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.

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.

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.

In this thesis the charge dynamics of correlated electron systems, in which a metallic phase lies in close proximity to an ordered phase, are investigated by means of angle resolved photoemission spectroscopy (ARPES). The analysis of the experimental data is complemented by electronic structure calculations within the framework of density functional theory (DFT). First the charge dynamics of the colossal magnetoresistant bilayer manganites are studied. The analysis of the ARPES spectra based on DFT calculations and a Peierls type charge density wave model, suggests that charge, orbital, spin and lattice degrees of freedom conspire to form a fluctuating two dimensional local order that produces a large pseudo gap of about 450 meV in the ferromagnetic metallic phase and that reduces the expected bilayer splitting. Next, the interplay of Kondo physics and (magnetic) order in the heavy fermion superconductor URu2Si2 is investigated. The low energy electronic structure undergoes strong changes at 17.5 K, where a second order phase transition occurs whose phenomenology is well characterized, but whose order parameter could not yet be unambigeously identified. Below THO, non-dispersive quasi particles with a large scattering rate suddenly acquire dispersion and start to hybridize with the conduction band electrons. Simultaniously the scattering rate drops sinificantly and a large portion of the Fermi surface vanishes due to the opening of a gap within the band of heavy quasi particles. The observed behaviour is in stark contrast to conventional heavy fermion systems where the onset of hybridization between localized and itinerant carriers happens in a crossover type transition rather than abruptly. These experimental results suggest that Kondo screening and the hidden order parameter work together to produce the unusual thermodynamic signatures observed in this compound. Finally, the influence of charge doping and impurity scattering on the superconducting porperties of the transition metal substituted iron pnictide superconductor Ba(Fe1-xTMx)2As2 (TM = Co, Ni) is studied. Here, resonant soft X-ray ARPES is applied to see element selective the contribution of the 3d states of the TM substitute to the Fe 3d host bands. The spectroscopic signatures of the substitution are found to be well reproduced by DFT supercell and model impurity calculations. Namely, the hybridization of the dopant with the host decreases with increasing impurity potential and the electronic states of the impurtiy become increasingly localized. Simultaniously, in all simulated cases a shift of the Fermi level due to electron doping is observed. The magnitude of the shift in the chemical potential that accurs in BaFe2As2, however, is in stark contrast to the marginal doping values obtained for the impurity model, where the shift of the chemical potential is largely compensated by the influence of the increasing impurity potential. This suggests that the rigid band behaviour of TM substituded BaFe2As2 is a peculiarity of the compound, which has strong implications for the developement of superconductivity. / In dieser Arbeit wird die Ladungstraegerdynamik in korrelierten Elektronensystemen, in denen eine metallische Phase in direkter Nachbarschaft zu einer geordneten Phase liegt, mit Hilfe von winkelaufgeloester Photoelektronenspektroskopie (ARPES) untersucht. Die Analyse der experimentellen Daten wird ergaenzt durch lektronenstrukturrechnungen im Rahmen der Dichtefunktionaltheorie (DFT). Zuerst wird die Ladungstraegerdynamik in gemischtvalenten zweischichtmanganaten mit kolossalem Magnetiwiderstand studiert. Die Analyse der Photoemissionsspektren basierend auf DFT Rechnungen und einem Peierls artigem Ladungsdichtewellenmodell, legt nahe, dass die Freiheitsgrade von Ladung, Orbitalen, Spin und des Ionengitters konspirieren, um eine fluktuierende zweidimensionale lokale Ordnung zu bilden, die verantwortlich ist fuer die beobachtete Pseudobandluecke von 450 meV, und die zur Reduktion der erwarteten Zweischichtaufspaltung beitraegt. Als naechstes wird das Zusammenspiel von Kondo Physik und (magnetischer) Ordung im Schwerfermionensupraleiter URu2Si2 untersucht. Die iedrigenergetische elektronische Struktur zeigt starke Veraenderungen bei 17.5 K, wo ein Phasenuebergang zweiter Ordnungstattfindet, der phenomenologisch gut charakterisiert ist, aber dessen Ordungsparameter nocht nicht eindeutig identifiziert werden konnte. Unterhalb von THOerlangen nicht dispergierende Quasiteilchen mit gro en Streuraten abrupt Dispersion und hybridisieren mit den Leitungselektronen. Gleichzeitig sinkt die Streurate und ein gro er Teil der Fermiflaeche verschwindet durch das Oeffnen einer Bandluecke innehalb des Bandes schwerer Quasiteilchen. Das beobachtete Verhalten steht in starkem Kontrast zu dem von konventionellen Schwerfermionensystemen, in denen die Hybridisierung zwischen lokalisierten und itineranten Ladungstraegern in einem kontinuierlichen Uebergang ablaeuft, anstatt abrubt. Diese experimentellen Befunde lassen den Schluss zu, dass das zusammenspiel zwischen Kondo Abschirmung und dem unbekannten Ordnungsparameter die ungewoehnlichen thermodynamischen Signaturen in dieser Verbindung hervorruft. Abschliessend wird das Zusammenwirken von Ladungstraegerdotierung und Streuung an Stoeratomen auf die Supraleitung uebergangsmetalldotierter Eisenpniktid Supraleiter Ba(Fe1-xTMx)2As2 (TM = Co, Ni) untersucht. Mit Hilfe von resonantem Weichenroentgen ARPES gelingt es, elementselektiv den Beitrag der 3d Zustaende des TM Substituenten zu den Eisen 3d Wirtsbaendern zu beobachten. Die spektroskopischen Signaturen der Substitution sind mit Hilfe von DFT Rechnungen und Modelrechnungen mit zufaellig verteilten Stoeratomen gut zu reproduzieren. Insbesondere nimmt die Hybridisierung des dotierten Uebergangsmetalls und der Eisenbaender mit zunehmender Kernladungszahl ab und die elektronischen Zustaende der Stoeratome werden zunehmen lokalisiert. Gleichzeitig wird in allen gerechneten Faellen eine Verschiebung des Fermi Niveaus durch Elektronendotierung beobachtet. Der Betrag der Verschiebung des chemischen Potentials in BaFe2As2 steht allerdings in starkem Kontrast zu den Werten, die man im Falle der Modellrechnungen erhaelt, wo die Verschiebung des Fermi Niveaus durch den Einfluss des Potentials der Stoeratome groesstenteils kompensiert wird. Dies legt nahe, dass das beobachtete "rigid band" Verhalten von TM substituiertem BaFe2As2 eine Besonderheit dieser Verbindung ist, welches starke Auswirkungen auf die Ausbildung von Supraleitung hat.

