During lithium-ion battery electrode fabrication, slurry drying conditions influence the resulting microstructure of electrodes. It has been found that the drying conditions can result in non-uniform cathode microstructures and material distributions. Accelerated drying, for example, is widely assumed to cause the binder in an electrode to migrate within the slurry, which can contribute to adhesion failure, and ultimately capacity fade and reduced battery life. While there are some conflicting studies regarding the aspects of accelerated drying that cause binder migration, there is not a widely used standard metric for measuring the gradient of binder across the thickness of an electrode. In this work, the vertical heterogeneity of electrodes, as measured using energy-dispersive X-ray spectroscopy (EDX), is correlated with different drying methods and rates. An improved metric for measuring the binder gradient in electrodes is proposed. For the electrodes in this study, binder migration is minimally affected by the drying method and the normalized binder gradient does not increase with increased drying rate. The results are compared to a drying physics model, and it is shown that further development of current models that predict binder gradient as a function of drying rate will need to be modified to more fully capture the physics of slurry drying.

Scanning Electron Microscopy combined with Energy Dispersive X-ray Spectroscopy (SEM-EDS) is a widely applied elemental microanalysis method. The integration of silicon drift detectors (SDDs) has notably enhanced EDS performance, enabling precise elemental identification due to its large sensitive area and low output capacitance.  Accurate simulations of SDDs can provide insights that enable the design and optimization of future models without the need for costly and time-consuming experimental iterations. Moreover, the current model-based quantification methods for EDS applications have reached their maximum predictive accuracy. As such, creating a more accurate simulation model could help achieve a higher level of precision in these quantification models, which would be immensely valuable for all EDS applications.  With this objective in mind, a simulation framework for modeling SDDs in EDS was developed based on Geant4, Allpix Squared, and COMSOL Multiphysics. The simulation encompasses the entire physics pipeline, including characteristic X-ray emission from the target sample and its absorption in the detector. The generated charge carriers within the detector are propagated through the internal electric field of the SDD, and their individual charge contribution is measured to simulate EDS spectra. The simulated model was compared to existing literature and in-house experimental measurements, showing strong agreement in the case of a well-tuned SDD. Limitations of the simulation framework are discussed, and further research to enhance accuracy and speed is explored.

The waters of the Indian Ocean southeast of Africa are a crucial junction for surface and deep ocean processes that serve as vital controls on Earth’s climate system. At the surface, the Agulhas Current, its retroflection, and Agulhas Leakage transfer water from the Indian and South Atlantic. The addition of this heat and salt to the Atlantic Basin helps drive the Meridional Overturning Circulation and the formation of deep water in the North Atlantic Ocean. On the timescales of centuries, the Meridional Overturning Circulation ultimately returns this water back to the Indian Ocean in the form of North Atlantic Deep Water. Proxy reconstructions show that the vigor of ocean overturning is immensely important to the global climate system, driving changes in atmospheric CO₂ concentrations and temperature and precipitation patterns across the planet. I use x-ray fluorescence core scanning, sediment provenance techniques, and core images from International Ocean Discovery Program Site U1474, located in the Natal Valley of the southwest Indian Ocean, to investigate past changes in the Agulhas Current and North Atlantic Deep Water. 40K/40Ar provenance ages measured on the clay fraction of sediment from Site U1474 indicate that, despite its great distance from the core site, the Zambezi is the most important factor influencing the deposition of terrigenous sediment in the Natal Valley. We present these results in a quantitative way, reinforcing the conclusions of previous studies. However, a comparison to newly available proxy records influenced by current speed and hydroclimate suggests that the strength of the Agulhas does not have a major influence on terrigenous sediment sources, at least at the headwaters of the Agulhas Current. Instead, I suggest that low-latitude hydrologic processes driven by zonal and meridional temperature gradients in conjunction with sea level are responsible for sediment source variability. In core photos, I found evidence for deep water variability in the Natal Valley in the form of millimeter-to-centimeter scale layers of olive-green sediment. To an overwhelming extent, these layers are formed during glacial periods, especially at their termination. I complement observations at Site U1474 with published proxy data for oxygen concentrations and measurements of total organic carbon percent in the Natal Valley and by extending our search for these green layers to core sites around the world. With these data, it is possible to confidently connect these layers to organic carbon concentrations in the sediment, reduction-oxidation processes in sediments following burial, and the local concentration of dissolved oxygen in the deep water. There are comparable fluctuations in the abundances of green layers in core sites in the path of North Atlantic Deep Water during glacial cycles, where more frequent and more intense green layer formation is driven by higher bottom water oxygen concentrations. Peaks in the abundance of green layers approximately 250 ka and 900 thousand years ago coincide with global scale excursions toward isotopically light benthic carbon isotopes. Connecting the green layers to the release of isotopically light organic carbon from sediments leads me to propose that long-observed fluctuations in the carbon cycle may be attributable to deep ocean oxygenation.

