Magnetic-field dependence of the spin states of the negatively charged exciton in GaAs quantum wells

We present high-field (<50 T) photoluminescence measurements of the binding energy of the singlet and triplet states of the negatively charged exciton in a 200-Angstrom quantum well. Comparing our data with those of other groups and with theoretical predictions we clearly show how the singlet, "bright" and "dark" triplet states may be identified according to the high-field dependence of their binding energies. We demonstrate that a very consistent behavior of the binding energy in a magnetic field has been observed in quantum wells of different widths by different groups and conclude that the triplet state found in this, as well as nearly all other experiments, is undoubtedly the bright triplet. By combining our data with that in the literature we are able to present the generic form of the binding energy of the spin states of the charged exciton in a magnetic field, which reveals the predicted singlet to dark triplet ground state transition at about 20 T

Wind bubbles within H II regions around slowly moving stars

Interstellar bubbles around O stars are driven by a combination of the star's wind and ionizing radiation output. The wind contribution is uncertain because the boundary between the wind and interstellar medium is difficult to observe. Mid-infrared observations (e.g., of the H II region RCW 120) show arcs of dust emission around O stars, contained well within the H II region bubble. These arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by density gradients and/or stellar motion. We present two-dimensional, radiation-hydrodynamics simulations investigating the evolution of wind bubbles and H II regions around massive stars moving through a dense (n=3000 cm^{-3}), uniform medium with velocities ranging from 4 to 16 km/s. The H II region morphology is strongly affected by stellar motion, as expected, but the wind bubble is also very aspherical from birth, even for the lowest space velocity considered. Wind bubbles do not fill their H II regions (we find filling factors of 10-20%), at least for a main sequence star with mass M~30 Msun. Furthermore, even for supersonic velocities the wind bow shock does not significantly trap the ionization front. X-ray emission from the wind bubble is soft, faint, and comes mainly from the turbulent mixing layer between the wind bubble and the H II region. The wind bubble radiates <1 per cent of its energy in X-rays; it loses most of its energy by turbulent mixing with cooler photoionized gas. Comparison of the simulations with the H II region RCW 120 shows that its dynamical age is <=0.4 Myr and that stellar motion <=4 km/s is allowed, implying that the ionizing source is unlikely to be a runaway star but more likely formed in situ. The region's youth, and apparent isolation from other O or B stars, makes it very interesting for studies of massive star formation and of initial mass functions.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and Astrophysics (new version corrects an error in the simulation postprocessing, figs 6,7,11 are modified slightly, conclusions unchanged

Evaluating Responses of Sugar Beet Cultivars to Fusarium Species in Greenhouse and Field Conditions

Fusarium yellows of sugar beet (Beta vulgaris L.) is becoming an important disease in Minnesota and North Dakota in the United States. One of the best ways to manage Fusarium yellows is to develop and use resistant cultivars. Responses of eight sugar beet cultivars to three Fusarium species were determined in the greenhouse and compared to the responses of the same eight cultivars grown in a field already infected by Fusarium yellows. There were significant and appreciable relationships between greenhouse and field studies for responses of eight sugar beet cultivars on Fusarium yellows. The estimated correlation coefficient for area under disease progress curve (AUDPC) between observations in fields and those in greenhouses was 0.987 (p<0.01). The mean AUDPC were significantly different among cultivars (p<0.01) in the greenhouse and in the field studies. Of the cultivars, Van der Have 46177 was the most susceptible, but Crystal R434 the most resistant. Crystal 820, Van der Have 66561, and Beta 4797R were resistant, and Beta 4818R, Seedex Magnum, and Hilleshog 2463Rz were moderately resistant. There was a strong negative relationship between the AUDPC and recoverable sucrose yield in the field experiments and the estimated coefficient of determination was 0.939 (p<0.01). It was concluded that greenhouse screening can act as a useful and reliable means to evaluate and select beet germplasms and/or accessions for resistance to Fusarium yellows.Peer reviewe

Strategic decision-making process (SDMP) in times of crisis:evidence from Greek banks

