Simulation der selbstorganisierten Wachstumskinetik von Quantenpunkten

Das heteroepitaktische Wachstum von Inseln im sub-monolagen Bereich wird mittels der Monte Carlo Technik simuliert. Die f&uumlr den Wachstumsprozess relevanten Parameter sind Deposition, Diffusion und n&aumlchste sowie &uumlbern&aumlchste Nachbarbindungen. Diffusionsprozesse geschehen mit einer Wahrscheinlichkeit, die &uumlber einen Arhenius Faktor bestimmt wird. Die f&uumlr gitterfehlangepasstes Wachstum typische, elastische Verspannung wird &uumlber ein selbstkonsistent generiertes Verspannungsfeld in das Programm eingebunden, wobei auch eine kubische Anisotropie ber&uumlcksichtigt werden kann. Der Einfluß der makroskopischen Wachstumsparameter wie Temperatur, Materialfluß zur Oberfl&aumlche w&aumlhrend der Deposition, Oberfl&aumlchenbedeckung und Dauer der Wachstumsunterbrechung auf die Grö&szligenordnung und r&aumlumliche Anordnung der Inseln wird betrachtet. Dazu wird ein optimaler Arbeitsbereich im Parameterraum bestimmt zu dem sowohl eine regul&aumlre r&aumlumliche Anordnung der Inseln als auch eine schmale Grö&szligenverteilung erzielt werden kann. Es wird der &Uumlbergang von kinetisch kontrollierten Wachstumsbedingungen, wie sie w&aumlhrend der Deposition vorherrschen, zu thermodynamisch kontrolliertem Wachstum, wie es nach langen Relaxationszeiten beobachtet wird, analysiert und ein &Uumlbergang zwischen den Wachstumsmodi beobachtet, bei dem sich die Grö&szligenverteilungen zu verschiedenen Temperaturen &uumlberschneiden. Es wurden Simulationen mit einem anisotropen elatischen Verspannungsfeld, welches dem von SiGe &aumlhnlich ist, angestellt um experimentell beobachtete Strukturen in der Form von Inselketten, die entlang der Si(100) Richtung orientiert sind, nachzuvollziehen. Die Simulationsergebnisse decken sich auf befriedigende Weise mit den experimentellen Befunden. Das Wachstum von gestapelten Quantenpunkt Schichten wird betrachtet, wobei das selbstkonsistent berechnete Verspannungsfeld vollst&aumlndig mitber&uumlcksichtig wird. Man findet einen &Uumlbergang von vertikal korreliertem Wachstum zu antikorreliertem Wachstum mit zunehmender Pufferschichtdicke sowie zunehmend bessere Grö&szligenordnung unter den Inseln mit zunehmender Anzahl der deponierten Schichten.The growth of sub-monolayer islands in heteroepitaxial semiconductor systems is simulated by means of the Monte Carlo method. As the relevant processes, deposition, diffusion and nearest- and next-nearest-neighbor bonding is included. Diffusion processes occur with Arrhenius-like probability. To account for the elastic strain inherent to lattice mismatched growth, a self-consistently calculated elastic strain field is incorporated. The influence of macroscopic growth parameters like temperature, flux to the surface during deposition, surface coverage and growth interruption time on size ordering and regular spatial arrangement of islands is analyzed. An optimal parameter range is identified to obtain both a regular spatial arrangement of dots and a narrow size distribution. The transition from kinetically controlled growth conditions to thermodynamically controlled growth reached after long equilibration times is analyzed and a crossover in island size distributions between both regimes is found for different temperatures. Simulations with anisotropic elastic strain parameters related to the SiGe system are performed to verify the experimentally observed formation of island chains oriented along Si(100) direction. Simulations are in good agreement with experimental data. The growth of stacked quantum dot layers is considered by fully taking into account the self-consistently calculated elastic strain field. A transition from vertically correlated growth to anti-correlated growth with increasing buffer layer thickness is observed as well as an improved ordering with respect to island sizes with increasing number of deposited layers

Myelination in the absence of UDP-galactose:ceramide galactosyl-transferase and fatty acid 2 -hydroxylase

<p>Abstract</p> <p>Background</p> <p>The sphingolipids galactosylceramide (GalCer) and sulfatide are major myelin components and are thought to play important roles in myelin function. The importance of GalCer and sulfatide has been validated using UDP-galactose:ceramide galactosyltransferase-deficient (<it>Cgt</it>^-/-) mice, which are impaired in myelin maintenance. These mice, however, are still able to form compact myelin. Loss of GalCer and sulfatide in these mice is accompanied by up-regulation of 2-hydroxylated fatty acid containing (HFA)-glucosylceramide in myelin. This was interpreted as a partial compensation of the loss of HFA-GalCer, which may prevent a more severe myelin phenotype. In order to test this hypothesis, we have generated <it>Cgt</it>^-/-mice with an additional deletion of the fatty acid 2-hydroxylase (<it>Fa2h</it>) gene.</p> <p>Results</p> <p><it>Fa2h</it>^-/-/Cgt^-/-double-deficient mice lack sulfatide, GalCer, and in addition HFA-GlcCer and sphingomyelin. Interestingly, compared to <it>Cgt</it>^-/-mice the amount of GlcCer in CNS myelin was strongly reduced in <it>Fa2h</it>^-/-/<it>Cgt</it>^-/-mice by more than 80%. This was accompanied by a significant increase in sphingomyelin, which was the predominant sphingolipid in <it>Fa2h</it>^-/-/<it>Cgt</it>^-/-mice. Despite these significant changes in myelin sphingolipids, compact myelin was formed in <it>Fa2h</it>^-/-/<it>Cgt</it>^-/-mice, and g-ratios of myelinated axons in the spinal cord of 4-week-old <it>Fa2h</it>^-/-/<it>Cgt</it>^-/-mice did not differ significantly from that of <it>Cgt</it>^-/-mice, and there was no obvious phenotypic difference between <it>Fa2h</it>^-/-/<it>Cgt</it>^-/-and <it>Cgt</it>^-/-mice</p> <p>Conclusions</p> <p>These data show that compact myelin can be formed with non-hydroxylated sphingomyelin as the predominant sphingolipid and suggest that the presence of HFA-GlcCer and HFA-sphingomyelin in <it>Cgt</it>^-/-mice does not functionally compensate the loss of HFA-GalCer.</p

