<p>Knowledge about the stability of phases and their relationships in the Fe-O system at high pressures and temperatures is essential in implications of the multifarious states of iron oxides for models of the evolution of the Earth. In this respect, the iron ferrite magnetite (FeFe₂O₄) plays a significant role since it participates in the control of geochemistry of ferric iron, and hence oxygen fugacity in the Earth`s deep interior.</p><p>High-pressure experiments on Fe₃O₄ were performed using the diamond anvil cell technique combined with the laser and electrical resistive heating. The approach based on the combination of the synchrotron x-ray diffraction with Raman spectroscopic measurements benefited from the complementarity of the two methods originating from the different sensitivity to a range of structural order. High-pressure transformation of magnetite to a dense polymorph of the CaTi₂O₄-type structure proceeds via an intermediate step of the decomposition into a mixture of oxides on a microscopic scale. The kinetic hindrance of the reaction of the decomposition effectively prevents a phase separation controlled by diffusion and restricts the formation of the daughter products to locally ordered structures in the strained lattice of magnetite.</p><p>Thermodynamic analysis of the observed phase transformations along with published results on the elastic properties and pressure-induced transitions of iron oxides has led to the reassessment of the phase diagram of Fe₃O₄. The pressure - temperature field of its stability with respect to the breakdown to a mixture of oxides FeO and Fe₂O₃, and to the transition to a high-pressure form, has been accordingly modified.</p>

<p>The role of the dimensionality and strain state of metallic lattices on the phase behavior of dissolved hydrogen was explored. Metallic superlattices with well defined hydrogen absorption potential on the nm scale, were utilized as test systems. The solubility isotherms of hydrogen in Fe/V(001), MoV/V(001), and Nb/W(110) superlattices were measured by a resistometric method, and the hydrogen-induced changes of the structures were measured by <i>in-situ</i> X-ray diffraction. In the V based superlattices, the long-ranged ordered bulk V hydride phase β-V₂H is absent, which is attributed to the finite-size of V lattice. The intrinsic strain-state of the hydrogen dissolving layers was found to have a strong effect on the interaction between metal and hydrogen as well as on the hydrogen-hydrogen (H-H) interaction. For low hydrogen content in the V layers, the compressive strain resulted in a strong enhancement of the H-H interaction, while a tensile strain appeared to diminish the H-H interaction. This is due to different site occupancy of hydrogen for different strain states, which depending on the relation between the symmetries of hydrogen induced global and local strain fields, gives rise to different elastic H-H interaction. Moderately strained V layers exhibited a strong attractive H-H interaction over a broad concentration range. In the concentration ranges where attractive H-H interaction was established, the hydrogen atoms appeared to be strongly correlated on a microscopic length scale. In the Nb based superlattices, the critical temperature for the α–α’ transition was found to be suppressed as a result of the clamping of the film plane by the film-substrate coupling. An exception from this could be noticed when the intrinsic compressive strain were reduced.</p>

Knowledge about the stability of phases and their relationships in the Fe-O system at high pressures and temperatures is essential in implications of the multifarious states of iron oxides for models of the evolution of the Earth. In this respect, the iron ferrite magnetite (FeFe2O4) plays a significant role since it participates in the control of geochemistry of ferric iron, and hence oxygen fugacity in the Earth`s deep interior. High-pressure experiments on Fe3O4 were performed using the diamond anvil cell technique combined with the laser and electrical resistive heating. The approach based on the combination of the synchrotron x-ray diffraction with Raman spectroscopic measurements benefited from the complementarity of the two methods originating from the different sensitivity to a range of structural order. High-pressure transformation of magnetite to a dense polymorph of the CaTi2O4-type structure proceeds via an intermediate step of the decomposition into a mixture of oxides on a microscopic scale. The kinetic hindrance of the reaction of the decomposition effectively prevents a phase separation controlled by diffusion and restricts the formation of the daughter products to locally ordered structures in the strained lattice of magnetite. Thermodynamic analysis of the observed phase transformations along with published results on the elastic properties and pressure-induced transitions of iron oxides has led to the reassessment of the phase diagram of Fe3O4. The pressure - temperature field of its stability with respect to the breakdown to a mixture of oxides FeO and Fe2O3, and to the transition to a high-pressure form, has been accordingly modified.