This paper investigates the strategic decision making process (SDMP) of Greek banks’ top management in the context of profound organisational changes introduced in 2012 due to the aftermath of the 2008 global financial crisis. It focuses on the impact of three key dimensions of the SDMP, namely, rationality, intuition and political behaviour, relating to four changes introduced, namely, mergers and acquisitions, branch network rationalisation, integration of information technology (IT) and downsizing of operations and personnel. A survey questionnaire was conducted, targeting Greek banks’ top management. Out of 140 questionnaires, 78 were returned, a 55.71% response rate. Data was analysed using structural equation modelling. Research findings identify rationality as a key dimension of SDMP for all organisational changes, as there was high focus on identifying and analysing all required information, use of external financial advisors, and reliance on multiple methods of information gathering. Decision-makers used their intuition in the form of past experience when making acquisition decisions, whilst their personal judgment and “inner voice” were neglected.Finally, political behaviour was not displayed during this process, as decision-makers were open with each other about their interests and preferences, and there was no bargaining, negotiation or use of power amongst them. One limitation was that of not considering all the factors that might help measure SDMP. Also, this study was conducted in a period of political and financial uncertainty for Greek banks, as well as for the Greek economy in general, so findings may not be generalizable to other industries and countries. Conducting interviews could have offered deeper insight as well. This study’s value lies in the fact that the organisational changes were determined by Greece’s leaders, and thus the Greek banks had to operate under a dynamic, inflexible and non-autonomous environment. Also, this study extends prior SDMP research by examining the impact of the three key SDMP dimensions on four types of organisational change

Quantum transport in weakly coupled superlattices at low temperature

We report on the study of the electrical current flowing in weakly coupled superlattice (SL) structures under an applied electric field at very low temperature, i.e. in the tunneling regime. This low temperature transport is characterized by an extremely low tunneling probability between adjacent wells. Experimentally, I(V) curves at low temperature display a striking feature, i.e a plateau or null differential conductance. A theoretical model based on the evaluation of scattering rates is developed in order to understand this behaviour, exploring the different scattering mechanisms in AlGaAs alloys. The dominant interaction in usual experimental conditions such as ours is found to be the electron-ionized donors scattering. The existence of the plateau in the I(V) characteristics is physically explained by a competition between the electric field localization of the Wannier-Stark electron states in the weakly coupled quantum wells and the electric field assisted tunneling between adjacent wells. The influence of the doping concentration and profile as well as the presence of impurities inside the barrier are discussed

On the stability of bow shocks generated by red supergiants: the case of IRC-10414

In this Letter, we explore the hypothesis that the smooth appearance of bow shocks around some red supergiants (RSGs) might be caused by the ionization of their winds by external sources of radiation. Our numerical simulations of the bow shock generated by IRC-10414 (the first-ever RSG with an optically detected bow shock) show that the ionization of the wind results in its acceleration by a factor of two, which reduces the difference between the wind and space velocities of the star and makes the contact discontinuity of the bow shock stable for a range of stellar space velocities and mass-loss rates. Our best fit model reproduces the overall shape and surface brightness of the observed bow shock and suggests that the space velocity and mass-loss rate of IRC-10414 are $\approx$50 ${\rm km} \, {\rm s}^{-1}$ and $\approx$$10^{-6}$ $M_\odot \, {\rm yr}^{-1}$, respectively, and that the number density of the local ISM is $\approx$3 ${\rm cm}^{-3}$. It also shows that the bow shock emission comes mainly from the shocked stellar wind. This naturally explains the enhanced nitrogen abundance in the line-emitting material, derived from the spectroscopy of the bow shock. We found that photoionized bow shocks are $\approx$15$-$50 times brighter in optical line emission than their neutral counterparts, from which we conclude that the bow shock of IRC-10414 must be photoionized.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter

Spectral Data for Generation of Molecular Complexity from Cyclooctatetraene: Preparation of Optically Active Protected Aminocycloheptitols and Bicyclo[4.4.1]undecatriene

Spectral data created in the course of the research project. Supports specific findings in Generation of Molecular Complexity from Cyclooctatetraene: Preparation of Optically Active Protected Aminocycloheptitols and Bicyclo[4.4.1]undecatriene . The racemic (6-cyclo-heptadienyl)Fe(CO)3+ cation ((±)-7), prepared from cyclooctatetraene, was treated with a variety of carbon and heteroatom nucleophiles. Attack took place at the less hindered C1 dienyl carbon and decomplexation of the (cycloheptadiene)Fe(CO)3 complexes gave products rich in functionality for further synthetic manipulation. In particular, a seven-step route was developed from racemic (6-styryl-2,4-cycloheptadien-1-yl)phthalimide ((±)-9 d) to afford the optically active aminocycloheptitols (−)-20 and (+)-20