1,297 research outputs found
Carrier and Light Trapping in Graded Quantum Well Laser Structures
We investigated the carrier and light trapping in GaInAs/AlGaAs single
quantum well laser structures by means of time resolved photoluminescence and
Raman spectroscopy. The influence of the shape and depth of the confinement
potential and of the cavity geometry was studied by using different AlGaAs/GaAs
short-period superlattices as barriers. Our results show that grading the
optical cavity improves considerably both carrier and light trapping in the
quantum well, and that the trapping efficiency is enhanced by increasing the
graded confining potential.Comment: PDF-format, 15 pages (including 4 figures), Applied Physics Letters
(June 2000
State-to-State Differential and Relative Integral Cross Sections for Rotationally Inelastic Scattering of H2O by Hydrogen
State-to-state differential cross sections (DCSs) for rotationally inelastic
scattering of H2O by H2 have been measured at 71.2 meV (574 cm-1) and 44.8 meV
(361 cm-1) collision energy using crossed molecular beams combined with
velocity map imaging. A molecular beam containing variable compositions of the
(J = 0, 1, 2) rotational states of hydrogen collides with a molecular beam of
argon seeded with water vapor that is cooled by supersonic expansion to its
lowest para or ortho rotational levels (JKaKc= 000 and 101, respectively).
Angular speed distributions of fully specified rotationally excited final
states are obtained using velocity map imaging. Relative integral cross
sections are obtained by integrating the DCSs taken with the same experimental
conditions. Experimental state-specific DCSs are compared with predictions from
fully quantum scattering calculations on the most complete H2O-H2 potential
energy surface. Comparison of relative total cross sections and state-specific
DCSs show excellent agreement with theory in almost all detailsComment: 46 page
Gamma-Ray Spectra & Variability of the Crab Nebula Emission Observed by BATSE
We report ~ 600 days of BATSE earth-occultation observations of the total
gamma-ray (30 keV to 1.7 MeV) emission from the Crab nebula, between 1991 May
24 (TJD 8400) and 1994 October 2 (TJD 9627). Lightcurves from 35-100, 100-200,
200-300, 300-400, 400-700, and 700-1000 keV, show that positive fluxes were
detected by BATSE in each of these six energy bands at significances of
approximately 31, 20, 9.2, 4.5, 2.6, and 1.3 sigma respectively per day. We
also observed significant flux and spectral variations in the 35-300 keV energy
region, with time scales of days to weeks. The spectra below 300 keV, averaged
over typical CGRO viewing periods of 6-13 days, can be well described by a
broken power law with average indices of ~ 2.1 and ~ 2.4 varying around a
spectral break at ~ 100 keV. Above 300 keV, the long-term averaged spectra,
averaged over three 400 d periods (TJD 8400-8800, 8800-9200, and 9200-9628,
respectively) are well represented by the same power law with index of ~ 2.34
up to ~ 670 keV, plus a hard spectral component extending from ~ 670 keV to ~
1.7 MeV, with a spectral index of ~ 1.75. The latter component could be related
to a complex structure observed by COMPTEL in the 0.7-3 MeV range. Above 3 MeV,
the extrapolation of the power-law continuum determined by the low-energy BATSE
spectrum is consistent with fluxes measured by COMPTEL in the 3-25 MeV range,
and by EGRET from 30-50 MeV. We interpret these results as synchrotron emission
produced by the interaction of particles ejected from the pulsar with the field
in different dynamical regions of the nebula system, as observed recently by
HST, XMM-Newton, and Chandra.Comment: To be published in the November 20, 2003, Vol 598 issue of the
Astrophysical Journa
A novel doxorubicin-glucuronide prodrug DOX-GA3 for tumour-selective chemotherapy: distribution and efficacy in experimental human ovarian cancer
