The continuous phase transition in thin magnetic films and superlattices has been studied using the magneto-optical Kerr effect (MOKE) and polarized neutron scattering (PNR).  It has been shown that the critical behavior of amorphous thin films belonging to the 2D XY universality class can be described within the same theory as crystalline sample. This means that quenched disorder only serves as a marginal perturbation in systems with this symmetry. The connection between interlayer exchange coupling and the observed critical behavior in Fe/V superlattices was explored. The results prove that the origin of unusually high values of the exponent β can be traced to a position dependence of the magnetization at elevated temperatures. The magnetization of the outermost layers within the superlattice shows a more pronounced decrease at lower temperatures, compared to the inner layers, which in turn have a more abrupt decrease in the vicinity of the critical temperature. This translates to a high exponent, especially when the layers are probed by a technique where more weight is given to the layers close to the surface, e.g.MOKE.  The interlayer exchange coupling as a function of spacer thickness and temperature was also studied in its own right. The data was compared to the literature, and a dependence on the thickness of the magnetic layers was concluded. The phase transition in amorphous FeZr/CoZr multilayers, where the magnetization emanates from ferromagnetic proximity effects, was investigated. Even though the determined exponents of the zero-field magnetization, the susceptibility and the critical isotherm did not correspond to any universality class, scaling plots displayed an excellent data collapse. Samples consisting of Fe δ-layers (0.3-1.4 monolayers) embedded in Pd were studied using element-specific resonant x-ray magnetic scattering. The magnetization of the two constituents showed distinctly different temperature dependences.

In many materials, changes in chemical composition, pressure or temperature can induce metal to insulator transitions. It is recently observed in yttrium hydride, for example, changes from a shiny mirror (YH2) to a transparent window (YH3), which has important technological application in optical devices. We have tuned the above transition by choosing pressure instead of composition. Our predicted finding is confirmed by recent experiments and opens a new way to design optical switches. The unique role that gold plays in society is to a large extent related to the fact that it is the most noble of all metals.We have studied the noble nature of gold by choosing pressure as tool. Our prediction shows that gold transforms from a face centered cubic to an hexagonal closed packed phase above 200 GPa whereas platinum, another noble metal, does not show any phase transition up to 500 GPa. This prediction has also been confirmed by experiments suggesting that platinum is more noble than gold. The growing concern about climate change and fossil fuel availability, the direct conversion of solar irradiation into electricity appears to be an ideal alternative to conventional energy sources. Power generation by solar cells is a direct method of solar energy conversion. We report a new cubic phase of TiO2 which can be stabilized at ambient conditions. This phase has an absorption three or four orders of magnitude larger than the conventional state-of-the-art solar cell based on anatase TiO2. Therefore, we are introducing a well established material with a new structure for future generation solar cells. The same effect is also observed in cubic SnO2. Electronic and optical properties of other materials such as BexZn1-xTe, RuO2 and IrO2 are also studied in present thesis. In particular, for BexZn1-xTe, we have used composition as a tool to tune the optical properties.

