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Topological phenomena known from solid state physics have been transferred to a variety of other classical and quantum systems. Due to the equivalence of the Hamiltonian matrix describing tight binding models and the grounded circuit Laplacian describing an electrical circuit we can investigate such phenomena in circuits. By implementing different Hermitian topological models general suggestions on designing those types of circuit are worked out with the aim of minimizing unwanted coupling effects and parasitic admittances in the circuit. Here the existence and the spatial profile of topological states as well as the band structure of the model can be determined.
Due to the complex nature of electric admittance the investigations can be directly expanded to systems with broken Hermiticity. The particular advantages of the experimental investigation of non-exclusively topological phenomena by means of electric circuits come to light in the realization of non-Hermitian and non-linear models. Here we find limitation of the Hermitian bulk-boundary correspondence principle, purely real eigenvalues in non-Hermitian PT-symmetrical systems and edge localization of all eigenstates in non-Hermitian and non-reciprocal systems, which in literature is termed the non-Hermitian skin effect.
When systems obeying non-linear equations are studied, the grounded circuit Laplacian based on the Fourier-transform cannot be applied anymore. By combination of the connectivity of a topological system together with non-linear van der Pol oscillators self-activated and self-sustained topological edge oscillations can be found. These robust high frequency sinusoidal edge oscillations differ significantly from low frequency relaxation oscillations, which can be found in the bulk of the system.
We report the results of a detailed investigation on the Te-stabilized (2 x 1) and the Cdstabilized c( 2 X 2) surfaces of ( 100) CdTe substrates. The investigation demonstrates for the first time that both laser illumination and, to a greater extent, high-energy electron irradiation increase the Te desorption and reduce the Cd desorption from ( 100) CdTe surfaces. Thus it is possible by choosing the proper growth temperature and photon or electron fluxes to change the surface reconstruction from the normally Te-stabilized to a Cd-stabilized phase.
Organic solar cells have great potential to become a low-cost and clean alternative to conventional photovoltaic technologies based on the inorganic bulk material silicon. As a highly promising concept in the field of organic photovoltaics, bulk heterojunction (BHJ) solar cells consist of a mixture of an electron donating and an electron withdrawing component. Their degree of intermixing crucially affects the generation of photocurrent. In this work, the effect of an altered blend morphology on polaron pair dissociation, charge carrier transport, and nongeminate recombination is analyzed by the charge extraction techniques time delayed collection field (TDCF) and open circuit corrected transient charge extraction (OTRACE). Different comparative studies cover a broad range of material systems, including polymer and small-molecule donors in combination with different fullerene acceptors. The field dependence of polaron pair dissociation is analyzed in blends based on the polymer pBTTT-C16, allowing a systematic tuning of the blend morphology by varying the acceptor type and fraction. The effect of both excess photon energy and intercalated phases are minor compared to the influence of excess fullerene, which reduces the field dependence of photogeneration. The study demonstrates that the presence of neat fullerene domains is the major driving force for efficient polaron pair dissociation that is linked to the delocalization of charge carriers. Furthermore, the influence of the processing additive diiodooctane (DIO) is analyzed using the photovoltaic blends PBDTTT-C:PC71BM and PTB7:PC71BM. The study reveals amulti-tiered alteration of the blend morphology of PBDTTT-C based blends upon a systematic increase of the amount of DIO. Domains on the hundred nanometers length scale in the DIO-free blend are identified as neat fullerene agglomerates embedded in an intermixed matrix. With the addition of the additive, 0.6% and 1% DIO already substantially reduces the size of these domains until reaching the optimum 3% DIO mixture, where a 7.1% power conversion efficiency is obtained. It is brought into connection with the formation of interpenetrating polymer and fullerene phases. Similar to PBDTTT-C, the morphology of DIO-free PTB7:PC71BM blends is characterized by large fullerene domains being decreased in size upon the addition of 3% DIO. OTRACE measurements reveal a reduced Langevin-type, super-second order recombination in both blends. It is demonstrated that the deviation from bimolecular recombination kinetics cannot be fully attributed to the carrier density dependence of the mobility but is rather related to trapping in segregated PC71BM domains. Finally, with regard to small-molecule donors, a higher yield of photogeneration and balanced transport properties are identified as the dominant factors enhancing the efficiency of vacuum deposited MD376:C60 relative to its solution processed counterpart MD376:PC61BM. The finding is explained by a higher degree of dimerization of the merocyanine dye MD376 and a stronger donor-acceptor interaction at the interface in the case of the vacuum deposited blend.
Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices. We analyze the impact of a systematically increased amount of the solvent additive diiodooctane (DIO) on the morphology of PBDTTT-C:PC71BM blends and related changes in free carrier formation and recombination by combining surface imaging, photophysical and charge extraction techniques. We identify agglomerates visible in AFM images of the 0% DIO blend as PC71BM domains embedded in an intermixed matrix phase. With the addition of DIO, a decrease in the size of fullerene domains along with a demixing of the matrix phase appears for 0.6% and 1% DIO. Surprisingly, transient absorption spectroscopy reveals an efficient photogeneration already for the smallest amount of DIO, although the largest efficiency is found for 3% DIO. It is ascribed to a fine-tuning of the blend morphology in terms of the formation of interpenetrating donor and acceptor phases minimizing geminate and nongeminate recombination as indicated by charge extraction experiments. An increase in the DIO content to 10% adversely affects the photovoltaic performance, most probably due to an inefficient free carrier formation and trapping in a less interconnected donor-acceptor network.
In this letter we study the influence of temperature and excitation power on the emission linewidth from site-controlled InGaAs/GaAs quantum dots grown on nanoholes defined by electron beam lithography and wet chemical etching. We identify thermal electron activation as well as direct exciton loss as the dominant intensity quenching channels. Additionally, we carefully analyze the effects of optical and acoustic phonons as well as close-by defects on the emission linewidth by means of temperature and power dependent micro-photoluminescence on single quantum dots with large pitches. (C) 2014 Author(s).
The second-order correlation function g\(^2\)(\(\tau\) = 0), input-output curves and pulse duration of the emission from a microcavity exciton-polariton system subsequent to picosecond-pulsed excitation are measured for different temperatures. At low temperatures a two-threshold behaviour emerges, which has been attributed to the onset of polariton lasing and conventional lasing at the first and the second threshold, respectively. We observe that polariton lasing is stable up to temperatures comparable with the exciton binding energy. At higher temperatures a single threshold displays the direct transition from thermal emission to photon lasing.
Semiconductor Quantum Dots (QDs) have been attracting immense interest over the last decade from both basic and application-orientated research because of their envisioned use as fundamental building blocks in non-classical device architectures. Their presumable ease of integration into existing semiconductor technology has bought them the reputation of being cost-efficiently scalable and renders them a place among the top candidates in a wide range of proposed quantum logic and quantum information processing schemes. These include the highly acclaimed use of QD as triggered sources of single pairs of entangled photons, which is a key ingredient of most of the intensivly investigated optical quantum cryptography operations. A big obstacle towards these goals are the pronounced asymmetries that are intrinsically present in all currently availabe semiconductor QD systems. They are a natural by-product that stems from the employed self-assembled growth methods and manifest in various forms such as shape-asymmetry, inhomogeneous strain distribution within the QD and concomittant piezo-elecric fields. These asymmetries in return give rise to distinct anisotropies in the optical properties of QDs, which in fact render their optical response classic. For device oriented research these anisotropies are therefore typically considered unwanted and actively researched to be controlled. They are, however, interesting from a fundamental point of view, as anisotropic QDs basically provide a testbed system for fundamental atom-like quantum physics with non-centrosymmetric potentials. As shall be shown in the current work, this gives rise to novel and interesting physics in its own right. Employing photoluminescence spectroscopy (PL) we investigate the optical anisotropy of the radiative recombination of excitons confined to CdSe/ZnSe QDs. This is done by angle-dependent polarization-resolved PL. We demonstrate experimentally that the electron-hole exchange interaction in asymmetric QDs gives rise to an effective conversion of the optical polarization from linear to circular and vice versa. The experiment is succesfully modeled in the frame of an exciton pseudospin-formalism that is based on the exchange induced finestructure splitting of the radiative excitonic states and unambiguously proves that the observed polarization conversion is the continuous-wave equivalent to quantum beats between the exchange split states in the time domain. These results indicate that QDs may offer extended functionality beyond non-classical light sources in highly integrated all-optical device schemes, such as polarization converters or modulators. In a further extension we apply the exciton pseudospin-formalism to optical alignment studies and demonstrate how these can be used to directly measure the otherwise hidden symmetry distribution over an ensemble of QDs. This kind of measurement may be used on future optical studies in order to link optical data more directly to structural investigations, as it yields valuable information on capped QDs that cannot be looked at directly by topological methods. In the last part of this work we study the influence of an in-plane magnetic field on the optical anisotropy. We find that the optical axis of the linear polarization component of the photoluminescence signal either rotates in the opposite direction to that of the magnetic field or remains fixed to a given crystalline direction. A qualitative theoretical analysis based on the exciton pseudospin Hamiltonian unambiguously demonstrates that these effects are induced by isotropic and anisotropic contributions to the heavy-hole Zeeman term, respectively. The latter is shown to be compensated by a built-in uniaxial anisotropy in a magnetic field B=0.4 T, resulting in an optical response that would be expected for highly symmetric QDs. For a comprehensive quantitative analysis the full heavy-hole exciton k.p-Hamiltonian is numerically calculated and the resulting optical polarization is modeled. The model is able to quantitatively describe all experimental results using a single set of parameters. From this model it is explicitly seen that a optical response characteristic for high symmetry QDs may be obtained from an ensemble of asymmetric QDs without a crossing of the zero-field bright exciton states, which was required for application of QDs in non-classical light sources. It is clearly demonstrated that any scheme using in-plane magnetic fields to symmetrize the optical response has to take into account at least four optically active states instead of the two observed in the absence of magnetic fields. These findings may explain some of the major disagreement on recent entanglement studies in asymmetric QDs, as models that do not take the above result into account cannot be a priori expected to provide reliable results on excitonic Bell states.
Background:
Inhibition of early platelet adhesion by blockade of glycoprotein-IB (GPIb) protects mice from ischemic stroke. To elucidate underlying mechanisms in-vivo, infarct development was followed by ultra-high field MRI at 17.6 Tesla.
Methods:
Cerebral infarction was induced by transient-middle-cerebral-artery-occlusion (tMCAO) for 1 hour in C57/BL6 control mice (N = 10) and mice treated with 100 mg Fab-fragments of the GPIb blocking antibody p0p/B 1 h after tMCAO (N = 10). To control for the effect of reperfusion, additional mice underwent permanent occlusion and received anti-GPIb treatment (N = 6; pMCAO) or remained without treatment (N = 3; pMCAO). MRI 2 h and 24 h after MCAO measured cerebral-blood-flow (CBF) by continuous arterial-spin labelling, the apparent-diffusion-coefficient (ADC), quantitative-T2 and T2-weighted imaging. All images were registered to a standard mouse brain MRI atlas and statistically analysed voxel-wise, and by cortico-subcortical ROI analysis.
