Mixed anion systems, such as oxynitrides and oxyfluorides, are an emerging class of interesting materials. The lower stability of mixed anion systems in comparison to oxide materials has had the consequence that this area of materials research is relatively less well explored. However, the development of new synthesis techniques has resulted in the preparation of many new mixed anion systems and so a detailed understanding of their structure and how this relates to their electronic and magnetic properties is necessary. Within this Thesis, several oxide, oxynitride and oxyfluoride systems are investigated with a particular focus on the magnetic behaviour of materials based on geometrically frustrated pyrochlore and kagome lattices. The Lu2Mo2O7 pyrochlore contains a geometrically frustrated network of vertex sharing Mo4+ (d2 S = 1) tetrahedra. Here, the solid state synthesis of Lu2Mo2O7−x is reported along with a discussion of the coexistence of two cubic pyrochlore phases that has been discovered in samples synthesised at 1600 ◦C. Powder neutron diffraction and thermogravimetric analysis have revealed that this two-phase behaviour originates from a miscibility gap between stoichiometric Lu2Mo2O7 and oxygen deficient Lu2Mo2O6.6. Magnetic susceptibility and muon spin relaxation measurements support the formation of a geometrically frustrated spin glass ground state in Lu2Mo2O7 with a spin freezing temperature Tf ∼ 16 K. Low temperature neutron diffraction has confirmed the absence of long range magnetic order and magnetic diffuse neutron scattering data have indicated the presence of competing nearest and next nearest neighbour antiferromagnetic exchange interactions in the spin glass state. The magnetic heat capacity of Lu2Mo2O7 follows a T2-dependence at the low temperatures, indicating that Lu2Mo2O7 is another rare example of an unconventional, topological spin glass, which is stable in the absence of significant chemical disorder. The magnetic properties of the oxygen deficient pyrochlore phase Lu2Mo2O6.6 are qualitatively similar to those of Lu2Mo2O7, but an increase in the spin freezing temperature Tf ∼ 20 K suggests that oxygen-vacancy disorder in Lu2Mo2O6.6 favours the onset of a glassy state at higher temperatures and enhances the degree of frustration. Oxynitride pyrochlores with the ideal composition R2Mo2O5N2 (R = rare earth) contain Mo5+ d1 S = 1 2 cations on the frustrated pyrochlore lattice and are thus ideal candidates to support exotic magnetic ground states. Here, the synthesis of oxynitride pyrochlores of the Lu2Mo2O7 system by thermal ammonolysis is discussed alongside powder neutron diffraction and susceptibility data that show no evidence for long range magnetic order and an absence of spin freezing down to at least 2 K despite the persistence of strong antiferromagnetic exchange (θ = −120 K). A comparison of the magnetic diffuse neutron scattering between the spin glass state of Lu2Mo2O7 and the oxynitride is given, which suggests that the majority of the magnetic scattering in the oxynitride system is inelastic. In addition, low temperature magnetic heat capacity shows an absence of magnetic phase transitions and a continuous density of states through a T-linear dependence down to 500 mK. [NH4]2[C7H14N][V7O6F18], diammonium quinuclidinium vanadium(III,IV) oxyfluoride or DQVOF, is a kagome bilayer system with a geometrically frustrated two-dimensional kagome network of V4+ d1 S = 1 2 cations and V3+ d2 S = 1 cations between the kagome layers. Here, low temperature magnetisation and heat capacity data are presented, which demonstrate that the interplane V3+ d2 cations are well decoupled from the kagome layers at low temperatures such that DQVOF is a good experimental realisation of a S = 1 2 kagome antiferromagnet. Despite significant antiferromagnetic exchange (θ = −60 K) within the kagome planes, muon spin relaxation data have confirmed the absence of spin freezing and the persistence of internal field fluctuations that are intrinsic to the kagome layers down to temperatures of 40 mK. The low temperature heat capacity of the V4+ kagome network follows T-linear behaviour down to the 300 mK, highlighting the absence of a spin gap in the low energy excitation spectrum of DQVOF. The low temperature magnetic study of DQVOF presented here thus strongly supports the formation of a gapless quantum spin liquid phase. In the final results chapter, a discussion of the anion ordering principles in oxynitride systems is given. A high temperature, high resolution neutron diffraction study of the oxynitride perovskite SrTaO2N has revealed that the partial anion order that results in segregated Ta-N zig-zag chains is stable up to 1100 ◦C. Furthermore, these anion ordering principles are extended to the d1 perovskite oxynitrides RVO2−xN1+x (R = La, Nd, Pr) in a variable temperature neutron diffraction study, which confirms that the anion chain ordering discovered in d0 SrTaO2N is robust to electron doping. The R = La analogue also provides an interesting example of a rhombohedral oxynitride perovskite phase which coexists with an orthorhombic phase over the 4−300 K temperature range of the neutron diffraction study.

