Andrea Damascelli

The electron-phonon interaction is ubiquitous in crystalline materials, leaving fingerprints on both physical and electronic properties. In the study of materials, angle-resolved photoemission spectroscopy (ARPES) can uniquely access the electronic band structure, and the electron interactions encoded within. However, disentangling the contributions from different degrees of freedom -- such as electron-electron and electron-phonon – can be very challenging. Extension of ARPES into the time domain via pump-probe spectroscopy allows one to access the electronic structure on an ultrafast timescale: this is advantageous as the intertwined interactions in equilibrium become separated in the time domain. In this thesis, we use time-resolved (TR)-ARPES to study the electron-phonon interaction in graphite. Specifically, we observe spectral features arising from the photoexcitation of electrons from the valence band to the conduction band, followed by quantized energy-loss processes corresponding to the emission of strongly-coupled optical phonons. The transfer of spectral weight from an identifiable initial state to a final state is the direct manifestation of a microscopic two-body scattering process from which we can extract the mode-projected electron-phonon matrix element. The spectral features observed in this study arise from the non-thermal (i.e. non-Fermi-Dirac) occupation of electrons. We use Boltzmann simulations to map out various regimes in graphite where non-thermal features arise. These non-thermal signatures are not unique to graphite but are ubiquitous in pump-probe experiments and intrinsically tied to the dominant scattering processes, their timescales, and corresponding bottlenecks. Our results were made possible by a custom-built state-of-the-art laser source featuring cavity-enhanced high-harmonic generation. The source has three key features: First, high photon energies capable of mapping the whole Brillouin zone of materials; second, a high repetition rate that minimizes the space-charge effect; and last, a balanced time and energy resolution capable of studying subtle spectral features. All three elements were crucial in discerning the spectral features related to electron-phonon scattering in graphite, the first observation of its kind. With these results, we demonstrate that the maturation of high-harmonic sources can now offer a tunable table-top source with unprecedented intensity, repetition rate and resolution.

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Graphene, the first truly 2D material to be isolated, is host to a wealth of remarkable properties. It can be modified in a variety of ways—strained, twisted, stacked, placed on a substrate, decorated with adatoms, etc.—to further enhance these properties or introduce new ones. In this thesis, we use several complementary surface characterization techniques to study two methods of modifying epitaxial graphene on a silicon carbide (SiC) substrate via the addition of other atoms.In the first method, we induce the Kekulé distortion—a periodic distortion of the bonds in graphene—using a small number of lithium atoms (
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The physics of strongly correlated materials is at the heart of current condensed matter research. The inclusion of interactions in these materials between electron themselves or with other excitations intertwines various degrees of freedom (orbital, spin, charge and lattice), leading to a number of novel phenomena like Mott-Hubbard and charge-transfer insulators, high-temperature superconductivity and mixed-valence and Kondo physics. This thesis focuses on the study of two classes of correlated materials: copper-oxide high-temperature superconductors, whose correlated physics is driven by the localized nature of the half-filled Cu 3d-orbitals, and the rare-earth hexaborides, which are characterized by the strongly correlated 4f-shell. Recently, it has been shown that the interplay between different mechanisms underlying the formation of the superconducting condensate in the hole-doped bi-layer Bi₂Sr₂CaCu₂O₈₊δcan be addressed in the time domain by means of time- and angle-resolved photoemission spectroscopy (TR-ARPES). Using this technique, the primary role of phase coherence has been established. By exploiting the same dynamical experimental approach, we show that such scenario also describes the ultrafast collapse of superconductivity in the single-layer compound Bi₂Sr₂CuO₆₊δ. Moreover, by performing a comprehensive study on different doping levels of both single- and bi-layer compounds, we provide new insights on the temperature evolution of the nodal quasiparticle spectral weight. The second part of the thesis focuses on electron-doped cuprates, addressing the putative relation between the spectroscopically observed pseudogap and the robust antiferromagnetic order. Employing TR-ARPES as a tool to perform a detailed temperature dependent investigation allows us to explicitly link the momentum-resolved pseudogap spectral features to the evolution of the short-range spin-fluctuations in the optimally-doped Nd₂-xCe₂CuO₄. Lastly, we make use of chemical substitution to investigate the mixed-valent character of the rare-earth hexaboride SmxLa₁-xB₆ series. Our combined ARPES and x-ray absorption measurements reveal a departure from a monotonic evolution of the Sm valence as a function of x and the possible emergence of a mixed-valent impurity regime.

