7,068 research outputs found
Exchange Current Corrections to Neutrino--Nucleus Scattering
Relativistic exchange current corrections to neutrino--nucleus cross sections
are presented assuming non--vanishing strange quark form factors for the
constituent nucleons. For charged current processes the exchange current
corrections can lower the impulse approximation results by 10\% while these
corrections are found to be sensitive to both the nuclear density and the
strange quark axial form factor of the nucleon for neutral current processes.
Implications on the LSND experiment to determine this form factor are
discussed.Comment: 11 pages, 2 figures, revtex 3.0, full postscript version of the file
and figures available at
http://www.nikhefk.nikhef.nl/projects/Theory/preprints/preprints.html To
appear in Phys. Rev. Lett
In-Plane Magnetic Field Induced Anisotropy of 2D Fermi Contours and the Field Dependent Cyclotron Mass
The electronic structure of a 2D gas subjected to a tilted magnetic field,
with a strong component parallel to the GaAs/AlGaAs interface and a weak
component oriented perpendicularly, is studied theoretically. It is shown that
the parallel field component modifies the originally circular shape of a Fermi
contour while the perpendicular component drive an electron by the Lorentz
force along a Fermi line with a cyclotron frequency given by its shape. The
corresponding cyclotron effective mass is calculated self-consistently for
several concentrations of 2D carriers as a function of the in-plane magnetic
field. The possibility to detect its field-induced deviations from the zero
field value experimentally is discussed.Comment: written in LaTeX, 9 pages, 4 figures (6 pages) in 1 PS file
(compressed and uuencoded) available on request from [email protected],
SM-JU-93-
Order-disorder transition in nanoscopic semiconductor quantum rings
Using the path integral Monte Carlo technique we show that semiconductor
quantum rings with up to six electrons exhibit a temperature, ring diameter,
and particle number dependent transition between spin ordered and disordered
Wigner crystals. Due to the small number of particles the transition extends
over a broad temperature range and is clearly identifiable from the electron
pair correlation functions.Comment: 4 pages, 5 figures, For recent information on physics of small
systems see http://www.smallsystems.d
Power-law carrier dynamics in semiconductor nanocrystals at nanosecond time scales
We report the observation of power law dynamics on nanosecond to microsecond
time scales in the fluorescence decay from semiconductor nanocrystals, and draw
a comparison between this behavior and power-law fluorescence blinking from
single nanocrystals. The link is supported by comparison of blinking and
lifetime data measured simultaneously from the same nanocrystal. Our results
reveal that the power law coefficient changes little over the nine decades in
time from 10 ns to 10 s, in contrast with the predictions of some diffusion
based models of power law behavior.Comment: 3 pages, 2 figures, compressed for submission to Applied Physics
Letter
Coulomb interaction effects on the electronic structure of radial polarized excitons in nanorings
The electronic structure of radially polarized excitons in structured
nanorings is analyzed, with emphasis in the ground-state properties and their
dependence under applied magnetic fields perpendicular to the ring plane. The
electron-hole Coulomb attraction has been treated rigorously, through numerical
diagonalization of the full exciton Hamiltonian in the non-interacting
electron-hole pairs basis. Depending on the relative weight of the kinetic
energy and Coulomb contributions, the ground-state of polarized excitons has
"extended" or "localized" features. In the first case, corresponding to small
rings dominated by the kinetic energy, the ground-state shows Aharonov-Bohm
(AB) oscillations due to the individual orbits of the building particles of the
exciton. In the localized regime, corresponding to large rings dominated by the
Coulomb interaction, the only remaining AB oscillations are due to the magnetic
flux trapped between the electron and hole orbits. This dependence of the
exciton, a neutral excitation, on the flux difference confirms this feature as
a signature of Coulomb dominated polarized excitons. Analytical approximations
are provided in both regimens, which accurate reproduce the numerical results.Comment: 9 pages, including 6 figure
Energy levels and far-infrared spectroscopy for two electrons in a semiconductor nanoring
The effects of electron-electron interaction of a two-electron nanoring on
the energy levels and far-infrared (FIR) spectroscopy have been investigated
