This thesis presents a set of measurements of spin asymmetries from the heavy atoms krypton and rubidium. These investigations allow examination of the spin orbit interaction for electron scattering from the target atoms. These measurements utilise spin polarized electrons in a crossed beam experiment to measure the Sherman function from krypton and the A2 parameter from the 52P state of rubidium. The measurements utilise a new spin polarized electron energy spectrometer which is designed to operate in the 20-200 eV range. The apparatus consists of a standard gallium arsenide polarized electron source, a 180 degrees hemispherical electron analyser to detect scattered electrons and a Mott detector to measure electron polarization. A series of measurements of the elastic Sherman function were performed on krypton at incident electron energies of 20, 50, 60, 65, 100, 150 and 200 eV. Scattered electrons are measured over an angular range of 30-130 degrees. These measurements are compared with calculations of the Sherman function which are obtained by solution of the Dirac-Fock equations. These calculations include potentials to account for dynamic polarization and loss of flux into inelastic channels. At the energies 50, 60 and 65 eV, experimental agreement with theory is seen to be extremely dependent on the theoretical model used. Measurement of the A2 parameter from the combined 52P1/2,3/2 state of rubidium are performed at an incident energy of 20 eV. The scattered electrons are measured over an angular range of 30-110 degrees. This measurement represents the first such measurement of this parameter for rubidium. Agreement with preliminary calculations performed using the R-matrix technique are good and are expected to improve with further theoretical development.

We correlated experimental results with theoretical estimations of the dielectric function ε(ω) in two contexts: the effect of an electric field in quantum wells and that of the spin polarization of an interacting electron-hole plasma in bulk semiconductors. In the first part, we recorded photoreflectance spectra from Ge/GeSi quantum wells of different widths but having comparable builtin electric fields caused by doping. The reason why the spectra differed in overall shape was difficult to understand by conventional methods, for example, by calculating the allowed transition energies or by fitting the data with lineshape functions at each transition energy. Instead, we computed the photoreflectance spectra from first-principles by using the confined electron and hole wavefunctions. This method showed that the spectra differ in overall shape because of the experimentally hitherto unobserved trend in quantum well electro-optical properties, from the quantum confined Franz-Keldysh effect to the bulk Franz-Keldysh effect, as the well width is increased. The second part develops a threeband microscopic theory for the optical properties due to spin-polarized carriers in quasiequilibrium. We show that calculations based on this theory reproduce all the trends observed in a recent circularly polarized pump-probe experiment reported in the literature. To make the computation less intensive, we proposed a simplified, two-band version of this theory which captured the main experimental features. Besides, we constructed a cw diode laser-based pump-probe setup for our own optical studies of spin-polarized carriers by Kerr rotation. We achieved a sensitivity of detection of Kerr rotation of 3 x 10¯ 8 rad, corresponding to an order of magnitude improvement over the best reports in the literature. The efficacy of our setup allowed for the demonstration of a pumpinduced spin polarization in bulk GaAs, under the unfavorable conditions of steady-state and room temperature.

One of the challenges facing condensed matter physics nowadays is to understand the electronic structure of high temperature superconductors. This dissertation compiles our contribution to the experimental information concerning this subject. Tunneling conductance spectroscopy a technique capable of probing the electronic density of states in hybrid structures was used to study the current and spin transport properties across junctions between metallic counterelectrodes and Bi2Sr2CaCu2O8- (BSCCO) crystals. Since in these structures the transport is mediated by transmission channels depending on superconductive characteristics, the energy resolved density of states is a signature of the mechanism of superconductivity. For instance, one can observe the superconductive energy gap and the behavior of subgap bound states due to phase sensitive Andreev reflections at the junction interface. In particular, tunneling spectroscopy makes possible the observation of the LOFF state characterized by the coexistence of superconductivity and magnetism. Cuprates like BSCCO are highly anisotropic materials and their superconductivity is almost two dimensional, being confined in the CuO2 planes. Therefore, our junctions combine monocrystals of underdoped samples of BSCCO with various thin film counterelectrodes normal metal (Ag), conventional superconductor (Pb) and ferromagnetic metal (Fe) deposited perpendicular onto the cuprate ab-plane (CuO2 plane). We performed measurements on Ag/BSCCO junctions for two current injection directions into the same crystal. We observed that, near the 110 crystal surface, the conductance spectra show a high zero bias peak (ZBCP) which is a manifestation of zero energy Andreev bound states due to an anisotropic superconductive order parameter. Near the 100 surface, the ZBCP is largely suppressed. This is consistent with a predominantly 2 2 x y d - -wave pairing symmetry. In some cases, the ZBCP splits or decreases in amplitude at low temperatures. This is consistent with the existence of a subdominant s-wave (or xy d ) resulting in a mixed d is + state which breaks time reversal symmetry (BTRS). Since we observe this phenomenon in the underdoped case, we do not confirm the possibility of a quantum critical point close to the optimal doping. Our Pb/BSCCO spectra contradict the theory explaining the BTRS by proximity effect. The Fe/BSCCO junctions measure the effect of spin polarization. We explain the recorded 4-peak asymmetric structure by the combined effect of a spin independent BTRS state and a spin filtering exchange energy in the barrier responsible for a large ZBCP splitting. The LOFF state was observed in the proximity region induced on the ferromagnetic side of multilayered-Fe/Ag/BSCCO structures. As expected for the LOFF order parameter, the spectra develops coherent damped oscillations with the Fe layer thickness probing different regions. The magnitude and sign of the oscillation depends on the energy. The conductances at energy zero or equal to the superconductive gap are modulated in antiphase proving that the order parameters takes successively positive and negative values. Changing the junction orientation with 4 p , results in an opposite behavior for the same distance. The maximal amplitudes in one direction is replaced by minima, showing that, besides space, the LOFF state modulation depends on the phase of the high temperature order parameter inducing the proximity