Complex networks are employed as a mathematical description of complex systems in many different fields, ranging from biology to sociology, economy and ecology. Dynamical processes in these systems often display phase transitions, where the dynamics of the system changes qualitatively. In combination with these phase transitions certain components of the system might irretrievably go extinct. In this case, we talk about absorbing transitions. Developing mathematical tools, which allow for an analysis and prediction of the observed phase transitions is crucial for the investigation of complex networks. In this thesis, we investigate absorbing transitions in dynamical networks, where a certain amount of diversity is lost. In some real-world examples, e.g. in the evolution of human societies or of ecological systems, it is desirable to maintain a high degree of diversity, whereas in others, e.g. in epidemic spreading, the diversity of diseases is worthwhile to confine. An understanding of the underlying mechanisms for emergence and persistence of diversity in complex systems is therefore essential. Within the scope of two different network models, we develop an analytical approach, which can be used to estimate the prerequisites for diversity. In the first part, we study a model for opinion formation in human societies. In this model, regimes of low diversity and regimes of high diversity are separated by a fragmentation transition, where the network breaks into disconnected components, corresponding to different opinions. We propose an approach for the estimation of the fragmentation point. The approach is based on a linear stability analysis of the fragmented state close to the phase transition and yields much more accurate results compared to conventional methods. In the second part, we study a model for the formation of complex food webs. We calculate and analyze coexistence conditions for several types of species in ecological communities. To this aim, we employ an approach which involves an iterative stability analysis of the equilibrium with respect to the arrival of a new species. The proposed formalism allows for a direct calculation of coexistence ranges and thus facilitates a systematic analysis of persistence conditions for food webs. In summary, we present a general mathematical framework for the calculation of absorbing phase transitions in complex networks, which is based on concepts from percolation theory. While the specific implementation of the formalism differs from model to model, the basic principle remains applicable to a wide range of different models.

The dissociation of hydrogen molecules at surfaces is the first step in the absorption process. If the absorbing material is covered by an oxide, this layer will determine the effective uptake rate of an underlying absorbing material. This effect is illustrated when determining the rate of transport of hydrogen through amorphous aluminium oxide layers. The transport rate was determined to be strongly thickness dependent. Hydrogen absorbed in a transition metal causes a volume expansion generated by a strain field around the absorbed hydrogen. This strain field causes a self-trapping of the hydrogen and a temperature dependent distribution in the atomic distances. The local strain field generated by the self-trapping process is found to be crucial for understanding both the hydrogen induced volume expansion as well as the diffusion of hydrogen. Ab-initio molecular dynamics simulations were used to reveal the temperature dependence of the unbinding of the hydrogen and the local strain field and its influence on the diffusion rate. The symmetry of the local strain field is also important for phase formation in metallic films and superlattices which are clamped to a substrate. As the thicknesses reduced from 50 to 10 nm thick vanadium films, substantial finite size effects become apparent in the phase diagrams. The volume change associated with the strain field cannot be accurately measured using x-ray diffraction because of its sensitivity to local arrangements of atoms. X-ray and neutron reflectivity were found to be more reliable probes of global effects of the sumof the local strainfields. Finite size effects in extremely thin V layers were also explored in metallic superlattices composed of iron and vanadium. The co-existence region, composed of a hydrogen gas and a solid-like phase, was found to be suppressed by at least 100 K to below 300 K. The hydrogen-hydrogen interaction can also be influenced by the electronic states in the non hydrogen absorbing layers, as demonstrated when comparing hydrogen absorption in Fe/V and Cr/V superlattices. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 728

No description available.

No description available.

