Dissertations / Theses on the topic 'Solids electronic structure'

In this thesis, we present results of studies of the electronic band structures and related electronic properties of some layered transition metal chalcogenides and their intercalation complexes. The materials investigated include group VIIc transition metal dichalcogenides, and 2H-TaS₂ and its lithium-, lead-, and tin-intercalated complexes, as well as dihafnium sulphide and selenide. Both experimental measurements and theoretical elect'onic band structure calculations have been carried out. The types of measurements conducted consist of reflectivity measurements in the energy range from 0.5 eV to 4.5 eV using the home-made reflectivity spectrometer, and electron energy loss measurements in the energy range up to 100 eV using the scanning transmission electron microscope as well as some characterization experiments (structural, chemical composition and thermal properties). The experimental investigations were restricted to the layered group VIIc metal dichalcogenides. All the electronic band structures are calculated using the linearized muffin-tin orbital (LMTO) method, and are reported for the first time except PdTe₂ and 2H-TaS₂. The obtained electronic band structures for the Ni-group metal dichalcogenides, and the semiconductor-metal shift in progression from PtS₂ through PtSe₂ to PtTe₂ are discussed in terms of the binding energies of the atomic valence orbitals of the constituent atoms, the local coordination of the metal atoms and the symmetry of the crystals as well as the charge transfer effects. A superlattice structural phase transition is proposed for PtSe₂, which may possibly explain the anomaly observed in the previous transport measurement. The previous photoemission spectra from NiTe₂, PdTe₂ and PtTe₂, and dHvA measurement on PdTe₂ are compared with their band structures in details, and a good agreement is found. Other available experimental data including the previous transport, optical and magnetic susceptibility measurements as well as the reflectivity and electron energy loss spectra measured in this work are also discussed in terms of these electronic structures. The band structure calculations for dihafnium chalcogenides predict that these materials are metals. They also suggest that there is a strong bonding between Hf atoms in the adjacent layers, thus giving rise to the rigidity in the c-direction which may preclude the intercalation of these materials. The results for 2H-TaS₂ and its intercalation complexes show that the rigid band model is essentially correct for 2H-LiTaS₂ but is an oversimplication for the post-transition metal intercalation compounds. Changes in the electronic structure upon intercalation are discussed in terms of the intercalant-host charge transfer and the hybridisation between the host states and the intercalation valence orbitals. Electrical conduction in 2H-PbTaS₂ and SnTaS₂ is found to be largely due to the p-valence electrons from the intercalant Pb (Sn) layers, resulting in the considerable increase in the superconducting transition temperature following intercalation. The results are also compared with the observed optical and transport properties and a broad agreement is found. The band structures and the electronic properties of other layered transition metal dichalcogenides and their intercalation complexes, as well as the band structure calculation techniques for the layered compounds are also reviewed in this thesis.

The present work gives an overview of the authors work in the field of electronic structure calculations. The main objective is to show how electronic structure methods in particular density functional theory (DFT) can be used for the description and interpretation of experimental results in order to enhance our understanding of physical and chemical properties of materials. The recently found superconductor MgB2 is an example where the electronic structure was the key to our understanding of the surprising properties of this material. The experimental confirmation of the predicted electronic structure from first principles calculations was very important for the acceptance of earlier theoretical suggestions. Molecular crystals build from magnetic clusters containing a few transition metal ions and organic ligands show fascinating magnetic properties at the nanoscale. DFT allows for the investigation of magnetic ordering and magnetic anisotropy energies. The magnetic anisotropy which results mainly from the spin-orbit coupling determines many of the properties which make the single molecule magnets interesting.

