In dieser Arbeit wurden Ergebnisse einer Analyse der elektronischen Struktur in Sr2CuO2Cl2 mittels winkelaufgelöster Photoemission dargelegt. Sie betreffen i) Sr2CuO2Cl2 als Untersuchungsgegenstand für die Grundlagenforschung an Hochtemperatursupraleitern (HTSL), ii) winkelaufgelöste Photoemission als Untersuchungsmethode zur Messung der elektronischen Struktur von HTSL und deren Muttersubstanzen, iii) Eigenschaften der elektronische Anregungen in Sr2CuO2Cl2 in unmittelbarer Nähe des chemischen Potenzials, sowie iv) das durch Korrelationen stark beeinflusste Dotierungsverhalten der Kuprate. 1. Sr2CuO2Cl2 eignet sich Dank seiner exakten Stöchiometrie und der strukturell perfekten Kupfer-Sauerstoff Ebenen hervorragend als Untersuchungssubstanz für die isolierende Phase der HTSL. 2. Mittels winkelaufgelöster Photoemission ist es möglich, die k-Raum- und Energieverteilung (Dispersion) von Anregungen nahe des chemischen Potenzials in Sr2CuO2Cl2 auszumessen. Dabei wurde eine starke Abhängigkeit der Intensität des Photoemissionssignales in Abhängigkeit von der Photonenenergie beobachtet. Es konnte nachgewiesen werden, dass sich diese Intensitätsoszillationen mit hoher Wahrscheinlichkeit durch eine Bragg-Interferenz der Photoelektronenewelle an der Schichtstruktur von Sr2CuO2Cl2 erklären und weiterhin, dass die gemessenen Dispersionskurven, von einer willkürlichen Normierung der Intensität abgesehen, unabhängig von der verwendeten Photoenenenergie sind. 3. Die Photoemissionsspektren von Sr2CuO2Cl2 zeigen die für Ladungstransfer-Isolatoren charakteristische Aufteilung des spektralen Gewichts (der besetzten Elektronenstruktur) in ein unteres Hubbardband, bindende und nichtbindende Sauerstoffbänder und der ZR-Singlet Struktur. Dabei konnte das untere Hubbardband identifiziert werden durch seine durch Resonanz hervorgerufene Intensitätsverstärkung (und dem daraus resultierenden d8-Charakter dieser Struktur), die Sauerstoffbänder und die ZR-Singlet Struktur durch eine Analyse ihres Symmetrieverhaltens unter Berücksichtigung der Polarisationsabhängigkeit des Photoemissionssignales. Das ZR-Singlet besitzt eine a1g-Symmetrie was sowohl die Konstruktion des ZR-Singlets nach Zhang und Rice als auch die Ergebnisse der Diagonalisierung der isolierten, mit zwei Löchern besetzten Plakette bestätigt. 4. Die Dispersion des ZR-Singlets ist parabolisch entlang Gamma-(Pi,Pi) mit einem absoluten Bindungsenergieminimum bei (0.5Pi,0.5Pi) und parabolisch entlang Gamma-(Pi,0) mit einem lokalen Bindungsenergieminimum bei (0.7Pi,0). Die Bandbreite entlang dieser beiden Richtungen beträgt etwa 300 meV beziehungsweise 200 meV, der Abstand der beiden minimalen Bindungsenergien entlang dieser Richtungen ist 72 meV. Das Spektralgewicht des ZR-Singlets ist keilförmig verteilt mit einem Maximum bei (0.5Pi,0.5Pi) und (0.7Pi,0), also jeweils in unmittelbarer Nähe der minimalen Bindungsenergien dieser Struktur. Es verschwindet bei Gamma und (Pi,Pi), hat jedoch einen endlichen Wert bei (Pi,0). Der Vergleich dieser Resultate favorisiert das erweiterte t-J-Modell zur Beschreibung der Dynamik des ZR-Singlets. Insbesondere i) die parabolische Dispersion des ZR-Singlets entlang Gamma-(Pi,Pi), ii) der geringe aber endliche energetische Abstand der Bindungsenergie-Minima entlang Gamma-(Pi,Pi) und Gamma-(Pi,0) und iii) die keilförmige Verteilung des Spektralen Gewichts entlang Gamma-(Pi,0) sind die experimentellen Indizien dafür. / A detailed ARPES study of the low binding-energy occupied electronic structure of Sr2CuO2Cl2 has been done. It corresponds to an investigation of the first electron-removal states of an undoped CuO2-plane: 1. The photoemission signal of the first electron-removal states at both (0.5Pi,0.5Pi) and (0.7Pi,0) exhibits a marked photon-energy dependence. The intensity profile shows strong oscillations with maxima near 16, 25, 35 and 49 eV, corresponding to final state crystal momenta kperp=0.82, 1.63, 2.40 and 3.12 A-1. 2. Along the high-symmetry directions Gamma-(Pi,Pi) and Gamma-(Pi,0) the first electron-removal states shows a strong polarization dependence. This can be linked to the strongly polarization-dependent matrix element, which in turn allows the determination of the symmetry of the first electron-removal state itself. For both high-symmetry directions we observe a polarization dependence in keeping with that expected for a Zhang-Rice singlet state in the framework of either a three-band or one-band model Hamiltonian. 3. Our data show that the dispersion of the first electron-removal states along both high symmetry directions (Gamma-(Pi,Pi) and Gamma-(Pi,0)) is parabolic-like and independent of the excitation energy. This, and the rather large difference in lowest binding energy of the first electron-removal state along these directions, shows the validity of the extended t-J model for describing the disperion relation of a single hole in an antiferromagnetic CuO2 plane. Thus, the inclusion of second (t2) and third (t3) neighbor hopping terms with realistic values of t2=-0.08 and t3=0.15 in units of the next neighbor hopping t=t1 are required. 4. Upon application of a simple fit procedure, we infer the momentum distribution of the spectral weight of the coherent and incoherent part of the first electron-removal state to have its maximum along Gamma-(Pi,Pi) at (0.5Pi,0.5Pi), being symmetrically suppressed away from this point. Along Gamma-(Pi,0) the spectral weights of both parts reach their maximum at (0.7Pi,0) and then drop fast. The ratio between the coherent and incoherent spectral weight is strongly photon-energy dependent, which, at first sight would appear to violate the physics of the spectral function: (i) the necessity for a more sophisticated framework in which to analyse the weight of the coherent and incoherent contributions to the spectral weight (ii) significant (hn-dependent) intensity due to extrinsic processes (iii) intensity in this energy region due to intrinsic electronic states other than the Zhang-Rice singlet.