In processing and data storage mainly ferromagnetic (FM) materials are being used. Approaching physical limits, new concepts have to be found for faster, smaller switches, for higher data densities and more energy efficiency. Some of the discussed new concepts involve the material classes of correlated oxides and materials with antiferromagnetic coupling. Their applicability depends critically on their switching behavior, i.e., how fast and how energy efficient material properties can be manipulated. This thesis presents investigations of ultrafast non-equilibrium phase transitions on such new materials. In transition metal oxides (TMOs) the coupling of different degrees of freedom and resulting low energy excitation spectrum often result in spectacular changes of macroscopic properties (colossal magneto resistance, superconductivity, metal-to-insulator transitions) often accompanied by nanoscale order of spins, charges, orbital occupation and by lattice distortions, which make these material attractive. Magnetite served as a prototype for functional TMOs showing a metal-to-insulator-transition (MIT) at T = 123 K. By probing the charge and orbital order as well as the structure after an optical excitation we found that the electronic order and the structural distortion, characteristics of the insulating phase in thermal equilibrium, are destroyed within the experimental resolution of 300 fs. The MIT itself occurs on a 1.5 ps timescale. It shows that MITs in functional materials are several thousand times faster than switching processes in semiconductors. Recently ferrimagnetic and antiferromagnetic (AFM) materials have become interesting. It was shown in ferrimagnetic GdFeCo, that the transfer of angular momentum between two opposed FM subsystems with different time constants leads to a switching of the magnetization after laser pulse excitation. In addition it was theoretically predicted that demagnetization dynamics in AFM should occur faster than in FM materials as no net angular momentum has to be transferred out of the spin system. We investigated two different AFM materials in order to learn more about their ultrafast dynamics. In Ho, a metallic AFM below T ≈ 130 K, we found that the AFM Ho can not only be faster but also ten times more energy efficiently destroyed as order in FM comparable metals. In EuTe, an AFM semiconductor below T ≈ 10 K, we compared the loss of magnetization and laser-induced structural distortion in one and the same experiment. Our experiment shows that they are effectively disentangled. An exception is an ultrafast release of lattice dynamics, which we assign to the release of magnetostriction. The results presented here were obtained with time-resolved resonant soft x-ray diffraction at the Femtoslicing source of the Helmholtz-Zentrum Berlin and at the free-electron laser in Stanford (LCLS). In addition the development and setup of a new UHV-diffractometer for these experiments will be reported. / In der Datenspeichertechnologie werden bisher hauptsächlich ferromagnetische Materialien eingesetzt. Da mit diesen aber physikalische Grenzen erreicht werden, werden neue Konzepte gesucht, um schnellere und kleinere Schalter, größere Datendichten und eine höherere Energieeffizienz zu erzeugen. Unter den diskutierten Materialklassen finden sich komplexen Übergangsmetalloxide und Materialien mit antiferromagnetischer Kopplung. Die Anwendbarkeit solcher Materialien hängt stark davon ab, wie schnell sich deren Eigenschaften verändern lassen und wieviel Energie dafür eingesetzt werden muss. Die vorliegende Arbeit beschäftigt sich mit ultraschnellen, Nicht-Gleichgewicht-Phasenübergängen genau in solchen Materialien. In Übergangsmetalloxiden führt die enge Kopplung zwischen den unterschiedlichen Freiheitsgraden zu einem effektiven niederenergetischen Anregungsspektrum. Diese Anregungen sind oft verknüpft mit spektakulären makroskopischen Eigenschaften, wie z.B. dem kolossalen Magnetowiderstand, Hochtemperatur-Supraleitung, Metall- Isolator-Übergang, die oft von nanoskaliger Ordnung von Spins, Ladungen, orbitaler Besetzung sowie Gitterverzerrungen begleitet sind. Dadurch werden diese Materialien interessant für Anwendbarkeit. Magnetit, ein Prototyp eines solchen funktionalen Materials zeigt einen Metall-Isolator-Übergang bei T = 123 K. Untersucht man die Ladungs- und orbitale Ordnung sowie die Struktur nach einer optischen Anregung, so findet man, dass die elektronische Struktur und Gitterverzerrung, die kennzeichnend für die Tieftemperaturphase sind, innerhalb der Zeitauflösung des Experiments von 300 fs zerstört wird. Der eigentliche Metall-Isolator-Übergang zeigt sich erst nach 1.5 ps. Die Ergebnisse zeigen, dass MITs in funktionalen Materialien bis zu tausend Mal schneller geschaltet werden können als in vorhandenen Halbleiter-Schaltern. Seit kurzem rücken auch ferrimagnetische und antiferromagnetische Materialen in den Fokus des Interesses. Es wurde im Ferrimagnet GdFeCo gezeigt, dass der Transfer von Drehimpuls zwischen zwei entgegengesetzten Subsystemen mit unterschiedlichen Zeitkonstanten zu einem Umschalten der Magnetisierung führt. Zudem wurde vorhergesagt, dass Demagnetisierungsdynamiken in antiferromagnetischen Materialien schneller ablaufen soll als in ferromagnetischen, da kein Drehimpuls aus dem Spinsystem abgeführt werden muss. Damit wir mehr über antiferromagnetische Dynamik erfahren haben wir zwei unterschiedliche Antiferromagneten untersucht, um sie mit den bekannten FM zu vergleichen. Im metallischen AFM Holmium fanden wir, dass die magnetische Ordnung schneller und zehnmal energieeffizienter zerstört werden kann als in vergleichbaren FM Metallen. In Europium-Tellurid, einem antiferromagnetischem Halbleiter, haben wir den Zerfall der magnetischen Ordnung im Hinblick auf Wechselwirkungen mit der Struktur untersucht. Wir fanden auf kurzen Zeitskalen eine eher entkoppelte Dynamik. Eine Ausnahme ist ein schneller Beitrag zur Gitterdynamik, den wir mit dem Wegfall von Magnetostriktion erklären. Die hier gezeigten Ergebnisse wurden mit Hilfe zeitaufgelöster resonanter weicher Röntgenbeugung an der Femtoslicing Strahlungsquelle des Helmholtz-Zentrums Berlin und am freien Elektronenlaser LCLS gemessen. Zusätzlich wird über die Entwicklung und den Bau eines UHV-Diffraktometers für diese Experimente berichtet.