Radiation chemistry is concerned with understanding the chemical kinetics following the application of ionising radiation. There are two main methods for modelling recom- bination and spin dynamics in radiation chemical systems: The Monte Carlo random flights algorithm, in which the trajectories of the diffusing species are followed ex- plicitly and the Independent Reaction Times (IRT) algorithm, where reaction times are sampled from appropriate marginal distribution functions. This thesis reports develop- ments to both methods, and applies them to better understand experimental findings, particularly spin relaxation effects. Chapter 4 introduces current simulation techniques and presents newly developed algorithms and simulation programs (namely Hybrid and Slice) for modelling spatially dependent spin effects. A new analytical approximation for accurately treating ion-pair recombination in low-permittivity solvents in also presented in this chapter. Chapter 5 explores the photodissociation of H₂O₂, where there is some controversy in the literature on the spin state of the precursor. This chapter explores the possibility of reproducing the observed spin polarisation phase using the Radical Pair Mechanism. Chapter 6 presents two new algorithms for treating reactive products in the IRT framework. These have been tested for two chemical systems: (i) photodissociation of H₂O₂ where the ·OH are scavengeable; (ii) water photolysis which produces H⁺, ·OH and e⁻__aq. In the latter case a careful handling of three body correlations is required. Chapter 7 presents simulation results which suggest a strong correlation between scavenging and ion recombination in low permittivity solvents. A path decomposition method has been devised that allows IRT simulations to be corrected for this effect. Chapter 8 presents evidence for spin-entanglement and cross-recombination to act as an extra source of relaxation for ion-recombination in low permittivity solvents. It is hypothesised this effect contributes to the anomalous relaxation times observed for certain cyclic hydrocarbons. Chapter 9 presents an extension of the IRT simulation method to micelles. The kinetics are shown to be accurately described using the mean reaction time and the exponential approximation.

This thesis is concerned with the muon-spin relaxation (musr) technique and its application in the study of a number of molecular magnetic systems that may be driven through a quantum phase transition at low temperatures through the application of a magnetic field or hydrostatic pressure. Musr is a highly sensitive probe of magnetism, but its utility can be severely limited by the lack of knowledge of the muon implantation site and the extent to which the muon perturbs its host. In a system of ionic fluorides, where partial information about the muon site is experimentally available, I demonstrate systematically that these problems can be addressed accurately using electronic-structure calculations. The F--$mu$--F complex formed by muons in many fluorides can be understood as an exotic molecule-in-a-crystal defect with a zero-point energy larger than that of any naturally-occurring triatomic molecule. I demonstrate the interesting possibility of controlling the magnetic dimensionality in a molecular magnet using applied pressure. musr and high-field magnetisation experiments under applied pressure on the coordination polymer CuF$_2$(H$_2$O)$_2$(pyrazine) show a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase. Density-functional theory calculations and calculations of the dipolar anisotropy complement the experiments. I describe how subtle differences in chemical composition can lead to starkly different structural and magnetic properties. [Cu(pyz)(H$_2$O)(gly)$_2$](ClO$_4$)$_2$ may be considered an antiferromagnetic chain that orders below 50 mK while the related compound [Cu(pyz)(gly)](ClO$_4$) is formed from Cu$^{2+}$ dimers and remains disordered down to 30 mK in zero field, but displays a field-temperature phase diagram consistent with the Bose-Einstein condensation of triplons. I also describe musr measurements on the strong-leg spin ladder DIMPY and on the molecular nanomagnets Cr$_8$Cd and Cr$_8$Mn which highlight some of the remaining challenges for longitudinal-field musr experiments.

