Phonon plasmon interaction in ternary group-III-nitrides

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 101, 041909 (2012) and may be found at https://doi.org/10.1063/1.4739415.Phonon-plasmon-coupling in the ternary group-III-nitrides InGaN and AlGaN is investigated experimentally and theoretically. Based on the observation of broadening and shifting of the A1(LO) mode in AlGaN upon Si-doping, a lineshape analysis was performed to determine the carrier concentration. The results obtained by this method are in excellent agreement to those from Hall measurements, confirming the validity of the employed model. Finally, neglecting phonon and plasmon damping, the Raman shift of the A1(LO) mode in dependence of the carrier concentration for AlGaN and InGaN is calculated. This enables a fast and contactless determination of carrier concentrations in the future.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Förderung von Vögeln der Agrarlandschaft durch die Neuanlage von Brut- und Nahrungshabitaten

Die zunehmende Verschlechterung von Brut- und Nahrungshabitaten für Vögel der Agrarlandschaft zählt heute zu den wichtigsten Ursachen für deren steten Rückgang. Mit der Bildung großflächiger Bewirtschaftungseinheiten mit einseitigen Fruchtfolgen verschlechtern sich die Nahrungsbedingungen für die Vögel auf dem Feld. Nach der politischen Wende bestand schon Anfang der 1990er Jahre Bedarf an der Neustrukturierung ausgeräumter Agrarflächen, besonders in Ostdeutschland. 1993 wurde auf einer Ackerfläche am südlichen Stadtrand von Berlin die sogenannte “Brandenburger Schichtholzhecke” als Modellprojekt angelegt, um einen ökologischen und ökonomischen Weg zur Neuanlage von Hecken und Feldrainen aufzuzeigen. Diese naturnahen Kleinstrukturen bieten in ihrer Kombination sowohl Brut- als auch ganzjährig Nahrungshabitate und zeigen insbesondere unter den heutigen Bedingungen Möglichkeiten auf, um die Lebensbedingungen der Agrarvögel zu verbessern. Dabei ist das Konzept der „Benjeshecke“ modifiziert worden, indem zwischen zwei parallel zueinander verlaufenden Gestrüppwällen aus Totholz heimische Bäume und Sträucher einreihig gepflanzt wurden. Im Frühjahr 1994 ist ein mindestens 5 m breiter Wildkräuterstreifen zwischen buhnenförmigen Querwällen aus Totholz entlang der 575 m langen Hecke etabliert worden und beendete damit die Gestaltungsphase. Regelmäßige Vogelbestandserfassungen von 1995 bis 1998 und im Jahr 2004 zeigten die kontinuierliche Nutzung der Hecke als Lebensraum durch Agrarvögel. Schon 1994 konnten die ersten Neuntöter an den Totholzwällen beobachtet werden. 1995 diente der Saum als Brutrevier für Goldammer, Neuntöter, Steinschmätzer und Schafstelze. 1998 waren insbesondere die Gehölzstrukturen schon so weit entwickelt, dass die Dorngrasmücke erstmalig in der Hecke nistete. Im Jahr 2004, zehn Jahre nach Anlage der Hecke, brüteten 7 Vogelarten mit 13 Brutpaaren im Saum (Goldammer, Neuntöter, Steinschmätzer, Schafstelze, Dorngrasmücke, Stieglitz, Rotkehlchen). Die Goldammer mit fünf Brutpaaren war der häufigste Brutvogel. Insgesamt erreichte die Zahl der Brutreviere einen Wert von 2,3 je 100 lfd. Meter Hecke. Untersuchungen zum Auftreten von Schwebfliegen im Saum und dem angrenzenden Feld haben gezeigt, dass deren Aktivität auf den Krautstreifen 1995 fünf mal und 2004 sieben mal höher war als im 5 m Bereich der angrenzenden Ackerkultur. Da Schwebfliegen den Hauptbestandteil der Nestlingsnahrung z. B. für die Goldammer darstellen, belegen diese Zahlen die Bedeutung des Saumes als Nahrungshabitat.Stichwörter: Vögel, Hecken, Feldraine, NahrungPromotion of farmland birds by recreation of nesting and feeding habitatsAbstractThe increasing degradation of nesting and feeding habitats for farmland birds, is one of the main causes of their steady decline. With the increasing formation of large-scale fields with uniform crops, the food conditions deteriorate for the birds on the field. After the political turn in the beginning of the 90ies, there was also a need for the restructuring of cleared agricultural landscape in East Germany. In 1993, the so-called ‚Brandenburg stackedwood hedge‘ was created on a fi eld south to Berlin as a model to show an ecological and economic way for the replanting of hedges and fi eld margins. These small structures providing in their combination of both breeding and foraging habitat throughout the year. Under the current condition they are showing a way to improve the living conditions for birds on farmland. The ‚Brandenburg stacked-wood hedge‘ consists of two brush barriers of dead wood arranged in parallel. Between them, there is one row of native-species trees and shrubs planted. The 575 m long stacked hedge is adjoined by a five-metre-wide herbaceous strip established by sowing a suitable mixture of seed in 1994. Regular recording of birds from 1995 to 1998 and in 2004 shows the continuous use of the hedge as a habitat for birds. Already in 1994, the first red-backed shrikes were observed in the dead wood. In 1995, the dead wood provided a breeding ground for yellow-hammer, red-backed shrike, northern wheatear und yellow wagtail. In 1998, the shrubs had grown to a suitable size to shelter for the fi rst time white throat. In 2004, ten years after establishment of the hedgerow, 13 bird pairs of 7 species nested in the hedgerow - yellowhammer, red-backed shrike, northern wheatear, yellow wagtail, whitethroat, eurasian goldfi nch, european robin. The yellow-hammer occurred most frequently with fi ve pairs. Density of breeding birds totalled 2.3 pairs per 100 m hedgerow. Investigations on the occurrence of hoverflies in the edge biotop and the adjacent field have shown, that there activity on the wild herb strips were five times higher in 1995 and seven times higher in 2004 than in the 5 m region of the adjacent arable crop. Since hoverflies representing the main part of the nestling food (yellowhammer), these figures demonstrate the importance of the edge biotop as foraging habitat.Keywords: birds, hedges, field margins, food sourc