Over the course of the last century, electronic structure theory (or, alternatively, computational quantum chemistry) has grown from being a fledgling field to being a ``full partner with experiment" [Goddard textit{Science} textbf{1985}, textit{227} (4689), 917--923]. Numerous instances of theory matching experiment to very high accuracy abound, with one excellent example being the high-accuracy textit{ab initio} thermochemical data laid out in the 2004 work of Tajti and co-workers [Tajti et al. textit{J. Chem. Phys.} textbf{2004}, textit{121}, 11599] and another being the heats of formation and molecular structures computed by Feller and co-workers in 2008 [Feller et al. textit{J. Chem. Phys.} textbf{2008}, textit{129}, 204105]. But as the authors of both studies point out, this very high accuracy comes at a very high cost. In fact, at this point in time, electronic structure theory does not suffer from an accuracy problem (as it did in its early days) but a cost problem; or, perhaps more precisely, it suffers from an accuracy-to-cost ratio problem. We can compute electronic energies to nearly any precision we like, textit{as long as we are willing to pay the associated cost}. And just what are these high computational costs? For the purposes of this work, we are primarily concerned with the way in which the computational cost of a given method scales with the system size; for notational purposes, we will often introduce a parameter, $N$, that is proportional to the system size. In the case of Hartree-Fock, a one-body wavefunction-based method, the scaling is formally $N^4$, and post-Hartree-Fock methods fare even worse. The coupled cluster singles, doubles, and perturbative triples method [CCSD(T)], which is frequently referred to as the ``gold standard" of quantum chemistry, has an $N^7$ scaling, making it inapplicable to many systems of real-world import. If highly accurate correlated wavefunction methods are to be applied to larger systems of interest, it is crucial that we reduce their computational scaling. One very successful means of doing this relies on the fact that electron correlation is fundamentally a local phenomenon, and the recognition of this fact has led to the development of numerous local implementations of conventional many-body methods. One such method, the DLPNO-CCSD(T) method, was successfully used to calculate the energy of the protein crambin [Riplinger, et al. textit{J. Chem. Phys.} textbf{2013}, textit{139}, 134101]. In the following work, we discuss how the local nature of electron correlation can be exploited, both in terms of the occupied orbitals and the unoccupied (or virtual) orbitals. In the case of the former, we highlight some of the historical developments in orbital localization before applying orbital localization robustly to infinite periodic crystalline systems [Clement, et al. textbf{2021}, textit{Submitted to J. Chem. Theory Comput.}]. In the case of the latter, we discuss a number of different ways in which the virtual space can be compressed before presenting our pioneering work in the area of iteratively-optimized pair natural orbitals (``iPNOs") [Clement, et al. textit{J. Chem. Theory Comput.} textbf{2018}, textit{14} (9), 4581--4589]. Concerning the iPNOs, we were able to recover significant accuracy with respect to traditional PNOs (which are unchanged throughout the course of a correlated calculation) at a comparable truncation level, indicating that our improved PNOs are, in fact, an improved representation of the coupled cluster doubles amplitudes. For example, when studying the percent errors in the absolute correlation energies of a representative sample of weakly bound dimers chosen from the S66 test suite [v{R}ez'{a}c, et al. textit{J. Chem. Theory Comput.} textbf{2011}, textit{7} (8), 2427--2438], we found that our iPNO-CCSD scheme outperformed the standard PNO-CCSD scheme at every truncation threshold ($tpno$) studied. Both PNO-based methods were compared to the canonical CCSD method, with the iPNO-CCSD method being, on average, 1.9 times better than the PNO-CCSD method at $tpno = 10^{-7}$ and more than an order of magnitude better for $tpno < 10^{-10}$ [Clement, et al. textit{J. Chem. Theory Comput.} textbf{2018}, textit{14} (9), 4581--4589]. When our improved PNOs are combined with the PNO-incompleteness correction proposed by Neese and co-workers [Neese, et al. textit{J. Chem. Phys.} textbf{2009}, textit{130}, 114108; Neese, et al. textit{J. Chem. Phys.} textbf{2009}, textit{131}, 064103], the results are truly astounding. For a truncation threshold of $tpno = 10^{-6}$, the mean average absolute error in binding energy for all 66 dimers from the S66 test set was 3 times smaller when the incompleteness-corrected iPNO-CCSD method was used relative to the incompleteness-corrected PNO-CCSD method [Clement, et al. textit{J. Chem. Theory Comput.} textbf{2018}, textit{14} (9), 4581--4589]. In the latter half of this work, we present our implementation of a limited-memory Broyden-Fletcher-Goldfarb-Shanno (BFGS) based Pipek-Mezey Wannier function (PMWF) solver [Clement, et al. textbf{2021}, textit{Submitted to J. Chem. Theory Comput.}]. Although orbital localization in the context of the linear combination of atomic orbitals (LCAO) representation of periodic crystalline solids is not new [Marzari, et al. textit{Rev. Mod. Phys.} textbf{2012}, textit{84} (4), 1419--1475; J`{o}nsson, et al. textit{J. Chem. Theory Comput.} textbf{2017}, textit{13} (2), 460--474], to our knowledge, this is the first implementation to be based on a BFGS solver. In addition, we are pleased to report that our novel BFGS-based solver is extremely robust in terms of the initial guess and the size of the history employed, with the final results and the time to solution, as measured in number of iterations required, being essentially independent of these initial choices. Furthermore, our BFGS-based solver converges much more quickly and consistently than either a steepest ascent (SA) or a non-linear conjugate gradient (CG) based solver, with this fact demonstrated for a number of 1-, 2-, and 3-dimensional systems. Armed with our real, localized Wannier functions, we are now in a position to pursue the application of local implementations of correlated many-body methods to the arena of periodic crystalline solids; a first step toward this goal will, most likely, be the study of PNOs, both conventional and iteratively-optimized, in this context.
Doctor of Philosophy
Increasingly, the study of chemistry is moving from the traditional wet lab to the realm of computers. The physical laws that govern the behavior of chemical systems, along with the corresponding mathematical expressions, have long been known. Rapid growth in computational technology has made solving these equations, at least in an approximate manner, relatively easy for a large number of molecular and solid systems. That the equations must be solved approximately is an unfortunate fact of life, stemming from the mathematical structure of the equations themselves, and much effort has been poured into developing better and better approximations, each trying to balance an acceptable level of accuracy loss with a realistic level of computational cost and complexity. But though there has been much progress in developing approximate computational chemistry methods, there is still great work to be done. textit{Many} chemical systems of real-world import (particularly biomolecules and potential pharmaceuticals) are simply too large to be treated with any methods that consistently deliver acceptable accuracy. As an example of the difficulties that come with trying to apply accurate computational methods to systems of interest, consider the seminal 2013 work of Riplinger and co-workers [Riplinger, et al. textit{J. Chem. Phys.} textbf{2013}, textit{139}, 134101]. In this paper, they present the results of a calculation performed on the protein crambin. The method used was DLPNO-CCSD(T), an approximation to the ``gold standard" computational method CCSD(T). The acronym DLPNO-CCSD(T) stands for ``domain-based local pair natural orbital coupled cluster with singles, doubles, and perturbative triples." In essence, this method exploits the fact that electron-electron interactions (``electron correlation") are a short-range phenomenon in order to represent the system in a mathematically more compact way. This focus on the locality of electron correlation is a crucial piece in the effort to bring down computational cost. When talking about computational cost, we will often talk about how the cost scales with the approximate system size $N$. In the case of CCSD(T), the cost scales as $N^{7}$. To see what this means, consider two chemical systems textit{A} and textit{B}. If system textit{B} is twice as large as system textit{A}, then the same calculation run on both systems will take $2^{7} = 128$ times longer on system textit{B} than on system textit{A}. The DLPNO-CCSD(T) method, on the other hand, scales linearly with the system size, provided the system is sufficiently large (we say that it is ``asymptotically linearly scaling"), and so, for our example systems textit{A} and textit{B}, the calculation run on system textit{B} should only take twice as long as the calculation run on system textit{A}. But despite the favorable scaling afforded by the DLPNO-CCSD(T) method, the time to solution is still prohibitive. In the case of crambin, a relatively small protein with 644 atoms, the calculation took a little over 30 days. Clearly, such timescales are unworkable for the field of biochemical research, where the focus is often on the interactions between multiple proteins or other large biomolecules and where many more data points are required. In the work that follows, we discuss in more detail the genesis of the high costs that are associated with highly accurate computational methods, as well as some of the approximation techniques that have already been employed, with an emphasis on local correlation techniques. We then build off this foundation to discuss our own work and how we have extended such approximation techniques in an attempt to further increase the possible accuracy to cost ratio. In particular, we discuss how iteratively-optimized pair natural orbitals (the PNOs of the DLPNO-CCSD(T) method) can provide a more accurate but also more compact mathematical representation of the system relative to static PNOs [Clement, et al. textit{J. Chem. Theory Comput.} textbf{2018}, textit{14} (9), 4581--4589]. Additionally, we turn our attention to the problem of periodic infinite crystalline systems, a class of materials less commonly studied in the field of computational chemistry, and discuss how the local correlation techniques that have already been applied with great success to molecular systems can potentially be applied in this domain as well [Clement, et al. textbf{2021}, textit{Submitted to J. Chem. Theory Comput.}].

