<div>Metallic components and metal-dielectric interfaces appear widely in modern electronics and the thermal management is an important issue. A very important feature that has been overlooked in the conventional Fourier's equations analyses is the nonequilibrium thermal transport induced by selective electron-phonon (e-p) coupling and phonon-phonon (p-p) coupling. It signicantly affects many processes such as laser heating and ignoring this phenomenon can lead to wrong or misleading predictions. On the other hand, as devices shrink into nano-scale, heat generation and dissipation at the interfaces between different components start to dominate the thermal process and present a challenge for thermal mitigations. Many unresolved issues also arise from interfaces, such as the unexpected high interfacial thermal conductance (ITC) at metal-diamond interfaces. Both of these require a deep understanding of the physics at interfaces.</div><div><br></div><div><div>Therefore in this work, I present multi-scale simulations in metals/dielectrics and interfaces based on two-temperature model (TTM) and establish the new multitemperature model (MTM). The methods are combined with Fourier's Law, molecular dynamics (MD), Boltzmann transport equations (BTE) and implemented to predict the thermal transport in several materials and interfaces where e-p coupling and p-p coupling are important. First-principles studies based on density functional theory (DFT) are also presented as predictive approaches to acquire the properties, as well as investigating the new physical phenomenon of non-local e-p coupling in metals. This research seeks to provide general, sophisticated but also simple simulation approaches which can help people accurately predict the thermal transport process. It also seeks to explore new physics which cannot be captured and predicted by conventional analyses based on Fourier's Law and can advance our understanding as well as providing new insights in the current thermal analysis paradigm.</div></div><div><br></div><div><div>The rst part of this thesis focuses on the non-equilibrium thermal transport in metals and across metal-dielectric interfaces based on TTM. First of all, nonequilibrium thermal transport in metal matrix composites (MMC) is investigated. Metal particle is usually added to polymer matrix for enhanced thermal performance. Here we apply TTM calculations and manifest a \critical particle size" above which the thermal conductivity of the composite material can be enhanced. MD simulations are performed to predict the thermal properties. TTM-Fourier and TTM-BTE calculations are conducted as comparisons. The widely used Au-SAM (self-assemblymonolayers) material pair is chosen to demonstrate our models. For a 1-D SAMAu-SAM sandwich system, the two calculation approaches present almost identical results, and the critical particle size is 10.7 nm. A general interpretation of thermal transport in sandwiched metal thin lms between two dielectric materials is also presented. It is found that when the lm thickness is on the order of several nanometers, due to strong e-p non-equilibrium the thermal transport is dominated by phonons</div><div>and electrons hardly contribute.</div></div><div><br></div><div><div>Then the e-p non-equilibrium thermal transport across metal-dielectric interfaces is investigated using TTM-MD. One possible explanation to the unexpected ITC at metal-diamond interfaces is the cross-interface e-p coupling mechanism, which is based on the hypothesis that electrons can couple to phonons within a certain distance rather than just those at the same location. Therefore we extend TTM-MD by modifying its governing equation to a non-local integral form. Two models are proposed to describe the coupling distance: the \joint-phonon-modes" model and the \phonon-wavelength" model. A case study of thermal transport across Cu-Si interfaces is presented, and both models predict similar coupling distances of 0.5 nm in Cu and 1.4 nm in Si near the interfaces. The cross-interface e-p coupling can increase the ITC by 20% based on our models. Based on the results, we construct a new mixed series-parallel thermal circuit. It is shown that such a thermal circuit is essential for understanding metal-nonmetal interfacial transport, while calculating a single resistance without solving temperature proles as done in most previous studies is generally incomplete.</div></div><div><br></div><div><div>Inspired by the previous work, we investigate further into the physics of nonlocal e-p coupling. First-principles calculations based on DFT is used due to their predictive feature without assumptions or adjustable parameters. By calculating the e-p coupling in metal lms of different sizes, we nd that e-p coupling has size effect which can only be explained by a non-local coupling picture. Results show that in Al, electrons and phonons can couple to each other in a range of up to 2 lattice-constants, or 0.8 nm. The coupling strength between electrons and phonons in adjacent atomic layers still has 75% of that in the same layer. Comparative studies are also performed on Cu and Ag. Results show that their non-local e-p coupling is not as signicant as in Al, with coupling distances of 0.37 nm for Cu and 0.49 nm for Ag. Similar results in Cu and Ag also indicate that materials with similar electronic structures have similar non-local e-p coupling properties.</div></div><div><br></div><div><div>In TTM, it is assumed that phonons are in thermal equilibrium and have a common temperature. In the second part of this thesis we go beyond TTM to investigate the non-equilibrium between phonons as well. TTM is extended to a general MTM with e-p coupling strength for each phonon branch. An averaged scattering lattice reservoir is dened to represent p-p scattering. The thermal transport process in single-layer graphene under constant and pulse laser irradiation is investigated. Results show that the phonon branches are in strong non-equilibrium. A comparison with TTM reveals that MTM can increase the thermal conductivity prediction by 50% and the hot electron relaxation time by 60 times. We also perform MTM simulations on Si-Ge interfaces to investigate the effect of non-equilibrium thermal transport on ITC. Results show that thermal non-equilibrium between phonons will introduce additional resistance at the interfaces, which is similar with e-p non-equilibrium's impact on ITC at metal-dielectric interfaces.</div></div>

