1,639 research outputs found
Exact solution of the Zeeman effect in single-electron systems
Contrary to popular belief, the Zeeman effect can be treated exactly in
single-electron systems, for arbitrary magnetic field strengths, as long as the
term quadratic in the magnetic field can be ignored. These formulas were
actually derived already around 1927 by Darwin, using the classical picture of
angular momentum, and presented in their proper quantum-mechanical form in 1933
by Bethe, although without any proof. The expressions have since been more or
less lost from the literature; instead, the conventional treatment nowadays is
to present only the approximations for weak and strong fields, respectively.
However, in fusion research and other plasma physics applications, the magnetic
fields applied to control the shape and position of the plasma span the entire
region from weak to strong fields, and there is a need for a unified treatment.
In this paper we present the detailed quantum-mechanical derivation of the
exact eigenenergies and eigenstates of hydrogen-like atoms and ions in a static
magnetic field. Notably, these formulas are not much more complicated than the
better-known approximations. Moreover, the derivation allows the value of the
electron spin gyromagnetic ratio to be different from 2. For
completeness, we then review the details of dipole transitions between two
hydrogenic levels, and calculate the corresponding Zeeman spectrum. The various
approximations made in the derivation are also discussed in details.Comment: 18 pages, 4 figures. Submitted to Physica Script
Theory of anomalous magnetic interference pattern in mesoscopic SNS Josephson junctions
The magnetic interference pattern in mesoscopic SNS Josephson junctions is
sensitive to the scattering in the normal part of the system. In this paper we
investigate it, generalizing Ishii's formula for current-phase dependence to
the case of normal scattering at NS boundaries in an SNS junction of finite
width. The resulting flattening of the first diffraction peak is consistent
with experimental data for S-2DEG-S mesoscopic junctions.Comment: 6 pages, 5 figures. Phys. Rev. B 68, 144514 (2003
Polarization-Sensitive Photodetectors Based on Directionally Oriented Organic Bulk-Heterojunctions
Polarized spectroscopic photodetection enables numerous applications in diverse areas such as sensing, industrial quality control, and visible light communications. Although organic photodetectors (OPDs) can offer a cost-effective alternative to silicon-based technology—particularly when flexibility and large-area arrays are desired—polarized OPDs are only beginning to receive due research interest. Instead of resorting to external polarization optics, this report presents polarized OPDs based on directionally oriented blends of poly(3-hexylthiophene) (P3HT) and benchmark polymer or nonfullerene acceptors fabricated using a versatile solution-based method. Furthermore, a novel postprocessing scheme based on backfilling and plasma etching is advanced to ameliorate high dark-currents that are otherwise inherent to fibrillar active layers. The resulting polarized P3HT:N2200 OPDs exhibit a broad enhancement across all principal figures of merit compared to reference isotropic devices, including peak responsivities of 70 mA W and up to a threefold increase in 3 dB bandwidth to 0.75 MHz under parallel-polarized illumination. Polarization ratios of up to 3.5 are obtained across a spectral range that is determined by the specific donor–acceptor combinations. Finally, as a proof-of-concept demonstration, polarized OPDs are used for photoelasticity analysis of rubber films under tensile deformation, highlighting their potential for existing and emerging applications in advanced optical sensing
Donor states in modulation-doped Si/SiGe heterostructures
We present a unified approach for calculating the properties of shallow
donors inside or outside heterostructure quantum wells. The method allows us to
obtain not only the binding energies of all localized states of any symmetry,
but also the energy width of the resonant states which may appear when a
localized state becomes degenerate with the continuous quantum well subbands.
The approach is non-variational, and we are therefore also able to evaluate the
wave functions. This is used to calculate the optical absorption spectrum,
which is strongly non-isotropic due to the selection rules. The results
obtained from calculations for Si/SiGe quantum wells allow us to
present the general behavior of the impurity states, as the donor position is
varied from the center of the well to deep inside the barrier. The influence on
the donor ground state from both the central-cell effect and the strain arising
from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure
Polarization-Sensitive Photodetectors Based on Directionally Oriented Organic Bulk-Heterojunctions
Polarized spectroscopic photodetection enables numerous applications in diverse areas such as sensing, industrial quality control, and visible light communications. Although organic photodetectors (OPDs) can offer a cost-effective alternative to silicon-based technology—particularly when flexibility and large-area arrays are desired—polarized OPDs are only beginning to receive due research interest. Instead of resorting to external polarization optics, this report presents polarized OPDs based on directionally oriented blends of poly(3-hexylthiophene) (P3HT) and benchmark polymer or nonfullerene acceptors fabricated using a versatile solution-based method. Furthermore, a novel postprocessing scheme based on backfilling and plasma etching is advanced to ameliorate high dark-currents that are otherwise inherent to fibrillar active layers. The resulting polarized P3HT:N2200 OPDs exhibit a broad enhancement across all principal figures of merit compared to reference isotropic devices, including peak responsivities of 70 mA W and up to a threefold increase in 3 dB bandwidth to 0.75 MHz under parallel-polarized illumination. Polarization ratios of up to 3.5 are obtained across a spectral range that is determined by the specific donor–acceptor combinations. Finally, as a proof-of-concept demonstration, polarized OPDs are used for photoelasticity analysis of rubber films under tensile deformation, highlighting their potential for existing and emerging applications in advanced optical sensing
Probing photo-ionization: Experiments on positive streamers in pure gasses and mixtures
