Dissertations / Theses on the topic 'Spin-resolved'
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Andres, Beatrice [Verfasser]. "Spin and Magnetization Dynamics Studied by Spin-Resolved Photoemission / Beatrice Andres." Berlin : Freie Universität Berlin, 2016. http://d-nb.info/1114138193/34.
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Lehmann, Hauke [Verfasser]. "Spin-resolved conductance quantization in InAs nanodevices / Hauke Lehmann." München : Verlag Dr. Hut, 2013. http://d-nb.info/103304170X/34.
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Krempasky, Juraj. "Angle- and spin-resolved photoemission on La²/3Sr1/3MnO3." Cergy-Pontoise, 2008. http://biblioweb.u-cergy.fr/theses/08CERG0398.pdf.
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La2/3Sr1/3MnO3 est un des oxydes de manganèse qui suscite un vif intérêt car on s’attend à ce qu’il soit un demi-métal. La spectroscopie de photoélectrons résolue en angle est une technique puissante de caractérisation de la structure électronique des systèmes complexes, dans lesquels les degrés de liberté de charge, d’orbitale et de spin déterminent des propriétés inattendues, comme la supraconductivité non conventionnelle ou la magnétorésistance colossale. Pour développer de nouveaux types de matériaux magnétiques adaptés à la spintronique, il est important d’accéder au degré de liberté de spin. Cette thése porte sur la spectroscopie haute résolution des photoélectrons, résolue en spin et en angle, de La2/3Sr1/3MnO3. Des échantillons sous forme de couches minces (< 100 nm) ont été préparés par ablation laser. Les spectres de photoélectrons ont été simulés à partir de calculs de structure de bandes en prenant en compte les effets de durées de vie du phototrou et l’élargissement de la composante perpendiculaire du vecteur d’onde. Les simulations reproduisent l’existence d’un gap au niveau de Fermi pour les électrons minoritaires et confirment que La2/3Sr1/3MnO3 a bien un caractère demi-métallique, en dépit du fait que la polarisation de spin mesurée est inférieure à 100%La2/3Sr1/3MnO3 is one of the manganese oxides that attracted a lot of interest because it is expected to be a half metal. Angle-resolved photoemission spectroscopy is a powerful technique for characterizing the electronic structure of complex systems, where charge, orbital, and spin degrees of freedom determine surprising properties, such as non-conventional superconductivity and colossal magnetoresistance. For the development of new types of magnetic materials suitable for spintronics, it is important to access to the spin degree of freedom. This thesis deals with high-resolution spin- and angle-resolved photoemission spectroscopy on La2/3Sr1/3MnO3. Thin film (< 100 nm) samples were prepared by laser ablation. The photoemission spectra were simulated from band structure calculations considering the photohole lifetime effects and the broadening of the perpendicular component of the wavevector. The simulations reproduce a gap between minority electrons at the Fermi level and confirm the half-metallic nature of La2/3Sr1/3MnO3, despite the fact that the measured spin polarization is lower than 100%
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Chen, Zhigang. "Spectrally resolved optical study of transient spin dynamics in semiconductors." Connect to online resource, 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3256384.
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Veenstra, Christian Neil. "Spin- and angle-resolved photoemission spectroscopy on unconventional superconductor strontium ruthenate." Thesis, University of British Columbia, 2013. http://hdl.handle.net/2429/44166.
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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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Schedin, Bernt Fredrik. "Angle-resolved spin-polarised inverse photoemission spectroscopy studies of Ni(110)." Thesis, University of Liverpool, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317198.
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Brand, Matthew Anthony. "Optical time resolved spin dynamics in III V semiconductor quantum wells." Thesis, University of Southampton, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.289510.
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Hari, Murali Krishnan. "Time-resolved domain wall motion in novel spin transfer torque devices." Thesis, University of Bath, 2016. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.687315.
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In the past decade, the interaction of spin polarised electrons with ferromagnetic domains and domain walls has stimulated an immense volume of research work in the field of spintronics worldwide. Perpendicular magnetic anisotropy (PMA) in thin film magnetic materials has been widely exploited as a building block for realising spintronic devices. Co/Pt multilayer thin films with PMA have been extensively investigated for their potential applications in magnetic recording media and MRAM-like devices. Much work has been done to develop ways to control the magnetic properties of these films. Ga+ focused ion beam (FIB) irradiation is a well-established technique for controlling the magnetic properties of systems with perpendicular magnetic anisotropy (PMA). Here we have systematically investigated the coercivity, magnetic anisotropy and surface roughness of Ta(4 nm)/Pt(3 nm)/Co(x nm)/Pt(1.8 nm) multilayer films as a function of Ga+ FIB irradiation dose. The influence of the thickness of Co and Pt layers on the coercivity and switching behaviour was systematically investigated and the conditions established for realising structures with medium coercivity (~100 Oe) and sharp switching that are well-suited for current-driven domain wall motion studies. An unexpected increase in the coercivity at very low Ga+ ion doses followed by a reduction at higher irradiation levels is observed. This correlates with an increase in surface roughness which we tentatively attribute to the ion induced formation of highly strained nanoscale regions of ordered CoPt alloy at the Co/Pt interface. The possibility to both increase and decrease coercivity with very low dose Ga+ ion irradiation could have important applications in the design of novel spintronic devices. In addition, annealing at relatively low temperatures (< 200 C) is shown to drastically increase the magnetic anisotropy and switching field for all Co thicknesses whereby the ratio of increase in coercivity and magnetic anisotropy is higher in thicker Co films. Optimised Co/Pt multilayer films were lithographically patterned into nanowire devices for time-resolved Extraordinary Hall Effect (EHE) measurements. The devices were based on 50 ohms coplanar waveguides incorporating single Hall cross structures. The coercivity of the region surrounding the Co/Pt Hall crosses was reduced by local Ga+ FIB irradiation, allowing the controlled nucleation of domain walls (DWs) at the edges of these regions by application of an appropriate field sequence. Domain imaging was carried out to study the evolution of artificial domains and DW dynamics under the application of magnetic fields and current pulses. DW creep measurements were carried out to study the effect of bias field on DW motion. Using pulsed currents, spin transfer torque-driven domain wall motion was demonstrated in these artificial domain structures and the DW position and velocity tracked using time-resolved EHE measurements.
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Maaß, Henriette [Verfasser], and Friedrich [Gutachter] Reinert. "Spin-dependence of angle-resolved photoemission from spin-orbit split surface states / Henriette Maaß ; Gutachter: Friedrich Reinert." Würzburg : Universität Würzburg, 2017. http://d-nb.info/1140435949/34.
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Völkl, Roland [Verfasser], and Christian [Akademischer Betreuer] Schüller. "Spin dynamics and spatially resolved spin transport phenomena in GaAs based structures / Roland Völkl. Betreuer: Christian Schüller." Regensburg : Universitätsbibliothek Regensburg, 2013. http://d-nb.info/1047236958/34.
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Hide, A. K. "Spin resolved electron spectroscopy studies of induced ferromagnetism in 4d transition metals." Thesis, University of Sussex, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263868.