The role of the dimensionality and strain state of metallic lattices on the phase behavior of dissolved hydrogen was explored. Metallic superlattices with well defined hydrogen absorption potential on the nm scale, were utilized as test systems. The solubility isotherms of hydrogen in Fe/V(001), MoV/V(001), and Nb/W(110) superlattices were measured by a resistometric method, and the hydrogen-induced changes of the structures were measured by in-situ X-ray diffraction. In the V based superlattices, the long-ranged ordered bulk V hydride phase β-V2H is absent, which is attributed to the finite-size of V lattice. The intrinsic strain-state of the hydrogen dissolving layers was found to have a strong effect on the interaction between metal and hydrogen as well as on the hydrogen-hydrogen (H-H) interaction. For low hydrogen content in the V layers, the compressive strain resulted in a strong enhancement of the H-H interaction, while a tensile strain appeared to diminish the H-H interaction. This is due to different site occupancy of hydrogen for different strain states, which depending on the relation between the symmetries of hydrogen induced global and local strain fields, gives rise to different elastic H-H interaction. Moderately strained V layers exhibited a strong attractive H-H interaction over a broad concentration range. In the concentration ranges where attractive H-H interaction was established, the hydrogen atoms appeared to be strongly correlated on a microscopic length scale. In the Nb based superlattices, the critical temperature for the α–α’ transition was found to be suppressed as a result of the clamping of the film plane by the film-substrate coupling. An exception from this could be noticed when the intrinsic compressive strain were reduced.

In this thesis, material properties have been examined under extreme conditions in computer-based calculations. The research on iron (Fe), nickel (Ni), and ferropericlase (Mg1-xFexO) are not only important for our understanding of the Earth, but also for an improved knowledge of these materials per se. An embedded-atom model for Fe demonstrated to reproduce properties such as structure factors, densities and diffusion constants, and was employed to evaluate temperature gradients at Earth core conditions. A similar interaction together with a two-temperature method was applied for the analysis of shock-induced melting of Ni. For Mg1-xFexO, the magnetic transition pressure was shown to increase with iron content. Furthermore, the C44 softening with pressure and iron composition supports the experimentally observed phase transition for Mg0.8Fe0.2O at 35 GPa. The properties of high density helium (He) is of great interest as the gas is one of the most abundant elements in the solar system. Furthermore, He and neon (Ne) are often used as pressure media in diamond anvil cells. The melting of He showed a possible fcc-bcc-liquid transition starting at T=340 K, P=22 GPa with a Buckingham potential, whereas the bcc phase was not seen with the Aziz form. For Ne, Monte Carlo calculations at ambient pressure showed very accurate results when extrapolating the melting temperatures to an infinite cluster limit. At high pressure, a one-phase ab initio melting curve showed a match with one-phase L-J potential results, which could imply a correspondence between ab initio/classical one-phase/two-phase calculations. In the search for hard materials, ab initio calculations for four TiO2 phases were compared. Just as imposed by experiment, the cotunnite phase was found to be very hard. The anomalous elastic behavior of the superconducting group-V metals V, Nb, Ta was found to be related to shrinking nesting vectors and the electronic topological transition (ETT).