A superfície (100) do Silício é estudada através de métodos de primeiros princípios, baseados na Teoria do Funcional da Densidade e no Método dos Pseud.opotenciais. Estudamos as propriedades eletrônicas e estruturais das principais reconstruções desta superfície. Obtivemos os principais sítios ligantes para pequenas estruturas de Germânio adsorvidas sobre ela. Para um átomo adsorvido o sítio mais estável encontrado foi o sítio M, semelhante a resultados existentes na literatura. A diferença está na configuração dos dímeros de Si da superfície. Observamos que diferentes configurações destes dímeros nos forneciam diferentes sítios metaestáveis. O deslocamento dos dímeros da superfície é tratado então como um novo grau de liberdade para a adsorção de átomos, além da posição destes sobre a superfície. Também foi estudada a adsorção de alguns trímeros de Ge sobre a superfície, sendo que a estrutura encontrada como sendo a mais estável concorda muito bem com outros resultados teóricos e experimentais. / The (100) surface of Silicon is studied using First Principles methods, based on the Density Functional Theory and on the Pseudopotential method. We obtained the electronic and structural properties of the most important reconstructions of this surface and we also obtained the main bonding sites for the adsorption of small Ge structures on the surface. Site M was found as being the most stable for the monomers adsorption, similar to previous results on the literature. The main difference is on the buckling of the silicon surface dimers. We observed that different configurations of these buckling gave us different metastable adsorption sites. We then insert a new degree of freedom for this kind of adsorption, related to the buckling of the silicon surface dimers. \\Me also studied the adsorption of Ge trimers on the surface, and the structure found as being the most stable agree very well with previous theoretical and experimetal results.

<p>The present thesis concerns method development and applications in the field of first principles electronic structure calculations.</p><p>Augmented planewaves combine the simple planewaves with exact solutions of the Schrödinger equation for a spherical potential. This combination yields a very good set of basis functions for describing the electronic structure everywhere in a crystal potential. In the present work, developments of the original augmented planewave (APW) method are presented. It is shown that the exact APW eigenvalues can be found using information from the eigenvalues of the APW secular matrix. This provides a more efficient scheme to solve the APW eigenvalue problem, than the traditional evaluation of the secular determinant. Further, a new way of linearizing the APW method is presented and compared to the traditional linearized APW method (LAPW). Using a combination of the original APW basis functions and the so called local orbitals (lo), the APW+lo linearization is found to reproduce the results of the LAPW method, but already at a smaller basis set size. Another advantage of the new linearization is a faster convergence of forces, with respect to the basis set size, as compared to the LAPW method.</p><p>The applications include studies of the non-collinear magnetic configuration in the fcc-based high-temperature phase of iron, γ-Fe. The system is found to be extremely sensitive to volume changes, as well as to a tetragonal distortion of the cubic unit cell. A continuum of degenerate spin spiral configurations, including the global energy minimum, are found for the undistorted crystal. The in-plane anisotropy of the ideal interface between a ferromagnetic layer of bcc Fe and the semiconducting ZnSe crystal is also investigated. In contrast to the four-fold symmetric arrangement of the atoms at the interface, the in-plane magnetic anisotropy displays a large uniaxiality. The calculated easy axes are in agreement with experiments for both Se and Zn terminated interfaces. In addition, calculations of the hyperfine parameters were performed for Li intercalated battery materials.</p>