Results:
Anti-GPIb treatment led to a relative increase of postischemic CBF vs. controls in the cortical territory of the MCA (2 h: 44.2 +/- 6.9 ml/100g/min versus 24 h: 60.5 +/- 8.4; p = 0.0012, F((1,18)) = 14.63) after tMCAO. Subcortical CBF 2 h after tMCAO was higher in anti-GPIb treated animals (45.3 +/- 5.9 vs. controls: 33.6 +/- 4.3; p = 0.04). In both regions, CBF findings were clearly related to a lower probability of infarction (Cortex/Subcortex of treated group: 35%/65% vs. controls: 95%/100%) and improved quantitative-T2 and ADC. After pMCAO, anti-GPIb treated mice developed similar infarcts preceded by severe irreversible hypoperfusion as controls after tMCAO indicating dependency of stroke protection on reperfusion.
Conclusion:
Blockade of platelet adhesion by anti-GPIb-Fab-fragments results in substantially improved CBF early during reperfusion. This finding was in exact spatial correspondence with the prevention of cerebral infarction and indicates in-vivo an increased patency of the microcirculation. Thus, progression of infarction during early ischemia and reperfusion can be mitigated by anti-platelet treatment.
The surface sublimation of Cd and Te atoms from the zinc blende (111)A CdTe surface has been investigated in detail by reflection high energy electron diffraction and x-ray photoelectron spectroscopy. These experiments verify that Te is much easier to evaporate than Cd. The experimental value for the Te activation energy from a Te stabilized (111)A CdTe surface is 1.41 ±0.1O eV, which is apparently inconsistent with recent theoretical results.
The role of elastic interactions, particularly for the self-organized formation of periodically faceted interfaces, was investigated in this thesis for archetype organic-metal interfaces. The cantilever bending technique was applied to study the change of surface stress upon formation of the interface between 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and Ag(111). This system is known to form a chemisorptive bonding. Indeed, the sign and the coverage-dependence of the surface stress change are in agreement to models and previous measurements of chemisorptive systems in literature. While the adsorption of molecules into the large domains is associated with a negative, i.e. compressive stress change, the formation of domain boundaries in the molecular layer induces a stress change of opposite sign, increasing the surface stress. The magnitude of the surface stress change of (-0.30 +- 0.10} N/m reflects a relatively weak binding of a PTCDA molecule to each individual single silver atom. It is emphasized, however, that if normalized to the surface stress change per molecule, this value corresponds to a stress change of (-2.2 +- 0.2) eV per molecule which is in the order of the suspected binding energy of this system. Therefore, these experiments reveal elastic interactions to be of significant order of magnitude for this system class. Thereby, they add a new point of view to the understanding of these interfaces. Besides, since the results are in agreement with the well-known properties of this interface, they establish the cantilever bending technique in the field of organic-metal interfaces. The mere existence of a bending of the sample implies an interesting detail for the PTCDA/Ag(111) interface in particular. It is the first experimental evidence for a structural change in the topmost substrate layers upon adsorption of PTCDA on Ag(111). Since such a modification has significant implications for the interpretation of other experimental results, a further investigation with more quantitative structural methods appears necessary. The main focus of this work, however, was on the investigation of the formation of the long-range ordered, self-organized faceted PTCDA/Ag(10 8 7) interface. Reciprocal space maps of this interface were recorded both by spot profile analysis low energy electron diffraction (SPA-LEED) and low energy electron microscopy (LEEM) in selected area LEED mode. Complementary to the reciprocal data, also microscopic real-space LEEM data were used to characterize the morphology of this interface. Six different facet faces ((111), (532), (743), (954), (13 9 5), and (542)) were observed for the preparation path of molecular adsorption on the substrate kept at 550 K. Facet-sensitive dark-field LEEM localized these facets to grow in homogeneous areas of microscopic extensions. If the pristine mesoscopic orientation locally deviates from the average orientation, e.g. in pristine step density, locally different facet types are formed, distorting the otherwise regular mesoscopic pattern. Hence, the original mesoscopic orientation of the substrate strongly determines the degree of order of the faceted surface and the facet species formed. The temperature-dependence of the interface formation was studied in a range between 418 K and 612 K in order to learn more about the kinetics of the process. Additional steeper facets of 27° inclination with respect to the (111) surface were observed in the low temperature regime. Furthermore, using facet-sensitive dark-field LEEM, spatial and size distributions of specific facets were studied for the different temperatures. The nucleation density of the facets did not depend on temperature and can therefore be concluded not to be limited by diffusion. Moreover, the facet dimensions were statistically analyzed. The total island size of the facets follows an exponential distribution, indicating a random growth mode in absence of any mutual facet interactions. While the length distribution of the facets also follows an exponential distribution, the width distribution is peaked, reflecting the high degree of lateral order. This anisotropy is temperature-dependent and occurs starting above 478 K substrate temperature during growth. The peaked distribution indicates the presence of a long-range interaction which leads to the structural order of the self-organized grating. The origin of this long-range interaction was investigated combining three complementary in-situ methods, all providing new insights into the formation of faceted organic-metal interfaces: the cantilever bending technique, high-resolution low energy electron diffraction (SPA-LEED), and microscopy (LEEM). The cantilever bending technique was applied for the first time to a faceting system at all. Below the faceting transition temperature the surface stress change associated with the formation of the PTCDA/Ag(10 8 7) interface resembles in shape and magnitude the one observed for the reference interface PTCDA/Ag(111). But above the transition temperature the absolute surface stress change of (-0.67 +- 0.10) N/m observed for the faceted PTCDA/Ag(10 8 7) interface is considerably larger than for the previous cases. Moreover, the stress change happens in distinguishable stages with a clearly resolvable fine structure of regimes of positive and negative stress changes. These different regimes of surface stress change can be correlated to different stages of the structural phase transition observed by the structural in-situ methods. Thereby, morphological objects (i.e. the facets) are assigned to a specific stress character. Thus, domains of different stress character can be identified on the surface. These stress domains are the prerequisite to apply continuum descriptions of the self-ordering process based on elastic interactions. Hence, the results are the first experimental verification that these continuum descriptions are indeed also applicable to the whole system class of faceting organic-metal interfaces. In conclusion, the results provide strong evidence for elastic interactions being the physical origin of long-range order for this system. In addition, the clear correlation of structural phase transition and surface stress change regimes suggests surface stress to play also an important role for the kinetics of the system. Indeed, the system seems to try to limit the overall stress change during the interface formation by forming facets of positive and negative stress character. Hence, the selection of specific facets could depend on the corresponding stress character. Furthermore, the system seems willing to re-facet at high coverages in order to prevent imperfect domain boundaries which are associated with an increase of surface stress. Finally, template-assisted growth of lateral, heterorganic nanostructures has been explored. Therefore, self-assembled monolayers as a second archetype class of molecules were grown on partially covered PTCDA/Ag(10 8 7) interfaces. Indeed, using standard surface science techniques, the basic principle of this growth scheme was confirmed to be successful.
The motivation for this work has been contributing a step to the advancement of technology. A next leap in technology would be the realization of a scalable quantum computer. One potential route is via topological quantum computing. A profound understanding of topological materials is thus essential. My work contributes by the investigation of the exemplary topological material HgTe. The focus lies on the understanding of the topological surface states (TSS) and new possibilities to manipulate them appropriately. Traditionally top gate electrodes are used to adjust the carrier density in such semi-conductor materials. We found that the electric field of the top gate can further alter the properties of the HgTe layer. The formation of additional massive Volkov-Pankratov states limits the accessibility of the TSS. The understanding of these states and their interplay with the TSS is necessary to appropriately design devices and to ensure their desired properties. Similarly, I observed the existence and stability of TSSs even without a bandgap in the bulk band structure in the inversion induced Dirac semi-metal phase of compressively strained HgTe. The finding of topological surface states in inversion-induced Dirac semi-metals provides a consistent and simple explanation for the observation reported for \(\text{Cd}_3\text{As}_2\).
These observations have only been possible due to the high quality of the MBE grown HgTe layers and the access of different phases of HgTe via strain engineering. As a starting point I performed Magneto-transport measurements on 67 nm thick tensilely strained HgTe layers grown on a CdTe substrate. We observed multiple transport channels in this three-dimensional topological insulator and successfully identified them. Not only do the expected topological surface states exist, but also additional massive surface states have been observed. These additional massive surface states are formed due to the electrical field applied at the top gate, which is routinely used to vary the carrier density in the HgTe layer. The additional massive surface states are called Volkov-Pankratov states after B. A. Volkov and O. A. Pankratov. They predicted the existence of similar massive surface states at the interface of materials with mutually inverted bands. We first found indications for such massive Volkov-Pankratov states in high-frequency compressibility measurements for very high electron densities in a fruitful collaboration with LPA in Paris. Magneto-transport measurements and \(k \cdot p\) calculations revealed that such Volkov-Pankratov states are also responsible for the observed whole transport. We also found indications for similar massive VPS in the electron regime, which coexist with the topological surface states. The topological surface states exist over the full investigated gate range including a regime of pure topological insulator transport. To increase the variability of the topological surface states we introduced a modulation doping layer in the buffer layer. This modulation doping layer also enabled us to separate and identify the top and bottom topological surface states.
We used the variability of the bulk band structure of HgTe with strain to engineer the band structure of choice using virtual substrates. The virtual substrates enable us to grow compressively strained HgTe layers that do not possess a bandgap, but instead linear crossing points. These layers are predicted to beDirac semi-metals. Indeed I observed also topological surface states and massive Volkov-Pankratov states in the compressively strained Dirac semi-metal phase. The observation of topological surfaces states also in the Dirac semi-metal phase has two consequences: First, it highlights that no bulk bandgap is necessary to observe topological surface states. Second, the observation of TSS also in the Dirac semi-metal phase emphasizes the importance of the underlying band inversion in this phase. I could not find any clear signatures of the predicted disjoint topological surface states, which are typically called Fermi-arcs. The presence of topological surface states and massive Volkov-Pankratov states offer a simple explanation for the observed quantum Hall effect and other two-dimensional transport phenomena in the class of inversion induced Dirac semi-metals, as \(\text{Cd}_3\text{As}_2\). This emphasizes the importance of the inherent bulk band inversion of different topological materials and provides a consistent and elegant explanation for the observed phenomena in these materials. Additionally, it offers a route to design further experiments, devices, and thus the foundation for the induction of superconductivity and thus topological quantum computing.