Als nicht-invasive Methode bietet die magnetische Kernspinresonanztomographie durch ihre Vielzahl an messbaren Größen wie Wassergehalt und Flussgeschwindigkeiten gute Voraussetzungen, um funktionelle Abläufe in Pflanzen und insbesondere Pflanzenwurzeln zu untersuchen. Für funktionelle NMR-Mikroskopie notwendige Hardware und Methoden wurden in dieser Arbeit entwickelt und angewendet. Aufgrund der starken Suszeptibilitätsunterschiede in den Proben und der notwendigen Zeitauflösung für funktionelle Studien, lag das Hauptaugenmerk dabei auf Turbospinechomethoden (auch als RARE bekannt). Im Rahmen des Hardwareaufbaus wurde ein neuartiges, modulares Probenkopfkonzept entwickelt. Außerdem war es notwendig geeignete Probengefäße und Pflanzenhandlingsysteme zu entwerfen, die die Anbringung einer HF-Spule im Wurzelbereich erlauben. Für die Auswertung gemessener Parameterkarten wurde eine Software geschrieben, mit der interaktiv Mittelwerte entlang geschlossener Pfade berechnet werden können, angepasst an den grob radialsymmetrische Aufbau der Pflanzenwurzeln. Als Grundlage für biologische Aussagen anhand von T1-, T2- und Spindichtekarten wurden aus einer umfangreichen Literaturrecherche die bekannten Zusammenhänge zwischen diesen Parametern und physiologischen Größen zusammengestellt. Ergänzend wurde das Verhalten einer monoexponentiellen Beschreibung der Relaxation von mehr-Kompartimentsystemen und von deren Durchmischung untersucht. Eine Computersimulation der Diffusion zwischen Volumenschichten mit unterschiedlichen Relaxationszeiten wurde implementiert. Damit konnte gezeigt werden, dass die Reichweite der Durchmischung der messbaren Relaxationszeiten bei freier Diffusion abhängig ist von der Diffusionsweite, die nach der Einstein-Smoluchowski-Gleichung aus der jeweils lokalen Relaxationszeit resultiert. Damit ergibt sich eine grundsätzliche Limitierung der räumlichen Auflösung von Relaxationszeitkarten und auch des jeweiligen Relaxationszeitkontrastes in NMR-Bildern. Daneben erklärt der Effekt der durch Diffusion vermittelten Relaxation auch den hellen Ring, der in NMR-Bildern die Wurzeln in Nährlösung umgibt. Die hauptsächliche Anwendung der entwickelten Methodik auf biologische Fragestellungen bestand in der Untersuchung der Reaktion von Maiswurzeln auf Trockenstress. Erstmals konnten dabei im Rahmen dieser Arbeit Kavitationen der Wassersäule im Xylem von Wurzeln sowie deren Wiederbefüllung nach Wiederbewässerung der Pflanzen direkt beobachtet werden. Bei der weiteren systematischen Untersuchung zu Kavitationen gelang es auch, die bislang unbekannte Geschwindigkeit zu bestimmen (Größenordnung 1mm/min) mit der die kavitierten Bereiche von unten mit einer neuen Wassersäule gefüllt werden. Außerdem konnte mit Hilfe von Flussgeschwindigkeitskarten nachgewiesen werden, dass Gefäße mit Kavitationen nach der Wiederbefüllung ihre volle Funktionalität wiedererlangen können. Aus solchen Flusskarten konnte auch der Volumenfluss berechnet und z.B. mit der Transpirationsrate verglichen werden. Die gemessenen T1- und Spindichtekarten bieten viele Hinweise auf die Funktion der unterschiedlichen Gewebetypen der Wurzel während des Trockenstresses und bei der Wiederbefüllung. Insbesondere T1 erwies sich als aussagekräftiger Parameter für die Beurteilung von aufgetretenen Gewebeschäden. Als Grundlage für zukünftige Studien wurden verschiedene Messungen mit Kontrastmittel im Umgebungsmedium der Wurzeln durchgeführt, sowie eine 3D-Turbospinechosequenz implementiert, mit der auch die interne Struktur der Wurzeln und ihrer Verzweigungen dargestellt werden konnte. / MR imaging methods allow the non-invasive determination of