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Dai, Ji. "Low-dimensional electron systems studied by angle- and spin-resolved photoemission spectroscopy." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS345.
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Les matériaux dans lesquels des interactions à plusieurs particules, un confinement de faible dimension et/ou un fort couplage spin-orbite sont présents témoignent d’une grande variété de phénomènes, mais sont encore mal compris. Des informations essentielles sur l’origine de tels phénomènes peuvent être obtenues en mesurant leur structure électronique. Cette thèse présente une étude expérimentale de la structure électronique de matériaux de faible dimension et/ou fortement corrélés présentant un intérêt fondamental actuel, en utilisant la spectroscopie par photoémission résolue en angle et en spin (ARPES et SARPES).Dans la partie introductive, je présente mon travail sur deux exemples de type "livre de texte", mais innovants, montrant comment les interactions affectent la structure de bande d'un matériau: le couplage des électrons avec des phonons dans une distribution de Debye dans un système électronique à deux dimensions (2DES) dans ZnO, semi-conducteur à oxyde à bande interdite large utilisé dans les applications photovoltaïques, et le dédoublement induit par un fort couplage spin-orbite (SOC) dans la bande de valence du ZnTe, un autre semi-conducteur important utilisé dans les dispositifs optoélectroniques. Ensuite, dans la suite de cette thèse, je discute de mes résultats originaux dans trois systèmes différents de basse dimensionnalité et d'intérêt actuel en recherche : 1.La réalisation d'un 2DES à la surface (110) de SnO₂, le premier du genre dans une structure rutile. L'ajustabilité de la densité de ses porteurs au moyen de la température ou du dépôt d'Eu, et la robustesse vis-à-vis les reconstructions de surface et l'exposition aux conditions ambiantes rendent ce 2DES prometteur pour les applications. Au moyen d'une simple réaction redox à la surface, ces travaux ont prouvé que les lacunes en oxygène pouvaient doper la bande de conduction à la surface de SnO₂, résolvant ainsi un problème longtemps débattu concernant le rôle desdites lacunes dans le dopage de type n dans SnO₂. 2.L'étude des états de surface topologiques dans M₂Te₂X (avec M = Hf, Zr ou Ti; et X = P ou As), une nouvelle famille de métaux topologiques en trois dimensions, provenant du SOC et étant protégés par la symétrie du renversement du temps. Leur structure électronique et leur texture de spin, étudiées par ARPES et SARPES, révèlent la présence de fermions de Dirac sans masse donnant naissance à des arcs de nœuds de Dirac. 3.L'étude du matériau YbNi₄P₂ à fermions lourds quasi unidimensionnel, qui présente une transition de phase quantique de second ordre d’une phase ferromagnétique à une phase paramagnétique de liquide de Fermi lors de la substitution partielle du phosphore par l'arséniure. Une telle transition ne devrait se produire que dans les systèmes zéro ou unidimensionnels, mais la mesure directe de la structure électronique des matériaux ferromagnétiques quantiques critiques faisait jusqu'à présent défaut. Grâce à une préparation et nettoyage méticuleux in situ de la surface des monocristaux YbNi₄P₂, qui sont impossibles à cliver, leur structure électronique a été mesurée avec succès au moyen de l'ARPES, dévoilant ainsi le caractère quasi-1D, nécessaire à la compréhension de la criticité quantique ferromagnétique, dans YbNi₄P₂. Le protocole utilisé pour rendre ce matériau accessible à l'ARPES peut être facilement généralisé à d'autres matériaux exotiques dépourvus de plan de clivageMaterials in which many-body interactions, low-dimensional confinement, and/or strong spin-orbit coupling are present show a rich variety of phenomena, but are still poorly understood. Essential information about the origin of such phenomena can be obtained by measuring their electronic structure. This thesis presents an experimental study of the electronic structure of some low-dimensional and/or strongly correlated materials of current fundamental interest, using angle- and spin-resolved photoemission spectroscopy (ARPES and SARPES). In the introductory part, I present my work on two innovative textbook examples showing how interactions affect the band structure of a material: the coupling of electrons with phonons in a Debye distribution in a two-dimensional electron system (2DES) in ZnO, a wide-band-gap oxide semiconductor used in photovoltaic applications, and the splitting induced by strong spin-orbit coupling (SOC) in the bulk valence band of ZnTe, another important semiconductor used in optoelectronic devices. Then, in the rest of this thesis, I discuss my original results in three different low-dimensional systems of current interest: 1.The realisation of a 2DES at the (110) surface of SnO₂, the first of its kind in a rutile structure. Tunability of its carrier density by means of temperature or Eu deposition and robustness against surface reconstructions and exposure to ambient conditions make this 2DES promising for applications. By means of a simple redox reaction on the surface, this work has proven that oxygen vacancies can dope the conduction band minimum at the surface of SnO₂, solving a long-debated issue about their role in n-type doping in SnO₂. 2.The study of topological surface states in M₂Te₂X (with M = Hf, Zr, or Ti; and X = P or As), a new family of three-dimensional topological metals, originating from SOC and being protected by time-reversal symmetry. Their electronic structure and spin texture, studied by ARPES and SARPES, reveal the presence of massless Dirac fermions giving rise to Dirac-node arcs. 3.The investigation of the quasi-one-dimensional heavy-fermion material YbNi₄P₂, which presents a second-order quantum phase transition from a ferromagnetic to a paramagnetic phase upon partial substitution of phosphorous by arsenide. Such a transition is expected to occur only in zero- or one-dimensional systems, but a direct measurement of the electronic structure of ferromagnetic quantum-critical materials was missing so far. By careful in-situ preparation and cleaning of the surface of YbNi₄P₂ single crystals, which are impossible to cleave, their electronic structure has been successfully measured by ARPES, thus effectively unveiling the quasi-one-dimensionality of YbNi₄P₂. Moreover, the protocol used to make this material accessible to ARPES can be readily generalised to other exotic materials lacking a cleavage plane
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DE, VITA ALESSANDRO. "PROBING BAND MAGNETISM IN DIFFERENT DIMENSIONS: ENERGY, SPIN AND TIME-RESOLVED STUDIES." Doctoral thesis, Università degli Studi di Milano, 2022. https://hdl.handle.net/2434/947210.
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This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano, that has been carried out since November 2019 at the Istituto Officina dei Materiali of the Consiglio Nazionale delle Ricerche (IOM-CNR) in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures and in the framework of the NFFA facility.
My experimental activity employed complementary spectroscopy and polarimetry techniques oriented to address the characterisation of electronic and spin properties of systems with decreasing dimensionality. This programme has been conducted by exploiting state-of-the-art infrastructures to generate visible, UV and EUV ultrashort pulses (tabletop lasers and HHG at NFFA-SPRINT laboratory) and soft X-ray synchrotron light (at Elettra, Diamond and ESRF synchrotron light sources).