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.

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The goal of this project was to study some unexplored optical and magneto-optical properties of the newest member of III-V ferromagnetic structures, InMnSb, as well as InSb films and InSb/AlInSb quantum wells. The emphasis was on dynamical aspects such as charge and spin dynamics in order to address several important issues of the spin-related phenomena. The objectives in this project were to: 1) understand charge/spin dynamics in NGS with different confinement potentials, 2) study phenomena such as interband photo-galvanic effects, in order to generate spin polarized current, 3) probe the effect of magnetic impurities on the spin/charge dynamics. This thesis describes three experiments: detection and measurement of spin polarized photocurrents in InSb films and quantum wells arising from the circular photogalvanic effect, and measurements of the carrier and spin relaxation in InSb and InMnSb structures by magneto-optical Kerr effect and differential transmission. The samples for our studies have been provided by Prof. Heremans at Virginia Tech, Prof. Santos at the University of Oklahoma, and Prof. Furdyna at the University of Notre Dame. / Ph. D.

This thesis presents investigations on hydrogenated graphene by scanning tunneling microscopy and spectroscopy (STM/STS) as well as the implementation of spin-polarized STM. Preparation processes for a magnetic standard sample and spin-sensitive chromium tips are developed. The measurements on graphene reveal specific hydrogen adsorption sites in low coverage and the formation of a pattern at higher coverage. Both is found to be in agreement with previous predictions and calculations. Upon hydrogenation, an impurity midgap state emerges in the density of states which is measured directly for the first time. Complementing angle resolved photoemission experiments confirm that this state is dispersionless over the whole Brillouin zone. A routine is developed to prepare the standard sample system of ultra-thin iron films on tungsten (Fe/W(110)). Investigations on this system confirm the magnetic properties known from literature, including the presence of a spin spiral, and prove that it is well suited for the characterization of spin-polarized tips. Different approaches for the preparation of tips from the antiferromagnetic material chromium are tested. Among these, a promising new method is presented: The coating of crystalline chromium tips with fresh chromium material suggests reproducibility of the tip characteristics. The performance of the produced tips in STM measurements is excellent in regard to a fixed spin-polarization, high resolution and stability. Especially, a recovery of the tip magnetization direction proposed in this thesis makes this new preparation method superior to all processes yielding antiferromagnetic tips reported so far. / Inhalt der vorliegenden Arbeit sind Untersuchungen von hydogeniertem Graphen mittels Rastertunnelmikroskopie und -spektroskopie (RTM/RTS) sowie die Einführung spin-polarisierter RTM. Im Rahmen dessen wurden Präparationsprozesse für magnetische Standardproben und spin-sensitive Chrom-Spitzen entwickelt. Die Messungen an Graphen zeigen spezifische Wasserstoff-Adsorptionsstellen bei geringer Bedeckung und die Ausbildung eines Musters bei höherer Bedeckung, jeweils in Übereinstimmung mit Vorhersagen und Berechnungen. Der durch Hydrogenierung entstehende Störstellenzustand in der Bandlücke der Zustandsdichte wurde zum ersten Mal direkt gemessen. Ergänzende winkelaufgelöste Photoelektronenspektroskopieexperimente bestätigen, dass dieser Zustand in der gesamten Brillouinzone dispersionsfrei ist. Ein Verfahren zur Herstellung magnetischer Standardproben aus ultradünnen Eisenfilmen auf Wolfram (Fe/W(110)) wurde entwickelt. RTM-Untersuchungen an diesem System bestätigen die bereits aus der Literatur bekannten magnetischen Eigenschaften, insbesondere das Vorhandensein einer Spinspirale. Damit ist Fe/W(110) hervorragend geeignet für die Charakterisierung spin-polarisierter Spitzen. Verschiedene Ansätze, die zur Herstellung von Spitzen aus dem antiferromagnetischen Material Chrom verfolgt wurden, werden präsentiert, darunter auch eine vielversprechende neue Methode: Das Aufwachsen eines frischen Chromfilms auf kristalline Spitzen desselben Materials verspricht eine Reproduzierbarkeit von Spitzeneigenschaften. Der Einsatz von so hergestellten Spitzen in RTMMessungen ist geprägt von einer festgelegten Spin-Polarisation, hohem Auflösungsvermögen und Stabilität. Insbesondere die mögliche Reproduzierbarkeit der Magnetisierungsrichtung, die in dieser Arbeit diskutiert wird, macht diese Methode allen bisher berichteten Herstellungprozessen antiferromagnetischer Spitzen überlegen.