Another possible path towards quantum computing has been proposed based on the chiral anomaly. The chiral anomaly is an apparent transport anomaly that manifests itself as an additional magnetic field-driven current in three-dimensional topological semimetals with a linear crossing point in their bulk band structure. I observed the chiral anomaly in compressively strained HgTe samples and performed multiple control experiments to identify the observed reduction of the magnetoresistance with the chiral anomaly. First, the dependence of the so-called negative magnetoresistance on the angle and strength of the magnetic field has been shown to fit the expectation for the chiral anomaly. Second, extrinsic effects as scattering could be excluded as a source for the observed negative MR using samples with different mobilities and thus impurity concentrations. Third, the necessity of the linear crossing point has been shown by shifting the electrochemical potential away from the linear crossing points, which diminished the negative magnetoresistance. Fourth, I could not observe a negative magnetoresistance in the three-dimensional topological insulator phase of HgTe. These observations together prove the existence of the chiral anomaly and verify compressively strained HgTe as Dirac semi-metal. Surprisingly, the chiral anomaly is also present in unstrained HgTe samples, which constitute a semi-metal with a quadratic band touching point. This observation reveals the relevance of the Zeeman effect for the chiral anomaly due to the lifting of the spin-degeneracy in these samples. Additionally to the chiral anomaly, the Dirac semi-metal phase of compressively strained HgTe showed other interesting effects. For low magnetic fields, a strong weak-antilocalization has been observed. Such a strong weak-anti-localization correction in a three-dimensional layer is surprising and interesting. Additionally, non-trivial magnetic field strength and direction dependencies have been observed. These include a strong positive magnetoresistance for high magnetic fields, which could indicate a metal-insulator transition. On a more device-oriented note, the semi-metal phase of unstrained HgTe constitutes the lower limit of the by strain engineering adjustable minimal carrier density of the topological surface states and thus of very high mobility.
To sum up, topological surface states have been observed in the three-dimensional topological insulator phase and the Dirac semi-metal phase of HgTe. The existence and accessibility of topological surface states are thus independent of the existence of a bandgap in the bulk band structure. The topological surface states can be accompanied by massive Volkov-Pankratov states. These VPS are created by electric fields, which are routinely applied to adjust the carrier density in semiconductor devices. The theoretical predicted chiral anomaly has been observed in the Dirac semi-metal phase of HgTe. In contrast to theoretical predictions, no indications for the Fermi-arc called disjoint surface states have been observed, but instead the topological and massive Volkov-Pankratov surface states have been found. These states are thus expected for all inversion-induced topological materials.
Surface systems attract great scientific attention due to novel and exotic properties. The atomically structured surfaces lead to a reduced dimensionality which alters electronic correlations, vibrational properties, and their impact on each other. The emerging physical phenomena are not observed for related bulk materials. In this thesis, ordered (sub)monolayers of metal atoms (Au and Sn) on semiconductor substrates (Si(111) and Ge(111)) and ultrathin intermetallic films (CePt5 and LaPt5) on metal substrate (Pt(111)) are investigated by polarized in situ surface Raman spectroscopy. The surface Raman spectra exhibit features of specific elementary excitations like surface phonons and electronic excitations, which are suitable to gain fundamental insights into the surface systems.
The Au-induced surface reconstructions (5x2) and (r3xr3) constitute quasi-one- and two-dimensional Au structures on the Si(111) substrate, respectively. The new reconstruction-related Raman peaks are analyzed with respect to their polarization and temperature behavior. The Raman results are combined with firstprinciples calculations to decide between different proposed structural models. The Au-(5x2)/Si(111) reconstruction is best described by the model of Kwon and Kang, while for Au-(r3xr3)/Si(111) the conjugate honeycomb-chained-trimer model is favored. The Sn-induced reconstructions with 1/3 monolayer on Ge(111) and Si(111) are investigated to reveal their extraordinary temperature behavior. Specific surface phonon modes are identified that are predicted within the dynamical fluctuation model. Contrary to Sn/Si(111), the corresponding vibrational mode of Sn/Ge(111) exhibits a nearly harmonic character. The reversible structural phase transition of Sn/Ge(111) from (r3xr3) to (3x3) is observed, while no phase transition is apparent for Sn/Si(111). Moreover, Raman spectra of the closely related systems Sn-(2r3x2r3)/Si(111) and thin films of a-Sn as well as the clean semiconductor surfaces Si(111)-(7x7) and Ge(111)-c(2x8) are evaluated and compared.
The CePt5/Pt(111) system hosts 4f electrons whose energy levels are modified by the crystal field and are relevant for a description of the observed Kondo physics. In contrast, isostructural LaPt5/Pt(111) has no 4f electrons. For CePt5/Pt(111), distinct Raman features due to electronic Raman scattering can be unambiguously related to transitions between the crystal-field states which are depth-dependent. This assignment is supported by comparison to LaPt5/Pt(111) and group theoretical considerations. Furthermore, the vibrational properties of CePt5 and LaPt5 reveal interesting similarities but also striking differences like an unusual temperature shift of a vibration mode of CePt5, which is related to the influence of 4f electrons.
The present thesis deals with surface treatment, material improvement, and the electronic structure of the diluted magnetic semiconductor (Ga,Mn)As. The two key issues are the preparation of clean surfaces and the observation of potential valence hybridizations in (Ga,Mn)As by means of photoemission spectroscopy. Several cleaning methods are applied individually to (Ga,Mn)As and their e ects are compared in detail by various methods. Based on the results of each method, a sophisticated recipe has been elaborated, which provides clean, stoichiometric, and reconstructed surfaces, even if the sample was exposed to air prior to preparation. Moreover, the recipe works equally well for intentionally oxidized surfaces. The individual advantages of ex-situ wet- chemical etching and in situ ion-milling and tempering can be combined in an unique way. In regard to the post-growth annealing in order to optimize the electronic and magnetic properties of (Ga,Mn)As, the effect of surface segregation of interstitial Mn was quantifed. It turns out that the Mn concentration at the surface increases by a factor 4.3 after annealing at 190 C for 150 h. The removal of the segregated and oxidized species by wet-chemical etching allows a tentative estimate of the content of interstitial Mn. 19-23% of the overall Mn content in as-grown samples resides on interstitial positions. The complementary results of core level photoemission spectroscopy and resonant photoemission spectroscopy give hints to the fact that a sizeable valence hybridization of Mn is present in (Ga,Mn)As. This outlines that the simple Mn 3d5-con guration is too naive to refect the true electronic structure of substitutional Mn in (Ga,Mn)As. Great similarities in the core level spectra are found to MnAs. The bonding is thus dominantly of covalent, not ionic, character. Transport measurements, in particular for very low temperatures (<10 K), are in agreement with previous results. This shows that at low temperature, the conduction is mainly governed by variable-range hopping which is in line with the presence of an impurity band formed by substitutional Mn. In the light of the presented results, it is therefore concluded that a double-exchange interaction is the dominant mechanism leading to ferromagnetic coupling in (Ga,Mn)As. The valence hybridization and the presents of an impurity band, both of which are inherent properties of substitutional Mn, are indications for a double-exchange scenario, being at variance to a RKKY-based explanation. Contributions from a RKKY-like mechanism cannot definitely be excluded, however, they are not dominant.
Transition metal oxides (TMO) represent a highly interesting material class as
they exhibit a variety of different emergent phenomena including multiferroicity and
superconductivity. These effects result from a significant interplay of charge, spin
and orbital degrees of freedom within the correlated d-electrons. Oxygen vacancies
(OV) at the surface of certain d0 TMO release free charge carriers and prompt the
formation of a two-dimensional electron gas (2DEG). Barium titanate (BaTiO3) is a
prototypical and promising d0 TMO. It displays ferroelectricity at room temperature
and features several structural phase transitions, from cubic over tetragonal (at
room temperature) and orthorhombic to rhombohedral. The spontaneous electric
polarization in BaTiO3 can be used to manipulate the physical properties of adjacent
materials, e.g. in thin films. Although the macroscopic properties of BaTiO3 are studied
in great detail, the microscopic electronic structure at the surface and interface of
BaTiO3 is not sufficiently understood yet due to the complex interplay of correlation
within the d states, oxygen vacancies at the surface, ferroelectricity in the bulk and
the structural phase transitions in BaTiO3.
This thesis investigates the electronic structure of different BaTiO3 systems by
means of angle-resolved photoelectron spectroscopy (ARPES). The valence band of
BaTiO3 single crystals is systematically characterized and compared to theoretical
band structure calculations. A finite p-d hybridization of titanium and oxygen states
was inferred at the high binding energy side of the valence band. In BaTiO3 thin films,
the occurrence of spectral weight near the Fermi level could be linked to a certain
amount of OV at the surface which effectively dopes the host system. By a systematic
study of the metallic surface states as a function of temperature and partial oxygen
pressure, a model was established which reflects the depletion and accumulation of
charge carriers at the surface of BaTiO3. An instability at T ~ 285K assumes a volatile
behavior of these surface states.
The ferroelectricity in BaTiO3 allows a control of the electronic structure at the interface
of BaTiO3-based heterostructures. Therefore, the interface electronic structure
of Bi/BaTiO3 was studied with respect to the strongly spin-orit coupled states in Bi by
also including a thickness dependent characterization. The ARPES results, indeed,
confirm the presence of Rashba spin-split electronic states in the bulk band gap of the ferroelectric substrate. By varying the film thickness in Bi/BaTiO3, it was able to modify
the energy position and the Fermi vector of the spin-split states. This observation
is associated with the appearance of an interface state which was observed for very
low film thickness. Both spectral findings suggest a significant coupling between the
Bi films and BaTiO3.
Masers as telecommunication amplifiers have been known for decades, yet their application is strongly limited due to extreme operating conditions requiring vacuum techniques and cryogenic temperatures. Recently, a new generation of masers has been invented based on optically pumped spin states in pentacene and diamond. In this study, we pave the way for masers based on spin S = 3/2 silicon vacancy (V\(_{Si}\)) defects in silicon carbide (SiC) to overcome the microwave generation threshold and discuss the advantages of this highly developed spin hosting material. To achieve population inversion, we optically pump the V\(_{Si}\) into their m\(_S\) = ±1/2 spin sub-states and additionally tune the Zeeman energy splitting by applying an external magnetic field. In this way, the prerequisites for stimulated emission by means of resonant microwaves in the 10 GHz range are fulfilled. On the way to realising a maser, we were able to systematically solve a series of subtasks that improved the underlying relevant physical parameters of the SiC samples. Among others, we investigated the pump efficiency as a function of the optical excitation wavelength and the angle between the magnetic field and the defect symmetry axis in order to boost the population inversion factor, a key figure of merit for the targeted microwave oscillator. Furthermore, we developed a high-Q sapphire microwave resonator (Q ≈ 10\(^4\)–10\(^5\)) with which we find superradiant stimulated microwave emission. In summary, SiC with optimized spin defect density and thus spin relaxation rates is well on its way of becoming a suitable maser gain material with wide-ranging applications.
Numerical simulations and an analytic approach based on transmission line theory are used to design splitters for nano-plasmonic signal processing that allow to arbitrarily adjust the ratio of transmission from an input into two different output arms. By adjusting the geometrical parameters of the structure, either a high bandwidth or a sharp transmission resonance is obtained. Switching between the two arms can be achieved by modulating the effective refractive index of the waveguide. Employing the instantaneous Kerr effect, switching rates in the THz regime are potentially feasible. The suggested devices are of interest for future applications in nanoplasmonic information processing.
The spatial uniformity of GaSb- and InAs substrate-based structures containing type II quantum wells was probed by means of large-scale photoluminescence (PL) mapping realized utilizing a Fourier transform infrared spectrometer. The active region was designed and grown in a form of a W-shaped structure with InAs and GaInSb layers for confinement of electrons and holes, respectively. The PL spectra were recorded over the entire 2-in. wafers, and the parameters extracted from each spectrum, such as PL peak energy position, its linewidth and integrated intensity, were collected in a form of two-dimensional spatial maps. Throughout the analysis of these maps, the wafers' homogeneity and precision of the growth procedure were investigated. A very small variation of PL peak energy over the wafer indicates InAs quantum well width fluctuation of only a fraction of a monolayer and hence extraordinary thickness accuracy, a conclusion further supported by high uniformity of both the emission intensity and PL linewidth.