The dramatic change in properties of water near its critical point (i.e. T = 374 °C and p = 22.1 MPa, note: 100 MPa = 0.1 GPa = 1 kbar ≈ 1000 atm) has been a subject of numerous studies and also lead to the development of various applications (e.g. in waste destruction, biomass processing, and the synthesis of advanced ceramic materials). However, comparatively little is known about the behavior of water at gigapascal pressures. The present study attempts to explore catalytical properties and reactivity of extreme water with respect to several oxide systems: SiO2, TiO2 and LiAlSiO4. “Extreme water” here is defined as existing at p,T conditions of 0.25–10 GPa and 200–1000 °C, thus considering both supercritical fluid and hot compressed ice. The study shows that extreme water can make high pressure mineral phases accessible at relatively mild T conditions. At the same time, high pressure aqueous environments appear efficient in stabilizing novel metastable structures and may be considered as a general route for synthesizing new materials. The hydrothermal treatment of SiO2 glass at 10 GPa and 300–550 °C yielded an unusual ultrahydrous form of stishovite with up to 3% of structural water. At the same time, the extreme water environment enhanced notably the kinetics of stishovite formation, making it accessible at unprecedentedly low temperatures. Thus, for the SiO2–H2O system water acts as both catalyst and reactant. For TiO2 a hydrothermal high pressure treatment proved to be of high importance for overcoming the kinetical hindrance of the rutile – TiO2-II transformation. 6 GPa and 650 °C were established as the mildest conditions for synthesizing pure TiO2-II phase in less than two hours. The crystallization of LiAlSiO4 glass in an extreme water environment yielded a number of different phases. In the low pressure region (0.25 – 2 GPa) mainly a zeolite (Li-ABW) and a dense anhydrous aluminosilicate (α-eucryptite) were obtained. At pressures above 5 GPa the formation of novel pyroxene-like structures with crystallographic amounts of structural water was observed. The overall conclusion of this study is that extreme water environments show a great potential for catalyzing phase transitions in oxide systems and for stabilizing novel structures via structural water incorporation. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

Cette thèse expérimentale explore les propriétés du point critique de la phase pseudogap dans le diagramme de phase des cuprates supraconducteurs. Dans une première partie, j’expose un état de l’art sur les connaissances du diagramme de phases température-dopage (T-p) de ces systèmes. Des études récentes montrent une chute importante de la densité de porteurs électroniques au voisinage du point critique suggérant une reconstruction de la surface Fermi. Pour comprendre la nature exacte de la transition de phases liée à cette reconstruction, j’ai réalisé des mesures complémentaires de transport thermique et de chaleur spécifique sous champ magnétique intense sur les familles La1.8-xSrxEu0.2CuO4 et La1.6-xSrxNd0.4CuO4.Dans une deuxième partie, après une introduction théorique sur la chaleur spécifique et le transport thermique, je détaille comment ces deux grandeurs ont été mesurées. En particulier, une technique originale de mesures de la chaleur spécifique a été mise au point pour combiner haute résolution et précision absolue en champ magnétique intense et basse température. Différents modèles thermiques et électroniques ont été développés pour comprendre et analyser les mesures et ont permis d’optimiser les différents montages de chaleur spécifique selon les gammes de température.Dans une troisième partie, je