Picosecond strain dynamics in Ge2_{2}Sb2_{2}Te5_{5} monitored by time-resolved x-ray diffraction

Coherent phonons (CP) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, non-thermal switching in phase-change materials such as Ge$_{2}$Sb$_{2}$Te$_{5}$(GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100 ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal, however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone implying the presence of transient non-equilibrium electron heating in addition to temperature driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase-transitions in GST.Comment: 7 pages, 4 figures, Phys. Rev. B, in pres

Picosecond strain dynamics in Ge2Sb2Te5 monitored by time-resolved x-ray diffraction

Coherent phonons (CPs) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, nonthermal switching in phase-change materials such as Ge2Sb2Te5 (GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100-ps x-ray pulses from the European Synchrotron Radiation Facility (ESRF) storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal; however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone, implying the presence of excited-state effects in addition to temperature-driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase transitions in GST

Single Crystalline 100% epi-Ge Layers on Engineered Oxide Heterostructures on Si

Single crystalline Germanium (Ge) has gained a lot of attention for applications as new material in microelectronics, photovoltaics and for photodetectors. The integration on the mature and predominating Silicon (Si) technology platform is a challenging technical task, which offers many basic scientific questions to be answered. This thesis is concerned with the integration of a functional Ge layer on the Si platform via an engineered oxide heterostructure, namely cubic PrO2. The oxide is incorporated to compensate for the 4% lattice constant mismatch of Ge and Si, with its lattice constant between the two semiconductors. An in situ reflection high energy electron diffraction (RHEED) monitoring of the layer deposition by molecular beam epitaxy (MBE) indicates that the initial growth mode of Ge on PrO2 follows a Volmer-Weber growth mode due to interface reactions, surface and strain energies. By properly tuning the growth parameters of MBE a growth recipe is developed, leading to the growth of atomically smooth single crystalline Ge (111) layers on the Pr2O3 (111) / Si (111) support system. The oxide is subject to a chemical reduction process during the Ge deposition, resulting in a Pr2O3 stoichiometry. The closed layers are not achieved by a change to van der Merwe growth, but by the adjustment of the growth kinetics, resulting in a smoothing out of the Volmer-Weber growth. The development of the recipe for the Ge layer growth is monitored with RHEED, ex situ x-ray reflectivity (XRR) and x-ray diffraction (XRD) measurements as well as scanning electron microscopy (SEM). These methods confirm the closed and smooth Ge surface and the sharp interface with the underlying Pr2O3. The closed layer stacks are investigated by synchrotron radiation x-ray diffraction under bulk sensitive and surface sensitive measurement conditions. This first study unveils a single crystalline type A / B / A stacking configuration of the Ge (111) / Pr2O3 (111) / Si (111) heterostack system. Driven by the results from the structural investigation a second study reveals the main defect mechanismsat work by XRD pole-figure measurements and reciprocal space maps (RSMs), supported by real space cross section transmission electron microscopy (TEM) images along a stacking sensitive direction. The defects limiting the long range order in the Ge layer are identified as stacking twins, microtwins and stacking faults (Fig. 2). The investigation of the thickness dependent behaviour discloses a threading behaviour of microtwins and stacking faults while stacking twins are confined to the interface. First results of high temperature UHV annealing experiments show the reduction of diffuse scattering by strain fields in defective Ge is possible, indicating a reduction of stacking faults, while microtwins as well as stacking twins are not nfluenced by the annealings. Future defect engineering approaches are required to improve the long range order of the epi-Ge layer for technological applications

Anti-inflammatory and general glucocorticoid physiology in skeletal muscles affected by Duchenne muscular dystrophy : exploration of steroid-sparing agents

In Duchenne muscular dystrophy (DMD), the activation of proinflammatory and metabolic cellular pathways in skeletal muscle cells is an inherent characteristic. Synthetic glucocorticoid intake counteracts the majority of these mechanisms. However, glucocorticoids induce burdensome secondary effects, including hypertension, arrhythmias, hyperglycemia, osteoporosis, weight gain, growth delay, skin thinning, cushingoid appearance, and tissue-specific glucocorticoid resistance. Hence, lowering the glucocorticoid dosage could be beneficial for DMD patients. A more profound insight into the major cellular pathways that are stabilized after synthetic glucocorticoid administration in DMD is needed when searching for the molecules able to achieve similar pathway stabilization. This review provides a concise overview of the major anti-inflammatory pathways, as well as the metabolic effects of glucocorticoids in the skeletal muscle affected in DMD. The known drugs able to stabilize these pathways, and which could potentially be combined with glucocorticoid therapy as steroid-sparing agents, are described. This could create new opportunities for testing in DMD animal models and/or clinical trials, possibly leading to smaller glucocorticoids dosage regimens for DMD patients

Van goudwaarde in verkiezingstijden? Electorale presidentialisering van het burgemeesterschap in Vlaanderen

Presidentialization involves a shift in focus from collective actors (such as parties) to one person, in this case the mayor. It is a specific form of personalization which focusses on one rather than on several individuals. Electoral presidentialization, which constitutes one of the diverse features of this phenomenon, was studied at the local level in Flanders (Belgium). By formulating hypotheses based on theoretical and empirical insights on personalization (referring to several politicians), we have tried to figure out whether presidentialization differs from personalization. From our results, it appears that mayors indeed attract a huge share of preferential votes, but also that most factors that influence personalization have an impact on presidentialization too. These include both individual variables (such as media attention and incumbency) and municipal variables (such as size of the municipality and number of parties). Although both trends differ in nature, the factors influencing them appear to be similar