Boron and boron rich solids are known to have a high concentration on intrinsic structural imperfections. From known structure data of real crystals and known band structure calculations of perfect ideal crystals a correlation between intrinsic structure defect concentration and electron deficit in the valence band is concluded. This correlation forms the basis for the following theses: 1. The electron deficit in the valence band of a perfect crystal is the driving force for the intrinsic structure defects in a real crystal. 2. The small electron deficit becomes compensated by the structure defects - this explains the semiconducting behavior. 3. The structure defects are the reason for the high density of localized electronic states in the band gap. The photoluminescence of beta-rhombohedral boron in the range 0.75eV to 1.4eV under interband excitation was investigated systematically and was interpreted using the one-dimensional Franck-Conden-Model. The only partially occupied B13-position in beta-rhombohedra l boron is suggested to be the reason for the localized electronic state, which is involved in the photoluminescence process. Together with an investigation of the time-depending photoconductivity under interband excitation the energy band schema of beta-rhombohedral boron is improved. The improved energy band schema is able to explain all known experimental data including the fatiguing of photoluminescence. An investigation of FIR-spectra of boron carbide and metal doped beta-rhombohedral boron by Kramers-Kronig-Analysis gives information on the main transport processes. Beside hopping conduction of localized electrons, band conduction of delocalized electrons were found. While holes in the valence band are the delocalized charge carriers in boron carbide, in vanadium doped beta-rhombohedral boron delocalized electrons in an extrinsic impurity band are suggested