The work contained within this thesis Is made up primarily of two pieces Both address active galactic nuclei And the galaxies that live nearby The obscured fraction of the population Is the topic of one publication And the type-II fraction in the optical regime In chapter four this is the theme I research the vicinity overdensity Around radio galaxies in chapter three, you’ll see I reduce some spectra at redshift one But not all observations in the end got done With the spectra I have I do what I can As if all target observations had actually ran In the end I conclude with results and the theme of research to be done further downstream. The works contained herein addresses two major topics in extragalactic astrophysics, namely the Type-II AGN fraction and the Overdensity-Radio power relation. Quantifying the Type-II AGN fraction has been attempted by many works in many different observational regimes, finding rather contrasting results. Accretion onto supermassive black holes contributes between 5 per cent and 20 per cent of the luminosity of the Universe, and seems to be closely linked to star formation processes. The large uncertainty on this value is due to the ill-determined contribution from obscured accretion, namely the Type-II fraction. In Chapters 3 and 4 I address this issue from a theoretical standpoint in the X-ray regime and an observational standpoint in the optical regime respectively. In Chapter 3 I show how crude X-ray spectroscopy of partially obscured AGN can lead to catastrophic underestimations of the intrinsic X-ray luminosity of these sources. Acting over an entire population, these partial obscurers can produce an obscured AGN fraction which decreases as a function of observed luminosity. The results are consistent with observations in the X-ray vs. IR luminosity of AGN classes. In Chapter 4 I select a statistically significant sample of AGN from an unbiased 250μm galaxy sample. After spectroscopic classification I find the optical Type- II AGN fraction to be consistent across several decades in [OIII] luminosity, a common proxy for intrinsic AGN luminosity. I also investigate the relation of AGN activity to host galaxy mass, as well as star formation activity and star formation history. Probing the environments of protoclusters will help to constrain the models of structure formation in the Universe. Until now, no dataset has been big enough to probe the environments of high redshift radio galaxies at a statistical level; While many believe that the feedback processes of high luminosity radio jets will have a direct impact on star formation in the surrounding medium it has not been tested. In Chapter 2 I investigate this on an statistical level, finding no meaningful correlation between radio galaxy radio power and source overdensity in the vicinities of these sources. In Chapter 5 I discuss the reduction of a 24μm sample at redshift z ∼ 1 for direct comparison with a local 12μm sample. With only a fraction of the target sample being observed, no statistically significant results could be derived, but the objects are spectroscopically classified and spectroscopic redshifts are measured where possible. Correlations in the data set are investigated and the limitations of the sample selection strategy are discussed.