This work consists of two parts. On the one hand, it describes simulation and
measurement of the effect of contaminations of the detector gas on the performance
of particle detectors, with special focus on Micromegas detectors. On the other
hand, it includes the setup of a production site for the finalization of drift panels
which are going to be used in the ATLAS NSW. The first part augments these
two parts to give an introduction into the theoretical foundations of gaseous particle
detectors.
We study the structure formation of 1,4,5,8-naphthalenetetracarboxylicacid-
dianhydride (NTCDA) multilayer films on Ag(111) surfaces by energy dispersive near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) and photoelectron spectroscopy. The time resolution of seconds of the method allows us to identify several sub-processes, which occur during the post-growth three-dimensional structural ordering, as well as their characteristic time scales. After deposition at low temperature the NTCDA molecules are preferentially flat lying and the films exhibit no long-range order. Upon annealing the molecules flip into an upright orientation followed by an aggregation in a transient phase which exists for several minutes. Finally, threedimensional islands are established with bulk-crystalline structure involving substantial mass transport on the surface and morphological roughening. By applying the Kolmogorov–Johnson–Mehl–Avrami model the activation energies of the temperature-driven sub-processes can be derived from the time evolution of the NEXAFS signal.
For determination of structures and structural dynamics of proteins organic fluorophores are a standard instrument. Intra- and intermolecular contact of biomolecular structures are determined in time-resolved and stationary fluorescence microscopy experiments by quenching of organic fluorophores due to Photoinduced Electron Transfer (PET) and dimerization interactions. Using PET we show in this work that end-to-end contact dynamics of serine-glycine peptides are slowed down by glycosylation. This slow down is due to a change in reaction enthalpy for end-to-end contact and is partly compensated by entropic effects. In a second step we test how dimerization of MR121 fluorophore pairs reports on end-to-end contact dynamics. We show that in aqueous solutions containing strong denaturants MR121 dimerization reports advantageously on contact dynamics for glycine-serine oligopeptides compared to the previously used MR121/tryptophane PET reporters. Then we analyze dimer interactions and quenching properties of different commercially available fluorophores being standards in Förster Resonance Energy Transfer (FRET) measurements. Distances in biomolecules are determinable using FRET, but for very flexible biomolecules the analysis of masurement data can be distorted if contact of the two FRET fluorophores is likely. We quantify how strong the quenching of fluorophore pairs with two different or two identical fluorophores is. Dimer spectra and association constants are quantified to estimate if fluophores are applicable in various applications, e.g. in FRET measurements with unstructured peptides and proteins.
The position of a single GaAs quantum dot (QD), which is optically active, grown by low-density droplet epitaxy (DE) (approximately 4 QDs/μm\(^{2}\)), was directly observed on the surface of a 45-nm-thick Al\(_{0.3}\)Ga\(_{0.7}\)As capping layer. The thin thickness of AlGaAs capping layer is useful for single photon sources with plasmonic optical coupling. A micro-photoluminescence for GaAs DE QDs has shown exciton/biexciton behavior in the range of 1.654 to 1.657 eV. The direct observation of positions of low-density GaAs DE QDs would be advantageous for mass fabrication of devices that use a single QD, such as single photon sources.
In Magnetic Resonance Imaging (MRI), acquisition of dynamic data may be highly complex due to rapid changes occurred in the object to be imaged. For clinical diagnostic, dynamic MR images require both high spatial and temporal resolution. The speed in the acquisition is a crucial factor to capture optimally dynamics of the objects to obtain accurate diagnosis. In the 90’s, partially parallel MRI (pMRI) has been introduced to shorten scan times reducing the amount of acquired data. These approaches use multi-receiver coil arrays to acquire independently and simultaneously the data.
Reduction in the amount of acquired data results in images with aliasing artifacts. Dedicated methods as such Sensitivity Encoding (SENSE) and Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA) were the basis of a series of algorithms in pMRI.
Nevertheless, pMRI methods require extra spatial or temporal information in order to optimally reconstruct the data. This information is typically obtained by an extra scan or embedded in the accelerated acquisition applying a variable density acquisition scheme.
In this work, we were able to reduce or totally eliminate the acquisition of the training data for kt-SENSE and kt-PCA algorithms obtaining accurate reconstructions with high temporal fidelity.
For dynamic data acquired in an interleaved fashion, the temporal average of accelerated data can generate an artifact-free image used to estimate the coil sensitivity maps avoiding the need of extra acquisitions. However, this temporal average contains errors from aliased components, which may lead to signal nulls along the spectra of reconstructions when methods like kt-SENSE are applied. The use of a GRAPPA filter applied to the temporal average reduces these errors and subsequently may reduce the null components in the reconstructed data. In this thesis the effect of using temporal averages from radial data was investigated. Non-periodic artifacts performed by undersampling radial data allow a more accurate estimation of the true temporal average and thereby avoiding undesirable temporal filtering in the reconstructed images. kt-SENSE exploits not only spatial coil sensitivity variations but also makes use of spatio-temporal correlations in order to separate the aliased signals. Spatio-temporal correlations in kt-SENSE are learnt using a training data set, which consists of several central k-space lines acquired in a separate scan. The scan of these extra lines results in longer acquisition times even for low resolution images. It was demonstrate that limited spatial resolution of training data set may lead to temporal filtering effects (or temporal blurring) in the reconstructed data.
In this thesis, the auto-calibration for kt-SENSE was proposed and its feasibility was tested in order to completely eliminate the acquisition of training data. The application of a prior TSENSE reconstruction produces the training data set for the kt-SENSE algorithm. These training data have full spatial resolution. Furthermore, it was demonstrated that the proposed auto-calibrating method reduces significantly temporal filtering in the reconstructed images compared to conventional kt-SENSE reconstructions employing low resolution training images. However, the performance of auto-calibrating kt-SENSE is affected by the Signal-to-Noise Ratio (SNR) of the first pass reconstructions that propagates to the final reconstructions.
Another dedicated method used in dynamic MRI applications is kt-PCA, that was first proposed for the reconstruction of MR cardiac data. In this thesis, kt-PCA was employed for the generation of spatially resolved M0, T1 and T2 maps from a single accelerated IRTrueFISP or IR-Snapshot FLASH measurement. In contrast to cardiac dynamic data, MR relaxometry experiments exhibit signal at all temporal frequencies, which makes their reconstruction more challenging. However, since relaxometry measurements can be represented by only few parameters, the use of few principal components (PC) in the kt-PCA algorithm can significantly simplify the reconstruction. Furthermore, it was found that due to high redundancy in relaxometry data, PCA can efficiently extract the required information from just a single line of training data.
It has been demonstrated in this thesis that auto-calibrating kt-SENSE is able to obtain high temporal fidelity dynamic cardiac reconstructions from moderate accelerated data avoiding the extra acquisition of training data. Additionally, kt-PCA has been proved to be a suitable method for the reconstruction of highly accelerated MR relaxometry data.
Furthermore, a single central training line is necessary to obtain accurate reconstructions. Both reconstruction methods are promising for the optimization of training data acquisition and seem to be feasible for several clinical applications.
The subject of this thesis is the control of strain in HgTe thin-film crystals. Such systems are members of the new class of topological insulator materials and therefore of special research interest. A major task was the experimental control of the strain in the HgTe films. This was achieved by a new epitaxial approach and confirmed by cristallographic analysis and magneto-transport measurements.
In this work, strain was induced in thin films by means of coherent epitaxy on substrate crystals. This means that the film adopts the lattice constant of the substrate in the plane of the substrate-epilayer interface. The level of strain is determined by the difference between the strain-free lattice constants of the substrate and epilayer material (the so-called lattice mismatch). The film responds to an in-plane strain with a change of its lattice constant perpendicular to the interface. This relationship is crucial for both the correct interpretation of high resolution X-ray diffraction (HRXRD) measurements, and the precise determination of the band dispersion. The lattice constant of HgTe is smaller than the lattice constant of CdTe. Therefore, strain in HgTe is tensile if it is grown on a CdTe substrate. In principle, compressive strain can be achieved by using an appropriate \(\text{Cd}_{1-x}\text{Zn}_{x}\text{Te}\) substrate. This concept was modified and applied in this work.
Epilayers have been fabricated by molecular-beam epitaxy (MBE). The growth of thick buffer layers of CdTe on GaAs:Si was established as an alternative to commercial CdTe and \(text{Cd}_{0.96}\text{Zn}_{0.04}\text{Te}\) substrates. The growth conditions have been optimized by an analysis of atomic force microscopy and HRXRD studies. HRXRD measurements reveal a power-law increase of the crystal quality with increasing thickness. Residual strain was found in the buffer layers, and was attributed to a combination of finite layer thickness and mismatch of the thermal expansion coefficients of CdTe and GaAs. In order to control the strain in HgTe epilayers, we have developed a new type of substrate with freely adjustable lattice constant.
CdTe-\(\text{Cd}_{0.5}\text{Zn}_{0.5}\text{Te}\) strained-layer-superlattices have been grown by a combination of MBE and atomic-layer epitaxy (ALE), and have been analyzed by HRXRD. ALE of the \(\text{Cd}_{0.5}\text{Zn}_{0.5}\text{Te}\) layer is self-limiting to one monolayer, and the effective lattice constant can be controlled reproducibly and straightforward by adjusting the CdTe layer thickness. The crystal quality has been found to degrade with increasing Zn-fraction. However, the effect is less drastic compared to single layer \(\text{Cd}_{1-x}\text{Zn}_{x}\text{Te}\) solid solutions. HgTe quantum wells (QWs) sandwiched in between CdHgTe barriers have been fabricated in a similar fashion on superlattices and conventional CdTe and \(\text{Cd}_{0.96}\text{Zn}_{0.04}\text{Te}\) substrates. The lower critical thickness of the CdHgTe barrier material grown on superlattice substrates had to be considered regarding the sample design. The electronic properties of the QWs depend on the strain and thickness of the QW. We have determined the QW thickness with an accuracy of \(\pm\)0.5 nm by an analysis of the beating patterns in the thickness fringes of HRXRD measurements and X-ray reflectometry measurements. We have, for the first time, induced compressive strain in HgTe QWs by an epitaxial technique (i.e. the effective lattice constant of the superlattice is lower compared to the lattice constant of HgTe). The problem of the lattice mismatch between superlattice and barriers has been circumvented by using CdHgTe-ZnHgTe superlattices instead of CdHgTe as a barrier material. Furthermore, the growth of compressively strained HgTe bulk layers (with a thickness of at least 50 nm) was demonstrated as well.