Das Verhalten der elektronischen Eigenschaften von gespaltenen, aus der Schmelze gezüchteten In2O3-(111) Kristallen wurde bei Deposition von Edelmetallen, In und Sn mittels winkelaufgelöster Photoelektronen-Spektroskopie untersucht. Die Stöchiometrie, strukturelle Qualität und Kristall-Orientierung, die Oberflächenmorphologie und die Elektronenkonzentration wurden jeweils mittels energiedispersiver Röntgenspektroskopie, Laue-Beugung, Raster Tunnel-Mikroskopie (STM) und Hall-Effekt untersucht. Die Ähnlichkeit der fundamentalen und Oberflächen-Bandlücken kann auf das fast flache Verhalten der Bänder auf der gespaltenen Oberfläche der Kristalle zurückgeführt werden. Die Grenzflächen von Ag und Au/In2O3 zeigen Schottky-Verhalten, während ein ohmscher in Cu, In und Sn /In2O3-Kontakten beobachtet wurde. Aufgrund der Übereinstimmung zwischen optischen und Oberflächen-Bandlücken, der Bildung eines Gleichrichterkontaktes und des Auftretens der Oberflächenphotospannung auf der frischen Kristalloberfläche kann gefolgert werden, dass SEAL nicht eine intrinsische Eigenschaft der gespaltenen Oberfläche der untersuchten Kristalle ist. Des Weiteren wurden bei dicker Au- und Cu-Beschichtung von In2O3 bei Raumtemperatur Shockley-artige Oberflächenzustände beobachtet. Zusätzlich wurde die erste Phase des Wachstums von Cu und In auf In2O3 von der Ausbildung eines 2-dimensionalen Elektrongases (2DEG) begleitet, welches bei dickeren Schichten verschwand, die von dem auf reinen Oberflächen von dünnen In2O3- Filmen gemessenen 2DEG verschieden sind. Nach Messung der Austrittarbeit von In2O3 und den jeweils untersuchten Metallen in situ und unter Verwendung der Schottky-Mott-Regel trat außer bei Ag/In2O3 eine deutliche Abweichung auf. Die experimentellen Ergebnisse stimmen auch mit fortgeschrittenen Theorien, die auf dem Elektronegativitätskonzept und MIGS–Modellen basieren, nicht überein. / The behavior of the electronic properties of as-cleaved melt-grown In2O3 (111) single crystals was studied upon noble metals, In and Sn deposition using angle-resolved photoemission spectroscopy. The stoichiometry, structural quality and crystal orientation, surface morphology, and the electron concentration were examined by energy dispersive X-ray spectroscopy, Laue diffraction, scanning tunneling microscopy (STM), and Hall-effect measurement, respectively. The similarity of the measured-fundamental and surface-band gaps reveals the nearly flat behavior of the bands at the as-cleaved surface of the crystals. Ag and Au/In2O3 interfaces show Schottky behavior, while an ohmic one was observed in Cu, In, and Sn/In2O3 contacts. From agreement of the bulk and surface band gaps, rectifying contact formation as well as the occurrence of photovoltage effect at the pristine surface of the crystals, it can be deduced that SEAL is not an intrinsic property of the as-cleaved surface of the studied crystals. Moreover, for thick Au and Cu overlayer regime at room temperature, Shockley-like surface states were observed. Additionally, the initial stage of Cu and In growth on In2O3 was accompanied by the formation of a two dimensional electron gas (2DEG) fading away for higher coverages which are not associated with the earlier-detected 2DEG at the surface of In2O3 thin films. The application of the Schottky-Mott rule, using in situ-measured work functions of In2O3 and the metals, showed a strong disagreement for all the interfaces except for Ag/In2O3. The experimental data also disagree with more advanced theories based on the electronegativity concept and metal-induced gap states models.