Nesta dissertação, fazemos a primeira investigação sobre flutuações em corrente e corrente média dependentes de spin em potenciais duplo e simples da estrutura Zn1-xMnxSe. Consideramos efeitos de campos magnético e elétrico externos à temperatura nula. Na presença de um campo magnético, a interação dos íons de Mn com elétrons de condução e valência (interação de troca sp-d) origina potenciais dependentes de spin para o transporte em Zn1-xMnxSe. Aqui, flutuações em corrente (\"shot noise\") e a corrente média são calculados usando o modelo quântico de transporte através do potencial dependente de spin é descrito por uma matriz s de espalhamento. Os elementos da matriz de espalhamento, i.e., as amplitudes de transmissão e reflexão, são determinados pelo método da matriz transferência. Nossos resultados indicam que estruturas de potenciais simples e duplos Zn1-xMnxSe agem como se fossem \"filtros de spin\" para corrente. Em determinadas faixas de parâmetros do sistema, \"shot noise\" pode complementar informações obtidas da corrente média / In this dissertation we investigation for the first time spin dependent-current and its fluctuations in double and single barrier potentials of the Zn1-xMn xSe structure sandwiched between ZnSe layers. We consider effects of external magnetic field, the interaction of the Mn ions with thew conduction and valence electrons (sp-d exchange interation) give rises to spin-dependent potentials for transport across the Zn1-xMn xSe layer. Here, the average current and its fluctuations are calculated using the quantum transport model in which transport across the spin-dependent potential is described via scattering matrix s. The elements of the scattering matrix, i.e., the transmission and reflection amplitudes, are determined through the transfer-matrix method. Our results indicate date single and double potentials of the Zn1-xMn xSe structure act as \"spin filters\" for the current. Within some system parameter range, shot noise can supplement the information contained in the average current

In this thesis, the interplay between electron transport and magnetization dynamics is explored in order to access to fundamental properties of ferromag- netic metal thin films. With the aim of extracting the influence of the electron surface scattering on the spin-dependent resistivities, thickness series of permal-loy (Ni80Fe20) films were grown and studied. In addition to standard electrical and magnetic measurements, a detailed study of the propagation of spin waves along these films was performed. Resorting to the current-induced spin-wave Doppler shift technique, the degree of spin-polarization of the electrical current was extracted. This degree of spin-polarization was found to decrease when the film thickness decreases, which suggests that the film surfaces contribute to the spin dependent resistivities and tend to depolarize the electrical current.

Nesta dissertação, fazemos a primeira investigação sobre flutuações em corrente e corrente média dependentes de spin em potenciais duplo e simples da estrutura Zn1-xMnxSe. Consideramos efeitos de campos magnético e elétrico externos à temperatura nula. Na presença de um campo magnético, a interação dos íons de Mn com elétrons de condução e valência (interação de troca sp-d) origina potenciais dependentes de spin para o transporte em Zn1-xMnxSe. Aqui, flutuações em corrente (\"shot noise\") e a corrente média são calculados usando o modelo quântico de transporte através do potencial dependente de spin é descrito por uma matriz s de espalhamento. Os elementos da matriz de espalhamento, i.e., as amplitudes de transmissão e reflexão, são determinados pelo método da matriz transferência. Nossos resultados indicam que estruturas de potenciais simples e duplos Zn1-xMnxSe agem como se fossem \"filtros de spin\" para corrente. Em determinadas faixas de parâmetros do sistema, \"shot noise\" pode complementar informações obtidas da corrente média / In this dissertation we investigation for the first time spin dependent-current and its fluctuations in double and single barrier potentials of the Zn1-xMn xSe structure sandwiched between ZnSe layers. We consider effects of external magnetic field, the interaction of the Mn ions with thew conduction and valence electrons (sp-d exchange interation) give rises to spin-dependent potentials for transport across the Zn1-xMn xSe layer. Here, the average current and its fluctuations are calculated using the quantum transport model in which transport across the spin-dependent potential is described via scattering matrix s. The elements of the scattering matrix, i.e., the transmission and reflection amplitudes, are determined through the transfer-matrix method. Our results indicate date single and double potentials of the Zn1-xMn xSe structure act as \"spin filters\" for the current. Within some system parameter range, shot noise can supplement the information contained in the average current