The control of the state of strain adds a new degree of freedom to the design of HgTe epilayers, which has a major influence on the band structure of QWs and bulk layers. Strain in bulk layers lifts the degeneracy of the \(\Gamma_8\) bands at \(\mathbf{k}=0\). Tensile strain opens an energy gap, compressive strain shifts the touching points of the valence- and conduction band to positions in the Brillouin zone with finite \(\mathbf{k}\). Such a situation has been realized for the first time in the course of this work. For QWs in the inverted regime, it is demonstrated that compressive strain can be used to significantly enhance the thermal energy gap of the two-dimensional electron gas (2DEG). In addition, semi-metallic and semiconducting behavior is expected in wide QWs, depending on the state of strain. An examination of the temperature dependence of the subband ordering in QWs revealed that the band gap is only temperature-stable for appropriate sample parameters and temperature regimes. The band inversion is always lifted for sufficiently high temperatures.
A large number of models investigate the influence of the band gap on the stability of the quantum-spin-Hall (QSH) effect. An enhancement of the stability of QSH edge state conductance is expected for enlarged band gaps. Furthermore, experimental studies on the temperature dependence of the QSH conductance are in contradiction to theoretical predictions. Systematic studies of these aspects have become feasible based on the new flexibility of the sample design.
Detailed low-temperature magnetotransport studies have been carried out on QWs and bulk layers. For this purpose, devices have been fabricated lithographically, which consist of two Hall-bar geometries with different dimensions. This allows to discriminate between conductance at the plane of the 2DEG and the edge of the sample. The Fermi energy in the 2DEG has been adjusted by means of a top gate electrode. The strain-induced transition from semi-metallic to semiconducting characteristics in wide QWs was shown. The magnitude of the semi-metallic overlap of valence- and conduction band was determined by an analysis of the two-carrier conductance and is in agreement with band structure calculations. The band gap of the semiconducting sample was determined by measurements of the temperature dependence of the conductance at the charge-neutrality point. Agreement with the value expected from theory has been achieved for the first time in this work. The influence of the band gap on the stability of QSH edge state conductance has been investigated on a set of six samples. The band gap of the set spans a range of 10 to 55 meV. The latter value has been achieved in a highly compressively strained QW, has been confirmed by temperature-dependent conductance measurements, and is the highest ever reported in the inverted regime. Studies of the carrier mobility reveal a degradation of the sample quality with increasing Zn-fraction in the superlattice, in agreement with HRXRD observations. The enhanced band gap does not suppress scattering mechanisms in QSH edge channels, but lowers the conductance in the plane of the 2DEG. Hence, edge state conductance is the dominant conducting process even at elevated temperatures. An increase in conductance with increasing temperature has been found, in agreement with reports from other groups. The increase follows a power-law dependency, the underlying physical mechanism remains open. A cause for the lack of an increase of the QSH edge state conductance with increasing energy gap has been discussed. Possibly, the sample remains insulating even at finite carrier densities, due to localization effects. The measurement does not probe the QSH edge state conductance at the situation where the Fermi energy is located in the center of the energy gap, but in the regime of maximized puddle-driven scattering. In a first set of measurements, it has been shown that the QSH edge state conductance can be influenced by hysteretic charging effects of trapped states in the insulating dielectric. A maximized conductance of \(1.6\ \text{e}^2/\text{h}\) was obtained in a \(58\ \mu\text{m}\) edge channel. Finally, measurements on three dimensional samples have been discussed. Recent theoretical works assign compressively strained HgTe bulk layers to the Weyl semi-metal class of materials. Such layers have been synthesized and studied in magnetotransport experiments for the first time. Pronounced quantum-Hall- and Shubnikov-de-Haas features in the Hall- and longitudinal resistance indicate two-dimensional conductance on the sample surface. However, this conductance cannot be assigned definitely to Weyl surface states, due to the inversion of \(\Gamma_6\) and \(\Gamma_8\) bands. If a magnetic field is aligned parallel to the current in the device, a decrease in the longitudinal resistance is observed with increasing magnetic field. This is a signature of the chiral anomaly, which is expected in Weyl semi-metals.
Mutual coupling and injection locking of semiconductor lasers is of great interest in non-linear dynamics and its applications for instance in secure data communication and photonic reservoir computing. Despite its importance, it has hardly been studied in microlasers operating at mu W light levels. In this context, vertically emitting quantum dot micropillar lasers are of high interest. Usually, their light emission is bimodal, and the gain competition of the associated linearly polarized fundamental emission modes results in complex switching dynamics. We report on selective optical injection into either one of the two fundamental mode components of a bimodal micropillar laser. Both modes can lock to the master laser and influence the non-injected mode by reducing the available gain. We demonstrate that the switching dynamics can be tailored externally via optical injection in very good agreement with our theory based on semi-classical rate equations. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near‐orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, 4,4′‐(9H,9′H‐[3,3′‐bicarbazole]‐9,9′‐diyl)bis(3‐(trifluoromethyl) benzonitrile) (pCNBCzoCF\(_{3}\)) is investigated, which shows intramolecular TADF but can also form exciplex states in combination with 4,4′,4′′‐tris[phenyl(m‐tolyl)amino]triphenylamine (m‐MTDATA). Orange emitting exciplex‐based OLEDs additionally generate a sky‐blue emission from the intramolecular emitter with an intensity that can be voltage‐controlled. Electroluminescence detected magnetic resonance (ELDMR) is applied to study the thermally activated spin‐dependent triplet to singlet up‐conversion in operating devices. Thereby, intermediate excited states involved in OLED operation can be investigated and the corresponding activation energy for both, intra‐ and intermolecular based TADF can be derived. Furthermore, a lower estimate is given for the extent of the triplet wavefunction to be ≥ 1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, the findings allow to draw a comprehensive picture of the spin‐dependent emission from intra‐ and intermolecular TADF OLEDs.
Semiconductors with strong spin–orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin–orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.
This work investigates the correlations between spin states and the light emission properties of organic light-emitting diodes (OLEDs), which are based on the principle of thermally activated delayed fluorescence. The spin-spin interactions responsible for this mechanism are investigated in this work using methods based on spin-sensitive electron paramagnetic resonance (EPR). In particular, this method has been applied to electrically driven OLEDs. The magnetic resonance has been detected by electroluminescence, giving this method its name: electroluminescence detected magnetic resonance (ELDMR).
Initial investigations on a novel deep blue TADF emitter were performed. Furthermore, the ELDMR method was used in this work to directly detect the spin states in the OLED. These measurements were further underlined by time-resolved experiments such as transient electro- and photoluminescence.
Within the scope of this thesis, spin related transport phenomena have been investigated in HgTe/HgCdTe quantum well structures. This material exhibits peculiar band structure properties, which result in a strong spin-orbit interaction of the Rashba type. An inverted band structure, i.e., a reversed ordering of the energy states in comparison to common semiconductors, is obtained for quantum well layers above a critical thickness. Furthermore, the band structure properties can be controlled in the experiments by moderate gate voltages. Most prominently, the type of carriers in HgTe quantum wells can be changed from n to p due to the narrow energy gap. Along with the inverted band structure, this unique transition is the basis for the demonstration of the Quantum Spin Hall state, which is characterized by the existence of two one-dimensional spin-polarized edge states propagating in opposite directions, while the Fermi level in the bulk is in the energy gap. Since elastic scattering is suppressed by time reversal symmetry, a quantized conductance for charge and spin transport is predicted. Our experiments provide the first experimental demonstration of the QSH state. For samples with characteristic dimensions below the inelastic mean free path, charge conductance close to the expected value of 2e^2/h has been observed. Strong indication for the edge state transport was found in the experiments as well. For large samples, potential fluctuations lead to the appearance of local n-conducting regions which are considered to be the dominant source of backscattering. When time reversal symmetry is broken in a magnetic field, elastic scattering becomes possible and conductance is significantly suppressed. The suppression relies on a dominant orbital effect in a perpendicular field and a smaller Zeeman-like effect present for any field direction. For large perpendicular fields, a re-entrant quantum Hall state appears. This unique property is directly related to the non-trivial QSH insulator state. While clear evidence for the properties of charge transport was provided, the spin properties could not be addressed. This might be the goal of future experiments. In another set of experiments, the intrinsic spin Hall effect was studied. Its investigation was motivated by the possibility to create and to detect pure spin currents and spin accumulation. A non-local charging attributed to the SHE has been observed in a p-type H-shaped structure with large SO interaction, providing the first purely electrical demonstration of the SHE in a semiconductor system. A possibly more direct way to study the spin Hall effects opens up when the spin properties of the QSH edge states are taken into account. Then, the QSH edge states can be used either as an injector or a detector of spin polarization, depending on the actual configuration of the device. The experimental results indicate the existence of both intrinsic SHE and the inverse SHE independently of each other. If a spin-polarized current is injected from the QSH states into a region with Rashba SO interaction, the precession of the spin can been observed via the SHE. Both the spin injection and precession might be used for the realization of a spin-FET similar to the one proposed by Datta and Das. Another approach for the realization of a spin-based FET relies on a spin-interference device, in which the transmission is controlled via the Aharonov-Casher phase and the Berry phase, both due to the SO interaction. In the presented experiments, ring structures with tuneable SO coupling were studied. A complex interference pattern is observed as a function of external magnetic field and gate voltage. The dependence on the Rashba splitting is attributed to the Aharonov-Casher phase, whereas effects due to the Berry phase remain unresolved. This interpretation is confirmed by theoretical calculations, where multi-channel transport through the device has been assumed in agreement with the experimental results. Thus, our experiments provide the first direct observation of the AC effect in semiconductor structures. In conclusion, HgTe quantum well structures have proven to be an excellent template for studying spin-related transport phenomena: The QSHE relies on the peculiar band structure of the material and the existence of both the SHE and the AC effect is a consequence of the substantial spin-orbit interaction. While convincing results have been obtained for the various effects, several questions can not be fully answered yet. Some of them may be addressed by more extensive studies on devices already available. Other issues, however, ask, e.g., for further advances in sample fabrication or new approaches by different measurements techniques. Thus, future experiments may provide new, compelling insights for both the effects discussed in this thesis and, more generally, other spin-orbit related transport properties.
Spin-Orbit Torques and Galvanomagnetic Effects Generated by the 3D Topological Insulator HgTe
(2021)
Nature shows us only the tail of the lion. But I have no doubt that the lion belongs with it even if he cannot reveal himself all at once. Albert Einstein
In my dissertation, I addressed the question of whether the 3D topological insulator mercury telluride (3D TI HgTe) is a suitable material for spintronics applications. This question was addressed by investigating the SOTs generated by the 3D TI HgTe in an adjacent ferromagnet (Permalloy) by using the ferromagnetic resonance technique (SOT-FMR).
In the first part of the dissertation, the reader was introduced to the mathematical description of the SOTs of a hybrid system consisting of a topological insulator (TI) and a ferromagnet (FM). Furthermore, the sample preparation and the measurement setup for the SOT-FMR measurements were discussed. Our SOT-FMR measurements showed that at low temperatures (T = 4.2 K) the out-of-plane component of the torque is dominant. At room temperature, both in-plane and out-of-plane components of the torque could be observed. From the symmetry of the mixing voltage (Figs. 3.14 and 3.15) we could conclude that the 3D TI HgTe may be efficient for the generation of spin torques in the permalloy [1]. The investigations reported here showed that the SOT efficiencies generated by the 3D TI HgTe are comparable with other existent topological insulators (see Fig. 3.17). We also discussed in detail the parasitic effects (such as thermovoltages) that can contribute to the correct interpretation of the spin torque efficiencies.
