Natural and synthetic SnO₂ samples were studied using scanning tunneling microscopy (STM). The SnO₂ surface flattens considerably following high temperature treatments up to 1500 K. The conductivity of the synthetic SnO₂ surface is significantly reduced following annealing at temperatures of approximately 1200-1500 K, making tunneling impossible. A decrease in conductivity was not observed for the natural SnO₂ sample following similar high temperature treatments, most likely due to impurities which act as dopants. No atomic scale images were collected on the SnO₂ surface which provided information regarding atomic positions and point defects on the surface. Water adsorption was studied on the stoichiometric Cr₂O₃ (101̲2) surface, using thermal desorption spectroscopy (TDS). Water was the only desorption product observed during TDS. Adsorption is primarily dissociative following exposure to water at 163 K. Approximately, 0.12 monolayers of water dissociate on the clean, nearly stoichiometric Cr₂O₃ (101̲2) surface. The first order kinetics observed for the recombination of dissociated water are not well understood. One possible explanation is that the rate limiting step for desorption involves the breaking of a Cr-O bond resulting in a freely diffusing OH species. The exchange of halogen and oxygen was studied on Cr₂O₃ (101̲2) using Auger electron spectroscopy (AES) and TDS. The exchange of chlorine and oxygen is completely reversible. Chlorine is removed from the Cr₂O₃ (101̲2) surface following exposure to oxygen. Exposure of CFCl₂CH₂Cl reduces the surface oxygen concentration to that of the clean, nearly stoichiometric Cr₂O₃ (101̲2) surface. The exchange of chlorine with oxygen appears to involve only chemisorbed surface oxygen, not bulk lattice oxygen. / Master of Science

The growth mechanism of chemical vapor deposition (CVD) grown homo-epitaxial diamond (110) and (111) films was studied using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). In addition, the field emission properties of diamond coated molybdenum microtips were studied as a function of exposure to different gases.

