3,238 research outputs found
Photoluminescence from silicon dioxide photonic crystal cavities with embedded silicon nanocrystals
One dimensional nanobeam photonic crystal cavities are fabricated in silicon
dioxide with silicon nanocrystals. Quality factors of over 9 x 10^3 are found
in experiment, matching theoretical predictions, with mode volumes of
1.5(lambda/n)^3 . Photoluminescence from the cavity modes is observed in the
visible wavelength range 600-820 nm. Studies of the lossy characteristics of
the cavities are conducted at varying temperatures and pump powers. Free
carrier absorption effects are found to be significant at pump powers as low as
a few hundred nanowatts.Comment: 13 pages 9 figure
Titania/alumina bilayer gate insulators for InGaAs metal-oxide-semiconductor devices
We describe the electrical properties of atomic layer deposited TiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> bilayer gate oxides which simultaneously achieve high gate capacitance density and low gate leakage current density. Crystallization of the initially amorphous TiO<sub>2</sub> film contributes to a significant accumulation capacitance increase (âŒ33%) observed after a forming gas anneal at 400â°C. The bilayer dielectrics reduce gate leakage current density by approximately one order of magnitude at flatband compared to Al<sub>2</sub>O<sub>3</sub> single layer of comparable capacitance equivalent thickness. The conduction band offset of TiO<sub>2</sub> relative to InGaAs is 0.6âeV, contributing to the ability of the stacked dielectric to suppress gate leakage conduction
Strict Limit on CPT Violation from Polarization of Gamma-Ray Bursts
We report the strictest observational verification of CPT invariance in the
photon sector, as a result of gamma-ray polarization measurement of distant
gamma-ray bursts (GRBs), which are brightest stellar-size explosions in the
universe. We detected the gamma-ray polarization of three GRBs with high
significance, and the source distances may be constrained by a well-known
luminosity indicator for GRBs. For the Lorentz- and CPT-violating dispersion
relation E_{\pm}^2=p^2 \pm 2\xi p^3/M_{Pl}, where \pm denotes different
circular polarization states of the photon, the parameter \xi is constrained as
|\xi|<O(10^{-15}). Barring precise cancellation between quantum gravity effects
and dark energy effects, the stringent limit on the CPT-violating effect leads
to the expectation that quantum gravity presumably respects the CPT invariance.Comment: 4 pages; accepted for publication in Physical Review Letters;
redshift estimates of GRBs changed (i.e z=0.382 was wrong for GRB 110721A)
and calculations of \xi limit improved from the previous versio
Measuring energy dependent polarization in soft gamma-rays using Compton scattering in PoGOLite
Linear polarization in X- and gamma-rays is an important diagnostic of many
astrophysical sources, foremost giving information about their geometry,
magnetic fields, and radiation mechanisms. However, very few X-ray polarization
measurements have been made, and then only mono-energetic detections, whilst
several objects are assumed to have energy dependent polarization signatures.
In this paper we investigate whether detection of energy dependent polarization
from cosmic sources is possible using the Compton technique, in particular with
the proposed PoGOLite balloon-experiment, in the 25-100 keV range. We use
Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus
X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6
and 35 hours were simulated, corresponding to a standard and a long duration
flight respectively. Both smooth and sharp energy variations of the
polarization are investigated and compared to constant polarization signals
using chi-square statistics. We can reject constant polarization, with energy,
for the Cygnus X-1 spectrum (in the hard state), if the reflected component is
assumed to be completely polarized, whereas the distinction cannot be made for
weaker polarization. For the accreting pulsar, constant polarization can be
rejected in the case of polarization in a narrow energy band with at least 50%
polarization, and similarly for a negative step distribution from 30% to 0%
polarization.Comment: 11 pages, 12 figures; updated to match version accepted for
publication in Astroparticle Physics (only minor changes
Recent Heavy-Flavor results at STAR
We present the recent results on non-photonic electron (NPE) yields from RHIC
run8 p+p collisions. The ratio as a function of in run8 with a
factor of 10 reduction of the inner detector material at STAR is found to be
consistent with those results from run3 taking into account the NPE from charm
leptonic decay and the difference of photonic electron yield from photon
conversion in detector material. \Jpsi spectra in \pp and \cucu collisions at
\sNN = 200 GeV with high sampled luminosity \Jpsi spectrum at high-\pT follows
scaling, but the scaling is violated at low \pT. -hadron
correlations in \pp collisions are studied to understand the \Jpsi production
mechanism at high . We observed an absence of charged hadrons accompanying
\Jpsi on the near-side, in contrast to the strong correlation peak in the
di-hadron correlations. This constrains the -meson contribution and jet
fragmentation to inclusive \Jpsi to be . Yields in
minimum-bias \cucu collisions are consistent with those in \pp collisions
scaled by the underlying binary nucleon-nucleon collisions in the measured \pT
range. Other measurements and future projects related to heavy-flavors are
discussed.Comment: 8 pages 4 figures, proceedings of the International Conference on
Strangeness in Quark Matter 2008 - Beijing, China, Oct. 6-10, 200
ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036
ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals
coherent X-ray pulsation from this source for the first time. The source was
first detected in the serendipitous Einstein observation and later identified
as a radio pulsar. Possible pulsation in the gamma-ray band has been detected
from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA
band is characterized by a hard power-law type emission with a photon index of
about 2.3, when fitted with a single power-law function modified with
absorption. An additional blackbody component of about 0.14 keV increases the
quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in
the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are
similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR
B0633+17 (Geminga), and PSR B0656+14.Comment: To appear in ApJ Letter
Observations of the supernova remnant W28 at TeV energies
The atmospheric Cerenkov imaging technique has been used to search for
point-like and diffuse TeV gamma-ray emission from the southern supernova
remnant, W28, and surrounding region. The search, made with the CANGAROO 3.8m
telescope, encompasses a number of interesting features, the supernova remnant
itself, the EGRET source 3EG J1800-2338, the pulsar PSR J1801-23, strong 1720
MHz OH masers and molecular clouds on the north and east boundaries of the
remnant. An analysis tailored to extended and off-axis point sources was used,
and no evidence for TeV gamma-ray emission from any of the features described
above was found in data taken over the 1994 and 1995 seasons. Our upper limit
(E>1.5 TeV) for a diffuse source of radius 0.25deg encompassing both molecular
clouds was calculated at 6.64e-12 photons cm^-2 s^-1 (from 1994 data), and
interpreted within the framework of a model predicting TeV gamma-rays from
shocked-accelerated hadrons. Our upper limit suggests the need for some cutoff
in the parent spectrum of accelerated hadrons and/or slightly steeper parent
spectra than that used here (-2.1). As to the nature of 3EG J1800-2338, it
possibly does not result entirely from pi-zero decay, a conclusion also
consistent with its location in relation to W28.Comment: 11 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysic
TeV observations of Centaurus A
We have searched for TeV gamma-rays from Centaurus A and surrounding region
out to +/- 1.0 deg using the CANGAROO 3.8m telescope. No evidence for TeV
gamma-ray emission was observed from the search region, which includes a number
of interesting features located away from the tracking centre of our data. The
3 sigma upper limit to the flux of gamma-rays above 1.5 TeV from an extended
source of radius 14' centred on Centaurus A is 1.28e-11 photons cm^-2 s^-1.Comment: 4 pages. Astroparticle Physics, accepted for publication. Some upper
limits overestimated by factor 2-4 in original version astro-ph/9901316. Now
correcte
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