2,111 research outputs found
Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields
Vertically coupled double quantum rings submitted to a perpendicular magnetic
field are addressed within the local spin-density functional theory. We
describe the structure of quantum ring molecules containing up to 40 electrons
considering different inter-ring distances and intensities of the applied
magnetic field. When the rings are quantum mechanically strongly coupled, only
bonding states are occupied and the addition spectrum of the artificial
molecules resembles that of a single quantum ring, with some small differences
appearing as an effect of the magnetic field. Despite the latter has the
tendency to flatten the spectra, in the strong coupling limit some clear peaks
are still found even when that can be interpretated from the
single-particle energy levels analogously as at zero applied field, namely in
terms of closed-shell and Hund's-rule configurations. Increasing the inter-ring
distance, the occupation of the first antibonding orbitals washes out such
structures and the addition spectra become flatter and irregular. In the weak
coupling regime, numerous isospin oscillations are found as a function of .Comment: 27 pages, 11 figures. To be published in Phys. Rev.
Circumstellar H-alpha from SN 1994D and future Type Ia supernovae: an observational test of progenitor models
Searching for the presence of circumstellar material is currently the only
direct way to discriminate between the different types of possible progenitor
systems for Type Ia supernovae. We have therefore looked for narrow H-alpha in
a high-resolution spectrum of the normal Type Ia supernova 1994D taken 10 days
before maximum and only 6.5 days after explosion. We derive an upper limit of
2.0E-16 erg cm^{-2} s^{-1} for an unresolved emission line at the local H II
region velocity. To estimate the limit this puts on wind density, we have made
time-dependent photoionization calculations. Assuming spherical symmetry we
find an upper limit of the mass loss rate which is roughly 1.5E-5 solar masses
per year for a wind speed of 10 km s^{-1}. This limit can exclude only the
highest-mass-loss-rate symbiotic systems as progenitors. We discuss the effect
of asymmetry and assess the relative merits of early optical, radio and X-ray
limits in constraining mass loss from Type Ia progenitors. We find that X-ray
observations can probably provide the most useful limits on the progenitor mass
loss, while high-resolution optical spectroscopy offers our only chance of
actually identifying circumstellar hydrogen.Comment: 7 pages, LaTeX, 3 Postscript figures, mn.sty, psfig.tex; MNRAS, in
pres
International perspectives on psychosocial working conditions, mental health, and stress of dairy farm operators
Dairy farm operators-farmers, workers, and family members-are faced with many demands and stressors in their daily work and these appear to be shared across countries and cultures. Dairy operators experience high psychosocial demands with respect to a hard work and production ethos, economic influences, and social and environmental responsibility. Furthermore, both traditional and industrial farms are highly dependent on external conditions, such as weather, fluctuating markets, and regulations from government authorities. Possible external stressors include disease outbreaks, taxes related to dairy production, and recent negative societal attitudes to farming in general. Dairy farm operators may have very few or no opportunities to influence and control these external conditions, demands, and expectations. High work demands and expectations coupled with low control and lack of social support can lead to a poor psychosocial work environment, with increased stress levels, ill mental health, depression, and, in the worst cases, suicide. Internationally, farmers with ill mental health have different health service options depending on their location. Regardless of location, it is initially the responsibility of the individual farmer and farm family to handle mental health and stress, which can be of short- or long-term duration. This paper reviews the literature on the topics of psychosocial working conditions, mental health, stress, depression, and suicide among dairy farm operators, farm workers, and farm family members in an international perspective
Temperature dependent effective mass renormalization in a Coulomb Fermi liquid
We calculate numerically the quasiparticle effective mass (m*)
renormalization as a function of temperature and electron density in two- and
three-dimensional electron systems with long-range Coulomb interaction. In two
dimensions, the leading temperature correction is linear and positive with the
slope being a universal density independent number in the high density limit.
We predict an enhancement of the effective mass at low temperatures and a
non-monotonic temperature dependence at higher temperatures (T/T_F ~ 0.1) with
the peak shifting toward higher temperatures as density decreases. In three
dimensions, we find that the effective mass temperature dependence is nonlinear
and non-universal, and depends on the electron density in a complicated way. At
very high densities, the leading correction is positive, while at lower
densities it changes sign and the effective mass decreases monotonically from
its zero temperature value with increasing temperature
The evolution of ultraviolet emission lines from the circumstellar material surrounding SN 1987A
The presence of narrow high-temperature emission lines from nitrogen-rich gas
close to SN 1987A has been the principal observational constraint on the evolu-
tionary status of the supernova's progenitor. A new analysis of the complete
five-year set of low and high resolution IUE ultraviolet spectra of SN 1987A
(1987.2--1992.3) provide fluxes for the N V 1240, N IV] 1486, He II 1640, OIII]
1665, NIII] 1751, and CIII] 1908 lines with significantly reduced random and
systematic errors and reveals significant short-term fluctuations in the light
curves. The N V, N IV] and N III] lines turn on sequentially over 15 to 20 days
and show a progression from high to low ionization potential, implying an ioni-
zation gradient in the emitting region. The line emission turns on suddenly at
83+/-4 days after the explosion, as defined by N IV]. The N III] line reaches
peak luminosity at 399+/-15 days. A ring radius of (6.24+/-0.20)E{17} cm and
inclination of 41.0+/-3.9 is derived from these times, assuming a circular
ring. The probable role of resonant scattering in the N V light curve
introduces systematic errors that leads us to exclude this line from the timing
analysis. A new nebular analysis yields improved CNO abundance ratios
N/C=6.1+/-1.1 and N/O=1.7+/-0.5, confirming the nitrogen enrichment found in
our previous paper. From the late-time behavior of the light curves we find
that the emission origi- nates from progressively lower density gas. We
estimate the emitting mass near maximum (roughly 400 days) to be roughly
4.7E{-2} solar masses, assuming a filling factor of unity and an electron
density of 2.6E4 cm^{-3}. These results are discussed in the context of current
models for the emission and hydrodynamics of the ring.Comment: 38 pages, AASTeX v.4.0, 13 Postscript figures; ApJ, in pres
Redundancy or GaAs? Two different approaches to solve the problem of SEU (Single Event Upset) in a digital optical link
The progenitor and early evolution of the Type IIb SN 2016gkg
We report initial observations and analysis on the Type IIb SN~2016gkg in the
nearby galaxy NGC~613. SN~2016gkg exhibited a clear double-peaked light curve
during its early evolution, as evidenced by our intensive photometric follow-up
campaign. SN~2016gkg shows strong similarities with other Type IIb SNe, in
particular with respect to the \he~emission features observed in both the
optical and near infrared. SN~2016gkg evolved faster than the prototypical
Type~IIb SN~1993J, with a decline similar to that of SN~2011dh after the first
peak. The analysis of archival {\it Hubble Space Telescope} images indicate a
pre-explosion source at SN~2016gkg's position, suggesting a progenitor star
with a mid F spectral type and initial mass \msun, depending on
the distance modulus adopted for NGC~613. Modeling the temperature evolution
within of explosion, we obtain a progenitor radius of
\rsun, smaller than that obtained from the analysis of the
pre-explosion images (\rsun).Comment: 7 pages, 5 figures. Submitted to ApJ Letter
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