Taking into account the technological demand for the controlled preparation of atomically precise graphene nanoribbons (GNRs) with well-defined properties, the present thesis is focused on the investigation of the role of the underlying metal substrate in the process of building GNRs using bottom-up strategy and on the changes in the electronic structure of GNRs induced by the GNR-metal interaction. The combination of surface sensitive synchrotron-radiation-based spectroscopic techniques and scanning tunneling microscopy with in situ sample preparation allowed to trace evolution of the structural and electronic properties of the investigated systems. Significant impact of the substrate activity on the growth dynamics of armchair GNRs of width N = 7 (7-AGNRs) prepared on inert Au(111) and active Cu(111) was demonstrated. It was shown that unlike inert Au(111) substrate, the mechanism of GNRs formation on Ag(111) and Cu(111) includes the formation of organometallic intermediates based on the carbon-metal-carbon bonds. Experiments performed on Cu(111) and Cu(110), showed that a change of the balance between molecular diffusion and intermolecular interaction significantly affects the on-surface reaction mechanism making it impossible to grow GNRs on Cu(110). It was demonstrated that deposition of metals on spatially aligned GNRs prepared on stepped Au(788) substrate allows to investigate GNR-metal interaction using angle-resolved photoelectron spectroscopy. In particular intercalation of one monolayer of copper beneath 7-AGNRs leads to significant electron injection into the nanoribbons, indicating that charge doping by metal contacts must be taken into account when designing GNR/electrode systems. Alloying of intercalated copper with gold substrate upon post-annealing at 200°C leads to a recovery of the initial position of GNR-related bands with respect to the Fermi level, thus proving tunability of the induced n-doping. Contrary, changes in the electronic structure of 7-AGNRs induced by the deposition of Li are not reversible.  It is demonstrated that via lithium doping 7-AGNRs can be transformed from a semiconductor into a metal state due to the partial filling of the conduction band. The band gap of Li-doped GNRs is reduced and the effective mass of the conduction band carriers is increased.