I used photoemission as the main tool in my investigation, supplementing my results with absorption spectroscopy. I focused on three materials, Fe(001)-p(1x1)O/MgO(001), EuSn2P2 and VI3, of high interest in modern and next-generation magnetic devices.
In the three systems I studied the electronic band structure to identify key features hinting at the bound electrons behaviour. I investigated the properties of the magnetically ordered phases and found evidence of the reduced dimensionality in the emergence of atypical spin ordering and the increasingly manifest electron correlation phenomena. The information retained by band electrons is critical to access the spin polarisation of the bands and to give insight into the effects of spatial confinement on the spin degree of freedom.
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Boll, Martin [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Spin and density resolved microscopy of Hubbard chains / Martin Boll ; Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2016. http://d-nb.info/1122435843/34.
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Strachan, John Paul. "Time-resolved x-ray imaging of magnetic nanostructures driven by spin-transfer torque /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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Chembrolu, Venkatesh. "Time-resolved X-ray imaging of magnetization dynamics in spin transfer torque devices /." May be available electronically:, 2008. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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Bawden, Lewis. "A spin- and angle-resolved photoemission study of coupled spin-orbital textures driven by global and local inversion symmetry breaking." Thesis, University of St Andrews, 2017. http://hdl.handle.net/10023/12049.
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The effect of spin-orbit coupling had once been thought to be a minor perturbation to the low energy band structure that could be ignored. Instead, a surge in recent theoretical and experimental efforts have shown spin-orbit interactions to have significant consequences. The main objective of this thesis is to investigate the role of the orbital sector and crystal symmetries in governing the spin texture in materials that have strong spin- orbit interactions. This can be accessed through a combination of spin- and angle-resolved photoemission spectroscopy (ARPES and spin-ARPES), both of which are powerful techniques for probing the one-electron band structure plus interactions, and supported by density functional theory calculations (DFT). We focus first on a globally inversion asymmetric material, the layered semiconductor BiTeI, which hosts a giant spin-splitting of its bulk bands. We show that these spin-split bands develop a previously undiscovered, momentum-space ordering of the atomic orbitals. We demonstrate this orbital texture to be atomic element specific by exploiting resonant enhancements in ARPES. These orbital textures drive a hierarchy of spin textures that are then tied to the constituent atomic layers. This opens routes to controlling the spin-splitting through manipulation of the atomic orbitals. This is contrasted against a material where inversion symmetry is globally upheld but locally broken within each monolayer of a two layer unit cell. Through our ARPES and spin-ARPES measurements of 2H-NbSe2, we discover the first experimental evidence for a strong out-of-plane spin polarisation that persists up to the Fermi surface in this globally inversion sym- metric material. This is found to be intrinsically linked to the orbital character and dimensionality of the underlying bands. So far, previous theories underpinning this (and related) materials' collective phases assume a spin- degenerate Fermi sea. We therefore expect this spin-polarisation to play a role in determining the underlying mechanism for the charge density wave phase and superconductivity. Through these studies, this thesis then develops the importance of global versus local inversion symmetry breaking and uncovers how this is intricately tied to the underlying atomic orbital configuration.
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Jamison, John S. "Time and Space Resolved Spin-Heat Transport in the Magnetic Insulator Yttrium Iron Garnet." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1586740671277489.
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Schlenk, Tobias [Verfasser]. "Spin-Resolved and Inelastic Scanning Tunneling Spectroscopy of Magnetic Atoms and Clusters / Tobias Schlenk." München : Verlag Dr. Hut, 2013. http://d-nb.info/1045126039/34.
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Page, Michael Roy. "Interactions between spin transport and dynamics studied using spatially resolved imaging and magnetic resonance." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480592093876192.
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Hilker, Timon [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Spin-resolved microscopy of strongly correlated fermionic many-body states / Timon Hilker ; Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2017. http://d-nb.info/115015943X/34.
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Ou, Yu-Sheng. "Ultrafast Study of Dynamic Exchange Coupling in Ferromagnet/Oxide/Semiconductor Heterostructures." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1490361750624291.
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Stepanov, Ivan Verfasser], Christoph [Akademischer Betreuer] Stampfer, and Gernot [Akademischer Betreuer] [Güntherodt. "Coherent electron Zitterbewegung : time-resolved electrical spin generation, manipulation and detection / Ivan Stepanov ; Christoph Stampfer, Gernot Güntherodt." Aachen : Universitätsbibliothek der RWTH Aachen, 2017. http://d-nb.info/116141181X/34.
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Ohara, Keishi. "CW and FT Time-Resolved EPR Studies on Spin and Reaction Dynamics of Short-Lived Paramagnetic Species." 京都大学 (Kyoto University), 1996. http://hdl.handle.net/2433/86231.
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Stepanov, Ivan [Verfasser], Christoph Akademischer Betreuer] Stampfer, and Gernot [Akademischer Betreuer] [Güntherodt. "Coherent electron Zitterbewegung : time-resolved electrical spin generation, manipulation and detection / Ivan Stepanov ; Christoph Stampfer, Gernot Güntherodt." Aachen : Universitätsbibliothek der RWTH Aachen, 2017. http://d-nb.info/116141181X/34.
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Khastehdel, Fumani Ahmad. "QUANTUM CONFINED STATES AND ROOM TEMPERATURE SPIN COHERENCE IN SEMICONDUCTOR NANOCRYSTAL QUANTUM DOTS." Case Western Reserve University School of Graduate Studies / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=case1449151739.
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Wang, Yihua. "Laser-Based Angle-Resolved Photoemission Spectroscopy of Topological Insulators." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10635.
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Topological insulators (TI) are a new phase of matter with very exotic electronic properties on their surface. As a direct consequence of the topological order, the surface electrons of TI form bands that cross the Fermi surface odd number of times and are guaranteed to be metallic. They also have a linear energy-momentum dispersion relationship that satisfies the Dirac equation and are therefore called Dirac fermions. The surface Dirac fermions of TI are spin-polarized with the direction of the spin locked to momentum and are immune from certain scatterings. These unique properties of surface electrons provide a platform for utilizing TI in future spin-based electronics and quantum computation. The surface bands of 3D TI can be directly mapped by angle-resolved photoemission spectroscopy (ARPES) and the spin polarization can be determined by spin-resolved ARPES. These types of experiments are the first to establish the 3D topological order, which demonstrates the power of ARPES in probing the surface of strongly spin-orbit coupled materials. Extensive investigation of TI has ranged from understanding the fundamental electronic and lattice structure of various TI compounds to building TI-based devices in search of more exotic particles such as Majorana fermions and magnetic monopoles. Surface-sensitive techniques that can efficiently disentangle the charge and spin degrees of freedom have been crucially important in tackling the multi-faceted problems of TI. In this thesis, I show that laser-based ARPES in combination with a time-of-flight spectrometer is a powerful tool to study the spin structure and charge dynamics of the Dirac fermions on the surface of TI. Chapter 1 gives a brief introduction of TI. Chapter 2 describes the basic principles behind ARPES and time-resolved ARPES (TrARPES). Chapter 3 provides a detailed account of the experimental setup to perform laser-based ARPES and TrARPES. In Chapters 4 and 5, how these two techniques are effectively applied to investigate two unique electronic properties of TI is elaborated. Through these studies, I have obtained a complete mapping of the spin texture of several prototypical topological insulators and have uncovered the cooling mechanism governing the hot surface Dirac fermions.Physics
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Khalil, Tarek. "New resonant behavior in the spin resolved photoionization of the rare gas atoms Kr 3d and Xe 4p." [S.l. : s.n.], 2002. http://deposit.ddb.de/cgi-bin/dokserv?idn=966656024.