A series of detailed experimental and computational studies on aspects of pressure-induced phase transitions in solids is described. The emphasis is on the nature of such transitions in tetrahedral semiconductors. It is found that the materials investigated exhibit previously unreported and very complex behaviour which is related to the structural transition. New high-pressure diffraction evidence suggests that the crystallographic structures of several III-V semiconductors are not in agreement with those suggested either by prior experimentation or total energy calculation. The high-pressure structures determined in this work are generally of lower symmetry than previously believed and in at least one case of much greater complexity. It is also argued that phase-transition-induced defects are created at the transition in many of these materials and that the transition may be sensitive to either the concentration or mobility of such defects. The development and implementation of a simple new x-ray detection method has allowed for the observation of these subtle processes for the first time. To a certain degree, these novel aspects of previously well-studied transitions are now amenable to theoretical and computational treatment through the advances made in modern computer technology and parallel algorithms. As a result, new ab-initio local density functional investigations of structural stability and possible transition routes in one of these materials have been performed and the results are in good agreement with observation. Also the issue of defects has been treated by Car-Parrinello-type pseudopotential calculations on defect energies. It is found, among other things that the defect formation energies are relatively low and that structural relaxation under the influence of Hellmann-Feynman forces is necessary in order to obtain accurate values of such energies. The remainder of the thesis is devoted to an exploration of pressure-induced metastable forms of group IV and III-V semiconductors. These studies unite and compare results from image plate experiments, ab-initio pseudopotential calculations as well as empirical calculations using pair potentials and are applied in an attempt to make speculations as to the relative structural stability of these phases at finite temperature.

Electronic processes in two different electroluminescent device structures, the forward biassed metal/thick insulator/semiconductor (MIS) diode and the high field metal/insulator/metal (MIM) panel, are investigated. Models are produced to explain the behaviour of two particular MIS systems which have been studied experimentally. One of these systems is the Au/cadmium stearate/n-GaP structure, where the insulator is deposited using Langmuir-Blodgett (LB) technology. The other is the Au/i-ZnS/n-ZnS structure. In the MIS devices electroluminescence occurs as a result of the recombination of electrons and holes in the semiconductor and so it is necessary to have an efficient minority carrier (hole) injection mechanism. Attention is paid to the impact excitation of the electron gas in the metal by the electrons injected from the semiconductor because this has been proposed by other workers as a process for producing holes in the metal that are energetically capable of entering the semiconductor valence band, provided they can traverse the insulator. The characteristics of the LB film devices are found to be best described by assuming the minority carrier injection to be limited by the hole transport through the insulator. Hopping between interface states on the successive LB layers is proposed as the transport mechanism. However, the device incorporating a II-VI semi-insulator is shown to be more characteristic of hole transport in the insulator valence band and a minority carrier injection which is limited by the supply of holes from the metal. In high field MIM panels the mechanism of electroluminescence is quite different with impurity centres being impact excited or impact ionised by injected electrons and subsequently luminescing. Such devices driven by a dc signal are susceptible to the formation of high current filaments which burn out and result in device failure. A model is developed which predicts that there is a voltage range over which the device can exist in either a low current state or two higher current states and the resultant instability is expected to be destructive. Current-voltage characteristics are produced using this model and their general features are found to be relatively insensitive to material and device parameters. In order to understand the evolution of the electrical state of the MIM device after switch-on, a time dependent theory of system behaviour is also developed. This is particularly important as the devices are usually driven by a pulsed signal. For an homogeneous system the current is found to converge to the lower current state of the steady state characteristic.

The basic properties of the rare earth metals, including single crystal growth, crystal and magnetic structures, and the relationship between electronic and magnetic structure, are reviewed. The problems encountered by the theoretical treatment of the partially occupied, but highly localised, lanthanide 4f levels as bands are discussed, and bandstructure calculations presented for the hexagonal close-packed rare earths. These are compared with available experimental and theoretical data. It is suggested that the exchange-splitting of the lanthanide valence bands may well persist in the paramagnetic state, and that account should be taken of the localised 4f moments in future calculations. The difficulties associated with the preparation of clean single crystal rare earth surfaces are described. The origin of the surface-orderdependent state seen in angle-resolved UV photoemission (ARUPS) spectra from rare earth (0/001) surfaces is discussed. (7 x 1) reconstructions of the (1120) surfaces of Ho, Er and Y are reported, with the resulting surface geometric and electronic structure being indistinguishable from those of the ideal (0001) structure. Momentum-resolved inverse photoernission measurements are presented for Y(000l), with results in good agreement with the calculated bandstructure. A comprehensive ARUPS study of the valence band of Ho(OOOl) is reported, and the results demonstrated to be entirely explicable in terms of emission from one-electron states. ARUPS data from Y(000l), Gd(000l) and Tb(000l) are presented, discussed in the light of the Ho results, and the conclusions of previous ARUPS studies of these surfaces revealed to be in error. Essentially similar ARUPS features are seen on all hcp rare earth (0001) surfaces so far studied and it is suggested that all other such surfaces will show the same features. The Ho(000l) 5p levels are shown to have significant band character, suggesting that further refinements to the band structure calculations are required.