The doxorubicin (DOX) prodrug N -[4-doxorubicin- N -carbonyl (oxymethyl) phenyl] O -β-glucuronyl carbamate (DOX-GA3) was synthesised for specific activation by human β-glucuronidase, which is released in necrotic areas of tumour lesions. This novel prodrug was completely activated to the parent drug by human β-glucuronidase with V max= 25.0 μmol min–1mg–1and K m= 1100 μM. The pharmacokinetics and distribution of DOX-GA3 in nude mice bearing human ovarian cancer xenografts (OVCAR-3) were determined and compared with DOX. Administration of DOX at 8 mg kg–1i.v. (maximum tolerated dose, MTD) to OVCAR-3-bearing mice resulted in a peak plasma concentration of the drug of 16.4 μM (t = 1 min). A 7.6-times lower peak plasma concentration of DOX was measured after injection of DOX-GA3 at 250 mg kg–1i.v. (50% of MTD). In normal tissues the prodrug showed peak DOX concentrations that were up to 5-fold (heart) lower than those found after DOX administration. DOX-GA3 activation by β-glucuronidase in the tumour yielded an almost 5-fold higher DOX peak concentration of 9.57 nmol g–1(P< 0.05) than the peak concentration of only 2.14 nmol g–1observed after DOX. As a consequence, the area under the curve of DOX calculated in tumour tissue after DOX-GA3 (13.1 μmol min–1g–1) was 10-fold higher than after DOX (1.31 μmol min–1g–1). The anti-tumour effects of DOX-GA3 and DOX were compared at equitoxic doses in OVCAR-3 xenografts at a mean tumour size of 125 mm3. The prodrug given i.v. at 500 mg kg–1weekly × 2 resulted in a maximum tumour growth inhibition of 87%, while the standard treatment with DOX at a dose of 8 mg kg–1i.v. weekly × 2 resulted in a maximum tumour growth inhibition of only 56%. Treatment with DOX-GA3 was also given to mice with larger tumours containing more necrosis. For tumours with a mean size of 400 mm3the specific growth delay by DOX-GA3 increased from 2.7 to 3.9. Our data indicate that DOX-GA3 is more effective than DOX and suggest that the prodrug will be specifically advantageous for treatment of advanced disease. © 2001 Cancer Research Campaign http://www.bjcancer.co
Quantum back-reaction of the superpartners in a large-N supersymmetric hybrid model
We study the supersymmetric hybrid model near and after the end of inflation.
As usual, we reduce the model to a purely scalar hybrid model on the level of
the classical fields. But on the level of quantum fluctuations and their
backreaction we take into account all superpartners of the waterfall field in a
large-N approximation. The evolution after slow roll displays two phases with a
different characteristic behaviour of the classical and fluctuation fields. We
find that the fluctuations of the pseudoscalar superpartner are of particular
importance in the late time phase. The motion of the waterfall field towards
its classical expectation value is found to be very slow and suggests a rather
flat potential and a stochastic force.Comment: 37 pages 19 figure
Piezoelectric exciton-acoustic phonon coupling in single quantum dots
Micro-photoluminescence spectroscopy at variable temperature, excitation
intensity and energy was performed on a single InAs/AlAs self-assembled quantum
dot. The exciton emission line (zero-phonon line, ZPL) exhibits a broad
sideband due to exciton-acoustic phonon coupling by the deformation potential
mechanism. Additionally, narrow low-energy sidebands at about 0.25 meV of the
ZPL are attributed to exciton-acoustic phonon piezoelectric coupling. In
lowering the excitation energy or intensity these bands gradually dominate the
emission spectrum of the quantum dot while the ZPL disappears. At high
excitation intensity the sidebands due to piezoelectric coupling decrease
strongly and the ZPL dominates the spectrum as a consequence of screening of
the piezoelectric coupling by the photocreated free carriers.Comment: 4 pages, 4 figure
Water bottle flipping physics
The water bottle flipping challenge consists of spinning a bottle, partially filled with water, and making it land upright. It is quite a striking phenomenon, since at first sight, it appears rather improbable that a tall rotating bottle could make such a stable landing. Here, we analyze the physics behind the water bottle flip, based on experiments and an analytical model that can be used in the classroom. Our measurements show that the angular velocity of the bottle decreases dramatically, enabling a nearly vertical descent and a successful landing. The reduced rotation is due to an increase in the moment of inertia, caused by the in-flight redistribution of the water mass along the bottle. Experimental and analytical results are compared quantitatively, and we demonstrate how to optimize the chances for a successful landing.</p
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