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The light-matter interaction is central to the photoemission process, with an ultraviolet photon providing the necessary impulse required to eject those electrons which we collect in an effort to understand the electronic structure of matter. As such, selection rules impose constraints on those electronic orbits to which one is sensitive. Photoemission-based techniques then present an opportunity to access information beyond spectroscopic characterization of a material's level structure; an orbital description of the underlying wavefunctions is also viable. We present here a numerical scheme within which such information can be garnered, with specific application to several experiments on candidate materials.The Fe-based superconductors are an ideal platform for application of this methodology. The electronic structure is characterized by a large number of closely spaced, moderately correlated states. The competition and cooperation between several competing energy scales pose a considerable challenge for both theory and experiment. The unique sensitivity to both spin and orbital degrees of freedom which photoemission provides therefore allow for a comprehensive exploration of the various energy scales in these compounds. Taking advantage of this sensitivity, we have mapped the momentum and energy dependence of spin-orbit entanglement in representative compounds, FeSe and LiFeAs.Despite the surface sensitivity which inhibits access to the crystal bulk in photoemission, there is a strong inclination to assert a correspondence between the bulk electronic structure, and that measured experimentally, a contentious claim which is frequently the cause of misinterpretation. We explore the surface issue in detail, and discover an interference mechanism which provides justification for the unanticipated success of valence-band photoemission in quasi two-dimensional materials. The surface issue is of specific relevance to the Fe-superconductors, where certain orbitals exhibit significant dispersion perpendicular to the surface. We examine the canonical Fe-superconductor LiFeAs, wherein a confluence of three-dimensional dispersion, spin-orbit coupling, and surface states have conspired to preclude identification of the low-energy electronic structure. We combine detailed photon-energy dependent measurements with results from a slab-projected model to unambiguously identify the three-dimensional Fermi surface of this material.

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Transition-metal oxides (TMOs) are a widely studied class of materials with fascinating electronic properties and a great potential for applications. Sr₂IrO₄ is such a TMO, with a partially filled 5d t₂g shell. Given the reduced Coulomb interactions in these extended 5d orbitals, the insulating state in Sr₂IrO₄ is quite unexpected. To explain this state, it has been proposed that SOC entangles the t₂g states into a filled jeff = 3/2 state and a half-filled jeff = 1/2 state, in which a smaller Coulomb interaction can open a gap. This new scheme extends filling and bandwidth, the canonical control parameters for metal-insulator transitions, to the relativistic domain. Naturally the question arises whether in this case, SOC can in fact drive such a transition. In order to address this question, we have studied the behaviour of Sr₂IrO₄ when substituting Ir for Ru or Rh. Both of these elements change the electronic structure and drive the system into a metallic state. A careful analysis of filling, bandwidth, and SOC, demonstrates that only SOC can satisfactorily explain the transition. This establishes the importance of SOC in the description of metal-insulator transitions and stabilizing the insulating state in Sr₂IrO₄.It has furthermore been proposed that the jeff = 1/2 model in Sr₂IrO₄ is an analogue to the superconducting cuprates, realizing a two-dimensional pseudo-spin 1/2 model. We test this directly by measuring the spin-orbital entanglement using circularly polarized spin-ARPES. Our results indicate that there is a drastic change in the spin-orbital entanglement throughout the Brillouin zone, implying that Sr₂IrO₄ can not simply be described as a pseudo-spin 1/2 insulator, casting doubt on direct comparisons to the cuprate superconductors. We thus find that the insulating ground state in Sr₂IrO₄ is mediated by SOC, however, SOC is not strong enough to fully disentangle the jeff = 1/2 state, requiring that Sr₂IrO₄ is described as a multi-orbital relativistic Mott insulator.