Thesis advisor: Ziqiang Wang / Angle-resloved photoemission spectroscopy (ARPES) is a powerful technique to study the electronic structure in solids. Its unique ability of resolving the energy and momentum information of electrons inside a solid provides an essential tool in measuring the electronic structure of solids. ARPES has made great contributions in the understanding of correlated system such as high-T_c superconductors and ruthenates. The Metal-insulator transition is a fundamental problem in condensed matter physics. The calcium substituted strontium ruthenate, Ca_2-xSr_xRuO₄, provides a good platform to study the metal-insulator transition in multi-orbital systems. This system has a complex phase diagram that evolves from a <italic>p</italic>-wave superconductor to a Mott insulator. One of important projects of this thesis focuses on Ca_2-xSr_xRuO₄ The growing evidence for coexistence of itinerant electrons and local moments in transition metals with nearly degenerate d orbitals suggests that one or more electron orbitals undergo a Mott transition while the others remain itinerant. We have observed a novel orbital selective Mott transition (OSMT) in Ca_1.8Sr_0.2RuO₄ by ARPES. While we observed two sets of dispersing bands and Fermi surfaces (FSs) associated with the doubly-degenerate d_yz and d_zx orbitals, the Fermi surface associated with the d_xy orbital which has a wider bandwidth is missing as a consequence of selective Mott localization. Our theoretical calculations have demonstrated that this unusual OSMT is mainly driven by the combined effects of inter-orbital carrier transfer, superlattice potentials and orbital degeneracy, whereas the bandwidth difference plays a less important role. Another important project of this thesis focuses on the recently discovered iron-pnictides superconductors. The idea of inter-FS scattering associated with the near-nesting condition has been proposed to explain the superconductivity in the pnictides. The near-nesting condition varies upon the carrier doping which shifts the chemical potential. We have performed a systematic photoemission study of the chemical potential shift as a function of doping in a pnictide system based on BaFe₂As₂. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by the renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) due to different Fermi velocities for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA from the band structure, which is fully consistent with the experimental phase diagram, strongly supports that inter-FS scattering over the near-nesting Fermi surfaces plays a vital role in the superconductivity of the iron pnictides. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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.

Nodal semimetals host either degenerate points (Dirac/Weyl points) or lines whose band topology in Brillouin zone can be classified either as trivial (normal nodal semimetals) or non trivial (topological nodal semimetals). This thesis investigates the electronic structure of two different categories of topological nodal semimetals probed by angleresolved photoemission spectroscopy (ARPES): The first material is Indium Bismuth (InBi). InBi is a semimetal with simple tetragonal structure with P4/nmm space group. This space group is predicted to host protected nodal lines along the perpendicular momentum direction at the high symmetry lines of the Brillouin zone boundary even under strong spin-orbit coupling (SOC) situation. As a semimetal with two heavy elements, InBi is a suitable candidate to test the prediction. The investigation by ARPES demonstrates not only that InBi hosts the nodal line in the presence of strong SOC, it also shows the signature of type-II Dirac crossing along the perpendicular momentum direction from the center of Brillouin zone. However, as the nodal line observed is trivial in nature, there is no exotic drumhead surface states observed in this material. This finding demonstrates that Dirac crossings can be protected in a non-symmorphic space group. The second material is NbIrTe₄ which is a semimetal that breaks inversion symmetry predicted to host only four Weyl points. This simplest configuration is confirmed by the measurement from the top and bottom surface of NbIrTe₄ showing only a pair of Fermi arcs each. Furthermore, it is found that the Fermi arc connectivity on the bottom surface experiences re-wiring as it evolves from Weyl points energy to the ARPES Fermi energy level. This change is attributed to the hybridisation between the surface and the bulk states as their projection lie within the vicinity of each other. The finding in this work demonstrates that although Fermi arcs are guaranteed in Weyl semimetals, their shape and connectivity are not protected and may be altered accordingly.

The purpose of this work is to explore a method that combines both top-down and bottom-up elements to fabricate electronic devices made from graphene, a single sheet of carbon atoms related to carbon nanotubes and graphite. This material has garnered interest in the semiconductor industry for many reasons, including its potential for ballistic conduction, natural ambipolar (both n- and p-type) carrier transport, and impermeability to nearly all elements. However, its lack of a band gap, and a lack of viable options for creating one in the material, suggests a limited future as a silicon replacement material. A solution to this problem is presented that uses a recently-reported technique of creating pre-patterned graphene features from the thermal decomposition of specially-structured silicon carbide (SiC) surfaces. We employ a combination of direct band structure measurements and electrical results to suggest that a semiconducting bent graphene nanostructure exists in this structured SiC system, creating a possible route toward a broad class of future graphene electronics.