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Körner, Helmut Sebastian [Verfasser], and Christian H. [Akademischer Betreuer] Back. "Time-resolved Kerr microscopy of spin waves propagating in magnetic nanostructures / Helmut Sebastian Körner ; Betreuer: Christian H. Back." Regensburg : Universitätsbibliothek Regensburg, 2018. http://d-nb.info/1179449037/34.
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Henn, Tobias [Verfasser], and Wolfgang [Gutachter] Ossau. "Hot spin carriers in cold semiconductors : Time and spatially resolved magneto-optical Kerr effect spectroscopy of optically induced electron spin dynamics in semiconductor heterostructures / Tobias Henn. Gutachter: Wolfgang Ossau." Würzburg : Universität Würzburg, 2015. http://d-nb.info/1102829730/34.
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Diniz, Ginetom S. "Electronic and Transport Properties of Carbon Nanotubes: Spin-orbit Effects and External Fields." Ohio University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1343143890.
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Helmstedt, Andreas [Verfasser]. "Probing the magnetic properties of [MnIII6CrIII]3+ single-molecule magnets by XAS, XMCD and spin-resolved electron spectroscopy / Andreas Helmstedt." Bielefeld : Universitätsbibliothek Bielefeld, 2014. http://d-nb.info/1068621702/34.
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Djeghloul, Fatima Zohra. "Study of organic semiconductor / ferromagnet interfaces by spin-polarized electron scattering and photoemission." Phd thesis, Université de Strasbourg, 2013. http://tel.archives-ouvertes.fr/tel-01062352.
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I studied organic semiconductor/ferromagnet interfaces by characterizing them by spin-polarized electron scattering and photoemission spectroscopy experiments. In the first part, a completely unexpected behaviour of the spin-dependent electron reflection properties of these interfaces is observed. In fact, sub-monolayer coverage of the organic molecules makes the electron reflection amplitude independent of the spin, i.e. both the reflectivity and the reflection phase become independent of the spin orientation of the incident electrons. Although I am not able at the moment to identify the cause of this phenomenon, I show that it is a very general phenomenon which is independent of the energy of the primary electrons, the choice of the ferromagnetic substrate, the choice of the organic molecule, and of the orientation of the initial spin polarization. It is not due to a change of the surface magnetization, a depolarization of the primary electrons, or a direct interaction of the molecules with the ferromagnetic substrate. Moreover, theory does not predict so far the experimental results and further research is required to unveil the physics behind these observations. In the second part of my thesis, spin-resolved photoemission experiments have been performed at the synchrotron SOLEIL. The main result is the observation of a highly spin-polarized molecule-induced electronic state close to the Fermi level. Measurements as a function of the organic layer thickness allow us to determine the interfacial character of this electronic state. Finally, these results are compared with theoretical calculations performed at the institute.
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Nontapot, Kanokwan. "Time-Resolved Studies of Magnetic and Non-Magnetic Narrow-Gap Semiconductors." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/28712.
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In recent years, spin relaxation, injection, and manipulation in semiconductors have attracted considerable interest because of several potential applications in "spintronic" devices and the necessity to understand and control spin-based phenomena. In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors (NGS). NGS based heterostructures are particularly interesting for spintronic applications due to their large spin-orbit coupling, which leads to considerable zero-field spin splitting. NGS are also candidates for electronic applications, such as high-speed and low-power microprocessors; as reported recently by Intel. Furthermore, as switching rates in electronic devices are pushed to even higher frequencies, it is important to understand dynamics in semiconductors such as NGS on femtosecond time-scales.
In this thesis, time-resolved studies of magnetic and non-magnetic NGS using ultrafast-laser spectroscopy techniques such as pump-probe spectroscopy and magneto-optical Kerr/Faraday effect, are reported. Our samples include: InSb-based quantum wells with different confinement potentials; InMnSb films, the newest III-V ferromagnetic semiconductors; and InAs films. The samples for these studies have been provided by the groups of Prof. Santos at the University of Oklahoma, Prof. Furdyna at the University of Notre Dame, and Prof. Guido at Virginia Tech.
The objectives in this thesis have been to: a) understand charge/spin dynamics in NGS with novel confinement potentials, b) probe the effect of magnetic impurities on the spin/charge dynamics, and c) develop concepts for spin based device applications. Several specific questions and concepts have been addressed including: the effect of large spin-orbit interaction in NGS on the dynamics, how large Rashba spin splitting in these materials affect the spin coherence life time, and carrier/spin dynamics in ferromagnetic semiconductor structures.Ph. D.
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Bhowmick, Mithun. "Time Resolved Spectroscopy in InAs and InSb based Narrow-Gap Semiconductors." Diss., Virginia Tech, 2012. http://hdl.handle.net/10919/77136.
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As the switching rates in electronic and optoelectronic devices are pushed to even higher frequencies, it is crucial to probe carrier dynamics in semiconductors on femtosecond timescales. Time resolved spectroscopy is an excellent tool to probe the relaxation dynamics of photoexcited carriers; where after the initial photoexcitation, the nonequilibrium population of electrons and holes relax by a series of scattering processes including carrier-carrier and carrier-phonon scattering. Probing carrier and spin relaxation dynamics in InAs and InSb based narrow-gap semiconductors is crucial to understand the different scattering mechanisms related to the systems. Similar studies in InSb quantum wells are also intriguing, especially for their scientifically unique features (such as small effective mass, large g-factor etc). Our time resolved techniques demonstrated tunability of carrier and spin dynamics which might be important for charge and spin based devices. The samples studied in this work were provided by the groups of Prof. Wessels (Northwestern University) and Prof. Santos (University of Oklahoma). Theoretical calculations were performed by the group of Prof. Stanton (University of Florida). The THz measurements were performed at Wright State University in collaboration with Prof. Jason Deibel. This work has been supported by the National Science Foundation through grants Career Award DMR-0846834, AFOSR Young Investigator Program 06NE231. A portion of this work was performed at the National High Magnetic Field Laboratory (in collaboration with Dr. Stephen McGill), which is supported by National Science Foundation Cooperative Agreement No. DMR-0654118, the State of Florida, and the U.S. Department of Energy.Ph. D.
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Chen, Jinghao [Verfasser], and Uwe [Akademischer Betreuer] Bovensiepen. "Ultrafast charge and spin dynamics at solid interfaces : Investigated with femtosecond time-resolved second harmonic spectroscopy / Jinghao Chen ; Betreuer: Uwe Bovensiepen." Duisburg, 2021. http://d-nb.info/1233966588/34.
