@phdthesis{Varykhalov2005,
  author    = {Varykhalov, Andrei},
  title     = {Quantum-size effects in the electronic structure of novel self-organized systems with reduced dimensionality},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5784},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {The Thesis is focused on the properties of self-organized nanostructures. Atomic and electronic properties of different systems have been investigated using methods of electron diffraction, scanning tunneling microscopy and photoelectron spectroscopy. Implementation of the STM technique (including design, construction, and tuning of the UHV experimental set-up) has been done in the framework of present work. This time-consuming work is reported to greater detail in the experimental part of this Thesis. The scientific part starts from the study of quantum-size effects in the electronic structure of a two-dimensional Ag film on the supporting substrate Ni(111). Distinct quantum well states in the sp-band of Ag were observed in photoelectron spectra. Analysis of thickness- and angle-dependent photoemission supplies novel information on the properties of the interface. For the first time the Ni(111) relative band gap was indirectly probed in the ground-state through the electronic structure of quantum well states in the adlayer. This is particularly important for Ni where valence electrons are strongly correlated. Comparison of the experiment with calculations performed in the formalism of the extended phase accumulation model gives the substrate gap which is fully consistent with the one obtained by ab-initio LDA calculations. It is, however, in controversy to the band structure of Ni measured directly by photoemission. These results lend credit to the simplest view of photoemission from Ni, assigning early observed contradictions between theory and experiments to electron correlation effects in the final state of photoemission. Further, nanosystems of lower dimensionality have been studied. Stepped surfaces W(331) and W(551) were used as one-dimensional model systems and as templates for self-organization of Au nanoclusters. Photon energy dependent photoemission revealed a surface resonance which was never observed before on W(110) which is the base plane of the terrace microsurfaces. The dispersion E(k) of this state measured on stepped W(331) and W(551) with angle-resolved photoelectron spectroscopy is modified by a strong umklapp effect. It appears as two parabolas shifted symmetrically relative to the microsurface normal by half of the Brillouin zone of the step superlattice. The reported results are very important for understanding of the electronic properties of low-dimensional nanostructures. It was also established that W(331) and W(551) can serve as templates for self-organization of metallic nanostructures. A combined study of electronic and atomic properties of sub-monolayer amounts of gold deposited on these templates have shown that if the substrate is slightly pre-oxidized and the temperature is elevated, then Au can alloy with the first monolayer of W. As a result, a nanostructure of uniform clusters of a surface alloy is produced all over the steps. Such clusters feature a novel sp-band in the vicinity of the Fermi level, which appears split into constant energy levels due to effects of lateral quantization. The last and main part of this work is devoted to large-scale reconstructions on surfaces and nanostructures self-assembled on top. The two-dimensional surface carbide W(110)/C-R(15x3) has been extensively investigated. Photoemission studies of quantum size effects in the electronic structure of this reconstruction, combined with an investigation of its surface geometry, lead to an advanced structural model of the carbide overlayer. It was discovered that W(110)/C-R(15x3) can control self-organization of adlayers into nanostructures with extremely different electronic and structural properties. Thus, it was established that at elevated temperature the R(15x3) superstructure controls the self-assembly of sub-monolayer amounts of Au into nm-wide nanostripes. Based on the results of core level photoemission, the R(15x3)-induced surface alloying which takes place between Au and W can be claimed as driving force of self-organization. The observed stripes exhibit a characteristic one-dimensional electronic structure with laterally quantized d-bands. Obviously, these are very important for applications, since dimensions of electronic devices have already stepped into the nm-range, where quantum-size phenomena must undoubtedly be considered. Moreover, formation of perfectly uniform molecular clusters of C60 was demonstrated and described in terms of the van der Waals formalism. It is the first experimental observation of two-dimensional fullerene nanoclusters with "magic numbers". Calculations of the cluster potentials using the static approach have revealed characteristic minima in the interaction energy. They are achieved for 4 and 7 molecules per cluster. The obtained "magic numbers" and the corresponding cluster structures are fully consistent with the results of the STM measurements.},
  subject      = {Nanostruktur},
  language  = {en}
}
@article{SternemannWilke2016,
  author    = {Sternemann, C. and Wilke, Max},
  title     = {Spectroscopy of low and intermediate Z elements at extreme conditions: in situ studies of Earth materials at pressure and temperature via X-ray Raman scattering},
  series = {High pressure research},
  volume    = {36},
  journal   = {High pressure research},
  publisher = {IOP Publ. Ltd.},
  address   = {Abingdon},
  issn      = {0895-7959},
  doi       = {10.1080/08957959.2016.1198903},
  pages     = {275 -- 292},
  year      = {2016},
