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Abstract: Functionalization of graphene on Ir(111) is a promising route to modify graphene by chemical means in a controlled fashion at the nanoscale. Yet, the nature of such functionalized sp3 nanodots remains unknown. Density functional theory (DFT) calculations alone cannot differentiate between two plausible structures, namely true graphane and substrate stabilized graphane-like nanodots. These two structures, however, interact dramatically differently with the underlying substrate. Discriminating which type of nanodots forms on the surface is thus of paramount importance for the applications of such prepared nanostructures. By comparing X-ray standing wave measurements against theoretical model structures obtained by DFT calculations we are able to exclude the formation of true graphane nanodots and clearly show the formation graphane-like nanodots.

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Abstract: The diamond and zinc-blende semiconductors are well-known and have been widely studied for decades. Yet, their electronic structure still surprises with unexpected topological properties of the valence bands. In this joint theoretical and experimental investigation, we demonstrate for the benchmark compounds InSb and GaAs that the electronic structure features topological surface states below the Fermi energy. Our parity analysis shows that the spin-orbit split-off band near the valence band maximum exhibits a strong topologically nontrivial behavior characterized by the Z 2 invariants ( 1 ; 000 ) . The nontrivial character is a consequence of the nonzero spin-orbit coupling and is imposed by the chosen constituents, in contrast to the conventional topological phase transition mechanism which relies on tuning parameters in the system Hamiltonian. Ab initio-based tight-binding calculations resolve topological surface states in the occupied electronic structure of InSb and GaAs, further confirmed experimentally by soft x-ray angle-resolved photoemission from both materials. Our findings are valid for all other materials whose valence bands are adiabatically linked to those of InSb, i.e., many diamond and zinc-blende semiconductors, as well as other related materials, such as half-Heusler compounds.

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Abstract: In this dissertation, we used a combination of synchrotron-based x-ray spectroscopic techniques such as angle-resolved photoelectron spectroscopy (ARPES), soft x-ray ARPES, hard x-ray photoelectron spectroscopy (HAXPES), and soft x-ray absorption spectroscopy (XAS) to investigate momentum-resolved and angle-integrated electronic structure of advanced three- and two-dimensional materials and interfaces. The results from the experiments were compared to several types of state-of-the-art first-principles theoretical calculations. In the first part of this dissertation we investigated the effects of spin excitons on the surface states of samarium hexaboride (SmB6), which has gained a lot of interest since it was proposed to be a candidate topological Kondo insulator. Here, we utilized high-resolution (overall resolution of approximately 3 meV) angle-resolved and angle-integrated valence-band photoemission measurements at cryogenic temperatures (1.2 K and 20 K) to show evidence for a V-shaped density of states of surface origin within the bulk gap of SmB6. Our temperature-dependent measurements of the valence-band spectra revealed a sharp feature appearing within the bulk gap of SmB6 at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6, which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The near-Fermi-energy spin-exciton-driven scattering is thermally activated and only appears below the temperature of about 25 K. This temperature is considerably lower than the temperature at which the bulk hybridization gap is first observed. Therefore, it is plausible that the formation of the Fermi-liquid, which is responsible for the surface conduction should only occur at very low temperatures and may be responsible for the plateau in the resistivity at 5 K. In the second part of this dissertation we investigated the electronic structure of a strongly-correlated oxide NdNiO3 grown along the unconventional pseudocubic [111] direction and buried under 4 unit cells (u.c.) of an insulating oxide LaAlO3. Over the last several decades transition-metal oxides (TMO) have been demonstrated to host a wide variety of strongly-correlated-electron phenomena, such as metal-insulator transitions and high-temperature superconductivity, induced by chemical doping and/or various external stimuli. Until now, majority of the work has been focused on systems grown along the pseudocubic [001] direction. Recently, however, several theoretical proposals have been put forward to utilize TMO heterostructures consisting of a few u.c. grown epitaxially along the [111] direction. Here, we realized the first momentum-resolved soft x-ray ARPES measurement of the valence-band electronic structure of artificial graphene-like Mott crystal NdNiO3 [111] buried under four u.c. of an insulating oxide LaAlO3. Our measurements of the buried Ni 3d states near the Fermi level exhibit excellent agreement with the first-principles calculations and establish the realization of an antiferro-orbital order in this artificial lattice. Such ‘engineered’ electronic structure is unique to this quazi-2D crystal and cannot be realized either in the bulk or thin-film nickelates grown along the conventional [001] direction. Our findings open the door for engineering novel polarized Mott-electronic ground states in rare-earth nickelates, as well as other strongly-correlated transition-metal oxides. From the technical perspective, we demonstrate that soft x-ray ARPES can be used to measure the momentum-resolved electronic structure of ultrathin (2 u.c.) layers, buried within complex oxide heterostructures.

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Abstract: It is possible to increase the charge capacity of transition metal oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2-Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4 e- are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.75O2 but displaced from the transition metal to the alkali metal layers, showing that vacancies in the transition metal layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li0.2Ni0.2Mn0.6]O2, is not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2 resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface localised O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilising the oxygen and avoiding oxygen loss.

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Abstract: The adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a large height of 3.37 Å and only a minute downward displacement of the functional anhydride groups (0.07 Å) occurs, yielding adsorption heights that agree with the sum of the involved van der Waals radii. Thus, already a single hBN layer leads to a decoupled (physisorbed) molecule, contrary to the situation on the bare Cu(111) surface.