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. / Mn5Ge3 attracts strong interest for spintronics applications because it has a high Curie-temperature (≈300 K) and it can be grown epitaxially on Ge(111) substrates, permitting direct injection of a spin-polarized current into the Ge semiconductor.Mn5Ge3(001) thin films grown on Ge(111)-c(2x8) reconstructed surface were studied by angle- and spin- resolved photoemission (ARPES, SARPES) using synchrotron radiation. ARPES results, obtained in the GALM and GAHK planes, are in agreement with simulations done with the help of band structure calculations based on the density functional theory, taking into account lifetime broadening and broadening caused by correlation effects.SARPES measurements done at several k-points of the GALM plane are also well acounted for by these simulations.Overall our results provide a remarkable validation of the band structure model for a proper description of the electronic properties of Mn5Ge3. Only the spectral intensity at the Fermi level is not well explained by the simulation. This departure is attributed to the 3D nature of the sample and to correlation effects.

Thesis advisor: Hong Ding / Thesis advisor: Ziqiang Wang / The discovery of the high-T_c superconductivity in iron-arsenic materials in 2008 immediately became one of the hottest topics in the condensed matter physics. This dissertation presents a systematic study on the pairing symmetry and electronic structure on the hole doped materials of BaFe₂As₂ (so called &ldquo;122&rdquo;-system), by angle-resolved photoemission spectroscopy (ARPES). In the early ARPES studies on &ldquo;122&rdquo;-pnictides, we observed two hole-like Fermi surfaces (FSs) centered at the Brillouin zone (BZ) center, (&Gamma;), and two electron-like FSs centered at the zone corner (M), which is (&pi;, &pi;) in the BZ or (&pi;, 0) in the unfolded BZ. The size of these FS sheets can be changed by carrier doping, which causes change of the chemical potential. In the superconducting state, temperature (<italic>T</italic>) and momentum (<italic>k</italic>) dependence of ARPES measurements reveals the Fermi-surface-dependent nodeless superconducting gaps in this system and shows that an <italic>s</italic>-wave symmetry is the most natural interpretation for our findings in terms of the pairing order parameter. The ratio 2&Delta;/k_BT_c switches from weak to strong coupling on different FS sheets. Large superconducting gaps are observed with a strong coupling coefficient (2&Delta;/k_BT_c) on the near-nested FSs connected by the antiferromagnetic (AF) wave vector ((&pi;, &pi;) in the BZ or (&pi;, 0) in the unfolded BZ). When T_c is suppressed in the heavily overdoped materials, the near-nesting condition vanishes, or more precisely, the (&pi;, &pi;) inter-FS scattering disappears due to the absence of either the hole-like or the electron-like FS at the Fermi energy (E_F). We have also performed ARPES measurements on k_z-dependence of the superconducting gap and band structure of the optimally hole doped sample Ba_0.6K_0.4Fe₂As₂. By varying the photon energy, we can tune k_z continuously. While significant k_z dispersion of the superconducting gaps is observed on the hole-like bands, much weaker k_z dispersion of the superconducting gaps is observed on the electron-like bands. Remarkably, we find that a 3D gap function based on short-range pairing can fit the superconducting gaps on all the FS sheets. Moreover, an additional hole-like FS (referred as the &alpha;<super>&lsquo;</super> FS) predicted by local density approximation (LDA) calculations is observed around the Z point. The disappearance of intensity of the &alpha;<super>&lsquo;</super> band near E_F at k_z = &pi;/2 suggests that the &alpha;<super>&lsquo;</super> band could either sink below E_F or be degenerate with the inner hole (&alpha;) band. The studies on the &alpha;<super>&lsquo;</super> band in the superconducting state reveal a nearly isotropic superconducting gap on this FS sheet. Underdoped samples Ba_0.75K_0.25Fe₂As₂ are used to study how the AF fluctuations and superconductivity interplay in the underdoped regime that is closer to the AF phase. we observe that the superconducting gap of the underdoped pnictides scales linearly with T_c. A distinct pseudogap develops upon underdoping and coexists with the superconducting gap. Remarkably, this pseudogap occurs mainly on the FS sheets that are connected by the AF wave vector, where the superconducting pairing is stronger as well. This suggests that both the pseudogap and the superconducting gap are driven by the AF fluctuations, and the long-range AF ordering competes with the superconductivity. The observed dichotomic behaviour of the pseudogap and the SC gap on different FS sheets in the underdoped pnictides shares similarities with those observed in the underdoped copper oxide superconductors, providing a possible unifying picture for both families of high-temperature superconductors. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