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Munson, Brian R. "Electronic to Vibrational Energy Transfer from Cl* (2P1/2) to CH4 and CD4." Wright State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=wright1239995101.
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Jones, Lee. "An investigation into the effect of a chiral adsorbate on surface magnetism using spin-resolved electron spectroscopy : tartaric acid on Ni{110}." Thesis, University of Liverpool, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.433770.
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Ndiaye, Waly. "Etude par photoémission résolue en angle et en spin de Mn5Ge3/Ge(111) en couches minces." Phd thesis, Université de Cergy Pontoise, 2013. http://tel.archives-ouvertes.fr/tel-00926486.
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Mn5Ge3 suscite de l'intérêt pour des applications dans le domaine de l'électronique de spin car il a une température de Curie élevée (≈300 K) et il peut croître épitaxialement sur des substrats Ge(111) permettant ainsi d'injecter directement dans le semi-conducteur Ge un courant polarisé en spin.Nous avons étudié par photoémission résolue en angle et en spin (ARPES, SARPES), utilisant le rayonnement synchrotron, des films minces de Mn5Ge3(001), obtenus par croissance sur la surface reconstruites Ge(111)-c(2x8).Les résultats ARPES, obtenus dans les plans GALM et GAHK, sont en accord avec des simulations faites sur la base de calculs de structure de bandes faisant appel à la théorie de la fonctionnelle de la densité.Les mesures SARPES faites en plusieurs points du plan GALM sont aussi bien reproduites par ces simulations.D'une façon globale, nos résultats apportent une validation remarquable de la description des propriétés électroniques de Mn5Ge3 par le modèle de bandes. Seule l'intensité spectrale au niveau de Fermi n'est pas bien expliquée par la simulation. Cette différence est attribuée à la nature tridimensionnelle de l'échantillon et à des effets de corrélation.
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Sánchez-Barriga, Jaime. "A photoemission study of quasiparticle excitations, electron-correlation effects and magnetization dynamics in thin magnetic systems." Phd thesis, Universität Potsdam, 2010. http://opus.kobv.de/ubp/volltexte/2010/4849/.
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This thesis is focused on the electronic, spin-dependent and dynamical properties of thin magnetic systems. Photoemission-related techniques are combined with synchrotron radiation to study the spin-dependent properties of these systems in the energy and time domains.
In the first part of this thesis, the strength of electron correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) and hcp Co(0001) is investigated by means of spin- and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the three-body scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. From this comparison it is demonstrated that the present state of the art many-body calculations, although improving the description of correlation effects in Fe and Co, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including nonlocal fluctuations.
In the second part, it is shown in detail monitoring by photoelectron spectroscopy how graphene can be grown by chemical vapour deposition on the transition-metal surfaces Ni(111) and Co(0001) and intercalated by a monoatomic layer of Au. For both systems, a linear E(k) dispersion of massless Dirac fermions is observed in the graphene pi-band in the vicinity of the Fermi energy. Spin-resolved photoemission from the graphene pi-band shows that the ferromagnetic polarization of graphene/Ni(111) and graphene/Co(0001) is negligible and that graphene on Ni(111) is after intercalation of Au spin-orbit split by the Rashba effect.
In the last part, a time-resolved x-ray magnetic circular dichroic-photoelectron emission microscopy study of a permalloy platelet comprising three cross-tie domain walls is presented. It is shown how a fast picosecond magnetic response in the precessional motion of the magnetization can be induced by means of a laser-excited photoswitch. From a comparision to micromagnetic calculations it is demonstrated that the relatively high precessional frequency observed in the experiments is directly linked to the nature of the vortex/antivortex dynamics and its response to the magnetic perturbation. This includes the time-dependent reversal of the vortex core polarization, a process which is beyond the limit of detection in the present experiments.Diese Dissertation beschäftigt sich mit den elektronischen, spinabhängigen und dynamischen Eigenschaften dünner magnetischer Systeme. Auf dem Photoeffekt basierende Untersuchungsmethoden werden zusammen mit Synchrotronstrahlung eingesetzt, um die spinabhängigen Eigenschaften dieser Systeme im Energie- und Zeitbereich zu untersuchen. Im ersten Teil dieser Arbeit wird mit spin- und winkelaufgelöster Photoemission die Stärke von Elektronenkorrelationseffekten in der spinabhängigen elektonischen Struktur von ferromagnetischerm bcc Fe(110) und hcp Co(0001) untersucht. Die experimentellen Ergebnisse werden verglichen mit theoreteischen Berechnungen im Rahmen der Näherung der Drei-Körper-Streuung und der dynamischen Molekularfeldtheorie, zusammen mit Berechnungen des Photoemissionsprozesses im Rahmen des Ein-Stufen-Modells. Ausgehend von diesem Vergleich wird gezeigt, dass die gegenwärtig fortgeschrittensten Rechnung, obgleich sie die Beschreibung von Korrelationseffekten in Fe und Co verbessern, zu kleine Massenrenormalisierungen und Streuraten ergeben, was zu der Forderung nach verfeinerten Vielteilchentheorien unter Einbeziehung von nichtlokalen Fluktuationen führt. Im zweiten Teil wird unter Kontrolle durch die Photoelektronenspektroskopie im Detail gezeigt, wie Graphen durch chemische Gasphasenabscheidung auf den Übergangsmetall-Oberflächen Ni(111) und Co(0001) aufgebracht und mit einer Monolage Au interkaliert werden kann. Für beide Systeme wird eine lineare E(k)-Dispersion masseloser Dirac-Fermionen im Graphen-pi-Band in der Nähe der Fermi-Energie beobachtet. Spinaufgelöste Photoemission des Graphen-pi-Bandes zeigt, dass die ferromagnetische Polarisation von Graphen/Ni(111) und Graphen/Co(0001) vernachlässigbar ist und dass Graphen/Ni(111) nach Interkalation mit Au eine Spin-Bahn-Aufspaltung aufgrund des Rashba-Effekts zeigt. Im letzten Teil wird eine zeitaufgelöste Studie des Röntgenzirkulardichroismus mit Photoelektronenmikroskopie präsentiert, die an einer Permalloy-Probe durchgeführt wurde, die drei als Stachelwände ausgebildete Domänenwände enthält. Es wird gezeigt, wie eine schnelle magnetische Antwort auf der Pikosekundenskala in der Präzessionsbewegung der Magnetisierung durch einen laserangesteuerten Photoschalter erzeugt werden kann. Durch Vergleich mit einer mikromagnetischen Rechnung wird gezeigt, dass die relativ hohe Präzessionsfrequenz, die im Experiment beobachtet wird, in unmittelbarer Beziehung steht zu den Eigenschaften der Vortex/Antivortex-Dynamik und ihrer Antwort auf die magnetische Störung. Das schließt die zeitabhängige Umkehr der Vortexkernpolarisation ein, einem Vorgang der jenseits der Nachweisgrenze der gegenwärtigen Experimente liegt.