présente l’ensemble des résultats obtenus en transport thermique et chaleur spécifique. Le transport thermique confirme la chute de la densité de porteur dans l’état normal (sans supraconductivité) des cuprates déjà observée en transport électrique sous champ intense. Par ailleurs, j ‘ai montré que cette chute existe également au sein de la phase supraconductrice (à champ magnétique nul), montrant qu’elle n’est influencée ni par la présence de la supraconductivité ni par le champ magnétique. Dans l’état normal, la loi de Wiedemann-Franz est respectée prouvant le caractère métallique de la phase pseudogap.La chaleur spécifique électronique montre un comportement non classique à proximité du point critique. Ce comportement anormal est caractérisé par une dépendance logarithmique en fonction de la température au dopage critique p* correspondant à la chute du nombre de porteurs. De plus, ces mesures suggèrent une divergence de la masse effective à p* en fonction du dopage. Ces deux observations sont la signature d’un point critique quantique localisé à T = 0 et p = p* dont l’origine est discutée dans la dernière partie. Les différentes classes d’universalités possibles sont discutées et une comparaison avec d’autres composés (fermions lourds, pnictures) possédant un point critique quantique est présentée. / This experimental PhD thesis explores the properties of the pseudogap critical point in the phase diagram of superconducting cuprates. In a first part, I present a state of the art on the knowledge of the temperature-doping (T-p) phase diagram of these systems. Recent studies show a dramatic drop in the electronic carrier density near the critical point, suggesting a Fermi surface reconstruction. To understand the exact nature of the phase transition related to this reconstruction, I performed complementary high magnetic field measurements of thermal transport and specific heat on La1.8-xSrxEu0.2CuO4 and La1.6-xSrxNd0.4CuO4 cuprates.In a second part, after a theoretical introduction on specific heat and thermal transport, I detail how these two quantities were measured. In particular, an original technique for measuring specific heat has been developed to combine high resolution and absolute accuracy in high magnetic field and low temperature. Different thermal and electronic models have been developed to understand and analyze the measurements in order to optimize the different set-ups according to the temperature range.In a third part, I present the results obtained in thermal transport and specific heat. Thermal transport confirms the drop in carrier density in the normal state (without superconductivity) of cuprates, already observed in high magnetic field electrical transport. Moreover, this drop also exists within the superconducting phase (in zero magnetic field), showing that it is neither influenced by the presence of superconductivity nor by the magnetic field. In the normal state, the Wiedemann-Franz low is satisfied, proving the metallic character of the pseudogap phase.Electronic specific heat shows non-classical behavior in the vicinity of the critical point. This abnormal behavior is characterized by a logarithmic dependence as a function of temperature at the critical doping p *, corresponding to the drop in the carrier density. Moreover, these measurements suggest a divergence of the effective mass at p * as a function of doping. These two observations are the signature of a quantum critical point located at T = 0 and p = p *, whose origin is discussed in the last part. I discuss the possible universality classes, and I compare with others compounds (heavy fermions, pnictides) which present a quantum critical point.