values such as flow velocity and water content. Therefore, these methods provide a perfect means for the in-vivo examination of plants together with their surrounding medium. It was the aim of this PhD thesis to develop necessary hardware and software methods for functional studies on plant roots. Because of the large susceptibility differences of these samples and to provide a sufficient time resolution for functional studies especially RARE sequences were used and optimized for the examination of roots. Corn plants were grown in specially designed glass tubes which provided a narrow part for the inclusion of an RF Helmholtz coil. A new, patented concept for modular MR probes was designed. It allows the construction of probes that are adapted to the needs of certain samples like climate chamber probes for plants, without the limitation caused by fixed frequencies for the RF channels. A method to average values of tissues that are composed of concentric layers, like corn roots, or exhibit radial value gradients was implemented. An intense literature research about the known dependency of T1 and T2 on physiologic aspects of plant tissue was performed, resulting in a compilation of relaxation influencing effects. This compilation serves as a basis for the interpretation of the biological experiments. Further theoretical considerations dealt with the effect of a mono exponential description of the relaxation within multi compartment systems and after mixing the contents of these compartments. A computer algorithm was implemented to simulate the free diffusion between volumes that exhibit different relaxation values. It could be shown that transition regions of "mixed" relaxation values occur on the edges of those volume regions. The width of these transition regions can be determined by the Einstein-Smoluchowski-Equation for the diffusion displacement, using the underlying relaxation values as the diffusion time. This effect is a fundamental limitation to the spatial resolution of relaxation maps and to the contrast that those relaxation values create in MR images. Based on this finding, the cause for the bright ring that surrounds roots within nutrient solution in MR images could be explained. It results from the diffusion driven in-flow of quickly relaxing magnetisation from the outer part of the roots into the free solution. The most important biological results of this PhD thesis concern the reaction of plant roots to drought stress and rewatering. In the context of these drought stress experiments, cavitations of the sap in the xylem vessels on the roots could be observed for the first time using MR. In addition, the refilling of these cavitated vessels was monitored systematically. It was possible to measure the velocity of the newly ascending sap level, which was calculated to be in the order of 1mm/min. Using flow imaging, it was also possible to confirm the regained functionality of the refilled vessels for the first time. Based on flow maps, also the volume flow could be calculated and e.g. could be compared to the transpiration rate of the plants. The T1 and spin density maps acquired during the stress experiments give information about the function of the different tissues during desiccation and rewatering. Especially a strong decrease in T1 in the cortex of certain roots could be correlated with an irreversible damage to the tissue. To provide a further basis for future studies, experiments on the uptake of the MR contrast agent Gd-DTPA by the roots were performed. Also a 3D RARE microscopy sequence was implemented and used to follow the inner structure of the branching of a side root.