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Schwöbel, Jörg [Verfasser], and Roland [Akademischer Betreuer] Wiesendanger. "Building a 300mK-17T-UHV scanning tunnelling microscopy system and spin-resolved investigation of individual bis(phthalocyaninato)terbium single-molecule magnets / Jörg Schwöbel ; Betreuer: Roland Wiesendanger." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2018. http://d-nb.info/1155304616/34.
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McCormick, Elizabeth Joan McCormick. "Optical Properties of Two Dimensional Semiconductors." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1531907387651019.
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Marino, Andrea. "Ultrafast investigation of electronic and structural dynamics in photomagnetic molecular solids." Thesis, Rennes 1, 2015. http://www.theses.fr/2015REN1S037/document.
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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 systèmeThe 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
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Ceeh, Hubert [Verfasser], Christoph Pascal [Akademischer Betreuer] [Gutachter] Hugenschmidt, Jonathan J. [Gutachter] Finley, and Friedrich [Gutachter] Reinert. "The new 2D-ACAR spectrometer for spin-resolved measurements of the electronic structure in correlated systems / Hubert Ceeh ; Gutachter: Jonathan J. Finley, Friedrich Reinert, Christoph Pascal Hugenschmidt ; Betreuer: Christoph Pascal Hugenschmidt." München : Universitätsbibliothek der TU München, 2015. http://d-nb.info/1120584221/34.
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Gonzalez, Hernandez Felix Guillermo. "Tempos de relaxação e decoerência em ensembles de pontos quânticos." [s.n.], 2007. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277852.
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Orientador: Gilberto Medeiros RibeiroTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Medidas experimentais foram realizadas para determinar as escalas de tempo de relaxação e decoerência do spin eletrônico como bit quântico. A estrutura dos estados de exciton foi investigada com o objetivo de servir como estados intermediários na manipulação do spin. O sistema utilizado para o estudo de decoerência é um ensemble de pontos quânticos auto-formados semicondutores. Dois temas servem como eixos centrais dos três experimentos desenvolvidos nesta tese: a polarização de spin e o fator g de Landé. No primeiro experimento, ao incluir o efeito do reservatório térmico, foi obtido o grau de polarização do spin (populações dos níveis up e down) para as camadas s e p. O desdobramento dos níveis orbitais em subníveis de spin permitiu obter a magnitude do fator g para estes estados. Mudando a orientação do campo magnético, foram observadas as anisotropias do tensor g e a sua relação com os detalhes do potencial de confinamento. Estas características permitiram inferir o tempo de relaxação T1. A medida da polarização resolvida no tempo foi realizada através de es-pectroscopia óptica de bombeio-prova. Os pulsos de luz e o campo magnético transverso permitem que uma polarização líquida seja inicializada. A rotação de Kerr permitiu observar oscilações desta polarização em torno do campo magnético com freqüência determinada pelo fator g. A perda da coerência de fase do spin resulta no decaimento destas oscilações numa escala de tempo T2. Medidas realizadas num ensemble de spins implicam em que o tempo de decoerência encontra-se limitado pela escala de defasagem T¤2< T2. Uma técnica semelhante à refocalização por spin-eco em experimentos de ressonância magnética nuclear, foi aplicada utilizando pulsos de laser para reverter a defasagem do ensemble. Tanto a possibilidade de medir o sinal de eco como o tempo de decoerência foram medidos como função da temperatura. A estrutura de níveis de exciton e a sua distribuição no ensemble foi estudada também com espectroscopia de bombeio-prova. Foram observados batimentos quânticos entre os níveis de estrutura fina do exciton para sis-temas 0D e 2D limitados pelo tempo de recombinação
Abstract: Experimental measurements were carried out to determine the scales of the relaxation and decoherence time for the electronic spin as quantum bit. The structure of the exciton states was investigated with the objective to serve as intermediate states in the spin manipulation. The system studied for the implementation of the quantum computation is an ensemble of self-assembled semiconductor quantum dots. Two subjects serve as central axes of the three experiments developed in this thesis: the spin polarization and the Landé g-factor. In the first experiment, when including the effect of the thermal reservoir, the degree of spin polarization (populations for the up and down levels) was measured for layers s and p. The splitting of the orbital levels in spin sublevels allowed to get the magnitude of factor g for these states. Changing the orientation of the magnetic field, the g-tensor anisotropies and its relation with the details of the confinement potential had been observed. These characteristics had allowed to infer the relaxation time T1. The time resolved polarization measurement was carried out by optical pump-probe spectroscopy. The pulses of light and the transverse magnetic field allow the initialization of a net polarization. The Kerr rotation allowed to observe oscillations of this polarization around the magnetic field with frequency determined for factor g. The loss of the spin phase coherence results in the decay of these oscillations in a time scale T2. Measurements carried out in an ensemble of spins imply that the decoherence time is limited by the ensemble dephasing time T¤2 < T2. A technique similar to the spin-echo refocalization in nuclear magnetic resonance experiments using laser pulses was applied to reverse the ensemble dephasing. The possibility to measure the echo signal and the decoherence time was measured as a function of the temperature. The structure of exciton levels and its distribution in ensemble were also studied with pump-probe spectroscopy. Quantum beats were observed be-tween the fine structure exciton levels for 0D and 2D systems, yet limited by the recombination time
Doutorado
Física da Matéria Condensada
Doutor em Ciências
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Bersweiler, Mathias. "From Sm1-xGdxAl2 electronic properties to magnetic tunnel junctions based on Sm1-xGdxAl2 and/or [Co/Pt] electrodes : Towards the integration of Zero Magnetization ferromagnets in spintronic devices." Thesis, Université de Lorraine, 2014. http://www.theses.fr/2014LORR0146/document.
