215 research outputs found
Theory of imaging a photonic crystal with transmission near-field optical microscopy
While near-field scanning optical microscopy (NSOM) can provide optical
images with resolution much better than the diffraction limit, analysis and
interpretation of these images is often difficult. We present a theory of
imaging with transmission NSOM that includes the effects of tip field,
tip/sample coupling, light propagation through the sample and light collection.
We apply this theory to analyze experimental NSOM images of a nanochannel glass
(NCG) array obtained in transmission mode. The NCG is a triangular array of
dielectric rods in a dielectric glass matrix with a two-dimensional photonic
band structure. We determine the modes for the NCG photonic crystal and
simulate the observed data. The calculations show large contrast at low
numerical aperture (NA) of the collection optics and detailed structure at high
NA consistent with the observed images. We present calculations as a function
of NA to identify how the NCG photonic modes contribute to and determine the
spatial structure in these images. Calculations are presented as a function of
tip/sample position, sample index contrast and geometry, and aperture size to
identify the factors that determine image formation with transmission NSOM in
this experiment.Comment: 28 pages of ReVTex, 14 ps figures, submitted to Phys. Rev.
Heating due to momentum transfer in low-energy positronium-antiproton scattering
Discovery that the elastic scattering cross sections in the antiproton-positronium system are very large close to threshold, particularly for excited positronium states.The implications for experiments aiming to use the positronium-antiproton system for antihydrogen formation are explored
In-beam gamma-ray spectroscopy of 35Mg and 33Na
Excited states in the very neutron-rich nuclei 35Mg and 33Na were populated
in the fragmentation of a 38Si projectile beam on a Be target at 83 MeV/u beam
energy. We report on the first observation of gamma-ray transitions in 35Mg,
the odd-N neighbor of 34Mg and 36Mg, which are known to be part of the "Island
of Inversion" around N = 20. The results are discussed in the framework of
large- scale shell-model calculations. For the A = 3Z nucleus 33Na, a new
gamma-ray transition was observed that is suggested to complete the gamma-ray
cascade 7/2+ --> 5/2+ --> 3/2+ gs connecting three neutron 2p-2h intruder
states that are predicted to form a close-to-ideal K = 3/2 rotational band in
the strong-coupling limit.Comment: Accepted for publication Phys. Rev. C. March 16, 2011: Replaced
figures 3 and 5. We thank Alfredo Poves for pointing out a problem with the
two figure
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
Racial residential segregation and colorectal cancer mortality in the Mississippi Delta Region
INTRODUCTION: Few studies have examined the effects of racial segregation on colorectal cancer (CRC) outcomes, and none has determined whether rurality moderates the effect of racial segregation on CRC mortality. We examined whether the effect of segregation on CRC mortality varied by rurality in the Mississippi Delta Region, an economically distressed and historically segregated region of the United States.
METHODS: We used data from the US Census Bureau and the 1999-2018 Surveillance, Epidemiology, and End Results (SEER) program to estimate mixed linear regression models in which CRC mortality rates among Black and White residents in Delta Region counties (N = 252) were stratified by rurality and regressed on White-Black residential segregation indices and 4 socioeconomic control variables.
RESULTS: Among Black residents, CRC mortality rates in urban counties were a function of a squared segregation term (b = 162.78, P = .01), indicating that the relationship between segregation and CRC mortality was U-shaped. Among White residents, main effects of annual household income (b = 29.01, P = .04) and educational attainment (b = 34.58, P = .03) were associated with CRC mortality rates in urban counties, whereas only annual household income (b = 19.44, P = .04) was associated with CRC mortality rates in rural counties. Racial segregation was not associated with CRC mortality rates among White residents.
CONCLUSION: Our county-level analysis suggests that health outcomes related to racial segregation vary by racial, contextual, and community factors. Segregated rural Black communities may feature stronger social bonds among residents than urban communities, thus increasing interpersonal support for cancer prevention and control. Future research should explore the effect of individual-level factors on colorectal cancer mortality
Charge Exchange and Chemical Reactions with Trapped Th
We have measured the reaction rates of trapped, buffer gas cooled Th
and various gases and have analyzed the reaction products using trapped ion
mass spectrometry techniques. Ion trap lifetimes are usually limited by
reactions with background molecules, and the high electron affinity of multiply
charged ions such as Th make them more prone to loss. Our results show
that reactions of Th with carbon dioxide, methane, and oxygen all occur
near the classical Langevin rate, while reaction rates with argon, hydrogen,
and nitrogen are orders of magnitude lower. Reactions of Th with oxygen
and methane proceed primarily via charge exchange, while simultaneous charge
exchange and chemical reaction occurs between Th and carbon dioxide.
Loss rates of Th in helium are consistent with reaction with impurities
in the gas. Reaction rates of Th with nitrogen and argon depend on the
internal electronic configuration of the Th.Comment: 9 pages, 10 figures, 4 tables. Submitted to PR
Mirror Energy Differences at Large Isospin Studied through Direct Two-Nucleon Knockout
The first spectroscopy of excited states in 52Ni (Tz=2) and 51Co (Tz=-3/2)
has been obtained using the highly selective two-neutron knockout reaction.
Mirror energy differences between isobaric analogue states in these nuclei and
their mirror partners are interpreted in terms of isospin nonconserving
effects. A comparison between large scale shell-model calculations and data
provides the most compelling evidence to date that both electromagnetic and an
additional isospin nonconserving interactions for J=2 couplings, of unknown
origin, are required to obtain good agreement.Comment: Accepted for publication in Physical Review Letter
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