Investigation of Ni2MnGa-based shape memory alloys

Compounds with the general composition Ni2+xMn1-xGa belong to the group of ferromagnetic shape memory alloys. These materials have recently attracted a considerable amount of interest due to their potential for applications. Ferromagnetic shape memory alloys, in contrast to conventional shape memory materials, offer the possibility to initiate a change of shape with the application of an external magnetic field of modest magnitude in addition to shape changes induced via a shift in temperature. In this thesis the magnetic and crystallographic properties of Ni2+xMn1-xGa alloys are investigated within the range 0 < x < 0.17. Structural investigations using both X-ray as well as neutron scattering have been carried out. The low temperature phase has been determined and structural information has been obtained within the transition region for the first time. Magnetic properties have been investigated using a SQUID magnetometer within the temperature range of T = 2 K to 360 K and using magnetic fields up to 5.5 Tesla. The locations of phase transitions were investigated using DSC and resistivity measurements for both heating as well as cooling. The composition dependence of the martensitic and ferromagnetic transition temperatures have been confirmed. The first order nature of the martensitic phase transformation has been confirmed in DSC, magnetisation and resistivity measurements

Investigation of new semiinsulating behavior of III-V compounds

The investigation of defect interactions and properties related to semiinsulating behavior of III-V semiconductors resulted in about twenty original publications, six doctoral thesis, one masters thesis and numerous conference presentations. The studies of new compensation mechanisms involving transition metal impurities have defined direct effects associated with deep donor/acceptor levels acting as compensating centers. Electrical and optical properties of vanadium and titanium levels were determined in GaAs, InP and also in ternary compounds InGaAs. The experimental data provided basis for the verification of chemical trends and the VRBE method. They also defined compositional range for III-V mixed crystals whereby semiinsulating behavior can be achieved using transition elements deep levels and a suitable codoping with shallow donor/acceptor impurities

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 355)

This bibliography lists 147 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during October, 1991. Subject coverage includes: aerospace medicine and psychology, life support systems and controlled environments, safety equipment, exobiology and extraterrestrial life, and flight crew behavior and performance

Energietransfer- und Energieumwandlungsmechanismen im pflanzlichen Lichtsammelkomplex II