  abstract  = {X-ray Raman scattering spectroscopy is an emerging method in the study of low and intermediate Z elements' core-electron excitations at extreme conditions in order to reveal information on local structure and electronic state of matter in situ. We discuss the capabilities of this method to address questions in Earth materials' science and demonstrate its sensitivity to detect changes in the oxidation state, electronic structure, coordination, and spin state. Examples are presented for the study of the oxygen K-, silicon L- and iron M-edges. We assess the application of both temperature and pressure in such investigations exploiting diamond anvil cells in combination with resistive or laser heating which is required to achieve realistic conditions of the Earth's crust, mantle, and core.},
  language  = {en}
}
@phdthesis{Rader2005,
  author    = {Rader, Oliver},
  title     = {Electron quantization and localization in metal films and nanostructures},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001912},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {Es ist seit einigen Jahren bekannt, dass Elektronen unter bestimmten Bedingungen in d{\"u}nne Filme eingeschlossen werden k{\"o}nnen, selbst wenn diese Filme aus Metall bestehen und auf Metall-Substrat aufgebracht werden. In Photoelektronenspektren zeigen diese Filme charakteristische diskrete Energieniveaus, und es hat sich herausgestellt, dass sie zu großen, technisch nutzbaren Effekten f{\"u}hren k{\"o}nnen, wie der oszillatorischen magnetischen Kopplung in modernen Festplatten-Lesek{\"o}pfen. In dieser Arbeit wird untersucht, inwieweit die der Quantisierung in zweidimensionalen Filmen zu Grunde liegenden Konzepte auf niedrigere Dimensionalit{\"a}t {\"u}bertragbar sind. Das bedeutet, dass schrittweise von zweidimensionalen Filmen auf eindimensionale Nanostrukturen {\"u}bergegangen wird. Diese Nanostrukturen sind zum einen die Terrassen auf atomar gestuften Oberfl{\"a}chen, aber auch Atomketten, die auf diese Terrassen aufgebracht werden, bis hin zu einer vollst{\"a}ndigen Bedeckung mit atomar d{\"u}nnen Nanostreifen. Daneben werden Selbstorganisationseffekte ausgenutzt, um zu perfekt eindimensionalen Atomanordnungen auf Oberfl{\"a}chen zu gelangen. Die winkelaufgel{\"o}ste Photoemission ist als Untersuchungsmethode deshalb so geeignet, weil sie das Verhalten der Elektronen in diesen Nanostrukturen in Abh{\"a}ngigkeit von der Raumrichtung zeigt, und unterscheidet sich darin beispielsweise von der Rastertunnelmikroskopie. Damit ist es m{\"o}glich, deutliche und manchmal {\"u}berraschend große Effekte der eindimensionalen Quantisierung bei verschiedenen exemplarischen Systemen zum Teil erstmals nachzuweisen. Die f{\"u}r zweidimensionale Filme wesentliche Rolle von Bandl{\"u}cken im Substrat wird f{\"u}r Nanostrukturen best{\"a}tigt. Hinzu kommt jedoch eine bei zweidimensionalen Filmen nicht vorhandene Ambivalenz zwischen r{\"a}umlicher Einschr{\"a}nkung der Elektronen in den Nanostrukturen und dem Effekt eines {\"U}bergitters aus Nanostrukturen sowie zwischen Effekten des Elektronenverhaltens in der Probe und solchen des Messprozesses. Letztere sind sehr groß und k{\"o}nnen die Photoemissionsspektren dominieren. Abschließend wird der Effekt der verminderten Dimensionalit{\"a}t speziell f{\"u}r die d-Elektronen von Mangan untersucht, die zus{\"a}tzlich starken Wechselwirkungseffekten unterliegen. Auch hierbei treten {\"u}berraschende Ergebnisse zu Tage.},
  language  = {en}
}
@phdthesis{Jay2020,
  author    = {Jay, Raphael Martin},
  title     = {Principles of charge distribution and separation},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xi, 162},
  year      = {2020},
  abstract  = {The electronic charge distributions of transition metal complexes fundamentally determine their chemical reactivity. Experimental access to the local valence electronic structure is therefore crucial in order to determine how frontier orbitals are delocalized between different atomic sites and electronic charge is spread throughout the transition metal complex. To that end, X-ray spectroscopies are employed in this thesis to study a series of solution-phase iron complexes with respect to the response of their local electronic charge distributions to different external influences. Using resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) at the iron L-edge, changes in local charge densities are investigated at the iron center depending on different ligand cages as well as solvent environments. A varying degree of charge delocalization from the metal center onto the ligands is observed, which is governed by the capabilities of the ligands to accept charge density into their unoccupied orbitals. Specific solvents are furthermore shown to amplify this process. Solvent molecules of strong Lewis-acids withdraw charge from the ligand allowing in turn for more metal charge to be delocalized onto the ligand. The resulting local charge deficiencies at the metal center are, however, counteracted by competing electron-donation channels from the ligand towards the iron, which are additionally revealed. This is interpreted as a compensating effect which strives to maintain local charge densities at the iron center. This mechanism of charge density preservation is found to be of general nature. Using time-resolved RIXS and XAS at the iron L-edge, an analogous interplay of electron donation and back-donation channels is also revealed for the case of charge-transfer excited states. In such transient configurations, the electronic occupation of iron-centered frontier orbitals has been altered by an optical excitation. Changes in local charge densities that are expected to follow an increased or decreased population of iron-centered orbitals are, however, again counteracted. By scaling the degree of electron donation from the ligand onto the metal, local charge densities at the iron center can be efficiently maintained. Since charge-transfer excitations, however, often constitute the initial step in many electron transfer processes, these findings challenge common notions of charge-separation in transition metal dyes.},