Positive streamers are thought to propagate by photo-ionization whose
parameters depend on the nitrogen:oxygen ratio. Therefore we study streamers in
nitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen, as well as in
pure oxygen and argon. Our new experimental set-up guarantees contamination of
the pure gases to be well below 1 ppm. Streamers in oxygen are difficult to
measure as they emit considerably less light in the sensitivity range of our
fast ICCD camera than the other gasses. Streamers in pure nitrogen and in all
nitrogen/oxygen mixtures look generally similar, but become somewhat thinner
and branch more with decreasing oxygen content. In pure nitrogen the streamers
can branch so much that they resemble feathers. This feature is even more
pronounced in pure argon, with approximately 10^2 hair tips/cm^3 in the
feathers at 200 mbar; this density could be interpreted as the free electron
density creating avalanches towards the streamer stem. It is remarkable that
the streamer velocity is essentially the same for similar voltage and pressure
in all nitrogen/oxygen mixtures as well as in pure nitrogen, while the oxygen
concentration and therefore the photo-ionization lengths vary by more than five
orders of magnitude. Streamers in argon have essentially the same velocity as
well. The physical similarity of streamers at different pressures is confirmed
in all gases; the minimal diameters are smaller than in earlier measurements.Comment: 28 pages, 14 figures. Major differences with v1: - appendix and
spectra removed - subsection regarding effects of repetition frequency added
- many more smaller change
An adaptive grid refinement strategy for the simulation of negative streamers
The evolution of negative streamers during electric breakdown of a
non-attaching gas can be described by a two-fluid model for electrons and
positive ions. It consists of continuity equations for the charged particles
including drift, diffusion and reaction in the local electric field, coupled to
the Poisson equation for the electric potential. The model generates field
enhancement and steep propagating ionization fronts at the tip of growing
ionized filaments. An adaptive grid refinement method for the simulation of
these structures is presented. It uses finite volume spatial discretizations
and explicit time stepping, which allows the decoupling of the grids for the
continuity equations from those for the Poisson equation. Standard refinement
methods in which the refinement criterion is based on local error monitors fail
due to the pulled character of the streamer front that propagates into a
linearly unstable state. We present a refinement method which deals with all
these features. Tests on one-dimensional streamer fronts as well as on
three-dimensional streamers with cylindrical symmetry (hence effectively 2D for
numerical purposes) are carried out successfully. Results on fine grids are
presented, they show that such an adaptive grid method is needed to capture the
streamer characteristics well. This refinement strategy enables us to
adequately compute negative streamers in pure gases in the parameter regime
where a physical instability appears: branching streamers.Comment: 46 pages, 19 figures, to appear in J. Comp. Phy
A short delirium caregiver questionnaire for triage of elderly outpatients with cognitive impairment:a development and test accuracy study
OBJECTIVES: Delirium is often missed in older outpatients. Caregivers can give valuable information that might improve identification rates. The aim of this study was to develop a short and sensitive delirium caregiver questionnaire (DCQ) for triage of elderly outpatients with cognitive impairment by telephone.DESIGN, SETTING, AND PARTICIPANTS: The pilot questionnaire was administered to 112 caregivers of patients who were referred for dementia screening to our clinic for geriatric psychiatry, and the final DCQ to 234 other caregivers.MEASUREMENTS: In phase I (2013-2014), we tested a pilot questionnaire with 17 items. Health professionals who established delirium diagnoses were blinded to the results. We then used the results and other information available at referral to construct the final DCQ with seven items. During phase II (2015-2016), we investigated the test accuracy of the final DCQ in a subsequent cohort. In both phases, the patients received a structured diagnostic workup. Time between referral and first visit was a secondary outcome.RESULTS: The final DCQ consisted of the following items: emergency visit required, sleeping disorder, fluctuating course, hallucinations, suspicious thoughts, previous delirium, and recent discharge from hospital. DCQ results indicated that urgent intake was required in 85 of 234 patients. Sensitivity was 73.5% (95% CI: 58.9-85.1%) and specificity 73.5% (95% CI: 66.5-79.7%). The mean number of days to first visit dropped from 31.6 to 11.2 in delirious patients (p = 0.001).CONCLUSIONS: Triage with the easy-to-use DCQ among patients referred for cognitive screening leads to earlier assessment and higher detection rates of delirium.</p
The first COMPTEL Source Catalogue
The imaging Compton telescope COMPTEL aboard NASA's Compton Gamma-Ray
Observatory has opened the MeV gamma-ray band as a new window to astronomy.
COMPTEL provided the first complete all-sky survey in the energy range 0.75 to
30 MeV. The catalogue, presented here, is largely restricted to published
results. It contains firm as well as marginal detections of continuum and line
emitting sources and presents upper limits for various types of objects. The
numbers of the most significant detections are 32 for steady sources and 31 for
gamma-ray bursters. Among the continuum sources, detected so far, are spin-down
pulsars, stellar black-hole candidates, supernova remnants, interstellar
clouds, nuclei of active galaxies, gamma-ray bursters, and the Sun during solar
flares. Line detections have been made in the light of the 1.809 MeV 26Al line,
the 1.157 MeV 44Ti line, the 847 and 1238 keV 56Co lines, and the neutron
capture line at 2.223 MeV. For the identification of galactic sources, a
modelling of the diffuse galactic emission is essential. Such a modelling at
this time does not yet exist at the required degree of accuracy. Therefore, a
second COMPTEL source catalogue will be produced after a detailed and accurate
modelling of the diffuse interstellar emission has become possible.Comment: 50 pages including 4 figures; accepted for publication in A&A
Supplement
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