Lithium isotope geochemistry of marine pore waters: Insights from cold seep fluids

Lithium concentration and isotope data (δ7Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ7Li (δ7Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ7Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite–illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a−1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure

Design of Comb Crack Resistant Milling Inserts: A Comparison of Stresses, Crack Propagation, and Deformation Behavior between Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 CVD Coatings

Investigations on comb crack resistance of milling inserts coated with chemical vapor deposition (CVD) Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 showed a distinct wear evolution in both systems. Wear studies revealed that the appearance of comb cracks is connected to the initial CVD cooling crack network. Micropillar compression tests indicated a brittle intergranular fracture mechanism for the Ti(C,N) layer and a transgranular fracture accompanied with signs of plastic deformation for the Zr(C,N) coating. Additionally, for the Zr(C,N) based system, a compressive stress condition in the temperature range of interest (200–600 ◦C) was determined by in-situ synchrotron X-ray diffraction. The set of residual compressive stresses together with the ability of the Zr(C,N) layer to deform plastically are key features that explain the enhanced resistance to comb crack wear of the Zr(C,N) based system in milling of cast iron

Controlled Stark shifts in Er3+^{3+}-doped crystalline and amorphous waveguides for quantum state storage

We present measurements of the linear Stark effect on the $^{4}$I$_{15/2} \to$ $^{4}$I$_{13/2}$ transition in an Er$^{3+}$-doped proton-exchanged LiNbO$_{3}$ crystalline waveguide and an Er$^{3+}$-doped silicate fiber. The measurements were made using spectral hole burning techniques at temperatures below 4 K. We measured an effective Stark coefficient $(\Delta\mu_{e}\chi)/(h)=25\pm1$kHz/Vcm$^{-1}$ in the crystalline waveguide and $(\bar{\Delta\mu_{e}}\chi)/(h)=15\pm1$kHz/Vcm$^{-1}$ in the silicate fiber. These results confirm the potential of Erbium doped waveguides for quantum state storage based on controlled reversible inhomogeneous broadening.Comment: 4 pages, 2 figures v2. typo in formula correcte

A multicomponent exercise intervention to improve physical functioning, cognition and psychosocial well-being in elderly nursing home residents: a study protocol of a randomized controlled trial in the PROCARE (prevention and occupational health in long-term care) project

Background Older adults, who are living in nursing homes that provide a high level of long-term nursing care, are characterized by multimorbidity and a high prevalence of dependency in activities of daily living. Results of recent studies indicate positive effects of structured exercise programs during long-term care for physical functioning, cognition, and psychosocial well-being. However, for frail elderly the evidence remains inconsistent. There are no evidence-based guidelines for exercises for nursing home residents that consider their individual deficits and capacities. Therefore, high-quality studies are required to examine the efficacy of exercise interventions for this multimorbid target group. The purpose of this study is to determine the feasibility and efficacy of a multicomponent exercise intervention for nursing home residents that aims to improve physical and cognitive functioning as well as quality of life. Methods A two-arm single-blinded multicenter randomized controlled trial will be conducted, including 48 nursing homes in eight regions of Germany with an estimated sample size of 1120 individuals. Participants will be randomly assigned to either a training or a waiting time control group. For a period of 16 weeks the training group will meet twice a week for group-based sessions (45–60 min each), which will contain exercises to improve physical functioning (strength, endurance, balance, flexibility) and cognitive-motor skills (dual-task). The intervention is organized as a progressive challenge which is successively adapted to the residents’ capacities. Physical functioning, cognitive performance, and quality of life will be assessed in both study groups at baseline (pre-test), after 16-weeks (post-treatment), and after 32-weeks (retention test, intervention group only). Discussion This study will provide information about the efficacy of a multicomponent exercise program in nursing homes (performance, recruitment). Results from this trial will contribute to the evidence of multicomponent exercises, which specifically focus on cognitive-motor approaches in the maintenance of mental and physical functioning. In addition, it will help to encourage older adults to actively engage in social life. Furthermore, the findings will lead to recommendations for health promotion interventions for frail nursing home residents. Trial registration The trial was prospectively registered at DRKS.de with the registration number DRKS00014957 on October 9, 2018

Isotopic evidence (⁸⁷Sr/⁸⁶Sr, δ⁷Li) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean [(Sr-87/Sr-86, delta Li-7) ]

The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and Sr-87/Sr-86 ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light delta Li-7 values, partly close to those of hydrothermal vent fluids (delta Li-7: +11.9 parts per thousand), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic Sr-87/Sr-86 ratios (similar to 0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic Sr-87/Sr-86 ratios (similar to 0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (&gt;200 degrees C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments. (C) 2009 Elsevier Ltd. All rights reserved