La capacité de photo-commuter les propriétés physico-chimiques des matériaux fonctionnels grâce à des transitions de phase induites par la lumière, ouvre des perspectives fascinantes pour diriger un matériau vers un nouvel état hors équilibre thermique. Cependant, il est fondamental de comprendre tous les phénomènes élémentaires, habituellement cachés dans une moyenne statistique lors des transformations à l'équilibre. Les études résolues en temps représentent une approche unique pour accéder à l'évolution des différents degrés de liberté du système et connaître les processus élémentaires mis en jeu lors de la commutation macroscopique. Les matériaux à transition de spin (SCO) sont d'un intérêt particulier car ce sont des systèmes photo-réversibles. Ces matériaux sont aussi des prototypes photomagnétiques et photochromiques qui commutent entre deux états de différente multiplicité de spin, nommés bas spin (LS) et haut spin (HS). Dans ce travail de thèse, nous étudions les dynamique ultrarapides électroniques et structurales de cette classe de solides moléculaires, en soulignant l'importance d'utiliser des sondes complémentaires sensibles à différents degrés de liberté. Les commutation photo-induite entre états de spin est ultra-rapide et initialement localisée à l'échelle moléculaire, où le couplage électron-phonon active des vibrations cohérentes intramoléculaires. Un transfert d'énergie ultra-rapide de la molécule au réseau, via un couplage phonon-phonon, permet de piéger efficacement le système dans le nouvel état photo-induit. Cependant, dans les solides moléculaires, l'excès d'énergie libérée de la molécule excitée résulte dans un aspect complexe multi-échelle impliquant plusieurs degrés de liberté à des échelles de temps différentes. Dans ce travail de thèse, nous avons étudié la dynamique multi-étape hors équilibre d'un système SCO présentant une brisure de symétrie entre la phase HS et la phase intermédiaire (IP) où une mise en ordre à longue distance des états HS et LS des molécules résulte en la formation d'une onde de concentration de spin (SSCW). La diffraction des rayons X résolue en temps combinée avec des études de spectroscopie optique donnent une description complète de la dynamique hors-équilibre de la SSCW hors-équilibre en mesurant l'évolution temporelle des deux paramètres d'ordre décrivant le système
The ability to photo-switch physical/chemical properties of functional materials through photo induced phase transition opens fascinating perspectives for driving the material towards new state out of thermal equilibrium. However, it is fundamental to disentangle and understand all the dynamical phenomena, otherwise hidden in statistically averaged macroscopic transformations. Arguably, time-resolved studies are unique approach to access the necessary information on the multiple degrees of freedom and elementary processes involved during the macroscopic switching. As photo-reversible molecular switches, spin crossover (SCO) materials are of particular interest. These photomagnetic and photochromic prototype materials undergo metastable photoinduced phase transition between two states of different spin multiplicity, namely low-spin (LS) and high-spin (HS). In this PhD work it will be presented the ultrafast electronic and structural dynamics of SCO molecular solids emphasizing the importance of using complementary probes sensitive to different degrees of freedom. The photoinduced spin state switching concerns initially only an ultrafast, but localized, molecular response which through strong electron-phonon coupling activates coherent intra-molecular vibrations. An ultrafast energy transfer from the molecule to the lattice, via phonon-phonon coupling, allows an efficient trapping of the system in the new photoinduced state. However in molecular solids, the excess of energy released from the absorber molecule results in a complex multi-scale aspect involving several degrees of freedom at different time scales. In this contest, we investigated the multi-step out-of equilibrium dynamics of a SCO system undergoing symmetry breaking between the HS phase and the intermediate (IP) phase where a long range ordering of HS and LS molecules results in a spin state concentration wave (SSCW), analogous to charge or spin density waves. Combined time-resolved X-ray diffraction and optical spectroscopy studies provide a complete overview of the out-of-equilibrium thermodynamics of the SSCW, investigating how the two order parameters describing the system evolve in time