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The physics of quantum materials is at the heart of current condensed matter research. The interactions in these materials between electrons themselves, with other excitations, or external fields can lead to a number of macroscopic quantum phases like superconductivity, the quantum Hall effect, or density wave orders. But the experimental study of these materials is often hindered by complicated structural and chemical properties as well as by the involvement of toxic elements.Graphene, on the other hand, is a purely two-dimensional material consisting of a simple honeycomb lattice of carbon atoms. Since it was discovered experimentally, graphene has become one of the most widely studied materials in a range of research fields and remains one of the most active areas of research today. However, even though graphene has proven to be a promising platform to study a plethora of phenomena, the material itself does not exhibit the effects of correlated electron physics.In this thesis, we show two examples of how epitaxially grown large-scale graphene can be exploited as a platform to design quantum phases through interaction with a substrate and intercalation of atoms. Graphene under particular strain patterns exhibits pseudomagnetic fields. This means the Dirac electrons in the material behave as if they were under the influence of a magnetic field, even though no external field is applied. We are able to create large homogeneous pseudomagnetic fields using shallow nanoprisms in the substrate, which allows us to study the strain-induced quantum Hall effect in a momentum-resolved fashion using angle-resolved photoemission spectroscopy (ARPES).In the second part, we show how the intercalation of gadolinium can be used to couple flat bands in graphene to ordering phenomena in gadolinium. Flat bands near the Fermi level are theorised to enhance electronic correlations, and in combination with novel ordering phenomena, play a key role in many quantum material families. Our ARPES and resonant energy-integrated X-ray scattering (REXS) measurements reveal a complex interplay between different quantum phases in the material, including pseudogaps and evidence for a density wave order.

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The work presented in this thesis is part of the design of the high resolution mass separator for the ARIEL facility under construction at TRIUMF, located in the UBC campus. This new facility, together with the existing ISAC facility, will produce rare isotope beams for nuclear physics experiments and nuclear medicine. The delivery of such beams requires a stage of separation after production to select the isotope of interest. The required separation is expressed in terms of resolving power defined as the inverse of the relative mass difference between two isotopes that need to be separated. The higher the mass the greater the resolving power required. The challenge is the separation of two isobars rather than two isotopes that by definition require a much lower resolving power. A resolving power of twenty thousand is the minimum required to achieve isobaric separation up to the uranium mass. The state of the art for existing heavy ion mass separators is a resolving power in the order of ten thousand for a transmitted emittance of less than three micrometers. The more typical long term operational value is well below ten thousand for larger emittances. The main goal of this project is to develop a mass separator that maintains an operational resolving power of twenty thousand. Different aspects influence the performance of the mass separator; the two main ones are the optics design and the field quality of the magnetic dipole(s) that provides the core functionality of the mass separator. In this thesis we worked from the hypothesis that minimizing the magnetic field integral variation with respect to the design mass resolution is equivalent to minimizing the aberration of the optical system. During this work we investigated how certain geometric parameters influence the field quality, as for example the dependency of the field flatness on the magnet pole gap. We also developed a new technique to control the mesh in the finite element analysis to facilitate particle tracking calculations. Beyond demonstrating our hypothesis, we ultimately produced a final magnet design where the field integral variation is less than one part in one hundred thousand.