Although the results reported here provide several indications that the 3D TI HgTe might be efficient in exerting spin-torques in adjacent ferromagnets [2], the reader was repeatedly made aware that parasitic effects might contaminate the correct writing and reading of the information in the ferromagnet. These effects should be taken into consideration when interpreting results in the published literature claiming high spin-orbit torque efficiencies [2–4]. The drawbacks of the SOT-FMR measurement method led to a further development of our measurement concept, in which the ferromagnet on top of the 3D TI HgTe was replaced by a
spin-valve structure. In contrast with our measurements, in this measurement setup, the current flowing through the HgTe is known and changes in the spin-valve resistance can be read via the GMR effect.
Moreover, the SOT-FMR experiments required the application of an in-plane magnetic field up to 300 mT to define the magnetization direction in the ferromagnet. Motivated by this fact, we investigated the influence of an in-plane magnetic field in the magnetoresistance of the 3D TI HgTe. The surprising results of these measurements are described in the second part of the dissertation. Although the TI studied here is non-magnetic, its transversal MR (Rxy) showed an oscillating behavior that depended on the angle between the in-plane magnetic field and the electrical current. This effect is a typical property of ferromagnetic materials and is called planar Hall effect (PHE) [5, 6]. Moreover, it was also shown that the PHE amplitude (Rxy) and the longitudinal resistance (Rxx) oscillate as a function of the in-plane magnetic field amplitude for a wide range of carrier densities of the topological insulator.
The PHE was already described in another TI material (Bi2−xSbxTe3) [7]. The authors suggested as a possible mechanism the scattering of the electron off impurities that are polarized by an in-plane magnetic field. We critically discussed this and other theoretical proposed mechanisms existent in the literature [8, 9].
In this thesis, we attempted to explain the origin of the PHE in the 3D TI HgTe by anisotropies in the band structure of this material. The k.p calculations based on 6-orbitals were able to demonstrate that an interplay between Rashba, Dresselhaus, and in-plane magnetic field deforms the Fermi contours of the camel back band of the 3D TI HgTe, which could lead to anisotropies in its conductivity. However, the magnetic fields needed to experimentally observe this effect are as
high as 40 T, i.e., one order of magnitude higher than reported in our experiments. Additionally, calculations of the DoS to assess if there is a difference in the states for Bin parallel and Bin perpendicular to the current were, so far, inconclusive. Moreover, the complicated dependence of Rashba in the p-conducting
regime of HgTe [10] makes it not straightforward the inclusion of this term in the band structure calculations.
Despite the extensive efforts to understand the origin of the galvanomagnetic effects in the 3D TI HgTe, we could not determine a clear mechanism for the origin of the PHE and the MR oscillations studied in this thesis. However, our work clarifies and excludes a few mechanisms reported in the literature as the origin of these effects in the 3D TI HgTe. The major challenge, which still needs to be overcome, is to find a model that simultaneously explains the PHE, the gate dependence, and the oscillations in the magnetoresistance of the 3D TI HgTe as a function of the in-plane magnetic field.
To conclude, the author would like to express her hope to have brought the reader closer to the complexity of the questions addressed in this thesis and to have initiated them into the art of properly conducting electrical transport measurements on topological insulators with in-plane magnetic fields.
The contribution of the present thesis consists of three parts. They are centered around investigating certain semiconductor heterointerfaces relevant to spin injection, exploring novel, diluted magnetic single barrier tunneling structures, and further developing diluted magnetic II-VI resonant tunneling diodes.
Spin- and angle-resolved photoelectron spectroscopy is the prime method to investigate
spin polarized electronic states at solid state surfaces. In how far the spin polarization
of an emitted photoelectron reflects the intrinsic spin character of an electronic state is
the main question in the work at hand. It turns out that the measured spin polarization
is strongly influenced by experimental conditions, namely by the polarization of the
incoming radiation and the excitation energy. The photoemission process thus plays a
non-negligible role in a spin-sensitive measurement. This work is dedicated to unravel
the relation between the result of a spin-resolved measurement and the spin character
in the ground state and, therefore, to gain a deep understanding of the spin-dependent
photoemission process.
Materials that exhibit significant spin-splittings in their electronic structure,
owing to a strong spin-orbit coupling, serve as model systems for the investigations in
this work. Therefore, systems with large Rashba-type spin-splittings as BiTeI(0001)
and the surface alloys BiAg2/Ag(111) and PbAg2/Ag(111) are investigated. Likewise,
the surface electronic structure of the topological insulators Bi2Te2Se(0001) and
Bi2Te3(0001) are analyzed.
Light polarization dependent photoemission experiments serve as a probe of the
orbital composition of electronic states. The knowledge of the orbital structure helps
to disentangle the spin-orbital texture inherent to the different surface states, when
in addition the spin-polarization is probed. It turns out that the topological surface
state of Bi2Te2Se(0001) as well as the Rashba-type surface state of BiTeI(0001) exhibit
chiral spin-textures associated with the p-like in-plane orbitals. In particular, opposite
chiralities are coupled to either tangentially or radially aligned p-like orbitals,
respectively. The results presented here are thus evidence that a coupling between
spin- and orbital part of the wave function occurs under the influence of spin-orbit
coupling, independent of the materials topology.
Systematic photon energy dependent measurements of the out-of-plane spin polarization
of the topological surface state of Bi2Te3(0001) reveal a strong dependence and
even a reversal of the sign of the photoelectron spin polarization with photon energy.
Similarly, the measured spin component perpendicular to the wave vector of the surface
state of BiAg2/Ag(111) shows strong modulations and sign reversals when the photon energy is changed. In BiAg2/Ag(111) the variations in the photoelectron spin
polarization are accompanied by significant changes and even a complete suppression
of the photoemission intensity from the surface state, indicating that the variations of
the spin polarization are strongly related to the photoemission cross section.
This relation is finally analyzed in detail by employing a simple model, which is
based on an evaluation of the transition matrix elements that describe the presented
experiments. The model shows that the underlying cause for the observed photoelectron
spin reversals can be found in the coupling of the spin structure to the spatial part
of the initial state wave function, revealing the crucial role of spin-orbit interaction
in the initial state wave function. The model is supported by ab initio photoemission
calculations, which show strong agreement with the experimental results.
This thesis addresses the identification and characterization of spin states in optoelectronic materials and devices using multiple spin-sensitive techniques. For this purpose, a systematic study focussing on triplet states as well as associated loss pathways and excited state kinetics was carried out. The research was based on comparing a range of donor:acceptor systems, reaching from organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) to organic photovoltaics (OPV) employing fullerene and multiple non-fullerene acceptors (NFAs). By developing new strategies, e.g., appropriate modeling, new magnetic resonance techniques and experimental frameworks, the influence of spin states in the fundamental processes of organic semiconductors has been investigated. Thereby, the combination of techniques based on the principle of electron paramagnetic resonance (EPR), in particular transient EPR (trEPR) and optically detected magnetic resonance (ODMR), with all-optical methods, such as transient electroluminescence (trEL) and transient absorption (TA), has been employed. As a result, excited spin states, especially molecular and charge transfer (CT) states, were investigated in terms of kinetic behavior and associated pathways, which revealed a significant impact of triplet states on efficiency-limiting processes in both optoelectronic applications.
Spin defects in solid-state materials are strong candidate systems for quantum information technology and sensing applications. Here we explore in details the recently discovered negatively charged boron vacancies (V\(_B\)\(^−\)) in hexagonal boron nitride (hBN) and demonstrate their use as atomic scale sensors for temperature, magnetic fields and externally applied pressure. These applications are possible due to the high-spin triplet ground state and bright spin-dependent photoluminescence of the V\(_B\)\(^−\). Specifically, we find that the frequency shift in optically detected magnetic resonance measurements is not only sensitive to static magnetic fields, but also to temperature and pressure changes which we relate to crystal lattice parameters. We show that spin-rich hBN films are potentially applicable as intrinsic sensors in heterostructures made of functionalized 2D materials.
The rich phase diagram of transition metal oxides essentially roots in the many body physics arising from strong Coulomb interactions within the underlying electron system.
Understanding such electronic correlation effects remains challenging for modern solid state physics, therefore experimental data is required for further progress in the field. For this reason, spectroscopic investigations of prototypical correlated materials are the scope of this thesis. The experimental methods focus on photoelectron spectroscopy, and the test materials are the correlated metal SrVO\(_3\) and the Mott insulator LaTiO\(_3\), both of which are fabricated as high quality thin films.
In SrVO\(_3\) thin films, a reduction of the film thickness induces a dimensional crossover from the metallic into the Mott insulating phase. In this thesis, an extrinsic chemical contribution from a surface over-oxidation is revealed that emerges additionally to the intrinsic change of the effective bandwidth usually identified to drive the transition. The two contributions are successfully disentangled by applying a capping layer that prevents the oxidation, allowing for a clean view on the dimensional crossover in fully stoichiometric samples. Indeed, these stoichiometric layers exhibit a higher critical thickness for the onset of the metallic phase than the bare and therefore over-oxidized thin films.
For LaTiO\(_3\) thin films, the tendency to over-oxidize is even stronger. An uncontrolled oxygen diffusion from the substrate into the film is found to corrupt the electronic properties of LaTiO\(_3\) layers grown on SrTiO\(_3\). The Mott insulating phase is only detected in stoichiometric films fabricated on more suitable DyScO\(_3\) substrates. In turn, it is demonstrated that a \(controlled\) incorporation of excess oxygen ions by increasing the oxygen growth pressure is an effective way of \(p\) doping the material which is used to drive the band filling induced Mott transition.
Gaining control of the oxygen stoichiometry in both materials allows for a systematic investigation of correlation effects in general and of the Mott transition in particular. The investigations are realized by various photoelectron spectroscopy techniques that provide a deep insight into the electronic structure. Resonant photoemission not only gives access to the titanium and vanadium related partial density of states of the valence band features, but also shows how the corresponding signal is enhanced by tuning the photon energy to the \(L\) absorption threshold. The enhanced intensity turns out to be very helpful for probing the Fermi surface topology and band dispersions by means of angular-resolved photoemission. The resulting momentum resolved electronic structure verifies central points of the theoretical description of the Mott transition, viz. the renormalization of the band width and a constant Luttinger volume in a correlated metal as the Mott phase is approached.
In this dissertation the electronic and high-energy optical properties of thin nanoscale
films of the magnetic topological insulator (MTI) (V,Cr)y(BixSb1-x)2-yTe3 are studied
by means of X-ray photoelectron spectroscopy (XPS) and electron energy-loss
spectroscopy (EELS). Magnetic topological insulators are presently of broad interest
as the combination of ferromagnetism and spin-orbit coupling in these materials
leads to a new topological phase, the quantum anomalous Hall state (QAHS), with
dissipation less conduction channels. Determining and controlling the physical
properties of these complex materials is therefore desirable for a fundamental understanding
of the QAHS and for their possible application in spintronics. EELS can
directly probe the electron energy-loss function of a material from which one can
obtain the complex dynamic dielectric function by means of the Kramers-Kronig
transformation and the Drude-Lindhard model of plasmon oscillations.