<p>The present thesis concerns applications of first principles electronic structure calculations in conjunction with methods of statistical mechanics for simulations of transition metal alloys both in the bulk and at surfaces.</p><p>A fully relativistic generalization of the exact muffin-tin orbitals (EMTO) method has been developed. The method accurately takes into account spin-orbit coupling and allows one to calculate orbital polarization and magneto-crystalline anisotropy in magnetic systems as well as increasing the range of applicability of the EMTO method to heavy elements. A new direct-exchange Monte Carlo (DEMC) method has been proposed, which is capable to tackling effectively statistical simulations of surface segregations in disordered and ordered alloys.</p><p>The applications of relativistic methods include calculations of spin and orbital magnetization in iron-cobalt disordered and partially ordered alloys, as well as computation of the core-level shifts (CLS) in transition metal alloys. It has been found, that relativistic corrections are important for CLS calculations in 5-d metal alloys. Properties of a Ni monolayer deposited on a Cu surface have been studied. The monolayer is found to be unstable in the top layer, and its magnetization depends greatly on the surface orientation. Two distinct energy levels have been found to exist Co/Cu/Ni trilayers deposited on the (100) Cu surface, which correspond to a completely paramagnetic trilayer and the case when only Ni is paramagnetic.</p><p>Vacancy ordering in substoichometric titanium carbides TiC_x have been simulated. Existence of three ordered phases in the range of carbon concentration x=0.5 ÷1.0 has been revealed and a theoretical phase diagram has been proposed. Surface segregations have been calculated in disordered Ni₅₀Pt₅₀ and Ni₅₀Pd₅₀ as well as in ordered NiPt alloys. Segregation reversal has been observed in the Ni₅₀Pt₅₀ alloy with Pt segregation at the (111) surface and Ni segregation at the (110). In the ordered NiPt alloys segregation behaviour is found to be affected greatly by small deviations from the exact stoichiometric composition in bulk. Surface magnetization in PdV and MoV bcc alloys have been studied. It has been found, that in PdV alloys surface segregations suppress magnetic order at the surface, while in MoV alloys magnetization is substantially enhanced due to the segregation.</p>

A first-principles linear scaling real-space method for investigating non-collinear magnetic behaviour of nanostructured materials has been developed. With this method, the magnetic structures of small supported transition metal clusters have been examined. The geometric constraints imposed on the clusters by the underlying surface is found to cause non-collinear behaviour for V, Cr, and Mn clusters on Cu(111). Fe clusters supported on Cu and Ni have been studied and both spin and orbital moments are found to be enhanced for the Fe atoms, which is attributed to the recuced symmetry present at the surface. Atoms in Co clusters have been found to order antiferromagnetically, and some times in a non-collinear fasion, when deposited on a W surface. Small clusters of fcc Fe embedded in Cu have been examined and a new type of ordering, not present in larger fcc Fe systems was found. Several theoretical studies of Fe and Co based nanostructures consisting of multilayers or embedded clusters have been conducted, with the aim of predicting high moment materials for use in data storage applications. In agreement with previous experiments an enhancement of the magnetic moment is found compared to the magnetic moment of bcc Fe. The enhancement has been shown to be caused by increased spin moments for Fe atoms in close proximity with Co atoms, and this enhancement depends on the number of Co neighbours. As a result of these studies, a possible method of increasing the magnetic moment of cluster based materials has been proposed. Fermi surface analysis have been performed both on bulk materials, in order to investigate mechanisms for stabilizing non-collinear magnetic states, and in layered structures where the effect of the Fermi surface on the interlayer exchange coupling has been investigated. In addition to the development of a real-space electronic structure method for non-collinear magnetism, a density matrix purification method has been implemented in the framework of linear muffin-tin orbitals.