<p>This thesis presents the application of some of the NMR methods in studying host-guest complexes, mainly in solution. The general focus of the work is on investigating the reorientational dynamics of some small molecules that are bound inside cavities of larger moieties. In the current work, these moieties belong to two groups: cryptophanes and cyclodextrins. Depending on the structure of the cavities, properties of the guest molecules and the formed complexes vary. Chloroform and dichloromethane are in slow exchange between the cage-like cavity of the cryptophanes and the solvent, on the chemical shift time scale, whereas adamantanecarboxylic acid, quinuclidine and 1,7-heptanediol in complex with cyclodextrins are examples of fast exchange. Kinetics and thermodynamics of complexation are studied by measuring exchange rates and translational self-diffusion coefficients by means of 1-dimenssional exchange spectroscopy and pulsed-field gradient (PFG) NMR methods, respectively. The association constants, calculated using the above information give estimates of the thermodynamic stability of the complexes. Carbon-13 spin relaxation data were obtained using conventional relaxation experiments, such as inversion recovery and dynamic NOE, and in some cases HSQC-type (Hetereonuclear Single Quantum Correlation Spectroscopy) experiments. Motional parameters for the free and bound guest, and the host molecules were extracted using different motional models, such as Lipari-Szabo, axially symmetric rigid body, and Clore models. Comparing the overall correlation times and the order parameters of the free and bound guest with the overall correlation time of the host molecule one can estimate the degree of the motional restriction, brought by the complexation, and the coupling between the motion of the bound guest and the reorientation of the host molecule. In one case, the guest motions were also investigated inside the cavities of a solid host material.</p>

This thesis presents the application of some of the NMR methods in studying host-guest complexes, mainly in solution. The general focus of the work is on investigating the reorientational dynamics of some small molecules that are bound inside cavities of larger moieties. In the current work, these moieties belong to two groups: cryptophanes and cyclodextrins. Depending on the structure of the cavities, properties of the guest molecules and the formed complexes vary. Chloroform and dichloromethane are in slow exchange between the cage-like cavity of the cryptophanes and the solvent, on the chemical shift time scale, whereas adamantanecarboxylic acid, quinuclidine and 1,7-heptanediol in complex with cyclodextrins are examples of fast exchange. Kinetics and thermodynamics of complexation are studied by measuring exchange rates and translational self-diffusion coefficients by means of 1-dimenssional exchange spectroscopy and pulsed-field gradient (PFG) NMR methods, respectively. The association constants, calculated using the above information give estimates of the thermodynamic stability of the complexes. Carbon-13 spin relaxation data were obtained using conventional relaxation experiments, such as inversion recovery and dynamic NOE, and in some cases HSQC-type (Hetereonuclear Single Quantum Correlation Spectroscopy) experiments. Motional parameters for the free and bound guest, and the host molecules were extracted using different motional models, such as Lipari-Szabo, axially symmetric rigid body, and Clore models. Comparing the overall correlation times and the order parameters of the free and bound guest with the overall correlation time of the host molecule one can estimate the degree of the motional restriction, brought by the complexation, and the coupling between the motion of the bound guest and the reorientation of the host molecule. In one case, the guest motions were also investigated inside the cavities of a solid host material.

We analyze a coincidence Stern-Gerlach measurement often discussed in connection with the derivation and illustration of Bell's theorem. The treatment is based on our recent analysis of the original Stern-Gerlach experiment (PCCP, 14, 1677‐1684 (2012)), where it is concluded that it is necessary to include a spin relaxation process to account for the experimental observations. We consider two limiting cases of a coincidence measurement using both an analytical and a numerical description. In on limit relaxation effects are neglected. In this case the correlation between the two spins present in the initial state is conserved during the passage through the magnets. However, at exit the z coordinate along the magnetic field gradient is randomly distributed between the two extreme values. In the other limit T2 relaxation is assumed to be fast relative to the time of flight through the magnet. In this case the z coordinate takes one of two possible values as observed in the original Stern‐Gerlach experiment. Due to the presence of a relaxation process involving transfer of angular momentum between particle and magnet the initially entangled spin state changes character leading to a loss of correlation between the two spins. In the original derivations of Bell's theorem based on a coincidence Stern‐Gerlach setup one assumes both a perfect correlation between the spins and only two possible values for the z‐coordinate on exit. According to the present calculations one can satisfy either of these conditions but not both simultaneously.

The phospholipid membrane is a self-assembled, dynamic molecular system that may exist alone in association with only water, or in complex systems comprised of multiple lipid types and proteins. In this dissertation the intra- and inter-molecular forces responsible for the atomistic, molecular and collective equilibrium structure and dynamics are studied by nuclear magnetic resonance spectroscopy (NMR). The multinuclear NMR measurements and various experimental techniques are able to provide data that enable the characterization of the hierarchical spatio-temporal organization of the phospholipid membrane. The experimental and theoretical studies conducted target membrane interactions ranging from model systems composed of only water and lipids, to multiple component domain forming membranes that are in association with peripheral and trans-membrane proteins. These measurements consisit of frequency spectrum lineshapes and nuclear-spin relaxation rates obtained using 2 H NMR, 13 C NMR, 31 P NMR and 1 H NMR. The changes of these experimental observables are interpreted within a statistical thermodynamic framework that allows the membrane structure, activation energies, and correlation times of motion to be determined. The cases presented demonstrate how fundamental principles of NMR spectroscopy may be applied to a host of membranes, leading to the biophysical characterization of membrane structure and dynamics.