The XPS core-level spectra in (V,Cr)y(BixSb1-x)2-yTe3 are analyzed in detail with
regards to inelastic background contributions. It is shown that the spectra can be
accurately described based on the electron energy-loss function obtained from an
independent EELS measurement. This allows for a comprehensive and quantitative
analysis of the XPS data, which will facilitate future core-level spectroscopy studies
in this class of topological materials. From the EELS data, furthermore, the bulk and
surface optical properties were estimated, and compared to ab initio calculations
based on density functional theory (DFT) performed in the GW approximation
for Sb2Te3. The experimental results show a good agreement with the calculated
complex dielectric function and the calculated energy-loss function. The positions of
the main plasmon modes reported here are expected to be generally similar in other
materials in this class of nanoscale TI films. Hence, the present work introduces
EELS as a powerful method to access the high-energy optical properties of TI
thin films. Based on the presented results it will be interesting to explore more
systematically the effects of stoichiometry, magnetic doping, film thickness and
surface morphology on the electron-loss function, potentially leading to a better
understanding of the complex interplay of structural, electronic, magnetic and
optical properties in MTI nanostructures.
The present thesis is devoted to the spectroscopic study of rare earth manganites RMnO3 (R = Gd, Dy, Tb, Eu(1 - x)Y(x)) in the submillimeter frequency range. A dynamic manifestation of a strong magnetoelectric coupling in these systems is the existence of electromagnons - spin waves excited by the electric component of the electromagnetic wave. The exact analytical solution of the Landau-Lifshitz equations obtained for cycloidal antiferromagnets builds the bridge between the inelastic neutron scattering and the optical experiments. A semi-quantitative agreement is achieved between the theory and the results by these two experimental techniques. Two suggested mechanisms of the magnetoelectric coupling, the inverse Dzyaloshinskii-Moriya (IDM) interaction and the symmetric Heisenberg exchange (HE) striction, are introduced in a perturbative manner. The qualitative conclusions regarding both static and dynamic electric properties are also in agreement with the experiment. GdMnO3 is the system in which the electromagnons were first detected at low frequencies. Far infrared measurements in GdMnO3 presented here have confirmed the existence of a second high frequency electromagnon at 75 reciprocal centimeter. The detection of an additional mode suggests the existence of at least short range ferroelectric order. Such order has not been observed in static experiments so far. The electromagnons in Eu(1 - x)Y(x)MnO3 helped to clarify the role of the rare earth magnetism. As the Y(3+) ions are diamagnetic and Eu(3+) ions possess Van Vleck paramagnetism only, it is the Mn subsystem that is primarily responsible for the magnetoelectric properties of rare earth manganites. The electromagnons in DyMnO3 and TbMnO3 do not change their excitation conditions upon the flop of the spin cycloid in external magnetic fields. This fact still lacks consistent theoretical explanation. Detailed measurements on TbMnO3 of different orientations have allowed to prove the existence of the IDM electromagnon. The study of DyMnO3 in external magnetic fields has shown that, depending on the Dy ordering, the electromagnons and static electric polarization can be either enhanced or suppressed. Thus, the magnetic order of rare earth moments still plays an important role. As a general result of the present work, the IDM interaction is capable to describe the static electric polarization and the weak electro-active excitation in the high-field phase of TbMnO3. The HE model is successful in explaining the high frequency electromagnon, including its excitation conditions and the spectral weight. However, both models are still unable to describe the energy and the spectral weight of the low frequency electromagnon. Further theoretical and experimental efforts are required in this direction.
In this thesis the electronic and magnetic structure of the transition metal oxyhalides TiOCl, TiOBr and VOCl is investigated. The main experimental methods are photoemission (PES) and x-ray absorption (XAS) spectroscopy as well as resonant inelastic x-ray scattering (RIXS). The results are compared to density-functional theory, and spectral functions from dynamical mean-field theory and different kinds of model calculations. Questions addressed here are those of the dimensionality of the magnetic and electronic interactions, the suitability of the oxyhalides as prototypical strongly correlated model systems, and the possibility to induce a filling-controlled insulator-metal transition. It turns out that TiOCl is a quasi-one-dimensional system with non-negligible two-dimensional coupling, while the one-dimensional character is already quite suppressed in TiOBr. In VOCl no signatures of such one-dimensional behavior remain, and it is two-dimensional. In all cases, frustrations induced by the crystal lattice govern the magnetic and electronic properties. As it turns out, although the applied theoretical approaches display improvements compared to previous studies, the differences to the experimental data still are at least partially of qualitative instead of quantitative nature. Notably, using RIXS, it is possible for the first time in TiOCl to unambiguously identify a two-spinon excitation, and the previously assumed energy scale of magnetic excitations can be confirmed. By intercalation of alkali metal atoms (Na, K) the oxyhalides can be doped with electrons, which can be evidenced and even quantified using x-ray PES. In these experiments, also a particular vertical arrangement of dopants is observed, which can be explained, at least within experimental accuracy, using the model of a so-called "polar catastrophe". However, no transition into a metallic phase can be observed upon doping, but this can be understood qualitatively and quantitatively within an alloy Hubbard model due to the impurity potential of the dopants. Furthermore, in a canonical way a transfer of spectral weight can be observed, which is a characteristic feature of strongly correlated electron systems. Overall, it can be stated that the transition metal oxyhalides actually can be regarded as prototypical Mott insulators, yet with a rich phase diagram which is far from being fully understood.
This thesis is aimed at establishing modalities of time-resolved photoelectron spectroscopy (tr-PES) conducted at a free-electron laser (FEL) source and at a high harmonic generation (HHG) source for imaging the motion of atoms, charge and energy at photoexcited hybrid organic/inorganic interfaces. Transfer of charge and energy across interfaces lies at the heart of surface science and device physics and involves a complex interplay between the motion of electrons and atoms. At hybrid organic/inorganic interfaces involving planar molecules, such as pentacene and copper(II)-phthalocyanine (CuPc), atomic motions in out-of-plane direction are particularly apparent. Such hybrid interfaces are of importance to, e.g., next-generation functional devices, smart catalytic surfaces and molecular machines. In this work, two hybrid interfaces – pentacene atop Ag(110) and copper(II)-phthalocyanine (CuPc) atop titanium disulfide (1T-TiSe2) – are characterized by means of modalities of tr-PES. The experiments were conducted at a HHG source and at the FEL source FLASH at Deutsches Elektronen-Synchrotron DESY (Hamburg, Germany). Both sources provide photon pulses with temporal widths of ∼ 100 fs and thus allow for resolving the non-equilibrium dynamics at hybrid interfaces involving both electronic and atomic motion on their intrinsic time scales. While the photon energy at this HHG source is limited to the UV-range, photon energies can be tuned from the UV-range to the soft x-ray-range at FLASH. With this increased energy range, not only macroscopic electronic information can be accessed from the sample’s valence and conduction states, but also site-specific structural and chemical information encoded in the core-level signatures becomes accessible. Here, the combined information from the valence band and core-level dynamics is obtained by performing time- and angle-resolved photoelectron spectroscopy (tr-ARPES) in the UV-range and subsequently performing time-resolved x-ray photoelectron spectroscopy (tr-XPS) and time-resolved photoelectron diffraction (tr-XPD) in the soft x-ray regime in the same experimental setup. The sample’s bandstructure in energy-momentum space and time is captured by a time-of-flight momentum microscope with femtosecond temporal and sub-Ångström spatial resolutions. In the investigated systems, out-of-equilibrium dynamics are traced that are connected to the transfer of charge and energy across the hybrid interfaces. While energetic shifts and complementary population dynamics are observed for molecular and substrate states, the shapes of involved molecular orbitals change in energy-momentum space on a subpicosecond time scale. In combination with theory support, these changes are attributed to iiiatomic reorganizations at the interface and transient molecular structures are reconstructed with sub-Ångström precision. Unique to the material combination of CuPc/TiSe2, a structural rearrangement on the macroscopic scale is traced simultaneously: ∼ 60 % of the molecules undergo a concerted, unidirectional in-plane rotation. This surprising observation and its origin are detailed in this thesis and connected to a particularly efficient charge transfer across the CuPc/TiSe2 interface, resulting in a charging of ∼ 45 % of CuPc molecules.
This thesis examines the electronic properties of two materials that promise the realization and observation of novel exotic quantum phenomena. For this purpose, angle-resolved photoemission forms the experimental basis for the investigation of the electronic properties. Furthermore, the magnetic order is investigated utilizing X-ray dichroism measurements.
First, the bulk and surface electronic structure of epitaxially grown HgTe in its three-dimensional topological insulator phase is investigated. In this study, synchrotron radiation is used to address the three-dimensional band structure and orbital composition of the bulk states by employing photon-energy-dependent and polarization-dependent measurements, respectively. In addition, the topological surface state is examined on in situ grown samples using a laboratory photon source. The resulting data provide a means to experimentally localize the bulk band inversion in momentum space and to evidence the momentum-dependent change in the orbital character of the inverted bulk states.
Furthermore, a rather new series of van der Waals compounds, (MnBi\(_2\)Te\(_4\))(Bi\(_2\)Te\(_3\))\(_n\), is investigated. First, the magnetic properties of the first two members of the series, MnBi\(_2\)Te\(_4\) and MnBi\(_4\)Te\(_7\), are studied via X-ray absorption-based techniques. The topological surface state on the two terminations of MnBi\(_4\)Te\(_7\) is analyzed using circular dichroic, photon-energy-dependent, and spin-resolved photoemission. The topological state on the (MnBi\(_2\)Te\(_4\))-layer termination shows a free-standing Dirac cone with its Dirac point located in the bulk band gap. In contrast, on the (Bi\(_2\)Te\(_3\))-layer termination the surface state hybridizes with the bulk valences states, forming a spectral weight gap, and exhibits a Dirac point that is buried within the bulk continuum. Lastly, the lack of unambiguous evidence in the literature showing a temperature-dependent mass gap opening in these magnetic topological insulators is discussed through MnBi\(_2\)Te\(_4\).