L-shell x-ray production cross sections are presented for Fe, Ni, Cu, Zn, Ga, and Ge by 0.5- to 5.0-MeV protons and by 0.5- to 8.0-MeV helium ions and Ca, Fe, Ni, Cu, and Ge by 0.75- to 4.5-MeV lithium ions. These measurements are compared to the first Born theory and the perturbed-stationary- state theory with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR). The results are also compared to previous experimental investigations. The high precision x-ray measurements were performed with a windowless Si(Li) detector. The efficiency of the detector was determined by the use of thin target atomic-field bremsstrahlung produced by 66.5 keV electrons. The measured bremsstrahlung spectra were compared to theoretical bremsstrahlung distributions in order to obtain an efficiency versus energy curve. The targets for the measurement were manufactured by the vacuum evaporation of the target element onto thin foils of carbon. Impurities in the carbon caused interferences inthe L-shell x-ray peaks. Special cleansing procedures were developed that reduced the impurity concentrations in the carbon foil, making the use of less than 5 μg/cm^2 targets possible. The first Born theory is seen to greatly overpredict the data at low ion energies. The ECPSSR theory matches the data very well at the high energy region. At low energies, while fitting the data much more closely than the first Born theory, the ECPSSR theory does not accurately predict the trend of the data. This is probably due to the onset of molecular-orbital effects, a mechanism not accounted for in the ECPSSR theory.

by Choy Wing Hong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 95-102). / Text in English; abstract also in Chinese. / by Choy Wing Hong. / ABSTRACT --- p.vi / ACKNOWLEDGEMENTS --- p.x / LIST OF FIGURES --- p.xi / LIST OF TABLES --- p.xiii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Surface passivation techniques --- p.2 / Chapter 1.2.1 --- Sulfide solution passivation --- p.2 / Chapter 1.2.2 --- Gas-phase sulfide passivation --- p.3 / Chapter 1.2.3 --- Ultra-violet and ozone exposure --- p.4 / Chapter 1.3 --- Surface structure of sulfide-passivated surface --- p.5 / Chapter 1.4 --- Surface structure of ultra-violet/ozone oxidation --- p.8 / Chapter 1.5 --- Objectives of present study --- p.10 / Chapter Chapter 2 --- Instrumentation --- p.12 / Chapter 2.1 --- Introduction --- p.12 / Chapter 2.2 --- Atomic force microscopy (AFM) --- p.12 / Chapter 2.2.1 --- The development of AFM --- p.12 / Chapter 2.2.2 --- Basic principles of AFM --- p.12 / Chapter 2.2.3 --- Forces and their relevance to atomic force microscopy --- p.13 / Chapter 2.2.3.1 --- Van Der Waals forces --- p.15 / Chapter 2.2.3.2 --- Repulsive forces --- p.15 / Chapter 2.2.3.3 --- Capillary forces --- p.15 / Chapter 2.2.4 --- Displacement sensor of AFM --- p.15 / Chapter 2.2.4.1 --- Electron tunneling --- p.16 / Chapter 2.2.4.2 --- Optical interference --- p.16 / Chapter 2.2.4.3 --- Laser beam deflection --- p.16 / Chapter 2.2.5 --- Instrument specification --- p.17 / Chapter 2.2.5.1 --- Contact mode AFM --- p.17 / Chapter 2.3 --- X-ray photoelectron spectroscopy --- p.19 / Chapter 2.3.1 --- The development of XPS --- p.19 / Chapter 2.3.2 --- Basic principles of XPS --- p.19 / Chapter 2.3.3 --- XPS experiments --- p.23 / Chapter 2.3.4 --- Quantitative analysis --- p.26 / Chapter 2.3.4.1 --- Atomic concentration of a homogenous materials --- p.26 / Chapter 2.3.4.2 --- Layer structure --- p.27 / Chapter 2.4 --- Rutherford backscattering spectrometry (RBS) --- p.29 / Chapter 2.4.1 --- Basic principles --- p.29 / Chapter 2.4.2 --- Kinematics --- p.29 / Chapter 2.4.3 --- Channeling --- p.31 / Chapter Chapter 3 --- Surface treatments --- p.32 / Chapter 3.1 --- Semiconductor wafer --- p.32 / Chapter 3.2 --- Cleaning procedures --- p.32 / Chapter 3.3 --- Polysulfide passivation --- p.34 / Chapter 3.4 --- UV/Ozone oxidation --- p.39 / Chapter Chapter 4 --- Surface roughness and oxide contents of sulfide passivation --- p.41 / Chapter 4.1 --- Introduction --- p.41 / Chapter 4.2 --- Experimental methodology --- p.42 / Chapter 4.3 --- Etching --- p.44 / Chapter 4.3.1 --- Etching effect of polysulfide solution --- p.45 / Chapter 4.3.2 --- Possible consequences of the etching effect --- p.45 / Chapter 4.4 --- Oxide contents --- p.47 / Chapter 4.4.1 --- Oxide gained during polysulfide solution treatment --- p.47 / Chapter 4.4.2 --- Oxide gained after polysulfide passivation --- p.47 / Chapter 4.5 --- Surface roughness --- p.49 / Chapter 4.5.1 --- Surface roughness after different passivation methods --- p.49 / Chapter 4.5.2 --- The sticking probability after different passivations --- p.51 / Chapter 4.6 --- The spiral ladder of solution-phase passivation --- p.55 / Chapter 4.7 --- Conclusions --- p.58 / Chapter Chapter 5 --- Sulfide on Ge/GaAs heterojunction --- p.59 / Chapter 5.1 --- Introduction --- p.59 / Chapter 5.1.1 --- Band structure of Ge/GaAs heteroj unction --- p.59 / Chapter 5.1.2 --- Lattice match of Ge/GaAs heteroj unction --- p.60 / Chapter 5.1.3 --- The growth of Ge on GaAs using molecular beam epitaxy --- p.62 / Chapter 5.2 --- The growth of Ge on GaAs using thermal pulse annealing --- p.63 / Chapter 5.3 --- Sulfide as an atomic interdiffusion barrier --- p.65 / Chapter 5.3.1 --- Experimental methodology --- p.65 / Chapter 5.3.2 --- Crystallinity of Ge --- p.67 / Chapter 5.3.3 --- Results and discussions --- p.67 / Chapter 5.3.3.1 --- RBS and XPS results --- p.67 / Chapter 5.3.3.2 --- AFM and I-V results --- p.71 / Chapter 5.4 --- Conclusions --- p.71 / Chapter Chapter 6 --- UV/03 on Ge/GaAs heterojunction --- p.72 / Chapter 6.1 --- Introduction of UV/o3 oxidation --- p.72 / Chapter 6.2 --- UV/o3 oxidation on GaAs --- p.74 / Chapter 6.3 --- Ge on UV/o3 treated GaAs --- p.76 / Chapter 6.3.1 --- Experimental methodology --- p.76 / Chapter 6.3.2 --- Crystallinity of Ge --- p.77 / Chapter 6.3.3 --- AFM results --- p.77 / Chapter 6.3.4 --- RBS results --- p.80 / Chapter 6.4 --- Diodes --- p.82 / Chapter 6.4.1 --- Fabrication of diode --- p.82 / Chapter 6.4.2 --- Diode characteristics --- p.84 / Chapter 6.4.3 --- I-V characteristics --- p.90 / Chapter 6.5 --- Conclusions --- p.90 / Chapter Chapter 7 --- Conclusion and future work --- p.93 / Chapter 7.1 --- Conclusions --- p.93 / Chapter 7.2 --- Future works --- p.94 / Reference --- p.95

Orientador: Marian Rosaly Davolos / Banca: José Clayston Melo Pereira / Banca: Sergio Antonio Marques de Lima / Banca: Yara Galvão Gobato / Banca: Andrea Simone Stucchi de Camargo Alvarez Bernardez / Resumo: Neste trabalho, foram obtidos e caracterizados complexos luminescentes com íons Ln3+ (Ln = Eu, Gd, Tb, Dy) homo e/ou heterobilantanídicos com o ligante ácido tiofeno-2- carboxílico, que atua eficientemente como antena, absorvendo energia e transferindo-a para o íon emissor. A presença do anel tiofênico nos ligantes confere características interessantes, como por exemplo, maior polarizabilidade por conter em sua estrutura um átomo de enxofre. Os complexos foram caracterizados por análise elementar, análise de Ln3+ por análise titulométrica com uso de edta, análise térmica, espectroscopia vibracional na região do infravermelho e DRX, que permitiram propor as estequiometrias [Ln(α-tpc)3(α-Htpc)2]n e [Ln(α-tpc)3(α-Htpc)] e que a coordenação se dá pelo modo bidentado. Pelas espectroscopias eletrônica UV-vis, fotoluminescência e de excitação por raios X, foram observadas as transições características dos íons Ln3+ envolvidos neste trabalho. São propostos mecanismos de transferência de energia em compostos puros e nos complexos heterobilantanídicos, à partir da elaboração de diagramas de energia, baseados nos resultados de fotoluminescência. Os complexos também tiveram suas propriedades magnéticas investigadas, em alguns casos, pela utilização da técnica de EPR, PPMS e VSM, sendo observado um comportamento predominantemente paramagnético para os complexos. Os complexos foram expostos à radiação ionizante por um tempo prolongado, a fim de verificar o comportamento e estabilidade química desses complexos homo e heterolantanídicos sob radiação ionizante e os resultados mostraram que os complexos são estáveis frente à excitação com raios-X. / Abstract: Monometallic and/or heterobimetallic Ln3+ (Ln = Eu, Gd, Tb, Dy) luminescent complex with thiophen-2-carboxylic acid was investigated. The ligand employed in this work, acts efficiently as antenna, absorbing and transfer energy to the emitter ion. The thiophen ring provides interesting features to the system, like