Thesis advisor: Ilija Zeljkovic / We used spectroscopic-imaging scanning tunneling microscopy (SI-STM) and spin-polarized STM (SP-STM) to unveil new electronic phenomena in several different quantum systems. We explored: (1) a potential topological superconductor heterostructure Bi₂Te₃/Fe(Te, Se), (2) high-Tc superconductors − Bi₂Sr₂CaCu₂O₈₊ₓ and Fe(Te, Se), and (3) doped spin-orbit Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. In Bi₂Te₃/Fe(Te, Se), we observed superconductivity (SC) on the surface of Bi₂Te₃ thin film, induced by the iron-based superconductor substrate. By annealing the optimally-doped cuprate superconductor Bi₂Sr₂CaCu₂O₈₊ₓ, we drastically lowered the surface hole doping concentration to detect a unidirectional charge stripe order, the first reported charge order on an insulating (defined by the spectral gap with zero conductance spanning the Fermi level) cuprates surface. In the high-Tc SC Fe(Te, Se) single crystal, we found local regions of electronic nematicity, characterized by C₂ quasiparticle interference (QPI) induced by Fermi surface anisotropy and inequivalent spectral weight of dyz and dxz orbitals near Fermi level. Interestingly, the nematic order is locally strongly anti-correlated with superconductivity. Finally, utilizing SP-STM, we observed a short-range antiferromagnetic (AF) order near the insulator-metal transition (IMT) in spin-orbital Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. The AF order inhomogeneity is found not to be strongly correlated with the charge gap. Interestingly, the AF order in the bi-layered Sr₃Ir₂O₇ shows residual memory behavior with temperature cycling. Overall, our work revealed new phenomena in a range of today’s most intriguing materials and set the stage for using SP-STM in other complex oxides. / Thesis (PhD) — Boston College, 2020. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Inventado no início dos anos 1980, o STM evoluiu para uma importante ferramenta na investigação das propriedades de superfícies e interfaces, com aplicações em várias áreas da ciência além da Física, como Ciências de Materiais e Química. O presente trabalho trata da microscopia de tunelamento, que se baseia no fenômeno do tunelamento quântico dos elétrons. No STM, a distância entre ponta e a amostra é reduzida até que as funções de onda dos elétrons na ponta e na superfície da amostra se sobreponham. Nessa situação, observa-se o fenômeno do tunelamento quântico de elétrons através da barreira formada entre os dois eletrodos (ponta e amostra). Como o valor da corrente de tunelamento é fortemente dependente da distância ponta-amostra, um microscópio STM pode ser utilizado para mapear a morfologia da superfície da amostra com alta resolução espacial. Além disso, outra importante capacidade do STM é a possibilidade de atuar no modo espectroscópico (STS). Por vezes, o estudo detalhado das propriedades de um sistema requer o uso de métodos não convencionais de microscopia STM. Um exemplo é o estudo do magnetismo de nanoestruturas por microscopia de tunelamento com resolução em spin (SP-STM). A implementação destes métodos não convencionais normalmente exigem recursos experimentais específicos, nem sempre disponíveis em equipamentos comercias. A versatilidade no controle das características funcionais do equipamento foi a principal razão que nos motivou a construir o microscópio STM. No primeiro capitulo faremos uma introdução geral ao tema da dissertação. Uma breve introdução ao método de STM será dada no capítulo 2, incluindo aspectos fundamentais do tunelamento quântico, bem como sua aplicação técngeralica. O capítulo 3 descreve o estudo das propriedades eletrônicas e magnéticas do grafeno preparado sobre a superfície vicinal Ni(977). O magnetismo observado na camada de grafeno induzido por um substrato ferromagnético é de grande interessante para o desenvolvimento de dispositivos spintrônicos. Relatamos a investigação das propriedades de uma monocamada de grafeno preparada sobre de Ni(977) por CVD, utilizando a microscopia de varredura por tunelamento (STM), dicroísmo circular magnético de raios-x (XMCD) e espectroscopia de fotoelétrons (XPS). No capítulo 4, apresentamos o detalhamento do projeto de construção de um STM para operar em ambiente de ultra alto vácuo (UHV). Características do STM como isolamento de vibração, desenho mecânico, sistema de varredura e processo de preparação de pontas são discutidos. Por fim, analisamos os resultados dos testes de operação no microscópio STM, dificuldades observadas ao longo do projeto e possíveis melhorias. / Invented in the early 1980s, STM has evolved into a standard tool to investigate the properties of surfaces and interfaces, with applications in various research fields, such as physics, material sciences and chemistry. The present work deals with scanning tunnelling microscopy, which refers to the quantum phenomenon of electron tunneling through a potential barrier. The distance between a conductive probe tip and sample is reduced until the electron wave functions of tip and sample surface have significant overlap, and electrons can tunnel through the vacuum barrier. As the so-detected tunneling current is strongly distance-dependant, it can be used to map the morphology of the sample surface with a resolution which goes far beyond the actual meaning of the term microscopy. Besides its unique spatial resolution, one strength of the STM is the possibility to perform local electronic spectroscopy. STM has evolved into a standard tool to investigate the properties of surfaces and interfaces, with applications in various research fields besides physics, such as material sciences, chemistry, or biology. Nevertheless, the detailed study of materials properties requires the use of non-conventional methods of STM microscopy. One example is the study of magnetism in nanostructures by spin polarized scanning tunneling microscope (Sp-STM). The implementation of such unconventional methods typically requires specific experimental features, not always available in commercial equipment. This versatility in controlling the functional characteristics of the equipment was one of the main reasons that motivated us to design and build our own STM microscope. A general introduction to the dissertation theme is presented in chapter 1. In chapter 2, a brief discussion about the method of STM is developed, including theoretical aspects on quantum tunneling. In chapter 3, the experimental study of electronic and magnetic properties of the graphene/Ni(977) is shown. The recent observation of magnetism in graphene layers induced by a ferromagnetic substrate is a very interesting issue and can impact the design of new carbon-based spintronic devices. Here we report on the investigation of the electronic and magnetic properties of the graphene/Ni(977) by using scanning tunneling microscope (STM), x-ray magnetic circular dichroism (XMCD) and x-ray photoelectron spectroscopy (XPS). In chapter 4, it was described the project and construction of a homemade STM operated in ultrahigh vacuum (UHV) condition. We discuss the details of main STM-UHV components: main and sample preparation chambers, STM head, tip scanner, and sample holder design. Characteristics of the mechanical and electronic design, vibration isolating system, tip and sample preparation are discussed. Finally, we report the results of testing experiments as well as discuss the encountered difficulties and some possible solutions.