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This thesis traces a path from conventional superconductivity in a bulk material to the introduction of superconductivity and other novel phenomena in graphene. Magnesium diboride is a conventional superconductor, where the pairing is mediated by the electron-phonon coupling. ARPES (angle resolved photoemission spectroscopy) is shown to be an excellent probe to quantitatively study the momentum dependence of the electron-phonon coupling, demonstrating the origin of the distribution of superconducting gap sizes previously observed with other experimental techniques.Next, we exploit our understanding of the electron-phonon coupling to study how it can be tuned in a low dimensional system. It is shown that the electron-phonon coupling in graphene can be strongly enhanced by the deposition of alkali adatoms. High resolution, low temperature ARPES measurements provide the first experimental evidence of superconductivity in this two-dimensional system, showing a temperature dependent pairing gap, and an estimated Tc of ~ 6K.Finally, we present a study of another graphene-adatom system expected to show novel physics. Thallium on graphene has been predicted to enhance the spin-orbit coupling, leading to a robust topological insulator state. From ARPES measurements characterizing this system, we disentangle the long-range and short-range scattering contributions and show that thallium atoms act as surprisingly strong short-range scatterers. Our results are consistent with theoretical predictions for this system, indicating it is a good place to search for a two-dimensional topological insulator.

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Charge segregation is very common in correlated oxides, spanning from the extreme limit of the Mott insulator, characterized by strong charge localization and suppressed charge dynamics, towards more mildly correlated states of matter, characterized by partial charge reorganization (charge-density-wave).In this thesis work I have investigated how the spatial organization of valence charge evolves with electrostatics (carrier doping) and chemistry (going down in the periodic table). For this reason, the focus is on the experimental study of the strongly-correlated 3d-oxides and the spin-orbit coupled 5d-oxides.These investigations have been performed with a bundle of state-of-the-art spectroscopic techniques in the field of quantum materials, namely: angle-resolved photoemission (ARPES), low-energy electron diffraction (LEED), resonant elastic X-ray scattering (REXS), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and optical spectroscopy. To support experimental data, we have used theoretical tools such as conventional density functional theory (DFT) for the 5d-oxides and developed ad-hoc approaches for the more complex 3d-materials.The all-around study of underdoped high-Tc Bi-based cuprates allowed us to shed new light on the universality and origin of charge-ordering instabilities in these materials and understand their interplay with superconductivity and pseudogap. These phenomena have been investigated in detail in underdoped samples of Bi2201, using a various experimental techniques (ARPES, REXS, XRD, LEED, STM) and various tailored theoretical approaches. The results of these studies are presented in Chapters 2 and 3.In the 5d-based iridates (in particular, Na₂IrO₃) we have revealed and characterized a novel form of Mott-Hubbard physics. This has been possible thanks to the combination of ARPES and optics for probing the electronic structure near the Fermi energy, and DFT for providing the theoretical framework to understand the electronic ground state in these materials. Ultimately, this approach helped demonstrate the crucial role of spin-orbit interaction in driving a novel Mott phase in materials where the Mott criterion might be violated (Chapter 4).Altogether, the resulting phenomena discovered in copper- and iridium based oxides have revealed novel unconventional aspects of the physics of correlated materials, thus paving the way for future explorations of the complex but fascinating jungle of transition metal oxides.