higher polarizability due the sulphur atom. The complex were characterized by elemental analysis, complexometric titrimetry with edta, in order to quantify Ln3+, thermal analysis, FTIR and XRD which allowed propose two different stoichiometries [Ln(α-tpc)3(α-Htpc)2]n and [Ln(α-tpc)3(α-Htpc)] and the coordination of the ligand to the Ln3+ occurs through the bidentate coordination mode. By the UV-vis, photoluminescence and X rays spectroscopies, were observed the Ln3+ transitions. Energy transfer mechanism were proposed in monometallic and heterobimetallic complexes, from the drafting of the energy diagram, based on results of fotoluminescence. The complex also have the magnetic properties investigated, by the EPR, PPMS and VSM techniques and the results shows a predominantly paramagnetic behavior to the complexes. The complexes were exposure to ionizing radiation during a extended time, in order to verify the chemical stability of the homo and hetero-lanthanide complex forward to ionizing radiation and is observed high stability over X-ray excitation. / Doutor

[Truncated abstract] Areal bone mineral density (BMD) is measured routinely in the clinic by a quantitative radiological technique, dual-energy X-ray absorptiometry (DXA). BMD is used widely to assess non-invasively but indirectly the mechanical fragility of bone and consequently is able to predict fracture risk. While BMD correlates well with in vitro measurements of bone strength it does not directly measure a mechanical property; half of incident minimally traumatic fractures in women occur with BMD values above the World Health Organisation defined threshold for osteoporosis. This arises partly because the mechanical strength of bone is dependent on its structural geometry and material strength as well as bone mineral mass. Essentially, bones fracture when load stresses exceed the mechanical capacity of the material to withstand them. The structural geometry (i.e., the amount of bone tissue and its complex three-dimensional arrangement within the macroscopic bone envelope) defines the stresses produced by a given load, while the intrinsic load capacity of the material is defined by the composition and microstructure of the bone tissue itself. Hip structural analysis (HSA) is a technique that elucidates the structural geometric component of bone strength; essentially combining information available from conventional DXA images of the proximal femur with a biomechanical beam model based on the stresses arising in a combination of pure bending and axial compression. A version of HSA has recently been released commercially, and has obtained US Food and Drug Administration approval for its clinical application. ... Given the acknowledged limitations of the HSA method when applied to 2-D projection images, a 3-D approach to structural geometry, using imaging modalities such as pQCT and QCT or a recently introduced version of DXA that mimics QCT, is indicated for the future. With that in mind and the possibility of the anthropometric phantom being adopted for future accuracy and precision assessments, improvements in the design of this phantom are recommended. Studies to better understand and verify Contents v the relevance of the 'local buckling' phenomenon as a structural geometric factor in the genesis of macro-fractures are also recommended. In summary, it is essential that superior (compared to BMD) non-invasively determined clinical predictors of bone fragility leading to fracture be investigated. Structural geometric variables are potential candidates. This has led to consideration of; (i) the need to progress beyond BMD for a more sensitive and specific bone strength measurement; (ii) theoretical advantages of structural geometry over BMD; (iii) limitations of the current HSA technique based on DXA, including those introduced by its restrictive assumptions; (iv) the value of HSA in longitudinal studies, exemplified by the 'normal' but rapid skeletal changes seen in human lactation, with possible implications for an analogous study of the menopause; and (v) an investigation, using a custom-designed anthropometric phantom, of the adaptation of HSA to certain emerging imaging modalities and methods able to resolve bone structural geometry in three dimensions.