In this thesis, non-collinear magnetism has been studied by using density functional theory and the augmented plane wave method with local orbitals (APW+lo). Two conditions for non-collinear instabilities have been identified in this thesis. First, the Fermi energy should cut through both spin up and down states. Secondly, strong nesting between the spin up and spin down Fermi surfaces is needed. The two criteria described here can be fulfilled by tuning the exchange-splitting and/or by modifying the volume. Calculations on several elements; bcc V, bcc and fcc Mn, bcc Fe, bcc and fcc Co, and bcc and fcc Ni show that a non-collinear state can be stabilized provided that the criteria discussed above are met. More complex materials have also been analyzed in terms of these two criteria. The substitutional alloys TlCo2Se2-xSx are found in experiments to possess spin spiral structures for x = {0-1.5} and at a concentration x = 1.75 the alloys become ferromagnetic. As S takes the place of Se in the crystal structure the distance between the Co layers is reduced and the turn angle of the spin spiral becomes smaller until it totally vanishes at x = 1.75. This thesis show that the evolution of the magnetic structure in these alloys is the consequence of a modification of the distance between Co layers, which induces a change in the interlayer exchange coupling. Fermi surfaces have been analyzed in TbNi5 in order to determine nesting features which would be responsible for the magnetic spin spiral observed in this material. The electronic structure of CeRhIn5 is also reported in this thesis. Furthermore, the 3-k magnetic structure of UO2 was investigated and the crystal field levels were calculated. Transition metal systems such as Fe in the superconducting high-pressure hcp phase and in the fcc crystal structure were also studied. The results obtained for fcc Fe are in accordance with previous reports. However the paramagnetic state in hcp Fe is found to be more stable than the antiferromagnetic configurations discussed earlier in the literature as being favored in the volume range where the hcp phase is stable and superconductivity appears (~ 15 GPa). The complex non-collinear magnetic structure in Mn3IrSi was calculated and the results are found to be in good agreement with experiments.

The present thesis concerns method development and applications in the field of first principles electronic structure calculations. Augmented planewaves combine the simple planewaves with exact solutions of the Schrödinger equation for a spherical potential. This combination yields a very good set of basis functions for describing the electronic structure everywhere in a crystal potential. In the present work, developments of the original augmented planewave (APW) method are presented. It is shown that the exact APW eigenvalues can be found using information from the eigenvalues of the APW secular matrix. This provides a more efficient scheme to solve the APW eigenvalue problem, than the traditional evaluation of the secular determinant. Further, a new way of linearizing the APW method is presented and compared to the traditional linearized APW method (LAPW). Using a combination of the original APW basis functions and the so called local orbitals (lo), the APW+lo linearization is found to reproduce the results of the LAPW method, but already at a smaller basis set size. Another advantage of the new linearization is a faster convergence of forces, with respect to the basis set size, as compared to the LAPW method. The applications include studies of the non-collinear magnetic configuration in the fcc-based high-temperature phase of iron, γ-Fe. The system is found to be extremely sensitive to volume changes, as well as to a tetragonal distortion of the cubic unit cell. A continuum of degenerate spin spiral configurations, including the global energy minimum, are found for the undistorted crystal. The in-plane anisotropy of the ideal interface between a ferromagnetic layer of bcc Fe and the semiconducting ZnSe crystal is also investigated. In contrast to the four-fold symmetric arrangement of the atoms at the interface, the in-plane magnetic anisotropy displays a large uniaxiality. The calculated easy axes are in agreement with experiments for both Se and Zn terminated interfaces. In addition, calculations of the hyperfine parameters were performed for Li intercalated battery materials.

