Photoinduzierter Ladungstransport in komplexen Oxiden

Komplexe Oxide weisen interessante, funktionelle Eigenschaften wie Ferroelektrizität, magnetische Ordnung, hohe Spinpolarisation der Ladungsträger, Multiferroizität und Hochtemperatursupraleitung auf. Diese große Vielfalt sowie die Realisierbarkeit des epitaktischen Wachstums von Heterostrukturen aus verschiedenen oxidischen Komplexverbindungen eröffnen zahlreiche technologische Anwendungsmöglichkeiten für die oxidbasierte Mikroelektronik. Der Schwerpunkt der vorliegenden Arbeit liegt auf der Untersuchung der Charakteristik des Ladungstransportes und insbesondere des Einflusses photogenerierter Ladungsträger auf diesen. Hierzu wurden die zwei vielversprechenden und momentan rege erforschten oxidischen Systeme La0,7Ce0,3MnO3 (LCeMO) und LiNbO3 (LNO) untersucht. Der erste Teil der vorliegenden Arbeit widmet sich der Untersuchung des photoinduzierten Ladungstransports in auf SrTiO3-Substrat gewachsenen LCeMO-Dünnfilmen. LCeMO ist als elektronendotierter Gegenpart zu den wohlbekannten und lochdotierten Manganaten wie La0,7Ca0,3MnO3 von großem Interesse für Anwendungen in der Spintronik so z.B. im spinpolarisierten p-n-übergang. Der Einfluss der Sauerstoffstöchiometrie, der chemischen Phasensegregation der Cer-Dotanden und der photogenerierten Ladungsträger auf die Manganvalenz und damit die Elektronenkonzentration in den LCeMO-Dünnfilmen wurde mittels Röntgenphotoelektronenspektroskopie (XPS) untersucht. Hierbei wurde eine Erhöhung der Elektronenkonzentration durch Reduktion des Sauerstoffgehalts oder durch Beleuchtung mit UV-Licht festgestellt. Messungen der Temperaturabhängigkeit des Widerstands haben einen photoinduzierten Isolator-Metall-übergang in den reduzierten LCeMO-Dünnfilmen gezeigt. Durch Auswertung der magnetfeldbedingten Widerstandsänderungen im beleuchteten und unbeleuchteten Zustand konnte dieser Isolator-Metall-übergang eindeutig auf eine Parallelleitung durch das SrTiO3-Substrat zurückgeführt werden. Der zweite Teil dieser Arbeit befasst sich mit dem Ladungstransport in Einkristallen des uniaxialen Ferroelektrikums LNO. Durch Vergleich der Volumenleitfähigkeit in eindomänigem LNO mit der Leitfähigkeit durch mehrdomänige Kristalle mit zahlreichen geladenen Domänenwänden konnte sowohl im abgedunkelten als auch im beleuchteten Zustand eine im Vergleich zur Volumenleitfähigkeit um mehrere Größenordnungen höhere Domänenwandleitfähigkeit festgestellt werden. Dabei ist die Domänenwandleitfähigkeit unter Beleuchtung mit Photonenenergien größer als der Bandlücke deutlich höher als im abgedunkelten Zustand.Complex oxides exhibit a variety of functional properties, such as ferroelectricity, magnetic ordering, high spin polarization of the charge carriers, multiferroicity and high-temperature superconductivity. This wide variety of functional properties of complex oxides combined with their structural compatibility facilitates epitaxial growth of oxide heterostructures with tailored functional properties for applications in oxide-based microelectronic devices. The focus of the present thesis lies on the characterization of the photoinduced charge transport in two intriguing complex oxides of current scientific interest, namely the electron doped mixed valence manganite La0,7Ce0,3MnO3 (LCeMO) and the ferroelectric LiNbO3 (LNO). The first part adresses the photoinduced charge transport in thin films of LCeMO grown on SrTiO3 substrates. LCeMO, being the electron doped counterpart to well known hole doped manganites like La0,7Ca0,3MnO3, is of current interest for spintronic applications like spin-polarized p-n-junctions. The influence of the oxygen stoichiometry, the chemical phase separation of cerium and of the photogenerated charge carriers on the manganese valence and hence the electron concentration in the LCeMO films were investigated with X-ray-photoelectron spectroscopy. This measurements revealed an increase in electron doping by reduction of the oxygen content or by illumination with UV-light. Measurements of the temperature dependence of the resistance of the reduced LCeMO films showed a photoinduced insulator-metal transition. Analysis of the magnetoresistive properties of the samples in the illuminated and dark state clearly revealed that this insulator-metal transition is caused by a parallel conduction through the SrTiO3 substrate. The second part of this thesis is dedicated to the charge transport in single crystals of the uniaxial ferroelectric LNO. A comparison of the bulk conductivity of single domain crystals with the conductivity of multidomain crystals with numerous charged domain walls revealed an several orders of magnitude higher domain wall conductivity as compared to the bulk conductivity. Such domain wall conductivity could be observed in the illuminated as well as in the dark state, although the domain wall conductivity was much higher for super-bandgap illumination

Physical Fitness Training in Patients with Subacute Stroke (PHYS-STROKE): multicentre, randomised controlled, endpoint blinded trial