This thesis is the result of investigations regarding the processes in InAs III-V semiconductor surfaces induced by additional charge incorporated by adsorbates. The aim of the project is to study the development of the accumulation layer on the metal/InAs(111)A/B surfaces and its electronic structure. InAs(111)A is indium-terminated and InAs(111)B is arsenic-terminated. In addition, InAs(100) is also studied. These three substrates are different; InAs(111)A has a (2x2)-termination, explained by an indium vacancy model, and the clean surface exhibits a two-dimensional electron gas (2DEG). InAs(111)B(1x1) is bulk-truncated and unreconstructed and does not host a 2DEG. InAs(100)(4x2)/c(8x2) exhibits a more covalent character of the surface bonds compared to InAs(111)A/B, and the surface is terminated by a complex reconstruction. Photoelectron spectroscopy and LEED (low energy electron diffraction) have been used as the main tools to study surfaces with sub-monolayer to monolayer amounts of adsorbates. A photoemission peak related to a two-dimensional electron gas appears close to the Fermi level. This 2DEG has in most cases InAs bulk properties, since it is located in the InAs conduction band. A systematic study of core levels and valence bands reveals that the appearance of the 2DEGs is a complex process connected to the surface order. Adsorption of lead, tin or bismuth on InAs(111)B(1x1) induces emission from a 2DEG, but only at monolayer coverage and when the surface is ordered. Cobalt reacts strongly with InAs forming InCo islands and no accumulation is observed. Examination of Cs/InAs(111)B does not reveal any 2DEG and the surface reaction is strongly related to the clean surface stabilization process. Examination of the In-terminated InAs(111)A(2x2) surface shows that In reacts strongly with cobalt and tin adatoms and with oxygen in cases of large exposure, which decreases the 2DEG intensity, while adatoms of cesium and small doses of oxygen enhance the emission from the 2DEG. InAs(100) is terminated with one kind of atom - the InAs(100)(4x2)/c(8x2) is indium terminated. Bismuth creates dimers on the surface and a 2DEG is observed. More generally, this thesis describes some of the general physical background applied to surface science and 2DEG. The first part is a general overview of the processes on the surface. The second part concentrates on the methods related to preparation of samples, and the third part on the measurement methods. The photoelectron spectroscopy part concerns the theory used in mapping electronic structure. The inserted figures are taken from different experiments, including results for InAs(111)A not previously published. / QC 20100910

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

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.

La motivation principale de cette thèse a été de déterminer les avantages et les inconvénients de l'utilisation de la spectroscopie de photoémission résolue en angle dans le domaine des rayons X mous.<br />L'étude d'un système bien connu, Ag(001) nous permet de discuter plusieurs questions telles que le rôle de la quantité de mouvement du photon, la pertinence de l'approximation d'électron libre à l'état final, et le rôle des phonons. La polarisation de la lumière incidente a aussi été exploitée. En choisissant un tel système, nous avons aussi voulu comparer les résultats expérimentaux avec des spectres calculés de photoémission résolue en angle dans cette gamme d'énergie.<br />Le comportement à basse température atypique des composés de Cérium est généralement imputé à l'effet Kondo. Des résultats originaux ont été obtenus en étudiant la bande de valence de trois composés monocristallins iso-structuraux de Cérium, CeCu2Ge2, CeNi2Ge2 et CeCo2Ge2. La position du seuil d'absorption M5 du Cérium dans la bande d'énergie des rayons X mous est exploitée pour isoler la contribution 4f à ces spectres. De plus, l'utilisation de photons incidents d'énergie relativement élevée permet de minimiser les effets de surface. Les spectres de photoémission présentés dans cette thèse incluent des études de dépendance en température, des spectres à la résonance, des spectres résolus en angle ou bien intégrés angulairement. Les premiers sont en accord avec le modèle d'impureté unique d'Anderson, alors que les derniers suggèrent qu'il est important de prendre en compte le réseau cristallin.

J'ai étudié les interfaces semi-conducteur organiques/ferromagnétique par la diffusion des électrons et la photoémission résolue en spin. Dans la première partie, un comportement inattendu de la réflexion d'électrons dépendante de spin à ces interfaces est observé. En fait, une couverture sous-monocouche des molécules organiques rend l’amplitude de réflexion d’électrons indépendante de spin, c.à.d. que la réflectivité ainsi que la phase de réflexion devient indépendante de l'orientation du spin des électrons incidents. Bien que je ne sois pas en mesure d'identifier la cause de ce phénomène, je montre qu'il s'agit d'un phénomène très général qui est indépendante de l'énergie des électrons primaires, du choix du substrat ferromagnétique, du choix de la molécule organique, et de l'orientation de la polarisation initiale. Il n'est pas du à un changement de l’aimantation de surface, à une dépolarisation des électrons primaires, ou à une interaction directe des molécules avec le substrat ferromagnétique. En outre, la théorie ne prédit pas les résultats expérimentaux et d'autres recherches sont donc nécessaires pour dévoiler la physique derrière ces observations. Dans la seconde partie de ma thèse, les expériences de photoémission résolue en spin sont réalisées au synchrotron SOLEIL. Le résultat principal est l'observation d'un état électronique induite par les molécules organiques près du niveau de Fermi qui est hautement polarisé en spin. Des mesures en fonction de l’épaisseur de la couche organique permettent d’identifier le caractère interfacial de cet état électronique. Enfin, ces résultats sont comparés avec des calculs théoriques effectués à l'institut. / 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.

No description available.

In this thesis, we have explored the possibilities to realize a Graphene Based Hybrid structures (GBHs) by the functionalization of a graphene layer on both sides. The first chapter gives a general introduction about graphene and a literature review of different metal intercalations on graphene. The second chapter explains the experimental techniques used in this work. In chapter 3, we studied the functionalization of epitaxial graphene on SiC(0001) by gold intercalation. With the help of Scanning Tunneling Microscopy, we have evidenced and characterized different intercalation modes such as the formation of aggregates of individual gold atoms and the formation of a continuous gold layer between the top graphene and the buffer layer. The free standing nature of the intercalated gold atoms was examined by differential charge density plot, projected density of states calculations and further by X-ray photoelectron spectroscopy. The band structure modification of graphene due to these intercalated gold atoms was evidenced by Angle-resolved photoemission spectroscopy, which reveals a strong Van Hove extension and an increase of the Fermi velocity. Extend to this research, to obtain an extended Van Hove singularity usually observed in highly doped graphene; we studied highly electron donor molecules, TetraThioFullvalene (TTF) on pristine and gold intercalated graphene and on graphite (chapter 4). The dependence of charge transfer of these molecules with their conformation and the reactivity of photochromic with conjugated molecules on graphene were also discussed. To understand the structural properties of these molecules photophysical measurements were performed in chapter 5.

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.