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This dissertation describes two related projects: the development of a coherent Lyman-α source and the implementation of a supersonic hydrogen beam.A two-photon resonance-enhanced four wave mixing process in krypton is used to generate high power coherent radiation at ωLy_α ⇒ 121.56 nm, the hydrogen Lyman-α line, to perform spectroscopy and cooling of magnetically trapped antihydrogen (1s − 2p transition). This is a tool to directly test both the Einstein Equivalence Principle and Charge, Parity, and Time inversion symmetry. The former can be tested by measuring the gravity interaction of matter and antimatter. Inversion symmetry can be tested by comparing the spectroscopic properties of hydrogen and antihydrogen. Both experiments require optically cooled antihydrogen. Under the current trapping conditions, optical cooling could be performed with nanosecond long pulses of 0.1 μJ of Lyman-α radiation at a repetition rate of 10 Hz.The process to generate Lyman-α radiation uses two wavelengths (ωR ⇒ 202.31 nm and ωT ⇒ 602.56 nm), which are mixed in a sum-difference scheme (ωLy_α = 2ωR−ωT ) with a two-photon resonance at (4s²4p⁵5p[1/2]₀ ← 4s²4p⁶(¹S₀) ). The source implemented produces 1.2 μW at the Lyman-α line and this was confirmed by performing spectroscopy of hydrogen. The design, implementation and characterization of the source are discussed in this dissertation.In the second part of the dissertation the implementation of the hydrogen beam and its characterization are discussed. The atomic hydrogen is generated with a thermal effusive source and it is entrained by an expanding noble gas. This process generates a cold beam of hydrogen atoms. Hydrogen is separated from the noble gas with a Zeeman bender that uses the forces generated by the Zeeman shift of low field seeking states of hydrogen and engineered magnetic field gradients. The hydrogen beam was characterized with a quadrupole mass spectrometer. The seed noble gas beam was characterized by colliding it with ultra-cold rubidium atoms in a magneto-optical trap. The trapped atoms loss rate resulting from these collisions can be used to measure the density of the atomic beam. This measurement demonstrates the potential of using magneto-optical traps as absolute flux monitors.

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Topological insulators (TIs), with a gapless surface state located in a large bulk band gap, define a new class of materials with strong application potential in quantum electronic devices. However, real TI materials have many critical problems, such as bulk conductivity and surface instability, which hinder us from utilizing their exotic topological states. Another fundamental question in the TI field is what the realistic spin texture of the topological surface states (TSSs) is; no conclusive answer has yet been reached, despite extensive studies.We present two studies of doping the prototypical TI materials via in situ potassium deposition at the surface of Bi₂Se₃ and by adding magnetic impurities into the bulk Bi₂Te₃ during crystal growth. We show that potassium deposition can overcome the instability of the surface electronic properties. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with tunable, reversible, and highly stable spin splitting. Our density functional theory (DFT) calculations reveal that these Rashba states are derived from quantum well states associated with a K-induced 5 nm confinement potential. The Mn impurities in Bi₂Te₃ have a dramatic effect on tailoring the spin-orbit coupling of the system, manifested by decreasing the size of the bulk band gap even at low concentrations (2%--5%). This result suggests an efficient way to induce a quantum phase transition from TI to trivial insulators.We also explicitly unveil the TSS spin texture in TI materials. By a combination of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and DFT slab calculations, we find that the surface states are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. We predict a way to probe the intrinsic spin texture of TSS, and to continuously manipulate the spin polarization of photoelectrons all the way from 0 to +/-100% by an appropriate choice of photon energy and linear polarization. Our spin-resolved ARPES experiment confirms these predictions and establishes a generic rule for the manipulation of photoelectron spin polarization. This work paves a new pathway towards the long-term goal of utilizing TIs for opto-spintronics.

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This thesis represents two bodies of work: a detailed look at what angle- resolved photoemission spectroscopy (ARPES) measures, as well as ARPES and circularly polarized photon spin- and angle-resolved photoemission spectroscopy (CPS-ARPES) measurements on the unconventional superconductor Sr₂RuO₄.In the first part I present a study of both established methods of ARPES analysis and some new variations on model spectral functions. This modelling was done in a realistic regime, yet far from the limits often assumed. Away from these limits I show that any "effective coupling" inferred from quasiparticle renormalizations differs drastically and unpredictably from the true coupling. Conversely, I show that perturbation theory retains good predictive power where expected, that the momentum dependence of the self-energy can be revealed via the relationship between velocity renormalization and quasiparticle strength, and that it is often possible to infer the self-energy and bare electronic structure through lineshape analysis.In the second part I present experimental ARPES and CPS-ARPES data on Sr₂RuO₄. Newly discovered and unexplained ARPES features are characterized and compared with a variety of different possible structural distortions through bulk and slab local-density approximation (LDA) band structure calculations. I thereby rule out phases driven by electronic interaction, such as Dirac- or Rashba-type surface states, and instead find that there exists a progressive structural modulation whereby both the surface and (at a minimum) sub-surface layers exhibit a (√2 x √2)R45° reconstruction.Through CPS-ARPES on Sr₂RuO₄ I also directly demonstrate that the effects of spin-orbit (SO) coupling are not limited to a modification of the band structure, but fundamentally entangle the spin and spatial parts of the wave-function. This must drive the superconducting state in Sr₂RuO₄ to be even more unconventional than is generally assumed, with mixing between singlet and triplet states that varies around the Fermi surface, and thereby offers a possible resolution to a number of experiments that clash with the categorization of Sr₂RuO₄ as a hallmark spin-triplet chiral p-wave superconductor.