The isolation of single layers of van der Waals materials has shown that their properties can be significantly different compared to their bulk counterparts. These observations, illustrates the importance of interface interactions for determining the materials properties even in weakly interacting materials and raise the question if materials properties of single layer van der Waals materials can be controlled by appropriate hetero-interfaces. To study interface effects in monolayer systems, surface science techniques, such as photoemission spectroscopy and scanning probe microscopy/spectroscopy, are ideally suited. However, before these characterization methods can be employed, approaches for the synthesis of hetero-van der Waals systems must be developed, preferably in-situ with the characterization methods, i.e. in ultra-high vacuum. Therefore, in this thesis, we explored novel approaches for creating van der Waals heterostructures and characterized fundamental structural and electronic properties of such systems. Specifically, we developed an approach to decouple graphene from a Ir(111) growth substrate by intercalation growth of a 2D-FeO layer, and we investigate van der Waals epitaxy of MoSe2 on graphite and other transition metal dichalcogenide substrates. For the Ir(111)/2D-FeO/graphene heterostructure system, we first demonstrated the growth of 2D-FeO on Ir(111). The FeO monolayer on Ir(111) exhibits a long range moiré structure indicating the locally varying change of the coordination of the Fe atoms with respect to the substrate Ir atoms. This variation also gives rise to modulations in the Fe2+-O2- separation, and thus in the monolayer dipole. We demonstrated that this structure can be intercalated underneath of graphene grown on Ir(111) by chemical vapor deposition. The modulation of the dipole in the 2D-FeO moiré structure consequently gives rise to a modulated charge doping in the graphene. This effect has been studied by C-1s core level broadening. In general, this study demonstrates that modulated substrates can be used to periodically modify 2D materials. Growth of transition metal dichalcogenides (TMDCs) by molecular beam epitaxy (MBE) is a very versatile approach for growing TMDC heterostructures. However, there may be unforeseen challenges in the synthesis of some of these materials. Here we show that in MBE growth of MoSe2, the formation of twin grain boundaries is very abundant. While this is detrimental in our efforts for characterizing interface properties of TMDC heterostructures, however the twin grain boundaries have exciting properties. Since the twin grain boundaries are aligned in an epitaxial film we were able to characterize their properties by angle resolved photoemission spectroscopy (ARPES), which may be the first time a material’s line defects could be studied by this method. We demonstrate that the line defects are metallic and exhibit a parabolic dispersing band. Because of the 1D nature of the metallic lines, embedded in a semiconducting matrix, the electronic structure follows a Tomonaga Luttinger formalism and our studies showed strong evidence of the predicted so-called spin charge separation in such 1D electron systems. Moreover, a metal-to-insulator Peierls transition has been observed in this system by scanning tunneling microscopy as well as in transport measurements. Finally, we have shown that the defect network that forms at the surface also lends itself for decoration with metal clusters. Although unexpected, the formation of grain boundary networks in MoSe2 marks the discovery of a new material with exciting quantum properties.

No description available.

In this thesis, the results of analyzing the electronic structure of two iron-based superconductors: FeSe and LiFeAs are presented. To access the electronic structure, angle-resolved photoemission spectroscopy was used. In our analysis, we focus on the structure of the superconducting gap and the influence of nematicity on the electronic structure. We have revealed changes in the electronic structure of FeSe caused by nematicity in all parts of the Brillouin zone. A scale of these changes is smaller than it was believed earlier. Also, we have observed an anomalous shift of the dispersions in opposite directions with temperature in this material. We have observed anisotropic superconducting gap on all sheets of the Fermi surfaces of both: FeSe and LiFeAs. We have shown that in LiFeAs, rotational symmetry is broken in the superconducting state, which manifests not only in the gap symmetry but also in the shapes of the Fermi surfaces sheets. This result indicates a realization of a novel phenomenon of superconductivity-induced nematicity:1 Iron-based superconductors 1.1 Introduction to iron-based superconductors 1.2 LiFeAs - special iron-based superconductor 1.3 FeSe - structurally simplest iron-based superconductor 2 Angle-Resolved Photoemission 3 Temperature evolution of the electronic structure of FeSe 3.1 Effects of nematicity from low-temperature measurements 3.2 Temperature dependent shift of the dispersions 3.3 Discussion and conclusions 4 Three-dimensional superconducting gap in FeSe 4.1 Superconducting gap on the electron-like pockets 4.2 Superconducting gap on the hole-like pocket 4.3 Discussion and conclusions 5 Superconductivity-induced nematicity in LiFeAs 5.1 Superconducting gap 5.2 Nematicity 5.3 Discussion and conclusions Summary

Topological insulators (TIs) are a class of quantum materials, which behave as insulators in the bulk, yet possess gapless spin-polarized surface states, which are robust against nonmagnetic impurities. The unique properties of TIs make them attractive not only for studying various fundamental phenomena in condensed matter and particle physics, but also as promising candidates for applications ranging from spintronics to quantum computation. Within the topological insulator realm, a great deal of focus has been placed on discovering new quantum materials, however, ideal multi-modal quantum materials have yet to be found. Here we study alpha-PdBi2, KFe2Te2, and DySb compounds including others within these families with high-resolution angle-resolved photoemission spectroscopy (ARPES) complimented by first principles calculations. We observe unique phase changes and phenomena across their transition temperatures. Our work paves a new direction in material discovery and application related to their unique electronic properties.