The light-harvesting complex of photosystem II (LHC-II) is the major antenna complex in plant photosynthesis. It accounts for roughly 30% of the total protein in plant chloroplasts, which makes it arguably the most abundant membrane protein on Earth, and binds about half of plant chlorophyll (Chl). The complex assembles as a trimer in the thylakoid membrane and binds a total of 54 pigment molecules, including 24 Chl a, 18 Chl b, 6 lutein (Lut), 3 neoxanthin (Neo) and 3 violaxanthin (Vio). LHC-II has five key roles in plant photosynthesis. It: (1) harvests sunlight and transmits excitation energy to the reaction centres of photosystems II and I, (2) regulates the amount of excitation energy reaching each of the two photosystems, (3) has a structural role in the architecture of the photosynthetic supercomplexes, (4) contributes to the tight appression of thylakoid membranes in chloroplast grana, and (5) protects the photosynthetic apparatus from photo damage by non photochemical quenching (NPQ). A major fraction of NPQ is accounted for its energy-dependent component qE. Despite being critical for plant survival and having been studied for decades, the exact details of how excess absorbed light energy is dissipated under qE conditions remain enigmatic. Today it is accepted that qE is regulated by the magnitude of the pH gradient (&#916;pH) across the thylakoid membrane. It is also well documented that the drop in pH in the thylakoid lumen during high-light conditions activates the enzyme violaxanthin de-epoxidase (VDE), which converts the carotenoid Vio into zeaxanthin (Zea) as part of the xanthophyll cycle. Additionally, studies with Arabidopsis mutants revealed that the photosystem II subunit PsbS is necessary for qE. How these physiological responses switch LHC-II from the active, energy transmitting to the quenched, energy-dissipating state, in which the solar energy is not transmitted to the photosystems but instead dissipated as heat, remains unclear and is the subject of this thesis. From the results obtained during this doctoral work, five main conclusions can be drawn concerning the mechanism of qE: 1. Substitution of Vio by Zea in LHC-II is not sufficient for efficient dissipation of excess excitation energy. 2. Aggregation quenching of LHC-II does not require Vio, Neo nor a specific Chl pair. 3. With one exception, the pigment structure in LHC-II is rigid. 4. The two X-ray structures of LHC-II show the same energy transmitting state of the complex. 5. Crystalline LHC-II resembles the complex in the thylakoid membrane. Models of the aggregation quenching mechanism in vitro and the qE mechanism in vivo are presented as a corollary of this doctoral work. LHC-II aggregation quenching in vitro is attributed to the formation of energy sinks on the periphery of LHC-II through random interaction with other trimers, free pigments or impurities. A similar but unrelated process is proposed to occur in the thylakoid membrane, by which excess excitation energy is dissipated upon specific interaction between LHC-II and a PsbS monomer carrying Zea. At the end of this thesis, an innovative experimental model for the analysis of all key aspects of qE is proposed in order to finally solve the qE enigma, one of the last unresolved problems in photosynthesis research.Der Lichtsammelkomplex des Photosystems II (light-harvesting complex II, LHC-II) stellt den Hauptantennenkomplex in der pflanzlichen Photosynthese dar. LHC-II fallen fünf Schlüsselrollen in der pflanzlichen Photosynthese zu: (1) Sammeln von Sonnenlicht und Weiterleitung der Anregungsenergie an die Reaktionszentren der Photosysteme I und II, (2) Regulierung der Menge an Anregungsenergie, welche beide Photosysteme erreicht, (3) Stabilisierung der Architektur der photosynthetischen Superkomplexe, (4) Beitrag zur dichten Packung der Grana-Thylakoidstapel in Chloroplasten und (5) Schutz des Photosyntheseapparates vor Lichtschäden durch die nicht-photochemische Löschung der Anregungsenergie (non photochemical quenching, NPQ). Einen großen Anteil am NPQ besitzt die energieabhängige Komponente qE. Obwohl ihre Notwendigkeit für das Überleben der Pflanze bekannt ist und die zugrunde liegenden Prozesse jahrzehntelang erforscht wurden, sind die exakten Mechanismen des Abführens von absorbierter Lichtenergie unter qE-Bedingungen weitgehend unbekannt. Es gilt als gesichert, dass qE durch den pH-Gradienten über die Thylakoidmembran reguliert wird. Weiterhin ist bekannt, dass infolge des pH-Abfalls im Thylakoidlumen unter Starklichtbedingungen das Enzym Violaxanthin-De-epoxidase (VDE) aktiviert wird, welches im Xanthophyllzyklus das Carotinoid Vio in Zeaxanthin (Zea) umwandelt. Desweiteren zeigten Studien an Arabidopsis Mutanten, dass PsbS, eine Untereinheit des Photosystems II, essentiell für qE ist. Inwiefern diese physiologischen Prozesse jedoch dazu beitragen, dass LHC-II von einem aktiven, Energie transferierenden, in einen Energie abführenden Zustand versetzt wird, in welchem Sonnenenergie nicht an die Photosysteme weitergeleitet, sondern in Wärme umgewandelt wird, ist ungeklärt und Gegenstand dieser Arbeit. Aus den Ergebnissen, die im Laufe dieser Doktorarbeit erhalten wurden, lassen sich fünf wesentliche Schlussfolgerungen in Bezug auf qE ableiten: 1. Der Austausch von Vio gegen Zea in LHC-II ist für ein effizientes Abführen von überschüssiger Anregungsenergie nicht ausreichend. 2. Energielöschung infolge von LHC-II Aggregation benötigt weder Vio und Neo, noch ein spezielles Chl-Paar. 3. Mit einer Ausnahme ist die Pigmentstruktur in LHC-II starr. 4. Beide Röntgenstrukturen von LHC-II zeigen denselben Energie abführenden Zustand des Komplexes. 5. Kristalliner LHC-II ähnelt dem Komplex in der Thylakoidmembran. Als Korollar aus dieser Doktorarbeit folgen Erklärungsmodelle sowohl für die Aggregations-verursachte Energielöschung in vitro, als auch für den qE-Mechanismus in vivo. Die infolge der Aggregation von LHC-II in vitro beobachtete Energielöschung wird der Bildung von Energiesenken in der Peripherie von LHC-II zugeschrieben, welche durch Interaktion mit anderen Trimeren, freien Pigmenten oder Unreinheiten hervorgerufen werden. Ein ähnlicher, jedoch nicht verwandter, Prozess wird für die Abführung überschüssiger Anregungsenergie in der Thylakoidmembran vorgeschlagen. Dieser beruht auf der spezifischen Interaktion zwischen LHC-II und einem PsbS-Monomer, welches Zea bindet

Monolithically integrated mode-locked ring lasers and Mach-Zehnder interferometers in AlGaInAs

In this thesis, monolithically integrated photonic devices for next generation optical telecommunications networks were investigated, namely semiconductor modelocked ring lasers and Mach-Zehnder interferometers operating at 1550 nm. Fabricated on the aluminium quaternary, the 2.3mm long passively mode-locked ring devices produced 1 ps pulses at a repetition rate of around 36GHz. It was found that the symmetrically placed dual saturable absorber configuration lead to the largest area of stable mode-locking, agreeing well with theoretical predictions in the literature. Optical harmonic injection mode-locking was found to improve the pulse timing stability, with a reduction in the radio frequency 3 dB linewidth from 1.4MHz down to 108 kHz, indicating a vast improvement in timing jitter. The sputtered SiO2 quantum-well intermixing technique allowed for the realisation of both symmetric and asymmetric arm length Mach-Zehnder interferometers, which were demonstrated as an electro-optic switch, tunable wavelength filter and optical code division multiple access encoder/decoder. The work concluded with the monolithic integration of a mode-locked ring laser and asymmetric Mach-Zehnder interferometer to demonstrate a simple, yet effective, photonic integrated circuit