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The puzzling physics of the high-temperature superconducting cuprates has led to many important questions and investigations regarding the mechanism of high-Tc's. This dissertation demonstrates that, through detailed experimental studies using angle-resolved photoemission (ARPES) and low-energy electron diffraction (LEED), a new type of periodic structural distortion exists in Bi₂Sr₂-xLaxCuO₆₊d (La-Bi2201), leading to the existence of multiple periodic lattice distortions (PLD) in the underdoped material. Photon-energy dependent ARPES reveals photoelectron diffraction effects that exhibit all relevant length scales as oscillations in the ARPES matrix element, leading to the observation of all of the diffraction-replica (DR) bands associated with the multiple PLD's. Furthermore, a charge-density-wave (CDW) associated with the new PLD at the crystal surface with a temperature-dependent wavelength is observed. A Ginzburg-Landau mean field model is shown to exhibit the same temperature-dependence from a combination of Fermi surface nesting and lattice commensurability affected by the temperature-dependent harmonic content of the CDW. A detailed temperature-dependence of the antinodal pseudogap reveals two simultaneous energy and temperature scales: one associated with the CDW, and the other in agreement with the magneto-optical Kerr effect and magnetic neutron diffraction results in the literature. Detailed nodal ARPES lineshapes reveal a peak in the real-part of the self-energy that coincides with band-crossing positions and the nodal ''kink'' energies for all dopings, providing evidence that these disparate features can be collectively explained by band hybridization allowed in the fully incommensurate crystal, which does not preserve parity. This provides one route to explain these phenomena - as a product of nested van-Hove singularities along the nodal line in underdoped La-Bi2201, which result in a Peierls distortion similar to the dichalcogenide CDW superconductors.

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Recent angle-resolved photoemission spectroscopy (ARPES) results, from experimentsperformed at the Swiss Light Source, Stanford Synchrotron RadiationLightsource, and Advanced Light Source synchrotrons, on the high-temperaturesuperconductors Tl₂Ba₂CuO₆+δ (Tl2201) and YBa₂Cu₃O (YBCO₆.₅) arepresented.An overdoped Tl2201 sample with a TC of 30K was found to have a Fermisurface, consisting of a large hole pocket centred at (Pi ,Pi ), which is approachinga topological transition. A superconducting gap consistent with a dχ₂−y₂ orderparameter was detected. In contrast with the underdoped HTSCs, where thequasiparticle (QP) linewidth at the top of the band is maximal in the antinodaldirection and minimal in the nodal direction, overdoped Tl2201 was revealed tohave a reverse nodal-antinodal anisotropy, with sharp QP peaks in the antinodalregion and broader peaks in the nodal region. The Tl2201 results establishTl2201 as a valuable material for exploring the overdoped side of the phasediagram with ARPES.Synchrotron experiments also yielded the first successful ARPES resultson underdoped YBa₂Cu₃O₇−δ (YBCO). Surface-sensitivetechniques were previously unsuccessful in studying YBCO becauseits cleaved surfaces are polar, resulting in an overdoped surface regardless of thedoping level of the bulk. By doping the cleaved surfaces with potassium, thesurface was progressively hole underdoped from as-cleaved continuously to thedoping of the bulk, and subsequent ARPES experiments performed revealed atransition from a holelike Fermi surface on the as-cleaved surface to disconnectedFermi arcs when the surface doping matched the bulk.In parallel with the synchrotron-based research, an in-house ARPES systemwas constructed at the University of British Columbia (UBC). Unlike conventionalARPES systems, the ARPES setup at UBC incorporates a molecularbeam epitaxy (MBE) system, which allows novel materials to be grown, characterized,and transferred to the ARPES chamber in vacuo. Novel design techniques to improve the accuracy of ARPES measurements are presented.The in-house ARPES system also serves as a prototype for an ARPES–MBEendstation being constructed at the Canadian Light Source (CLS). Design studiesof some potential improvements to the in-house system, to be implementedat the CLS, are also presented.