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Le contexte général de ce travail est le développement et l'intégration de nouveaux matériaux magnétiques ayant des propriétés originales et d'intérêt potentiel pour la spintronique. En tant que matériau ferromagnétique d’aimantation nulle, le composé Sm1-xGdxAl2 (SGA) suscite un intérêt particulier, puisqu’il est capable, dans son état magnétique compensé, de polariser en spin un courant d’électrons. Dans un premier temps, des expériences de photoémission résolues en angle et en spin sur synchrotron ont permis d’effectuer une analyse précise de la structure électronique selon diverses directions de la zone de Brillouin et d’estimer de manière directe la polarisation de spin au niveau de Fermi du composé SGA. Dans un second temps, une attention particulière a été portée aux multicouches [Co/Pt] et aux JTMs à base de [Co/Pt]. Les multicouches [Co/Pt] constituent la seconde électrode des JTMs à base de SGA. Leurs propriétés magnétiques (en particulier l'anisotropie perpendiculaire et l'aimantation à saturation) ont été soigneusement étudiées en fonction de l'épaisseur de Pt et de la nature de la couche tampon (Pt, MgO ou Al2O3), et en liaison avec leurs caractéristiques structurales. Leur intégration dans des JTMs à base de [Co/Pt] a permis ensuite de remonter d’une part à la polarisation tunnel effective des multicouches [Co/Pt] et d’autre part aux configurations magnétiques des différentes électrodes, configurations parfaitement expliquées et reproduites par des simulations micro-magnétiques. Dans un troisième temps, les résultats de magnéto-transport au sein des JTMs SGA/MgO/[Co/Pt] sont présentés et discutésThe general context of this work is the development and integration of new magnetic materials with original properties of potential interest for spintronic applications. In this field, the Sm1-xGdxAl2 (SGA) compound drives a particular attention, as a zero-magnetization ferromagnet that can exhibit a spin polarization in its magnetic compensated state. In a first step, synchrotron-based angle and spin resolved photoemission spectroscopy experiments have permitted to perform an accurate analysis of the electronic structure along various directions of the Brillouin Zone and to get a direct estimation of the spin polarization at the Fermi level. In a second step, a special attention has been the paid to [Co/Pt] multilayers and to [Co/Pt]-based MTJs. The [Co/Pt] multilayers would constitute the second electrode in SGA-based MTJs. Their magnetic properties (especially the perpendicular anisotropy and the saturation magnetization) have been carefully investigated as a function of Pt thickness and nature of the buffer layer (Pt, MgO or Al2O3), and in close connection with structural characteristics. Their integration in [Co/Pt]-based MTJs has permitted to determine the [Co/Pt] effective tunnel polarization and to unravel the magnetic configurations of both electrodes which are perfectly explained and reproduced by micromagnetic simulations. In a third step, the results concerning the magneto-transport experiments in SGA/MgO/[Co/Pt] MTJs are presented and discussed
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Pincelli, T. "PROBING ELECTRON CORRELATION DYNAMICS: A MULTI-TECHNIQUE STUDY APPLIED TO THE HALF-METALLIC OXIDE LA1-XSRXMNO3." Doctoral thesis, Università degli Studi di Milano, 2017. http://hdl.handle.net/2434/543731.
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This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano that has been carried out, starting in November 2014, mostly at the Laboratorio TASC of IOM-CNR in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures, in the framework of the NFFA and APE-beamline facilites, as well as by accessing international large scale infrastructures and laboratories.
The activity has addressed the development of experimental methodologies and novel instrumentation oriented to the study of the dynamical properties of highly correlated materials after high energy excitation. The science programme has been carried out by exploiting ultrafast femtosecond probes from the optical regime (Ti-Sa lasers, fibre laser oscillators) to the extreme UV-soft X rays at FERMI, to the picosecond hard X-rays from the SPring-8 and Diamond synchrotron radiation source. The sample synthesis of correlated oxides and its characterization has been performed within the NFFA facility and APE-group collaboration in Trieste as well as the design and construction of the all new laser High Harmonic Generation beam line NFFA-SPRINT and its end station for time resolved vectorial electron spin polarimetry.
This report concentrates on the main scientific concern of my work that has been the relaxation of external perturbations in a correlated electron material both in the time and space domain.
I have employed Photoelectron Spectroscopy (PES) mostly in the Hard X-ray regime (HAXPES), pushing the boundaries of its application to achieve a coherent perspective. The material I have mainly focused on is La0.67Sr0.33MnO3 (LSMO), of high interest for spintronics. This system is prototypical, yielding the highest simplicity in the class of transition metal oxides.
In the spatial investigation, I have controlled with high precision the PES probing depth and I have observed the evolution of one spectral feature. I have identified it as probe of electronic hybridization and long-range ordering. I have studied LSMO films of 40 nm in three substrate-induced strain states (1% tensile in-plane, relaxed, 1% compressive in-plane) and a 18 nm film of (Ga,Mn)As (GMA), a well-studied diluted magnetic semiconductor. I have found that the electronic properties to be modified at significant distances from the surface, 4 nm for LSMO and 1.2 nm for GMA, while strain had no detectable effects.
In the temporal study, I have employed HAXPES in pump-probe mode (TR-HAXPES) to observe the evolution of the electronic structure after intense optical excitation. A detailed dynamical characterization with optical techniques has allowed me to identify the characteristic time of the collapse of long-range magnetic order to be significantly longer than the one of elemental transition metals. I have ascribed this effect to the half-metallic character of LSMO. With TR-HAXPES I have observed that the whole electronic band-structure evolution is bottlenecked by the slow response of the magnetization, proceeding on hundreds of picoseconds timescales.
Finally, I have described the techniques and the instrumentation that can be used to push these investigations to shorter spatial and temporal scales. This has been realized in the form of the NFFA-SPRINT laboratory, a facility open to users, which I participated in designing and developing.
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Kremer, Geoffroy. "Installation d’un nouveau dispositif de photoémission résolue en angle et en spin, et étude des propriétés électroniques de matériaux artificiels aux propriétés remarquables." Thesis, Université de Lorraine, 2018. http://www.theses.fr/2018LORR0293/document.
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Dans ce travail de thèse, nous illustrons la pertinence de la technique de photoémission pour l'étude des propriétés électroniques des matériaux. Dans la première partie, nous détaillons le développement et la phase de tests d'un nouveau bâti expérimental composé d'une chambre d'épitaxie par jets moléculaires (MBE) ainsi que d'une chambre de photoémission résolue en angle et en spin (SR-ARPES), connecté au tube Daum à l'Institut Jean Lamour. Les hautes performances de ce nouveau dispositif sont d'une part évaluées par une série de mesures expérimentales sur un système connu de la littérature (état de Shockley à la surface de l'Au(111)), et d'autre part illustrées par l'analyse de matériaux originaux (isolants topologiques, effet Kondo moléculaire …). Les valeurs de résolution en énergie sont inférieures à 2 meV et 300 meV pour la photoémission utilisant les rayonnements UV (UPS) et X (XPS) respectivement. La résolution angulaire est quant à elle meilleure que 0,2° et la température minimale atteignable est de 8,7 K. Finalement, des premières mesures de SR-ARPES ont démontré la capacité de ce nouveau bâti à mesurer les détails les plus fins de la structure de bandes polarisée en spin, se rapprochant ainsi de l'état de l'art dans le domaine. Ce nouveau dispositif est donc pleinement opérationnel. La seconde partie est consacrée à l'étude d'un oxyde de silicium ultra-mince bidimensionnel (2D) à la surface d'un substrat monocristallin de Ru(0001). Nous étudions tous les stades de croissance en partant du substrat nu de Ru(0001) jusqu'à une bicouche cristalline de cet oxyde, par XPS haute résolution (rayonnement synchrotron) et photoémission résolue en angle (ARPES). Nous confirmons la structure atomique établie dans la littérature pour ce système à la