Since its physical isolation via the "scotch tape method," graphene (a monolayer of graphite) has attracted much attention from both the solid-state and high-energy scientific communities because its elementary excitations mimic relativistic chiral fermions. This has allowed graphene to act as a testbed for exploring exotic forms of symmetry breaking and for verifying certain longstanding theoretical predictions dating back to the very first formulation of relativistic quantum mechanics. In this dissertation I describe scanning tunneling microscopy and spectroscopy experiments that visualize ordered electronic states in graphene that originate from its unique chiral structure. Two detailed investigations of chemical vapor deposition graphene grown on copper are presented. In the first, a heretofore unrealized phase of graphene with broken chiral symmetry called the Kekulé distortion is directly visualized. In this phase, the graphene bond symmetry breaks and manifests as a (√3×√3)R30° charge density wave. I show that its origin lies in the interactions between individual vacancies ("ghost adatoms") in the crystalline copper substrate that are mediated electronically by the graphene. These interactions induce the formation of a hidden order in the positions of the ghost adatoms that coincides with Kekulé bond order in the graphene itself. I then show that the transition temperature for this ordering is 300K, suggesting that Kekulé ordering occurs via enhanced vacancy diffusion at high temperature. In the second, Klein tunneling of electrons is visualized for the first time. Here, quasi-circular regions of the copper substrate underneath graphene act as potential barriers that can scatter and transmit electrons. At certain energies, the relativistic chiral fermions in graphene that Klein scatter from these barriers are shown to fulfill resonance conditions such that the transmitted electrons become trapped and form standing waves. These resonant modes are visualized with detailed spectroscopic images with atomic resolution that agree well with theoretical calculations. The trapping time is shown to depend critically on the angular momenta quantum number of the resonant state and the radius of the trapping potential, with smaller radii displaying the weakest trapping.

Iron-based high temperature superconductors are a large family of materials that exhibit unconventional superconductivity and arise from antiferromagnetically-ordered parent compounds. One of the grand challenges in understanding the behavior of these materials is determining the physical mechanisms responsible for the transition into the superconducting state. This thesis describes two recent investigations to explore the magnetic and superconducting properties of NaFeAs in response to changes in temperature and nickel dopants. The peculiar interplay of magnetism and superconductivity in nickel-doped NaFeAs is elucidated using muon spin rotation. Our experimental findings on this novel system are supported with both computational and theoretical calculations. The second investigation describes an improvement to the analysis framework to the scanning tunneling microscopy technique that leverages recent advances in nonconvex optimization. This novel approach is applied directly to microscopy images of NaFeAs to provide unprecedented phase-sensitive access to the quasiparticle scattering spectrum in the material. These results place constraints on theoretical models that describe the local electronic structure and physics of NaFeAs.

Scanning tunneling microscopy has been a powerful tool in expanding our understanding in the study of condensed matter physics. Many of the exotic materials of interest exhibit rich phases of matter at different temperatures and pressures. In order to probe the rich array of phases we developed a novel technique of combining scanning tunneling microscopy with tunable temperature and tunable mechanical strain in ultra high vacuum conditions. The mechanisms that give rise to high temperature superconductivity has been a long standing problem in physics. The discovered of iron-based high temperature superconductors (pnictides) have spurred much research into the mechanisms that give rise to the different exotic states observed in these new materials in hopes to better understand the underlying nature of unconventional superconductivity. Here we present a detailed study of the Nematic ordered phase in the prototypical iron- based high temperature superconductor, NaFeAs. Using our novel strain, temperature, scanning tunneling microscopy technique, we can attain an atomic-resolution view of the effects of the nematic phase on the local density of states along with the effects of anisotropic strain on the electronic structure. We further systematically study NaFeAs along both axes of the phase diagram, tuning temperature and Cu doping. We probe the material from the parent compound to beyond the supercon- ducting dome with increased Cu doping and from superconducting temperatures towell above the structural transition temperatures. Using our novel strain, temperature, scanning tunneling microscopy technique we nanoscopically identified the region of long-range nematic order and the region of nematic fluctuations in the phase diagram and find that true long range nematic order sets in at the tetragonal to orthorhombic structural transition temperature but nematic fluctuations continue at higher temperatures and also into the overdoped regime, then seemingly disappearing at the edge of the superconducting dome. We further find that our applied stain increasing the amplitude of the nematic fluctuations showing strong nonlinear coupling between strain and electronic nematicity. The power of our novel strain, temperature, scanning tunneling microscopy tech- nique in probing quasiparticle interference proves ideal for studying the topological, Weyl semimetal 1T’-MoTe 2 . In it’s orthorombic phase the material has topologically nontrivial protected surface Fermi arcs. By measuring quasiparticle interference in this material at different temperatures we can probe both topologically nontrivial phase (orthorhombic phase) and the topologically trivial phase (monoclinic phase). In the topologically nontrivial phase we see quasiparticle interference measurements in good agreement with angular resolved photoemission spectroscopy and theoretical calculations. In the topologically trivial phase we see the lack of the quasiparticle interference coming from the trivial surface state.

Graduation date: 1996