Universiẗat, Diss., 2002--Freiburg (Breisgau). / Erscheinungsjahr an der Haupttitelstelle: 2002.

Orientador: Guillermo Gerardo Cabrera Oyarzún / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin" / Made available in DSpace on 2018-08-16T15:57:09Z (GMT). No. of bitstreams: 1 Rosa_PriscilaFerrariSilveira_M.pdf: 3441309 bytes, checksum: ac0fe3faa7642e9a7ef8330049e33761 (MD5) Previous issue date: 2010 / Resumo: A descrição da dinâmica quântica de sistemas de muitos corpos é um ingrediente chave para a computação quântica. No presente projeto, propomos o estudo da dinâmica em cadeias finitas de spin 1/2 na presença de impurezas ou defeitos. O modelo adotado será o de Ising quântico com campo transverso, do qual é possível obter o espectro de forma exata na presença de uma impureza. A dinâmica do sistema é movida exclusivamente por flutuações quânticas, cuja origem é o Princípio da Incerteza. Investigamos a relaxação de estados iniciais caracterizados por uma magnetização espacialmente não homogênea e que não possuam hipóteses sobre a proximidade com o estado de equilíbrio. Dessa forma, a matriz densidade inicial será dependente apenas de uma única coordenada espacial. A investigação então é realizada através da grandeza de interesse, a saber, a evolução temporal do valor médio das componentes de Fourier da magnetização, (SzQ)t. Soluções exatas, tanto analíticas quanto numéricas, são obtidas. Um dos objetivos iniciais deste trabalho consiste na busca de processos de relaxação lentos. Para os casos de solução analítica (impurezas periódica e antiperiódica) observamos relaxações oscilatórias e amortecidas por uma lei de potência no tempo do tipo (t/tQ)-vQ, onde tQ e vQ são dois parâmetros livres e Q é o número de onda associado a cada componente de Fourier. Há uma criticalidade no expoente vQ, o qual muda de 3/2 para 1/2 para certos valores de Q críticos. Por outro lado, para os casos de solução numérica (impurezas arbitrárias), os processos de relaxação são distintos daqueles citados acima. Os estados iniciais analisados são do tipo produto direto ferromagnético com um único spin virado, próximo ou distante da impureza. Neste caso, as evoluções temporais oscilam em torno deum valor médio não nulo e há uma larga faixa de valores de Q na qual os modos não se extinguem completamente / Abstract: The description of many body systems quantum dynamics is a key ingredient for quantum computation. In the present project we study finite spin-1/2 chains dynamic properties in the presence of impurities or defects. We adopt the quantum Ising model with transverse field, of which it is possible to obtain the energy spectrum by exact calculations in the presence of one impurity. The system dynamics is driven exclusively by quantum fluctuations, whose origin is the Uncertainty Principle. We investigate the relaxation of initial states characterized by spatially inhomogeneous magnetization without any hypothesis about the proximity with the equilibrium state. Thus, the initial density matrix will be dependent of only one spatial coordinate. The investigation then is realized through the temporal evolution of the magnetization's Fourier components. Exact solutions, analytical and numerical, are obtained. One of the goals of this work consist in the search of slow relaxation processes. For the analytical cases (periodic and anti-periodic impurities) we observe oscillatory relaxations with a decay given by a power law in time (t/tQ)-vQ, where tQ and vQ are two free parameters and Q is the wave number associated to a Fourier component. There is a criticality in the exponent vQ: its value changes from 3/2 to 1/2 for certain critical values of Q. On the other hand, for the numerical cases (arbitrary impurities), the relaxation processes are distinct from the cases cited above. The initial state analyzed is a ferromagnetic direct product with only one flipped spin, near or far from the impurity. In this case, the temporal evolutions oscillate around a finite mean value and there is a large interval of Q values in which the modes do not extinguish completely / Mestrado / Física da Matéria Condensada / Mestra em Física