Effects of hydrogen anneals on oxygen deficient SrTiO3−x single crystals

The influence of hydrogen gas anneals on the electrical properties of nominally undoped, oxygen-deficient SrTiO3−x single crystals was investigated. Titanium getter layers and vacuum anneals were used to obtain oxygen-deficient SrTiO3−x with a low electrical resistivity. These crystals showed an optical absorption peak at 2.92 eV and strong midinfrared absorption. Subsequent anneals at 800 °C in forming gas, which contained 10% hydrogen, returned the crystals into the insulating, transparent state. The mechanisms by which hydrogen anneals can compensate for the effects of oxygen vacancies in SrTiO3−x are discussed. The results show that forming gas anneals of stoichiometric SrTiO3 can lead to complex electrical conduction behavior

The ReaxFF reactive force-field : development, applications and future directions

The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method

Two-particle correlations in azimuthal angle and pseudorapidity in inelastic p + p interactions at the CERN Super Proton Synchrotron

Results on two-particle ΔηΔϕ correlations in inelastic p + p interactions at 20, 31, 40, 80, and 158 GeV/c are presented. The measurements were performed using the large acceptance NA61/SHINE hadron spectrometer at the CERN Super Proton Synchrotron. The data show structures which can be attributed mainly to effects of resonance decays, momentum conservation, and quantum statistics. The results are compared with the Epos and UrQMD models.ISSN:1434-6044ISSN:1434-605

Micromagnetic study of self-organized magnetic nanostructures

In der vorliegenden Arbeit wurden die mikromagnetische Struktur sowie das Ummagnetisierungsverhalten epitaktisch gewachsener MnAs Filme auf dem Substrat GaAs untersucht. Im Mittelpunkt steht die mikromagnetischen Struktur von anisotrop erspannten MnAs Filmen auf GaAs(001). Die Verspannung führt zur selbstorganisierten Anordnung ferromagnetischer Streifen. Ihre Domänenstruktur wurde mittels MFM (magnetischer Kraftmikroskopie) bestimmt und mit den Resultaten der XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy) verglichen. Um eine vollständige Charakterisierung der mikromagnetischen Eigenschaften der Streifenstruktur zu erreichen, wurden die MFM Experimente in einem äusseren Magnetfeld durchgeführt. Die Beantwortung der zentralen Frage nach der Domänenstruktur ist mit der Entwicklung eines mikromagnetischen Simulators für dreidimensionale magnetische Strukturen auf mesoskopischer Skala gelungen. Die Stabilität der dreidimensionalen mikromagnetischen Struktur hängt von den Eigenschaften der selbstorganisierten Streifenstruktur ab, d.h. sowohl von der Filmdicke als auch vom Verhältnis ihrer Breite zur Filmdicke - und damit der Temperatur. Durch die Erkenntnis, dass eine magnetische Struktur in der Tiefe des Streifens vorhanden ist, können die verbleibenden Unterschiede in den XMCDPEEM- und MFM-Resultaten erklärt werden. Durch die Simulationsergebnisse in Kombination mit den Experimenten wird eine widerspruchsfreie Deutung der mikromagnetischen Struktur sowie deren Ummagnetisierungsverhalten ermöglicht. Zudem wird die mikromagnetische Struktur von MnAs auf GaAs(111) simuliert und damit das Verständnis der mikromagnetischen Strukturen auf alle vorhandenen Substratorientierungen vervollständigt.In the present thesis the micromagnetic structure, as well as the magnetization reversal, of epitaxial MnAs films on GaAs substrates are studied. The investigation is focused on the micromagnetic structure of anisotropically strained MnAs films on GaAs(001). The strain originates a selforganized array of ferromagnetic stripes. The magnetic domains were investigated using MFM (magnetic force microscopy) and the results were compared with XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy). To completely characterize the micromagnetic properties of the stripe structure, MFM experiments were performed in the presence of an external field. To unambiguously determine the domain structure a three-dimensional micromagnetic simulator was developed capable to calculate magnetic structures with mesoscopic dimensions. The stability of the three-dimensional micromagnetic structure depends on the properties of the selforganized stripe structure, i.e., on the film thickness as well as on the ratio of the stipe width to thickness - and thus the temperature. Taking into account the magnetization distribution in-depth, the remaining differences between the XMCDPEEM and the MFM results can be explained by the disturbing effect of the MFM tip. The results of the micromagnetic simulations, in combination with the experimental results, allow for a determination of the micromagnetic structure in an applied field throughout the phase coexistence regime. Moreover, the micromagnetic structure of MnAs films on GaAs(111) is simulated and thus the understanding of the micromagnetic properties have been extended on all substrate orientations