CASTRO JUNIOR, Manoel Carvalho. Transições de fase em perovskitas politípicas. 2013. 153 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2013. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-10-29T18:31:41Z No. of bitstreams: 1 2013_tese_mccastrojunior.pdf: 7600147 bytes, checksum: e5ee8745b29672a07d4666c3a0c48753 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-11-03T19:46:16Z (GMT) No. of bitstreams: 1 2013_tese_mccastrojunior.pdf: 7600147 bytes, checksum: e5ee8745b29672a07d4666c3a0c48753 (MD5) / Made available in DSpace on 2015-11-03T19:46:16Z (GMT). No. of bitstreams: 1 2013_tese_mccastrojunior.pdf: 7600147 bytes, checksum: e5ee8745b29672a07d4666c3a0c48753 (MD5) Previous issue date: 2013 / In this thesis, vibrational spectroscopy was used to investigate the structural transitions of polytypic perovskites. Representative compounds of two polytypes were studied: the multiferroic 3C-type perovskite Pb(Fe1/2Nb1/2)O3 (PFN) and the 6H-type perovskites Ba3B2+Sb2O9 (B = Mg, Ca e Sr). Based on anomalies or deviations from normal behavior of the adjustable parameters of the temperature dependence of the phonon spectra of these compounds, it was possible to identify structural transformations. In PFN, it was observed, evidences of two transitions between ferroelectric phases, in addition to a transition between different magnetic ordering states (paramagnetic → antiferromagnetic) and different polar states (paraelectric → ferroelectric). These results provide stronger evidences than previous Raman Spectroscopy reports in literature due to a better resolution, which allowed us to observe, for example, a new phonon at the Néel temperature. In the case of the 6H perovskites, besides to the usual observation of temperature dependence of the phonon spectra, which provides evidences of the transition of a hexagonal to monoclinic phase at 140 K in the Ba3MgSb2O9 (BMS) and of a monoclinic to triclinic phase at 240 K in the Ba3MgSb2O9 (BCS), we classify the phonons of the three compounds using two methods. First we applied the Hartree-Fock ab-initio method to calculate the normal modes of the Sb2O9 dimer; and assuming that the unit cell of BMS is constituted of Sb2O9 dimers and isolated ions of Ba and Mg, we correlate, using group theory methods, the calculated modes of Sb2O9 with the internal modes of BMS. Later we used the FG Wilson method to confirm the previous results, being obtained a good agreement between the two methods. / Nesta tese a espectroscopia vibracional foi empregada para investigar as transições estruturais de perovskitas politípicas. Compostos representativos de dois polítipos foram estudados: a perovskita multiferróica tipo 3C Pb(Fe1/2Nb1/2)O3 (PFN) e as perovskitas tipo 6H Ba3B2+Sb2O9 (B = Mg, Ca e Sr). A partir de anomalias ou desvios do comportamento normal dos parâmetros ajustáveis com a temperatura do espectro de fônons destes compostos foram identificadas transformações estruturais. No PFN foram observados indícios de duas transições entre fases ferroelétricas, além de uma transição entre diferentes fases de ordenamento magnético (estado paramagnético → estado antiferromagnético) e uma transição entre um estado paraelétrico e um ferroelétrico. Estes resultados são expostos de forma bem mais clara do que em trabalhos de espectroscopia Raman anteriormente informados na literatura, devido a melhor resolução obtida, o que propiciou até a observação da origem de um novo modo no entorno da temperatura de Néel. Nas perovskitas 6H, além da observação usual do comportamento dos fônons em função da temperatura, o que rendeu a observação da transição de uma fase hexagonal para uma fase monoclínica em 140K no Ba3MgSb2O9 (BMS) e de uma fase monoclínica para uma fase triclínica em 240 K no Ba3MgSb2O9 (BCS), procurou-se classificar os fônons dos três compostos utilizando dois métodos computacionais. Primeiramente utilizou- se o método ab initio de Hartree-Fock para calcular os modos normais do dímero Sb2O9; e assumindo que a célula unitária do BMS é formada de dímeros Sb2O9 e íons isolados de Ba e Mg, conseguiu-se correlacionar, utilizando teoria de grupos, os modos calculados do Sb2O9 com os modos internos do BMS. Posteriormente utilizou-se o método FG de Wilson para corroborar os resultados anteriormente obtidos, sendo observado um bom acordo entre ambos métodos.