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The overarching theme of this thesis is the growth and characterization of thin ferromagnetic oxide films. This is a very broad project, and as a result, this thesis covers a variety of related topics. These include technical aspects, like the design and commissioning of a combined film growth and analysis system (Chapter 2), the development of an algorithm to build up surface diffraction patterns from single-reflection high-energy electron diffraction images (Chapter 3) and a chapter detailing methods to measure and correct for various non-linearities in the response of electron analyzers used in photo-electron spectroscopy (Chapter 4). An intermediate chapter (Chapter 5) deals with theoretical calculations to determine the effect of substituting pnictogens (nitrogen, phosphorous and arsenic, specifically) for oxygen in EuO. In particular, it is determined which systems are most likely to synthesize without phase separating and how the system reacts to the addition of acceptor sites from the pnictogen. This information motivates the experimental work in Chapter 6 on nitrogen-substituted EuO, which uses a novel growth technique to produce the first example of a mixed valent europium system. Both the development of a novel growth technique and the study of a new type of ferromagnetic semiconductor are important first steps in building future spintronic devices. The final chapter (Chapter 7) details attempts to grow nitrogen-substituted SrO in an attempt to induce ferromagnetic ordering in a normally non-magnetic oxide by spin polarizing the p-states. The results in this last chapter demonstrate that p-state derived magnetism is present in SrO(₁₋x)Nx.

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The primary objective of this research is to develop and study newer ways to control electronic properties of correlated oxide systems using impurities. This goal has been achieved by introducing dilute localized 3d Mn impurities in place of a delocalized 4d Ru sites in a 2-dimensional Ru-O matrix and doping an electronically reconstructed polar surface of YBCO via surface impurities.The first part of the work concentrates on X-ray Absorption Spectroscopy (XAS) andResonant Soft X-ray Scattering (RSXS) studies on lightly Mn doped Sr₃Ru₂O₇ . Our goalis to understand the electronic structure of the material both at room and low temperatureand ultimately to understand the mechanism behind the low temperature metal-insulatortransition in this compound. With XAS, we zoom into the local electronic structure ofthe impurities themselves and discovered unusual valence and crystal field level inversionin the Mn impurities. With the help of density functional theory and cluster multipletcalculations, we developed a model to describe the hierarchy of the crystal eld levels ofthe Mn impurities. Long range magnetic properties of the compound has been probedwith Resonant Soft X-ray Scattering (RSXS) on the Mn and Ru L-edges. We have foundand analyzed the (¼,¼,0) structurally forbidden diffraction peak and connected it to an antiferromagnetic instability in the parent compound. Doping dependent scattering studies revealed the magnetic structure of the low temperature insulating phase and ultimately the mechanism behind the metal-insulator transition itself.Angle Resolved Photoemission Spectroscopy (ARPES) has been used to investigate the low energy electronic structure of underdoped high-Tc superconductor YBCO. It has been revealed that due to the presence of polar surfaces there is electronic reconstruction on the surface of a cleaved YBCO sample. A novel technique has been devised that allows one to control the surface doping level in-situ by evaporating Potassium (K) on the YBCO surface. We showed that K tunes the surface doping level by donating electrons and thereby makes it possible to continuously tune the surface doping level throughout the phase diagram.

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