C-type related order in the defective fluorites La2Ce2O7 and Nd2Ce2O7 studied by neutron scattering and ab initio MD simulations

This work presents a structural investigation of La2-xNdxCe2O7 (x = 0.0, 0.5, 1.0, 1.5, 2.0) using X-ray powder diffraction and total scattering neutron powder diffraction, analysed using Rietveld and the reverse Monte Carlo method (RMC). Ab initio molecular dynamics (MD) modelling is also performed for further investigations of the local order. The main intensities in the neutron diffraction data for the La2-xNdxCe2O7 series correspond to the fluorite structure. However, additional C-type superlattice peaks are visible for x > 0 and increase in intensity with increasing x. The Nd-containing compositions (x > 0) are best fitted with Rietveld analysis by using a combination of oxygen deficient fluorite and oxygen excess C-type structures. No indications of cation order are found in the RMC or Rietveld analysis, and the absence of cation order is supported by the MD modelling. We argue that the superlattice peaks originate from oxygen vacancy ordering and associated shift in the cation position away from the ideal fluorite site similar to that in the C-type structure, which is seen from the Rietveld refinements and the observed ordering in the MD modelling. The vacancies favour alignments in the , and especially the direction. Moreover, we find that such ordering might also be found to a small extent in La2Ce2O7, explaining the discernible modulated background between the fluorite peaks. The observed overlap of the main Bragg peaks between the fluorite and C-type phase supports the co-existence of vacancy ordered and more disordered domains. This is further supported by the observed similarity of the radial distribution functions as modelled with MD. The increase in long range oxygen vacancy order with increasing Nd-content in La2-xNdxCe2O7 corresponds well with the lower oxide ion conductivity in Nd2Ce2O7 compared to La2Ce2O7 reported earlier