The present thesis concerns applications of first principles electronic structure calculations in conjunction with methods of statistical mechanics for simulations of transition metal alloys both in the bulk and at surfaces. A fully relativistic generalization of the exact muffin-tin orbitals (EMTO) method has been developed. The method accurately takes into account spin-orbit coupling and allows one to calculate orbital polarization and magneto-crystalline anisotropy in magnetic systems as well as increasing the range of applicability of the EMTO method to heavy elements. A new direct-exchange Monte Carlo (DEMC) method has been proposed, which is capable to tackling effectively statistical simulations of surface segregations in disordered and ordered alloys. The applications of relativistic methods include calculations of spin and orbital magnetization in iron-cobalt disordered and partially ordered alloys, as well as computation of the core-level shifts (CLS) in transition metal alloys. It has been found, that relativistic corrections are important for CLS calculations in 5-d metal alloys. Properties of a Ni monolayer deposited on a Cu surface have been studied. The monolayer is found to be unstable in the top layer, and its magnetization depends greatly on the surface orientation. Two distinct energy levels have been found to exist Co/Cu/Ni trilayers deposited on the (100) Cu surface, which correspond to a completely paramagnetic trilayer and the case when only Ni is paramagnetic. Vacancy ordering in substoichometric titanium carbides TiCx have been simulated. Existence of three ordered phases in the range of carbon concentration x=0.5 ÷1.0 has been revealed and a theoretical phase diagram has been proposed. Surface segregations have been calculated in disordered Ni50Pt50 and Ni50Pd50 as well as in ordered NiPt alloys. Segregation reversal has been observed in the Ni50Pt50 alloy with Pt segregation at the (111) surface and Ni segregation at the (110). In the ordered NiPt alloys segregation behaviour is found to be affected greatly by small deviations from the exact stoichiometric composition in bulk. Surface magnetization in PdV and MoV bcc alloys have been studied. It has been found, that in PdV alloys surface segregations suppress magnetic order at the surface, while in MoV alloys magnetization is substantially enhanced due to the segregation.

First-principles calculations such as density functional theory (DFT) employ numerical approaches to solve the Schrodinger equation of a system. Standard functionals employed to determine the cohesive system energy, specifically the local density and generalized gradient approximations (LDA and GGA), underestimate the correlation of 5f electrons to their ions in AO₂ systems (A=U/Pu/Np). The standard correction, the "Hubbard +U," causes the multidimensional energy surface to develop a large number of local minima which do not correspond to the ground state (global minimum). Because all useful energy values derived from DFT calculations depend on small differences between relatively large cohesive energies, comparing systems wherein one or more of the samples are not in the ground state has the potential to introduce large errors. This work presents an analysis of the fundamental issues of metastable states in both pure and binary AO₂ systems, investigates novel methods of handling them, and describes why current literature approaches which appear to work well for the pure compounds are not well-suited for systems containing multiple actinide species.

Metallic fuels have an important role to play in "fast breeder" Gen-IV type nuclear reactors, and U-Pu-Zr is one of the prototypical systems. Because of the variability in fuel chemistry during burn-up, it is important to understand the effect of minor actinides and fission products on phase stability. Within this framework, we present a study on phase equilibria in the binary Np-Zr alloy system on which little work has been published. To resolve the contradictory reports on the ordering/ clustering trends of the bcc phase, a thermodynamic study is performed using the CALPHAD method. The calculated Np-Zr phase diagram is consistent with two sets of data: formation enthalpies of the bcc phase that are calculated with ab initio KKR-ASA-CPA electronic-structure method and lattice stabilities of solution phases obtained from first-principles technique. Another important feature in the Np-Zr alloy system is the non-stoichiometric delta-NpZr2 phase that forms in a hexagonal C32 structure similar to the delta-phase in the U-Zr system and the w-phase in pure Zr. An increase in the homogeneity range of the delta-phase when going from Pu to Np and to U is attributed to a lowering of its heat of formation that is caused by an increase in d-band occupation. Two different possibilities for the stability of the delta- and w- phases have been proposed in the present work. Additionally, calculated changes in enthalpy versus temperature are plotted for two alloy compositions of the Np-Zr system to guide future experimental work in resolving important issues in this system. Finally, an ab initio study, implemented with the L(S)DA U formalism, is performed for pure Np that reveals a transition from a non-magnetic to a magnetic state at a critical U parameter.