The microscopic origin of ultrafast demagnetization, i.e. the quenching of the magnetization of a ferromagnetic metal on a sub-picosecond timescale after laser excitation, is still only incompletely understood, despite a large body of experimental and theoretical work performed since the discovery of the effect more than 15 years ago. Time- and element-resolved x-ray magnetic circular dichroism measurements can provide insight into the microscopic processes behind ultrafast demagnetization as well as its dependence on materials properties. Using the BESSY II Femtoslicing facility, a storage ring based source of 100 fs short soft x-ray pulses, ultrafast magnetization dynamics of ferromagnetic NiFe and GdTb alloys as well as a Au/Ni layered structure were investigated in laser pump – x-ray probe experiments. After laser excitation, the constituents of Ni50Fe50 and Ni80Fe20 exhibit distinctly different time constants of demagnetization, leading to decoupled dynamics, despite the strong exchange interaction that couples the Ni and Fe sublattices under equilibrium conditions. Furthermore, the time constants of demagnetization for Ni and Fe are different in Ni50Fe50 and Ni80Fe20, and also different from the values for the respective pure elements. These variations are explained by taking the magnetic moments of the Ni and Fe sublattices, which are changed from the pure element values due to alloying, as well as the strength of the intersublattice exchange interaction into account. GdTb exhibits demagnetization in two steps, typical for rare earths. The time constant of the second, slower magnetization decay was previously linked to the strength of spin-lattice coupling in pure Gd and Tb, with the stronger, direct spin-lattice coupling in Tb leading to a faster demagnetization. In GdTb, the demagnetization of Gd follows Tb on all timescales. This is due to the opening of an additional channel for the dissipation of spin angular momentum to the lattice, since Gd magnetic moments in the alloy are coupled via indirect exchange interaction to neighboring Tb magnetic moments, which are in turn strongly coupled to the lattice. Time-resolved measurements of the ultrafast demagnetization of a Ni layer buried under a Au cap layer, thick enough to absorb nearly all of the incident pump laser light, showed a somewhat slower but still sub-picosecond demagnetization of the buried Ni layer in Au/Ni compared to a Ni reference sample. Supported by simulations, I conclude that demagnetization can thus be induced by transport of hot electrons excited in the Au layer into the Ni layer, without the need for direct interaction between photons and spins. / Der mikroskopische Ursprung der ultraschnellen Entmagnetisierung, d.h. des Rückgangs der Magnetisierung eines ferromagnetischen Metalls innerhalb einer Pikosekunde nach Laseranregung, ist bisher nur unvollständig verstanden, trotz umfangreicher experimenteller und theoretischer Arbeiten, die seit der Entdeckung des Effekts vor mehr als 15 Jahren durchgeführt wurden. Zeit- und elementaufgelöster Röntgenzirkulardichroismus kann Einblick in die mikroskopischen Prozesse hinter der ultraschnellen Entmagnetisierung sowie deren Materialabhängigkeit gewähren. Am BESSY II Femtoslicing, einer speicherringbasierten Quelle für 100 fs kurze Röntgenpulse, wurde ultraschnelle Magnetisierungsdynamik von ferromagnetischen NiFe- und GdTb-Legierungen sowie einer Au/Ni-Schichtstruktur in Anregungs-Abfrage-Experimenten untersucht. Nach Laseranregung zeigen die Konstituenten von Ni50Fe50 und Ni80Fe20 deutlich unterscheidbares Verhalten und damit entkoppelte Dynamik, trotz starker Austauschkopplung der Ni- und Fe-Untergitter im Gleichgewichtszustand. Weiterhin variieren die Werte der Zeitkonstanten der Entmagnetisierung von Ni und Fe für Ni50Fe50 und Ni80Fe20, und auch für die jeweiligen reinen Elemente. Diese Unterschiede