Growth of high-quality SrTiO3 films using a hybrid molecular beam epitaxy

A hybrid molecular beam epitaxy approach for atomic-layer controlled growth of high-quality SrTiO3 films with scalable growth rates was developed. The approach uses an effusion cell for Sr, a plasma source for oxygen, and a metal-organic source titanium tetra isopropoxide for Ti. SrTiO3 films were investigated as a function of cation flux ratio on 001 SrTiO3 and LaAlO30.3Sr2AlTaO60.7 LSAT substrates. Growth conditions for stoichiometric insulating films were identified. Persistent 180 oscillations reflection high-energy electron diffraction oscillation characteristic of layer-by-layer growth were observed. The full widths at half maximum of x-ray diffraction rocking curves were similar to those of the substrates, i.e., 34 arc sec on LSAT. The film surfaces were nearly ideal with root mean square surface roughness values of less than 0.1 nm. The relationship between surface reconstructions, growth modes, and stoichiometry is discussed

Growth of high-quality SrTiO3 films using a hybrid molecular beam epitaxy

A hybrid molecular beam epitaxy approach for atomic-layer controlled growth of high-quality SrTiO3 films with scalable growth rates was developed. The approach uses an effusion cell for Sr, a plasma source for oxygen, and a metal-organic source titanium tetra isopropoxide for Ti. SrTiO3 films were investigated as a function of cation flux ratio on 001 SrTiO3 and LaAlO30.3Sr2AlTaO60.7 LSAT substrates. Growth conditions for stoichiometric insulating films were identified. Persistent 180 oscillations reflection high-energy electron diffraction oscillation characteristic of layer-by-layer growth were observed. The full widths at half maximum of x-ray diffraction rocking curves were similar to those of the substrates, i.e., 34 arc sec on LSAT. The film surfaces were nearly ideal with root mean square surface roughness values of less than 0.1 nm. The relationship between surface reconstructions, growth modes, and stoichiometry is discussed

Toward ultraclean correlated metal CaVO3

We report the synthesis and electronic properties of the correlated metal CaVO3, grown by hybrid molecular beam epitaxy. Films were grown on (100) LaAlO3 substrates at a temperature of 900 °C by supplying a flux of elemental Ca through a thermal effusion cell and metalorganic precursor, vanadium oxitriisopropoxide, as a source of vanadium. The presence of a self-regulated growth regime was revealed by the appearance of a specific surface reconstruction detected by reflection high-energy electron diffraction. Films grown within the growth window were characterized by atomically flat surfaces. X-ray reciprocal space maps revealed that the films were coherently strained to the substrate and inherited its twinned microstructure. Despite the presence of twin walls, CaVO3 thin films, grown within the stoichiometric growth window, revealed very low electrical resistivities at low temperatures, with residual resistivity ratios exceeding 90, while films grown at either Ca- or V-excess show deteriorated transport properties, attributed to the presence of extrinsic defects arising from the non-stoichiometry present in these films

Effects of hydrogen anneals on oxygen deficient SrTio3-x single crystals

The influence of hydrogen gas anneals on the electrical properties of nominally undoped, oxygen-deficient SrTiO3−x single crystals was investigated. Titanium getter layers and vacuum anneals were used to obtain oxygen-deficient SrTiO3−x with a low electrical resistivity. These crystals showed an optical absorption peak at 2.92 eV and strong midinfrared absorption. Subsequent anneals at 800 °C in forming gas, which contained 10% hydrogen, returned the crystals into the insulating, transparent state. The mechanisms by which hydrogen anneals can compensate for the effects of oxygen vacancies in SrTiO3−x are discussed. The results show that forming gas anneals of stoichiometric SrTiO3 can lead to complex electrical conduction behavior