OBJECTIVE: To determine the safety and efficacy of aerobic exercise on activities of daily living in the subacute phase after stroke. DESIGN: Multicentre, randomised controlled, endpoint blinded trial. SETTING: Seven inpatient rehabilitation sites in Germany (2013-17). PARTICIPANTS: 200 adults with subacute stroke (days 5-45 after stroke) with a median National Institutes of Health stroke scale (NIHSS, range 0-42 points, higher values indicating more severe strokes) score of 8 (interquartile range 5-12) were randomly assigned (1:1) to aerobic physical fitness training (n=105) or relaxation sessions (n=95, control group) in addition to standard care. INTERVENTION: Participants received either aerobic, bodyweight supported, treadmill based physical fitness training or relaxation sessions, each for 25 minutes, five times weekly for four weeks, in addition to standard rehabilitation therapy. Investigators and endpoint assessors were masked to treatment assignment. MAIN OUTCOME MEASURES: The primary outcomes were change in maximal walking speed (m/s) in the 10 m walking test and change in Barthel index scores (range 0-100 points, higher scores indicating less disability) three months after stroke compared with baseline. Safety outcomes were recurrent cardiovascular events, including stroke, hospital readmissions, and death within three months after stroke. Efficacy was tested with analysis of covariance for each primary outcome in the full analysis set. Multiple imputation was used to account for missing values. RESULTS: Compared with relaxation, aerobic physical fitness training did not result in a significantly higher mean change in maximal walking speed (adjusted treatment effect 0.1 m/s (95% confidence interval 0.0 to 0.2 m/s), P=0.23) or mean change in Barthel index score (0 (-5 to 5), P=0.99) at three months after stroke. A higher rate of serious adverse events was observed in the aerobic group compared with relaxation group (incidence rate ratio 1.81, 95% confidence interval 0.97 to 3.36). CONCLUSIONS: Among moderately to severely affected adults with subacute stroke, aerobic bodyweight supported, treadmill based physical fitness training was not superior to relaxation sessions for maximal walking speed and Barthel index score but did suggest higher rates of adverse events. These results do not appear to support the use of aerobic bodyweight supported fitness training in people with subacute stroke to improve activities of daily living or maximal walking speed and should be considered in future guidelines. TRIAL REGISTRATION: ClinicalTrials.gov NCT01953549

Polaron-Mediated Luminescence in Lithium Niobate and Lithium Tantalate and Its Domain Contrast

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on doped LNO that made the characterization of the formed polaron states possible by their absorption bands. Next, we proceed by reporting on the two polaron-mediated photoluminescence bands that have been observed in LNO: (1) A near-infrared luminescence band in the range of 1.5 eV shows a mono-exponential decay and a strong dependence on iron doping. This luminescence is emitted by bound polarons returning from an excited state to the ground state. (2) A luminescence band at visible wavelengths with a maximum at 2.6 eV shows a stretched-exponential decay and is strongly enhanced by optical damage resistant doping around the doping threshold. This luminescence stems from the recombination of free electron and hole polarons. The next major topic of this review are domain contrasts of the visible photoluminescence that have been observed after electrical poling of the substrate, as singly inverted domains show a slightly reduced and faster decaying luminescence. Subsequent annealing results in an exponential decrease of that domain contrast. We show that this contrast decay is strongly related to the mobility of lithium ions, thus confirming the role of polar defect complexes, including lithium vacancies, for these domain contrasts. Finally we discuss the extension of our investigations to lithium tantalate (LTO) samples. While the results on the domain contrast and its decay are similar to LNO, there are remarkable differences in their luminescence spectra

Physical fitness training in Subacute Stroke (PHYS-STROKE) - study protocol for a randomised controlled trial

BACKGROUND: Given the rising number of strokes worldwide, and the large number of individuals left with disabilities after stroke, novel strategies to reduce disability, increase functions in the motor and the cognitive domains, and improve quality of life are of major importance. Physical activity is a promising intervention to address these challenges but, as yet, there is no study demonstrating definite outcomes. Our objective is to assess whether additional treatment in the form of physical fitness-based training for patients early after stroke will provide benefits in terms of functional outcomes, in particular gait speed and the Barthel Index (co-primary outcome measures) reflecting activities of daily living (ADL). We will gather secondary functional outcomes as well as mechanistic parameters in an exploratory approach. METHODS/DESIGN: Our phase III randomised controlled trial will recruit 215 adults with moderate to severe limitations of walking and ADL 5 to 45 days after stroke onset. Participants will be stratified for the prognostic variables of “centre”, “age”, and “stroke severity”, and randomly assigned to one of two groups. The interventional group receives physical fitness training delivered as supported or unsupported treadmill training (cardiovascular active aerobic training; five times per week, over 4 weeks; each session 50 minutes; total of 20 additional physical fitness training sessions) in addition to standard rehabilitation treatment. The control intervention consists of relaxation sessions (non-cardiovascular active; five times per week week, over 4 weeks; each session 50 minutes) in addition to standard rehabilitation treatment. Co-primary efficacy endpoints will be gait speed (in m/s, 10 m walk) and the Barthel Index (100 points total) at 3 months post-stroke, compared to baseline measurements. Secondary outcomes include standard measures of quality of life, sleep and mood, cognition, arm function, maximal oxygen uptake, and cardiovascular risk factors including blood pressure, pulse, waist-to-hip ratio, markers of inflammation, immunity and the insulin-glucose pathway, lipid profile, and others. DISCUSSION: The goal of this endpoint-blinded, phase III randomised controlled trial is to provide evidence to guide post-stroke physical fitness-based rehabilitation programmes, and to elucidate the mechanisms underlying this intervention. TRIAL REGISTRATION: Registered in ClinicalTrials.gov with the Identifier NCT01953549