I have been one of the main responsible for building a new laboratory for Bose-Einstein condensation with 87Rb. In particular, the experimental setup has been designed for performing experiments with Bose-Einstein condensates load into optical lattices of variable geometries. All parts essential for Bose-Einstein condensation are in place. Atoms are collected in a magneto-optical trap, transferred to another vacuum chamber, with better vacuum, and trapped in another magneto-optical trap. Atoms are successfully transferred to a dark magnetic trap, and system for diagnostics with absorption imaging has been realized. We have not yet been able to form a Bose-Einstein condensate, due to a range of technical difficulties. Equipment for alignment of optical lattices with flexible geometry has been designed, built, and tested. This tool has been proven to work as desired, and there is a great potential for a range of unique experiments with Bose-Einstein condensates in optical lattices of various geometries, including superlattices and quasi-periodic lattices. Numerical studies have been made on anisotropic optical lattices, and the existence of a transition between a 2D superfluid phase and a 1D Mott-insulating phase has been confirmed. We have shown that the transition is of Berezinskii-Kosterlitz-Thouless type. In another numerical study it has been shown that using stimulated Raman transitions is a practical method for transferring atoms between states in a double optical lattice. Thus, it will be possible to transfer populations between the lattices, with further applications in qubit read/write operations.

The preparation of layered magnets needs the knowledge of growth techniques which are focused on the growth of Fe/V(001) superlattices. Such films have been structurally investigated by X-rays reflectivity and diffraction. The magnetic investigations have been carried out by magneto-optic Kerr effect (MOKE), Superconducting Quantum Interference Device (SQUID) magnetometry and polarized neutron reflectivity (PNR). This latter technique has been used in cooperation with the Institute Laue Langvin (Grenoble, France) and Ruhr Universität (Bochum, Germany). The cross-over in universality class is shown in a series of layered magnets where a δ-doping layer of Fe has been embedded between two layers of Pd showing that the magnetization depends on the effective magnetic thickness of the polarized Pd. A model for the cross-over has been developed in terms of magnetic excitations. The interlayer exchange coupling (IEC) mediated by a non-magnetic spacer has been reviewed focusing the attention on the recent theoretical and experimental works based on Fe/V(001) superlattices. The IEC can be tailored at will by reversibly alloying of the spacer with H: this has been proved in Fe/V(001) double layers showing that in the two dimensional limit, the universality class is not affected by the coupling. The magnetic order-disorder transitions in Fe/V(001) superlattices do not seem to belong to any universality class. A phenomenological model which accounts for the effective coupling at the boundaries has been developed. The influence of the inherent ordering temperatures of single magnetic layers has been investigated in Fe/V(001) superlattices proving that the weakest ferromagnetic layer affects the overall magnetic ordering. A new kind of layered magnet has been developed to increase the effect of the boundaries. PNR measurements show that the universality class depends on which length-scale is investigated.

Thermal fluctuations and disorder strongly influence the behaviour of hightemperature superconductors. In particular the vortices play a key role in determining their properties. In this thesis the main focus lies on phase transitions, both in ultra-thin films and in three-dimensional systems, which are driven by vortex fluctuations. The last paper concerns the influence of antiferromagnetism on superconductivity in a simple model. A brief review of these topics is given in the introductory part. The main results are: The phase transition in ultra-thin superconducting/superfluid films is studied within the two-dimensional Coulomb gas model, which is known to have a Berezinskii-Kosterlitz-Thouless transition at low vortex densities. We construct the phase diagram from grand canonical Monte Carlo simulations on a continuum, without any restrictions on the vortex density. The dynamical universality classes for vortices in superconductors in zero magnetic field are studied by means of Monte Carlo simulations, with particular attention to the role of screening of the vortex interaction. We construct a formula for the k = 0 helicity modulus directly in terms of the vortex line fluctuations, which can serve as a useful way to detect superconducting coherence in model calculations. A method for simulating vortex lines on a continuum is developed, and used to study the melting of the Abrikosov vortex lattice. We study the critical dynamics for vortices in the presence of columnar defects. The linear resistivity and current-voltage characteristics are calculated in Monte Carlo simulations, and the critical behaviour extracted using finite size scaling. We reconsider the scaling properties as the magnetic field is tilted away from the direction of the columns. The influence of antiferromagnetic correlations on the superconducting properties is studied in a simplified lattice fermion model for superconductivity in the presence of an antiferromagnetic background. We find that the superconducting critical temperature is enhanced by antiferromagnetic order, and that a gap with dx2-y2-wave symmetry is the most stable. / QC 20100512