  language  = {en}
}
@phdthesis{Born2021,
  author    = {Born, Artur},
  title     = {Electronic structure, quasi-particle interaction and relaxation in 3d-elements from X-ray spectroscopy},
  school      = {Universit{\"a}t Potsdam},
  pages     = {123},
  year      = {2021},
  abstract  = {Any physical system can be described on the level of interacting particles, thus it is of fundamental importance to improve the scientific understanding of interacting many-body systems. This thesis experimentally addresses specific quasi-particle interactions, namely interactions be- tween electrons and between electrons and phonons. It describes the consequential effects of those processes on the electronic structure and the core-hole relaxation pathways in 3d metals. Despite the great amount of experimental and theoretical studies of these interactions and their impact on the behavior of solid-state matter, there are still open questions concerning the cor- responding physical, chemical and mechanical properties of solid-state matter. Especially, the study of 3d metals and their compounds is a great experimental challenge, since those exhibit a variety of spectral features originating from many-body effects such as multiplet splitting, shake up/off satellites, vibrationally excited states or more complex effects like superconductivity and ultrafast demagnetization. In X-ray spectroscopy, these effects often produce overlapping fea- tures, complicating the analysis and limiting the understanding. In this thesis, to overcome the limitations set by conventional X-ray spectroscopy, two different experimental approaches were successfully refined, namely Auger electron photoelectron coincidence spectroscopy (APECS) and temperature-dependent X-ray emission spectroscopy (tXES), which enabled the separation of different core-hole relaxation pathways and the isolation of the impact of specific many-body interactions in the experimental spectra. APECS was utilized at the new Coincidence electron spectroscopy for chemical analysis (Co- ESCA) station at BESSY II to study the core-hole decay and electron-correlation effects in single- crystal Ni, Cu and Co. The observation of photoelectrons in coincidence with Auger electrons allows for the separation of the initial and final state effects in the Auger electron spectra. The results show that a Cu LV V Auger spectrum can be represented by broadened atomic multiplets confirming the localized nature of the intermediate core-hole states. In contrast, the Co LV V Auger spectrum is band-like and can be represented by the self-convolution of the valence band. Ni behaves mixed, localized and itinerant. Thus, the Ni Auger spectrum can only be represented by a mixture of atomic multiplet peaks and the self-convoluted valence band. In the case of Ni, the LV V Auger electrons in coincidence with the 6 eV satellite photoelectrons were also stud- ied. Utilizing the core-hole clock method, the lifetime of the localized double-hole intermediate 2 p53d9 states of 1.8 fs could be determined. However, a fraction of these states delocalizes before the Auger decay contributing to the main peak. A similar delocalization was observed for the double-hole states produced by the L2L3M4,5 Coster-Kronig process. Additionally, the influence of surface oxidation on the Ni(111) 3p levels was studied with APECS. The Ni 3p PES spectrum is broad and featureless, due to overlapping many-body effects and gives little chance for exact analysis using conventional photoelectron spectroscopy. Utilizing APECS or precisely the final state selectivity of the method, the spectral width of the 3p levels could be narrowed and their positions and the spin-orbit splitting were determined. Moreover, due to the surface sensitivity of the method, the chemically shifted 3p photoelectron peaks originating from the oxidized surface and the bulk Ni were disentangled. For the study of the atomic electron-phonon spin-flip scattering in 3d metals as a spin-relaxation channel, the tXES method at the SolidFlexRIXS station was developed. The atomic spin-flip scat- tering was studied in single-crystal Ni, Cu, Co and in FeNi alloys, which show considerable dif- ferences in their behavior. The scattering rate in Ni increases with temperature, whereas the rate in Cu and Co remains constant within the measured temperature range up to 1000 K. In FeNi alloys, our results reveal that the spin-flip scattering is restricted by sublattice exchange energies J. The electron-phonon scattering driven spin-flips only appear in the case where the thermal energy ex- ceeds the exchange energy kT > J. This thresholding is an important microscopic process for the description of the sublattice dynamics in alloys, but as shown also relevant for elemental magnetic systems. Overall, the results strongly indicate that the spin-flip probability is correlated with the exchange energy, which might become an important parameter in the ultrafast demagnetization debate. Taken together, the applied experimental approaches allowed to study complex many-body effects in 3d metals. The results show that utilizing APECS enabled the distinction and clear assignment of otherwise overlapping features in AES or PES spectra of Ni, Cu, Co and NiO. This is of fundamental importance for the basic understanding of photoionization and core-hole decay processes but also for the chemical analysis in applied science. The measurement of the atomic electron-phonon spin-flip scattering rate utilizing tXES shows that the electron-phonon spin-flip scattering is a relevant atomic process for the macroscopic demagnetization process. Additionally, a temperature-dependent thresholding mechanism was discovered, which introduces an important dynamic factor into the electron-phonon spin-flip model.},
  language  = {en}
}