xix, 142 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.
Transparent amorphous oxide semiconductors are a relatively new class of materials which show significant promise for electronic device applications. The electron mobility in these materials is at least ten times greater than that of the current dominant material for thin-film transistors: amorphous silicon. The density of states within the gap of a semiconductor largely determines the characteristics of a device fabricated from it. Thus, a fundamental understanding of the electronic structure within the mobility gap of amorphous oxides is crucial to fully developing technologies based around them. Amorphous zinc tin oxide (ZTO) and indium gallium zinc oxide (IGZO) were investigated in order to determine this sub-gap structure. Junction-capacitance based methods including admittance spectroscopy and drive level capacitance profiling (DLCP) were used to find the free carrier and deep defect densities. Defects located near insulator-semiconductor interfaces were commonly observed and strongly depended on fabrication conditions. Transient photocapacitance spectroscopy (TPC) indicated broad valence band-tails for both the ZTO and IGZO samples, characterized by Urbach energies of 110±20 meV. These large band-tail widths imply that significant structural disorder exists in the atomic lattice of these materials. While such broad band-tails generally correlate with poor electronic transport properties, the density of states near the conduction band is more important for devices such as transistors. The TPC spectra also revealed an optically active defect located at the insulator-semiconductor junction. Space-charge-limited current (SCLC) measurements were attempted in order to deduce the density of states near the conduction band. While the SCLC results were promising, their interpretation was too ambiguous to obtain a detailed picture of the electronic state distribution. Another technique, modulated photocurrent spectroscopy (MPC), was then employed for this purpose. Using this method narrow conduction band-tails were determined for the ZTO samples with Urbach energies near 10 meV. Thus, by combining the results of the DLCP, TPC and MPC measurements, a quite complete picture of the density of states within the mobility gap of these amorphous oxides has emerged. The relationship of this state distribution to transistor performance is discussed as well as to the future development of device applications of these materials.
Committee in charge: Stephen Kevan, Chairperson, Physics; J David Cohen, Member, Physics; David Strom, Member, Physics; Jens Noeckel, Member, Physics; David Johnson, Outside Member, Chemistry

Cette thèse porte sur la RMN haute résolution de composés paramagnétiques solides. Elle présente de nouvelles méthodes permettant d'obtenir de tels spectres, ainsi que l'exploitation des spectres ainsi obtenus pour la caractérisation structurale de complexes paramagnétiques. Nous avons dans un premier temps développé des méthodes qui permettent de surmonter les difficultés liées au paramagnétisme. Ces méthodes inclues du transfert d'aimantation par recouplage dipolaire, l'adaptation d'impulsions adiabatiques pour les solides en rotation à très haute vitesse et la compréhension des phénomènes mis en cause dans leur efficacité. Nous présentons ensuite l'exploitation des données ainsi obtenues. Notamment pour la caractérisation de la structure électronique d'un catalyseur à base de fer à haut spin, la démonstration de l'absence de relaxation dite "de Curie" dans les solides ainsi que le développement d'un nouvel outil de cristallographie par RMN de poudres
This thesis is about high-resolution solid-state NMR of paramagnetic molecules. It exposes new methods to obtain high-resolution NMR spectra of paramagnetic solids. These methods gave us access to the structural information born by the electronic paramagnetism. In the first part, we propose new tools to overcome the difficulties associated with NMR of paramagnetic solids. These methods include proton to carbon magnetization transfer via dipolar recoupling, the use of adiabatic pulses with paramagnetic solids rotating at high MAS speeds, the development of a theory for a better understanding of the physics of such pulses. The second part exposes the interpretation of the high quality spectra obtained throught those methods. We characterized the electronic structure of high-spin iron (II) catalyst, we tackled the absence of the so-called "Curie relaxation" mechanism in the solid-state an we developed a new tool for crystallography thanks to proton NMR of paramagnetic powedrs

Nous etudions les proprietes spectrales et la diffusion quantique dans diverses structures quasiperiodiques dans le cadre d'un hamiltonien de liaisons fortes. Nous developpons une approche par groupe de renormalisation (g. R. ) pour calculer la fonction de green de chaines quasiperiodiques 1d. Une nouvelle analyse par g. R. Est introduite pour le pavage octogonal 2d, qui ameliore des resultats plus anciens. Nous prouvons une relation entre le comportement d'echelle de la mesure du spectre et celui de la fonction de green. La diffusion quantique est egalement etudiee dans differents systemes quasiperiodiques a 1, 2 et 3 dimensions. Des resultats errones anterieurs, dus a des contributions logarithmiques ignorees, sont corriges. A 2 et 3 d, une transition est trouvee entre un comportement ballistique et sous-ballistique en fonction de l'intensite de la modulation quasiperiodique. Enfin, un modele fractal du silicium poreux est introduit et resolu du point de vue de son spectre d'excitations