Phase transitions and their critical scaling properties, especially in systems with disorder, are important both for our theoretical understanding of our environment, but also for their practical use in applications and materials in our everyday life. This thesis presents results from finite size scaling analysis of critical phenomena in systems with disorder, using high-precision Monte Carlo simulations and state of the art numerical methods. Specifically, theoretical models suitable for simulations in the presence of uncorrelated or correlated disorder are studied. Uncorrelated strong disorder, as present in the two dimensional gauge glass model to study the vortex glass phase of high temperature superconductors in an applied magnetic field is shown to lack a finite temperature phase transition. Further, results from dynamic quantities, such as resistance and autocorrelation functions, indicate the existence of two distinct diverging correlation times, one associated with local relaxation and one associated with vortex phase slips. Correlated disorder is studied both in the superfluid transition of helium-4 and in the anisotropic critical scaling of a transverse Meissner-like transition in an experimental setup of a high temperature superconductor. For the superfluid helium transition, it is shown that the presence of fractally correlated disorder presumably alters the universality class of the pure model. Also, a comparison with experimental data suggests that the critical scaling theory describing the heat capacity of helium-4 may need to be modified in the presence of the disorder. In the case of superconductors, analyzing experimental data from resistance measurements in a system with columnar defects together with an anisotropy in the applied magnetic field, reveals a fully anisotropic scaling regime. Finally, a data analysis is presented from simulations of a charged particle gas system in three dimensions, where the normal Coulomb interaction between charges is changed into a logarithmic interaction. Previous work indicates the possibility of a transition similar to the Kosterlitz-Thouless transition in certain two dimensional systems. On the contrary, our simulations seem to favor a system whose critical scaling behavior is consistent with a transition occurring only at zero critical temperature. Overall, disorder in the model systems studied leads to important modifications of the critical scaling properties of pure systems, and thereby also to possible changes of the corresponding universality classes. This results in interesting predictions with experimentally relevant consequences. / QC 20100811

Spin systems are powerful mathematical models widely used and studied in Statistical Physics and Computer Science. This thesis focuses the study of spin systems on colorings and weighted independent sets (the hard-core model). In many spin systems, there exist phase transition phenomena: there is a threshold value of a parameter such that when the parameter is on one side of the threshold, the system exhibits the so-called spatial decay of correlation, i.e., the influence from a set of vertices to another set of vertices diminishes as the distance between the two sets grows; when the parameter is on the other side, long range correlations persist. The uniqueness problem and the reconstruction problem are two major threshold problems that are concerned with the decay of correlations in the Gibbs measure from different perspectives. In Computer Science, the study of spin systems mainly focused on finding an efficient algorithm that samples the configurations from a distribution that is very close to the Gibbs measure. Glauber dynamics is a typical Markov chain algorithm for performing sampling. In many systems, the convergence time of the Glauber dynamics also exhibits a threshold behavior: the speed of convergence experiences a dramatic change around the threshold of the parameter. The first two parts of this thesis focus on making connections between the phase transition of the convergence time of the dynamics and the phase transition of the reconstruction phenomenon in both colorings and the hard-core model on regular trees. A relatively sharp threshold is established for the change of the convergence time, which coincides with the reconstruction threshold. A general technique of upper bounding the conductance of the dynamics via analyzing the sensitivity of the reconstruction algorithm is proposed and proven to be very effective for lower bounding the convergence time of the dynamics. The third part of the thesis provides an innovative analytical method for establishing a strong version of the decay of correlation of the Gibbs distributions for many two spin systems on various classes of graphs. In particular, the method is applied to the hard-core model on the square lattice, a very important graph that is of great interest in both Statistical Physics and Computer Science. As a result, we significantly improve the lower bound of the uniqueness threshold on the square lattice and extend the range of parameter where the Glauber dynamics is rapidly mixing.