Dans cette thèse, nous avons exploré les possibilités de réaliser des structures hybrides à base graphène (GBHS) par la fonctionnalisation bilatérale du graphène. Le premier chapitre donne une introduction générale sur le graphène et sur la littérature concernant les différentes méthodes d’intercalations de métaux dans le graphène. Le chapitre 2 décrit les techniques expérimentales utilisées. Le chapitre 3 concerne la fonctionnalisation du graphène épitaxié sur SiC (0001) par intercalation d'atome d’or. Les différents modes d'intercalation de l’or ont été mis en évidence par microscopie tunnelle (formation d'agrégats d'atomes d'or individuels et formation d'une couche d'or continue). La nature de ces atomes d'or intercalées a été examinée par des calculs d’image de densité de charge, et par Spectroscopie de Photoélectrons X (XPS). La modification de la structure de bande du graphène a aussi été mise en évidence par Spectroscopie de Photoélectrons UV Résolue Angulairement (ARPES) par des expériences sur synchrotron. Ces études ont révélé une forte extension de la singularité de Van Hove et une augmentation de la vitesse de Fermi. Afin d’agir sur cette extension de la singularité de Van Hove, des molécules fortement donneuses d'électrons, telle que la molécule de TetraThioFulvalene (TTF) ont été déposée sur graphène intercalé or et sur graphite (chapitre 4). La dépendance du transfert de charge de ces molécules avec leur conformation et la réactivité photochromique de ces molécules conjuguées sur le graphène ont également été abordés. Pour comprendre les propriétés structurales de ces molécules, des mesures photophysiques ont été effectuées qui apparaissant dans le chapitre 5. / 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.

Cette thèse présente une étude expérimentale des propriétés électroniques de deux matériaux topologiques, notamment l’isolant topologique tridimensionnel irradié Bi₂Te₃ et le super-réseau topologique naturel Sb₂Te. Les deux systèmes ont été étudiés par des techniques basées sur la spectroscopie de photoémission. Les composés Bi₂Te₃ ont été irradiés par des faisceaux d’électrons de haute énergie. L’irradiation avec des faisceaux d’électrons est une approche très prometteuse pour réaliser des matériaux qui sont vraiment isolants dans le volume, afin de mettre en évidence le transport quantique dans les états de surface protégés. En étudiant une série d’échantillons de Bi₂Te₃ par la technique de spectroscopie de photoémission résolue en temps et en angle (trARPES), nous montrons que les propriétés topologiques des états de surface de Dirac sont conservées après irradiation électronique, mais leurs dynamiques ultra-rapides de relaxation sont très sensibles aux modifications reliées aux propriétés du volume. De plus, nous avons étudié la structure électronique des bandes occupées et inoccupées du Sb₂Te. En utilisant la microscopie de photoémission d’électrons à balayage (SPEM), nous avons constamment trouvé diverses régions non équivalentes sur la même surface après avoir clivé plusieurs monocristaux de Sb₂Te. Nous avons pu identifier trois terminaisons distinctes caractérisées par différents rapports stœchiométriques de surface Sb/Te et possédant des différences claires dans leurs structures de bandes. Pour la terminaison dominante riche en tellure, nous avons également fourni une observation directe des états électroniques excités et de leurs dynamiques de relaxation en ayant recours à la technique trARPES. Nos résultats indiquent clairement que la structure électronique de surface est fortement affectée par les propriétés du volume du super-réseau. Par conséquent, pour les deux systèmes, nous montrons que la structure électronique de surface est absolument connectée aux propriétés du volume. / This thesis presents an experimental study of the electronic properties of two topological materials, namely, the irradiated three-dimensional topological insulator Bi₂Te₃ and the natural topological superlattice phase Sb₂Te. Both systems were investigated by techniques based on photoemission spectroscopy. The Bi₂Te₃ compounds have been irradiated by high-energy electron beams. Irradiation with electron beams is a very promising approach to realize materials that are really insulating in the bulk, in order to emphasize the quantum transport in the protected surface states. By studying a series of samples of Bi₂Te₃ using time- and angle-resolved photoemission spectroscopy (trARPES) we show that, while the topological properties of the Dirac surface states are preserved after electron irradiation, their ultrafast relaxation dynamics are very sensitive to the related modifications of the bulk properties. Furthermore, we have studied the occupied and unoccupied electronic band structure of Sb₂Te. Using scanning photoemission microscopy (SPEM), we have consistently found various nonequivalent regions on the same surface after cleaving several Sb₂Te single crystals. We were able to identify three distinct terminations characterized by different Sb/Te surface stoichiometric ratios and with clear differences in their band structure. For the dominating Te-rich termination, we also provided a direct observation of the excited electronic states and of their relaxation dynamics by means of trARPES. Our results clearly indicate that the surface electronic structure is strongly affected by the bulk properties of the superlattice. Therefore, for both systems, we show that the surface electronic structure is absolutely connected to the bulk properties.