There are significant differences between experiment and theoretical calculations of the electronic structure of GaSb/GaAs self-assembled quantum dots. Using a multi-band effective mass approximation it is shown that the influence of size and geometry of quantum dots has little or no effect in determining the hydrostatic strain. Furthermore, the valenceband ground state energies of the quantum dots studied are surprisingly consistent. This apparent paradox attributed to the influence of biaxial strain in shaping the heavy-hole and light-hole potentials. Consequently, it is shown that a simple, hydrostatically derived potential is insufficient to accurately describe the electronic structure of such quantum dots. In addition, using the latest experimental results measuring the conductionband offset, it has been shown that much better experimental contact may be achieved for the magnitude of the transition energies derived compared to theoretically derived transition energies. The transition energies of Si/Ge self-assembled quantum dots has also been calculated. In particular, a range of quantum dot structures have been proposed that are predicted to have an optical response in the 3-5 micron range.

Le problème de la métallisation de l'hydrogène, posé il y a près de 80 ans, a été désigné comme la troisième question ouverte en physique du XXIe siècle. En effet, en raison de sa légèreté et de sa réactivité, les informations expérimentales sur l'hydrogène à haute pression sont limitées et extrêmement difficiles à obtenir. Il est donc essentiel de mettre au point des méthodes précises pour guider les expériences. Dans cette thèse, nous nous concentrons sur l'étude de la structure électronique, y compris les phénomènes d'état excité, en utilisant les techniques de Monte Carlo quantique (QMC). En particulier, nous développons une nouvelle méthode de calcul pour le gap accompagnée d'un traitement précis de l'erreur induit par la taille finie de la cellule de simulation. Nous établissons un lien formel entre l'erreur de la taille finie et la constante diélectrique du matériau. Avant d'étudier l'hydrogène, la nouvelle méthode est testée sur du silicium cristallin et du carbone de diamant, pour lesquels des informations expérimentales sur l'écart sont disponibles. Nos résultats montrent que le biais dû à la supercellule de taille finie peut être corrigé, de sorte que des valeurs précises dans la limite thermodynamique peuvent être obtenues pour les petites supercellules sans avoir besoin d'une extrapolation numérique. Comme l'hydrogène est un matériau très léger, les effets quantiques nucléaires sont importants. Une description précise des effets nucléaires peut être réalisée par la méthode de Monte Carlo à ions et électrons couplés (CEIMC), une méthode de simulation des premiers principes basée sur le QMC. Nous utilisons les résultats de la méthode CEIMC pour discuter les effets quantiques et thermiques des nuclei sur des propriétés électroniques. Nous introduisons une méthode formelle de traitement du gap électronique et de la structure des bandes à température finie dans l'approximation adiabatique et discutons des approximations qui doivent être faites. Nous proposons également une nouvelle méthode pour calculer des propriétés optiques à basse température, qui constituera une amélioration par rapport à l'approximation semi-classique couramment utilisée. Enfin, nous appliquons tout le développement méthodologique de cette thèse pour étudier la métallisation de l'hydrogène solide et liquide. Nous constatons que pour l'hydrogène moléculaire dans sa structure cristalline parfaite, le gap QMC est en accord avec les calculs précédents de GW. Le traitement des effets quantiques nucléaires entraîne une forte réduction du gap (2 eV). Selon la structure, le gap indirect fondamental se ferme entre 380 et 530 GPa pour les cristaux idéaux et 330-380 GPa pour les cristaux quantiques, ce qui dépend moins de la symétrie cristalline. Au-delà de cette pression, le système entre dans une phase de mauvais métal où la densité des états au niveau de Fermi augmente avec la pression jusqu'à 450-500 GPa lorsque l'écart direct se ferme. Notre travail soutient en partie l'interprétation des récentes expériences sur l'hydrogène à haute pression. Nous explorons également la possibilité d'utiliser une représentation multidéterminante des états excités pour modéliser les excitations neutres et calculer la conductivité via la formule de Kubo. Nous avons appliqué cette méthodologie à l'hydrogène cristallin idéal et limité au niveau de Monte Carlo variationnel de la théorie. Pour l'hydrogène liquide, la principale conclusion est que la fermeture du gap est continue et coïncide avec la transition de dissociation moléculaire. Nous avons été en mesure d'étalonner les fonctions de la théorie fonctionnelle de la densité (DFT) en nous basant sur la densité QMC des états. En utilisant les valeurs propres des calculs Kohn-Sham corrigé par QMC pour calculer les propriétés optiques dans le cadre de la théorie de Kubo-Greenwood , nous avons constaté que l'absorption optique théorique calculée précédemment s'est déplacée vers des énergies plus faibles
The hydrogen metallization problem posed almost 80 years ago, was named as the third open question in physics of the XXI century. Indeed, due to its lightness and reactivity, experimental information on high pressure hydrogen is limited and extremely difficult to obtain. Therefore, the development of accurate methods to guide experiments is essential. In this thesis, we focus on studying the electronic structure, including excited state phenomena, using quantum Monte Carlo (QMC) techniques. In particular, we develop a new method of computing energy gaps accompanied by an accurate treatment of the finite simulation cell error. We formally relate finite size error to the dielectric constant of the material. Before studying hydrogen, the new method is tested on crystalline silicon and carbon diamond, systems for which experimental information on the gap is available. Although finite-size corrected gap values for carbon and silicon are larger than the experimental ones, our results demonstrate that the bias due to the finite size supercell can be corrected for, so precise values in the thermodynamic limit can be obtained for small supercells without need for numerical extrapolation. As hydrogen is a very light material, the nuclear quantum effects are important. An accurate capturing of nuclear effects can be done within the Coupled Electron Ion Monte Carlo (CEIMC) method, a QMC-based first-principles simulation method. We use the results of CEIMC to discuss the thermal renormalization of electronic properties. We introduce a formal way of treating the electronic gap and band structure at a finite temperature within the adiabatic approximation and discuss the approximations that have to be made. We propose as well a novel way of renormalizing the optical properties at low temperature, which will be an improvement upon the commonly used semiclassical approximation. Finally, we apply all the methodological development of this thesis to study the metallization of solid and liquid hydrogen. We find that for ideal crystalline molecular hydrogen the QMC gap is in agreement with previous GW calculations. Treating nuclear zero point effects cause a large reduction in the gap (2 eV). Determining the crystalline structure of solid hydrogen is still an open problem. Depending on the structure, the fundamental indirect gap closes between 380 and 530 GPa for ideal crystals and 330–380 GPa for quantum crystals, which depends less on the crystalline symmetry. Beyond this pressure, the system enters into a bad metal phase where the density of states at the Fermi level increases with pressure up to 450–500 GPa when the direct gap closes. Our work partially supports the interpretation of recent experiments in high pressure hydrogen. However, the scenario where solid hydrogen metallization is accompanied by the structural change, for example, a molecular dissociation, can not be disproved. We also explore the possibility to use a multideterminant representation of excited states to model neutral excitations and compute the conductivity via the Kubo formula. We applied this methodology to ideal crystalline hydrogen and limited to the variational Monte Carlo level of the theory. For liquid hydrogen, the main finding is that the gap closure is continuous and coincides with the molecular dissociation transition. We were able to benchmark density functional theory (DFT) functionals based on the QMC density of states. When using the QMC renormalized Kohn-Sham eigenvalues to compute optical properties within the Kubo-Greenwood theory, we found that previously calculated theoretical optical absorption has a shift towards lower energies