werden durch die magnetischen Momente der Untergitter erklärt, die sich in den Legierungen gegenüber den reinen Elementen ändern, sowie durch die Stärke der Austauschkopplung zwischen den Untergittern. GdTb zeigt Entmagnetisierung in zwei Stufen, was typisch für Seltene Erden ist. Die Zeitkonstante der langsameren zweiten Stufe wurde kürzlich mit der Stärke der Spin-Gitter-Kopplung in reinem Gd und Tb in Verbindung gebracht, wobei die stärkere, direkte Spin-Gitter-Kopplung in Tb zu schnellerer Entmagnetisierung führt. In GdTb folgt die Entmagnetisierung von Gd auf allen Zeitskalen der von Tb. Dies beruht auf einer verstärkten Kopplung der magnetischen Momente von Gd an das Gitter, über die indirekte Austauschkopplung an die Tb-Momente. Dadurch kann Spindrehimpuls schneller an das Gitter abfließen. Zeitaufgelöste Messungen der Entmagnetisierung einer Ni-Schicht unter einer Au-Deckschicht, deren Dicke ausreichend ist um den anregenden Laserpuls praktisch vollständig zu absorbieren, zeigen eine leicht verzögerte aber trotzdem ultraschnelle Entmagnetisierung im Vergleich mit einer Ni-Referenzprobe. Unterstützt durch Simulationen zeigt sich, dass Entmagnetisierung durch den Transport heißer Elektronen von der Au-Deckschicht in die Ni-Schicht ausgelöst wird, ohne dass direkte Wechselwirkung zwischen Photonen und Spins notwendig ist.

This work presents a study of the electronic structure of four transition metal oxides (TMOs) using spectroscopic data and a variety of theoretical models. TMOs are a class of materials made from d-block metals in the periodic table, and one or more oxygen atoms. The nature of d-electrons is examined and theoretical models used to treat d-electron systems are tested against experimental data. Background theory of condensed matter physics is outlined. An overview of density functional theory (DFT) as a theoretical model for calculating the electronic structure of materials is presented. A variety of exchange-correlation (XC) functionals used within the DFT framework are outlined and tested for their applicability to the TMO systems in question. X-ray spectroscopy is briefly outlined and used to test the validity of the different XC functionals. All four compounds, AgO, Ag2O, CuO, and Cu2O require a Hubbard U term in the XC functional to most accurately reproduce experimental results. The effects of varying the value of U is examined in depth. The oxygen K-edge X-ray emission spectra (XES) exhibits a“two peak” structure for all compounds; the effect of varying the U value is to change the intensity ratio of the two peaks. The ratio of the two peaks as a function of U shows a linear trend in all compounds. A simple line is fit to the peak ratio vs. U curve. A common line between all compounds would provide an important metric with which to predict the appropriate U value needed in similar materials based on simple experimental data. However, the parameters of the fitted line were not common between the four compounds and any metric derived from this method would be system-dependent and not widely applicable to other systems. There are, however, interesting trends in the data when the U value is varied that provide subjects for future research. A number of fundamental quantities are determined both from experiment and theoretical calculations. Calculated bandgap values are shown to be lower than the experimental values for most functionals tested. This is not unexpected as DFT methods are known to predict much smaller bandgaps than expected. The Heyd-Scuseria-Ernzerhof (HSE) functional used for Ag2O and Cu2O does predict the bandgaps very accurately. The core-hole effect is estimated and proven to be negligible in these systems. Charge transfer and on-site Coulomb repulsion energies, important quantities in the electronic behaviour of TMOs, are determined and compared to previously reported values.