monocouche, avec en particulier l'existence de deux types de liaisons inéquivalentes Si-O-Ru révélées par des mesures inédites d’XPS haute résolution au niveau de la raie de cœur de l'O1s. En outre, nos mesures ARPES mettent en évidence l'existence d'états dispersifs bidimensionnels propres à ce matériau 2D. Alors que la monocouche est fortement connectée au substrat de ruthénium (liaisons covalentes), la bicouche en est déconnectée (liaisons de van der Waals). Notre étude confirme l'existence d’une telle transition avec des signatures claires à la fois en XPS et en ARPES, démontrant notamment la disparition des liaisons Si-O-Ru. Nous démontrons également la robustesse de ce système, qui une fois cristallisé peut être remis à l'air sans modifications majeures de ses propriétés électroniques, lui donnant ainsi un fort potentiel de fonctionnalisation (par exemple au sein d'hétérostructures 2D complexes comme couche isolante). Finalement, dans une troisième partie nous nous intéressons aux aspects théoriques de la photoémission résolue en angle. Alors que la structure de bandes est périodique dans l'espace réciproque, ce n'est pas le cas de l'intensité de photoémission, qui peut présenter des variations complexes dépendant de nombreux paramètres. Ces aspects sont généralement mal compris par les expérimentateurs. Nous présentons ici un modèle simple récemment proposé qui s'inscrit dans une description en trois étapes du processus de photoémission, et qui permet d'évaluer les éléments de matrice à un électron. Ces éléments de matrice représentent l'ingrédient essentiel permettant de comprendre la répartition du poids spectral en photoémission. Nous démontrons que dans ce modèle ils sont proportionnels à la transformée de Fourier de l'état de Wannier du système considéré, ainsi qu'à un terme de polarisation contenant les effets géométriques inhérents à toute expérience de photoémission. Nous appliquons alors cette approche à des systèmes physiques comme le graphène, ou encore au cas de mesures de dichroïsme circulaire réalisées au niveau des états d et de l'état de Shockley d'un monocristal de Cu(111), mettant ainsi en évidence ses succès et ses limitationsIn this work, we highlight the relevance of photoemission spectroscopy for investigating the electronic properties of materials. In the first part, we tackle the development and the test phase of a new experimental setup which is composed of a molecular beam epitaxy (MBE) and a spin and angle resolved photoemission (SR-ARPES) chambers, connected to the tube at the Institut Jean Lamour. The high performances of this new setup are evaluated. On one hand by measuring well known system from the litterature (Shockley state at the Au(111) surface) and on the other hand by studying materials with novel properties (topological insulators, molecular Kondo effect …). Energy resolution is better than 2 meV for UV photoemission (UPS) and 300 meV for X-ray photoemission (XPS). We also have an angular resolution better than 0.2° and a lowest sample temperature of 8.7 K. Finally, first SR-ARPES measurements demonstrate the ability of this new installation to measure finest details of the spin polarized band structure. In short, this new setup is fully operationnal. The second part is dedicated to the study of a two dimensionnal (2D) ultra thin silicon oxide at the surface of a cristalline Ru(0001) substrate. Both growth and electronic properties are studied by high resolution XPS and ARPES. We confirm the structural model accepted for the system in the litterature for the monolayer case. In particular we confirm the existence of two inequivalent Si-O-Ru bonds with unprecedented high resolution XPS measurements on the O1s core level. In addition, our ARPES measurements highlight new dispersives states with 2D character which are unambiguously attributed to this oxide. While the monolayer is strongly connected to the ruthenium substrate (covalent bonds), the bilayer is disconnected from this latter one (van der Waals). Our work confirms the existence of such a transition with unambiguous signatures both in XPS and ARPES, in particular with the breaking of Si-O-Ru bonds. We also demonstrate the robustness of this system which, after being cristallised, can go to atmosphere without fundamental modification of his electronic properties. That gives a lot of potential applications to this 2D cristalline oxide, which could play in the futur the role of a wide band gap insulator in 2D heterostructures. In the last part, we focus on the theoretical aspects of photoemission. While band structure is periodic in the reciprocal space, it is not the case of photoemission intensity which can depend on a lot of parameters. We are motivated by the fact that these considerations are generally not well understood by experimentalists. Here, we present a simple model recently proposed in the three step approach of the photoemission process. With this model we can evaluate the one-electron matrix elements which play a key role to understand the variations of spectral weight in photoemission. In this approach, one-electron matrix elements are proportionnal to both Fourier transform of the Wannier state of the system and to a polarization term. We apply this model to « real » systems, in particular to graphene and to circular dichroism measurements on Cu(111) sample, highlighting sucess and limitations of this model
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Berntsen, Magnus H. "Consequences of a non-trivial band-structure topology in solids : Investigations of topological surface and interface states." Doctoral thesis, KTH, Material- och nanofysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-121974.
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The development and characterization of experimental setups for angle-resolved photoelectron spectroscopy (ARPES) and spin- and angle-resolved photoelectron spectroscopy (SARPES) is described. Subsequently, the two techniques are applied to studies of the electronic band structure in topologically non-trivial materials. The laser-based ARPES setup works at a photon energy of 10.5 eV and a typical repetition rate in the range 200 kHz to 800 kHz. By using a time-of-flight electron energy analyzer electrons emitted from the sample within a solid angle of up to ±15 degrees can be collected and analyzed simultaneously. The SARPES setup is equipped with a traditional hemispherical electron energy analyzer in combination with a mini-Mott electron polarimeter. The system enables software-controlled switching between angle-resolved spin-integrated and spin-resolved measurements, thus providing the possibility to orient the sample by mapping out the electronic band structure using ARPES before performing spin-resolved measurements at selected points in the Brillouin zone. Thin films of the topological insulators (TIs) Bi2Se3, Bi2Te3 and Sb2Te3 are grown using e-beam evaporation and their surface states are observed by means of ARPES. By using a combination of low photon energies and cryogenic sample temperatures the topological states originating from both the vacuum interface (surface) and the substrate interface are observed in Bi2Se3 films and Bi2Se3/Bi2Te3 heterostructures, with total thicknesses in the ultra-thin limit (six to eight quintuple layers), grown on Bi-terminated Si(111) substrates. Band alignment between Si and Bi2Se3 at the interface creates a band bending through the films. The band bending is found to be independent of the Fermi level (EF) position in the bulk of the substrate, suggesting that the surface pinning of EF in the Si(111) substrate remains unaltered after deposition of the TI films. Therefore, the type and level of doping of the substrate does not show any large influence on the size of the band bending. Further, we provide experimental evidence for the realization of a topological crystalline insulator (TCI) phase in the narrow-band semiconductor Pb1−xSnxSe. The TCI phase exists for temperatures below the transition temperature Tc and is characterized by an inverted bulk band gap accompanied by the existence of non-gapped surface states crossing the band gap. Above Tc the material is in a topologically trivial phase where the surface states are gapped. Thus, when lowering the sample temperature across Tc a topological phase transition from a trivial insulator to a TCI is observed. SARPES studies indicate a helical spin structure of the surface states both in the topologically trivial and the TCI phase.QC 20130507
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Matveev, Sergey M. "Photophysics and photochemistry of diiodomethane and hexabromoiridate - paradigm molecules for organic and inorganic chemistry - studied with sub-50-fs broadband pump-probe spectroscopy." Bowling Green State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1467909513.
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