Thinning mechanisms of heterogeneous continental lithosphere

The mechanisms responsible for the formation of extremely thinned continental crust (<10 km thick) and lithosphere during rifting remains debated. Observations from present-day and fossil passive margins highlight the role of deep-seated deformation, likely controlled by heterogeneities within the continental lithosphere, such as changing lithologies, mechanical anisotropies and inherited structures. We investigate the mechanisms of lithospheric thinning by exploring the role of pre-existing heterogeneities on the architecture and evolution of rifted margins. We estimate pre-rift pressure conditions (P0) vs. depth diagrams of crustal to lithospheric sections, to quantify rift-related modifications on inherited lithostatic pressure gradients. Two field examples from the Alpine Tethys margins in the Eastern and Southern Alps (SE Switzerland and N Italy) were selected to characterize: (1) the pre-rift architecture of the continental lithosphere; (2) the localization of rift-related deformation in distinct portions of the lithosphere; and (3) the interaction between pre-existing heterogeneities of the lithosphere and rift-related structures. These observations are compared with high-resolution, two-dimensional thermo-mechanical numerical models. The design of the models takes into account pre-existing mechanical heterogeneities representing the initial pre-rift architecture of the continental lithosphere. Extensional structures consist of high-angle and low-angle normal faults, anastomosing shear-zones and decoupling horizons. Such structures accommodate the lateral extraction of mechanically stronger levels derived from the middle to lower crust. As a result, the extremely thinned continental crust in Tethyan passive margins represents the juxtaposition and amalgamation of distinct strong levels of the crust separated by major extensional structures identified by sharp pressure gradients. Future work should determine the applicability of these results to other present-day and fossil rifted margins

Assessment of the inverse dispersion method for the determination of methane emissions from a dairy housing

Methane (CH4) emissions from dairy housings, mainly originating from enteric fermentation of ruminating animals, are a significant source of greenhouse gases. The quantification of emissions from naturally ventilated dairy housings is challenging due to the spatial distribution of sources (animals, housing areas) and variable air exchange. The inverse dispersion method (IDM) is a promising option, which is increasingly used to determine gaseous emissions from stationary sources, as it offers high flexibility in the application at reasonable costs. We used a backward Lagrangian stochastic model combined with concentration measurements by open-path tunable diode laser spectrometers placed up- and downwind of a naturally ventilated housing with 40 dairy cows to determine the CH4 emissions. The average emissions per livestock unit (LU) were 317 (±44) g LU−1 d−1 and 267 (±43) g LU−1 d−1 for the first and second campaign, in September – October and November – December, respectively. For each campaign, inhouse tracer ratio measurements (iTRM) were conducted in parallel during two subperiods. For simultaneous measurements, IDM showed average emissions which were lower by 8% and 1% than that of iTRM, respectively, for the two campaigns. The differences are within the uncertainty range of any of the two methods. The IDM CH4 emissions were further analysed by wind direction and atmospheric stability and no differences in emissions were found. Overall, IDM showed its aptitude to accurately determine CH4 emissions from dairy housings or other stationary sources if the site allows adequate placement of sensors up- and downwind in the prevailing wind direction. To acquire reliable emission data, depending on the data loss during measurements due to quality filtering or instrument failure, a measuring time of at least 10 days is required

Polarity Control in Group-III Nitrides beyond Pragmatism

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. GaN or ZnO on sapphire are prominent examples for that. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. In this work, we study the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy as an example. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers

Antibiotic use during pregnancy increases offspring asthma severity in a dose‐dependent manner

Background: The use of antibiotics during pregnancy is associated with increased allergic asthma risk in the offspring, and given that approximately 25% of pregnant women are prescribed antibiotics, it is important to understand the mechanisms contributing to this phenomenon. Currently, there are no studies that directly test this association experimentally. Our objective was to develop a mouse model in which antibiotic treatment during pregnancy results in increased offspring asthma susceptibility. Methods: Pregnant mice were treated daily from gestation day 8-17 with an oral solution of the antibiotic vancomycin, and three concentrations were tested. At weaning, offspring were subjected to an adjuvant-free experimental asthma protocol using ovalbumin as an allergen. The composition of the gut microbiome was determined in mothers and offspring with samples collected from five different time points; shortchain fatty acids were also analyzed in allergic offspring. Results: We found that maternal antibiotic treatment during pregnancy was associated with increased offspring asthma severity in a dose-dependent manner. Furthermore, maternal vancomycin treatment during pregnancy caused marked changes in the gut microbiome composition in both mothers and pups at several different time points. The increased asthma severity and intestinal microbiome changes in pups were also associated with significantly decreased cecal short-chain fatty acid concentrations. Conclusion: Consistent with the "Developmental Origins Hypothesis," our results confirm that exposure to antibiotics during pregnancy shapes the neonatal intestinal environment and increases offspring allergic lung inflammation