based on a model calculation which is performed within the exactly numerical
diagonalization. It is found that the interaction changes the energy spectra
dramatically, and also shows significant influence on the FIR spectroscopy. The
crossings between the lowest spin-singlet and triplet states induced by the
coulomb interaction are clearly revealed. Our results are related to the
experiment recently carried out by A. Lorke et al. [Phys. Rev. Lett. 84, 2223
(2000)].Comment: 17 pages, 6 figures, revised and accepted by Phys. Rev. B (Dec. 15
Optical excitations of a self assembled artificial ion
By use of magneto-photoluminescence spectroscopy we demonstrate bias
controlled single-electron charging of a single quantum dot. Neutral, single,
and double charged excitons are identified in the optical spectra. At high
magnetic fields one Zeeman component of the single charged exciton is found to
be quenched, which is attributed to the competing effects of tunneling and
spin-flip processes. Our experimental data are in good agreement with
theoretical model calculations for situations where the spatial extent of the
hole wave functions is smaller as compared to the electron wave functions.Comment: to be published in Physical Review B (rapid communication
Muon capture on nuclei with N > Z, random phase approximation, and in-medium renormalization of the axial-vector coupling constant
We use the random phase approximation to describe the muon capture rate on
Ca,Ca, Fe, Zr, and Pb. With
Ca as a test case, we show that the Continuum Random Phase
Approximation (CRPA) and the standard RPA give essentially equivalent
descriptions of the muon capture process. Using the standard RPA with the free
nucleon weak form factors we reproduce the experimental total capture rates on
these nuclei quite well. Confirming our previous CRPA result for the
nuclei, we find that the calculated rates would be significantly lower than the
data if the in-medium quenching of the axial-vector coupling constant were
employed.Comment: submitted to Phys. Rev.
Neuropsychological deficits in disordered screen use behaviours : A systematic review and meta-analysis
Over the last few decades, excessive and disordered screen use has become more prevalent, prompting investigations into its associated consequences. The extent to which disordered screen use behaviours impact neuropsychological functioning has been reportedly mixed and at times inconsistent. This review sought to synthesise the literature and estimate the magnitude of overall cognitive impairment across a wide range of disordered screen use behaviours. We also sought to determine the cognitive domains most impacted, and whether the observed impairments were moderated by the classification of screen-related behaviours (i.e., Internet or gaming) or the format of cognitive test administration (i.e., paper-and-pencil or computerised). A systematic search of databases (Embase, PsycINFO, MEDLINE) identified 43 cross-sectional articles that assessed neuropsychological performance in disordered screen use populations, 34 of which were included in the meta-analysis. A random-effects meta-analysis revealed significant small/medium (g = .38) cognitive deficits for individuals with disordered screen use behaviours relative to controls. The most affected cognitive domain with a significant medium effect size (g = .50) was attention and focus followed by a significant reduction in executive functioning (g = .31). The classification of disordered screen use behaviours into Internet or gaming categories or the format of cognitive testing did not moderate these deficits. Additionally, excluding disordered social media use in an exploratory analysis had little effect on the observed outcomes. This study highlights a number of methodological considerations that may have contributed to disparate findings and shows that disordered screen use can significantly impact cognitive performance. Recommendations for future research are also discussed. Data for this study can be found at https://osf.io/upeha/
Electro-elastic tuning of single particles in individual self-assembled quantum dots
We investigate the effect of uniaxial stress on InGaAs quantum dots in a
charge tunable device. Using Coulomb blockade and photoluminescence, we observe
that significant tuning of single particle energies (~ -0.5 meV/MPa) leads to
variable tuning of exciton energies (+18 to -0.9 micro-eV/MPa) under tensile
stress. Modest tuning of the permanent dipole, Coulomb interaction and
fine-structure splitting energies is also measured. We exploit the variable
exciton response to tune multiple quantum dots on the same chip into resonance.Comment: 16 pages, 4 figures, 1 table. Final versio
- …