A good permanent magnet should possess a large saturation magnetisation (Ms), large mag- netocrystalline anisotropy energy (MAE) and a high Curie temperature (TC). A difficult but important challenge to overcome for a sustainable permanent magnet industry is to find novel magnetic materials, exhibiting a large MAE, without the use of scarcely available elements such as rare-earth metals. The purpose of this thesis is to apply computational methods, including density functional theory and Monte Carlo simulations, to assess the three above mentioned permanent magnet properties and in particular to discover new replacement materials with large MAE without the use of critical materials such as rare-earths. One of the key results is the theoretical prediction of a tetragonal phase of Fe1−xCox-C with large Ms and significantly increased MAE which is later also experimentally confirmed. Furthermore, other potential materials are surveyed and in particular the properties of a number of binary alloys in the L10 structure, FeNi, CoNi, MnAl and MnGa, are thoroughly investigated and shown to posses the desired properties under certain conditions.

This thesis describes the development and testing of a new technique for the measurement of structure factors based on the matching of theoretical calculations with experimental, energy-filtered zone-axis Convergent Beam Electron Diffraction (CBED) patterns. The sum-of-squares difference between a set of experimental diffraction intensities and a theoretical calculation is minimised by varying a set of low-order structure factors until a best fit is obtained. The basic theory required for the simulation of zone-axis CBED patterns is given. Additional theory is developed specifically for the pattern matching method in order to improve the efficiency of the matching calculation. This includes the development of analytic expressions for the gradient of the sum-of-squares with respect to each of the fitting parameters, and the addition of beams to the pattern calculation by second-order perturbation theory. The effects of random and systematic errors are considered by fitting to simulated `noisy' data. A wide range of potential systematic error effects are investigated and limits are found for errors in the accelerating voltage, Debye-Waller factor and lattice parameter which reduce systematic errors to acceptable levels. These tests also investigate the sensitivity of the method to structure factor variations, which gives an indication of how many structure factors can be measured. Finally, the method is applied to the measurement of low-order structure factors from experimental Si [110] zone-axis patterns. The results are compared to the best X-ray Pendellösung measurements available, and the bonding charge densities obtained from both the zone-axis and X-ray measurements are constructed