Use of Protein Crosslinking and Tandem Mass Spectrometry to Study the PsbO, PsbP and PsbQ Extrinsic Proteins of Higher Plant Photosystem II

Photosystem II (PSII) is a light-driven, water plastoquinone oxidoreductase present in all oxygenic photosynthetic organisms. The oxygen evolution process is catalyzed by the Mn4CaO5 cluster and an ensemble of intrinsic and extrinsic proteins which are associated with the photosystem. This metal cluster is stabilized and protected from exogenous reductants by the extrinsic proteins, PsbO, PsbP and PsbQ in higher plants, which are present on the lumenal face of PSII. No crystal structure for the higher plant PSII is currently available; consequently, the binding locations of these extrinsic proteins in PSII remain elusive. We have used chemical-crosslinkers Bis (sulfosuccinimidyl) suberate (BS3) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) to crosslink the extrinsic proteins in their bound state to PSII followed by identification of the crosslinked products by tandem mass spectrometry. BS3 crosslinking identified the interacting domain of PsbP with PsbQ involving the PsbP residues 93Y, 96K and 97T (located in the 17-residue loop 3A, 89G-105S) which are in close proximity (\u3c11.4Å) to the N-terminal 1E residue of PsbQ. We also found that this PsbP assumes a compact structure from the nine independent crosslinked residues between the N- and C-terminus of PsbP. This suggests that the N-terminus of PsbP, 1A-11K (which is not resolved in the current crystal structures), is closely associated with the C-terminal domain 170K-186A. Additionally, interacting domains of two PsbQ copies from different PSII monomers were identified. The residue pairs 98K-133Y and 101K-133Y of PsbQ were crosslinked. These residues are \u3e30 Å apart when mapped onto the PsbQ crystal structure. Since BS3 can only crosslink residues which are within 11.4 Å, these residues are hypothesized as inter-molecular crosslinks of PsbQ. Furthermore, EDC crosslinking provided structural information pertaining to the organization of the N-terminus, absent in the cyanobacterial-PsbO. In this study, twenty-four crosslinked residues located in the N-terminal, loop and the β-barrel region of PsbO were identified. The models incorporating crosslinking data suggests several differences in cyanobacterial- and higher plant-PsbO. The results on extrinsic proteins provide significant new information concerning the association of the extrinsic proteins with PSII and are valuable while proposing overall models of higher plant PSII

Electronic structure study of copper-containing perovskites

This thesis concerns the computational study of copper containing perovskites using electronic structure methods. We discuss an extensive set of results obtained using hybrid exchange functionals within Density Functional Theory (DFT), in which we vary systematically the amount of exact (Hartree-Fock, HF) exchange employed. The method has enabled us to obtain accurate results on a range of systems, particularly in materials containing strongly correlated ions, such as Cu2+. This is possible because the HF exchange corrects, at least qualitatively, the spurious self-interaction error present in DFT. The materials investigated include two families of perovskite-structured oxides, of potential interest for technological applications due to the very large dielectric constant or for Multi-Ferroic behaviour. The latter materials exhibit simultaneously ferroelectric and ferromagnetic properties, a rare combination, which is however highly desirable for memory device applications. The results obtained using hybrid exchange functionals are highly encouraging. Initial studies were made on bulk materials such as CaCu3Ti4O12 (CCTO) which is well characterised by experiment. The inclusion of HF exchange improved, in a systematic way, both structural and electronic results with respect to experiment. The confidence gained in the study of known compounds has enabled us to explore new compositions predictively. Interesting results have been obtained, and we have been able to identify new materials of potential interest, which represent clear new targets for future experimental studies

NASA/Howard University Large Space Structures Institute

Basic research on the engineering behavior of large space structures is presented. Methods of structural analysis, control, and optimization of large flexible systems are examined. Topics of investigation include the Load Correction Method (LCM) modeling technique, stabilization of flexible bodies by feedback control, mathematical refinement of analysis equations, optimization of the design of structural components, deployment dynamics, and the use of microprocessors in attitude and shape control of large space structures. Information on key personnel, budgeting, support plans and conferences is included

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 378)

This bibliography lists 185 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during Aug. 1993. Subject coverage includes: aerospace medicine and physiology, life support systems and man/system technology, protective clothing, exobiology and extraterrestrial life, planetary biology, and flight crew behavior and performance