Within the framework of this thesis the mechanisms of growth and reorganisation of surfaces within the first few layers were investigated that are the basis for the fabrication of high quality thin films and interfaces. Two model systems, PTCDA/Ag(111) and CdSe/ZnSe quantum dots (QD), were chosen to study such processes in detail and to demonstrate the power and improvements of the aberration corrected spectromicroscope SMART [1] simultaneously. The measurements benefit especially from the enhanced transmission of the microscope and also from its improved resolution. SMART, the first double–aberration corrected instrument of its kind [2], provided comprehensive methods (LEEM/PEEM, μ–LEED, μ–XPS) to study in–situ and in real time the surface reorganisation and to determine morphology, local structure and local chemical composition of the resulting thin film. Complementarily, a commercial AFM [3] was used ex–situ. XPEEM and μ–XPS measurements were made possible by attaching SMART to the high flux density beamline of the soft–X–ray source BESSY–II [4]. PTCDA/Ag(111) – Growth and structure of the first two layers Although PTCDA/Ag(111) is one of the most intensely studied model systems for the growth of organic semiconductor thin films, it still offers new insights into a complex growth behaviour. This study enlightens the temperature dependant influence of morphological features as small as monatomic Ag steps on the growth process of the first two layers. At low temperatures, single Ag steps act as diffusion barriers. But interdiffusion was observed already for the 2nd layer whereas domain boundaries in the 1st PTCDA–layer persist for crystallite growth in the 2nd layer. 1st layer islands are more compact and the more dendritic development of the 2nd layer indicates reduced interaction strength between 2nd and 1st layer. These findings were explained by a model consisting of structural and potential barriers. The second part of the PTCDA study reveals a variety of phases that appears only if at least two layers are deposited. Besides the six known rotational domains of the interface system PTCDA/Ag(111) [5], a further manifold of structures was discovered. It does not only show a surprising striped image contrast, but the 2nd layer also grows in an elongated way along these so–called ’ripples’. The latter show a rather large period and were found in a wide temperature range. Additionally the μ-LEED pattern of such a domain shows a new super–superstructure as well. This phase is explained by a structural model that introduces a rotated, more relaxed domain in the 2nd layer that does not exist in the first layer. Its structural parameters are similar to those of the bulk unitcells of PTCDA. The model is confirmed by the observation of two different rotational domains that grow on top of one single ’substrate’ domain in the 1st layer. The orientations of the ripple phases fit as well to the predictions of the model. The growth direction along the ripples corresponds to the short diagonal of the super–superstructure unitcell with diamond–like shape. CdSe/ZnSe – Inverse structuring by sublimation of an α-Te cap With the second model system the formation of CdSe quantum dots (QD) from strained epi-layers was investigated. In this case the structures do not form during deposition, but rather during sublimation of the so–called ‘ignition cap’. For these pilot experiments not only the process of QD formation itself was of interest, but also the portability of the preparation and the prevention of contaminations. It was found that the α-Se is well suited for capping and the last step of the QD preparation, the sublimation of the α-Te cap, needs a sufficiently high rate in rise of temperature. Subsequently the cap, the process of desorption and the final surface with the quantum structures were investigated in detail. The cap was deposited by the MBE-group in Würzburg as an amorphous Te layer but was found to contain a variety of structures. Holes, cracks, and micro–crystallites within an α-Te matrix were identified. Sublimation of the “ignition cap” was observed in real–time. Thus the discovered cap-structures could be correlated with the newly formed features as, e.g., QDs on the bare CdSe surface. Since CdSe/ZnSe QDs prefer to form in the neighbourhood of the Te μ–crystallites, Te was found to play a major role in their formation process. Different explanations as the impact of Te as a surfactant, an enhanced mobility of adatoms or as stressor nuclei are discussed. The spectromicroscopic characterisation of the CdSe surface with QDs revealed the crystallographic directions. An increased Cd signal of the film was found at positions of former holes. Several possibilities as segregation or surface termination are reviewed, that might explain this slight Cd variation. Therewith, an important step to a detailed understanding of the complex reorganisation process in coating systems could be achieved.
Carbonaceous aerosols are responsible for large uncertainties in climate models, degraded visibility, and adverse health effects. The Carbonaceous Aerosols and Radiative Effects Study (CARES) was designed to study carbonaceous aerosols in the natural environment of the Central Valley, California, and learn more about their atmospheric formation and aging. This paper presents results from spectro-microscopic measurements of carbonaceous particles collected during CARES at the time of a pollution accumulation event (27-29 June 2010), when in situ measurements indicated an increase in the organic carbon content of aerosols as the Sacramento urban plume aged. Computer-controlled scanning electron microscopy coupled with an energy dispersive X-ray detector (CCSEM/EDX) and scanning transmission X-ray microscopy coupled with near-edge X-ray absorption spectroscopy (STXM/NEXAFS) were used to probe the chemical composition and morphology of individual particles. It was found that the mass of organic carbon on individual particles increased through condensation of secondary organic aerosol. STXM/NEXAFS indicated that the number fraction of homogenous organic particles lacking inorganic inclusions (greater than similar to 50 nm equivalent circular diameter) increased with plume age, as did the organic mass per particle. Comparison of the CARES spectro-microscopic dataset with a similar dataset obtained in Mexico City during the MILAGRO campaign showed that fresh particles in Mexico City contained three times as much carbon as those sampled during CARES. The number fraction of soot particles at the Mexico City urban site (ranging from 16.6 to 47.3 %) was larger than at the CARES urban site (13.4-15.7%), and the most aged samples from CARES contained fewer carbon-carbon double bonds. Differences between carbonaceous particles in Mexico City and California result from different sources, photochemical conditions, gas phase reactants, and secondary organic aerosol precursors. The detailed results provided by these spectro-microscopic measurements will allow for a comprehensive evaluation of aerosol process models used in climate research.
The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.
The Berezinskii-Kosterlitz-Thouless (BKT) theorem predicts that two-dimensional bosonic condensates exhibit quasi-long-range order which is characterized by a slow decay of the spatial coherence. However previous measurements on exciton-polariton condensates revealed that their spatial coherence can decay faster than allowed under the BKT theory, and different theoretical explanations have already been proposed. Through theoretical and experimental study of exciton-polariton condensates, we show that the fast decay of the coherence can be explained through the simultaneous presence of multiple modes in the condensate.
Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate
(2019)
We describe a setup for time-resolved photoemission electron microscopy (TRPEEM) with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump–probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity (LEER) measurements. Finally, we demonstrate the in-situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.
In this thesis, soft x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) studies of the electronic structure of selected organic molecules and liquids were carried out. The first part focuses on the used experimental techniques and the development of the instrumentation necessary for these studies, namely a soft x-ray spectrometer, and a temperature-controlled flow-through liquid cell. The former was optimized by a special analytical ray tracing method developed exclusively for this purpose. Due to its high transmission, the spectrometer facilitates a novel experimental approach of recording comprehensive 'RIXS maps', which are 2-dimensional plots of x-ray scattering intensities as a function of both, excitation and emission photon energy. The liquid cell extends these possibilities to the study of liquids, especially the interaction of molecules in liquids and their chemical reactions under well-controlled conditions. Organic molecules have attracted considerable attention in the last decade. The intense research activities related to these materials have two main motivations: on the one hand, organic molecules have a technological application as building blocks of organic semiconductors, while, on the other hand, organic molecules are the functional elements in biological systems. In order to cost-effectively produce optimized organic electronic devices, a fundamental knowledge of the electronic properties of the organic molecules interface is necessary. Therefore, many studies of the electronic structure of potential candidates for organic electronics exist. Two of these candidates, namely C60 and well-ordered multilayers PTCDA on a Ag(111) surface are investigated in this thesis. For the study of C60 molecules, a comprehensive 'RIXS map' was recorded and analyzed. The RIXS map taken in only 25 minutes allows a quantitative analysis of energy losses, yielding for example the HOMO-LUMO distance. It also identifies a core-excitonic state and facilitates a quantitative comparison of its binding energy with that of valence excitons in C60. Furthermore, decay channel-selective partial fluorescence yield XAS spectra can be extracted from the RIXS map, yielding information on the population of the core-excitonic state as a function of excitation energy. As a second model system of organic molecules relevant for organic electronics, PTCDA was chosen. The complex electronic structure of the occupied states of a highly ordered, flat-lying PTCDA multilayer on a Ag(111) surface was investigated by symmetry-resolved resonant x-ray emission spectroscopy. The rapidly occurring beam damage effects were characterized on the basis of irradiation-time dependent series of C and O x-ray emission spectra. Upon varying the excitation energy and emission geometry, atom- and symmetry-specific carbon K emission spectra with negligible beam damage effects were obtained that allow to distinguish between electronic states with sigma and pi symmetry. A density functional theory calculation of the PTCDA molecule reproduces the energy positions of the most prominent emission features remarkably well. In addition, the energy positions of the sigma and pi emissions agree well with the calculated energies of the respective orbitals. In order to shed light on the second aspect of organic molecules, namely their role in biological systems, first a detailed investigation of the electronic structure and proton dynamics of liquid water as the medium of most chemical and biochemical reactions was carried out. Therefore, a comprehensive oxygen K RIXS map of liquid water was recorded and analyzed in great detail. A temperature-dependent comparison with XAS and RIXS data of D2O, NaOH, and NaOD leads to the conclusion, that ultra-fast dissociation takes place in liquid water on the timescale of the oxygen 1s core hole lifetime, resulting in a characteristic spectral contribution in the RIXS spectra. The dissociation is promoted by intact hydrogen bonds with neighboring molecules. In consequence, the rate of dissociation directly depends on the initial hydrogen bond configuration. In the next step towards biologically relevant systems, the nitrogen K edges of the amino acids glycine and histidine were investigated in powderous form as well as in their native environment, namely in aqueous solution. X-ray absorption and emission spectra of the aqueous solutions were analyzed at pH-values of 6 and for glycine also at pH 12 and compared to the spectra of powders. A pH-value of 12 causes deprotonation of the amino group, leading to significant changes in the nitrogen spectra as compared to pH 6. The results from these four examples demonstrate that a wealth of novel information can be obtained by using the new experimental tools developed in this thesis, namely a highly sensitive x-ray spectrometer and a flow-through liquid cell.
Soft x-ray spectroscopic study of methanol and glycine peptides in different physical environments
(2017)
Ion-specific effects occur in a huge variety of aqueous solutions of electrolytes and larger molecules like peptides, altering properties such as viscosity, enzyme activity, protein stability, and salting-in and salting-out behavior of proteins. Typically, these type of effects are rationalized in terms of the Hofmeister series, which originally orders cations and anions according to their ability to enhance or suppress the solubility of proteins in water. This empirical order, however, is still not understood yet. Quite some effort was made to gain a molecular level understanding of this phenomenon, yet no consensus has been found about the underlying mechanisms and the determination and localization of the interaction sites.
Resonant inelastic soft x-ray scattering (RIXS) combines x-ray emission (XES) and absorption spectroscopies (XAS), probing the partial local density of states of both occupied and unoccupied electronic states and is thus a promising candidate to shed more light onto the issue. The studies presented in this work are directed towards an improved understanding of the interaction between salts and peptides. In order to address this topic, the impact of different physical environments on the electronic structure of small molecules (i.e., methanol and glycine derived peptides) is investigated systematically using soft x-ray spectroscopic methods, corroborated with density functional theory (DFT) calculations.
In a first step, molecules without any interactions to the surrounding are investigated, using gas-phase methanol as a model system. Thereby, the local and element specific character of RIXS is demonstrated and used to separately probe the local electronic structure of methanol’s hydroxyl and methyl group, respectively. The attribution of the observed emission features to distinct molecular orbitals is confirmed by DFT calculations, which also quantitatively explain the different relative intensities of the emission features. For resonant excitation of the O K pre-edge absorption resonance, strong isotope effects are found that are explained by dynamical processes at the hydroxyl group. This serves as an excellent example for possible consequences of a local change in the geometric structure or symmetry of a molecule on its electronic structure.
In the following, the sample system is expanded to the amino acid glycine and its smallest derived peptides diglycine and triglycine. As a first step, they are studied in their crystalline form in solid state. Again, a comprehensive picture of the electronic structure is developed by measuring RIXS maps at the oxygen and nitrogen K absorption edge, corroborated by DFT calculations. Similar to the case of methanol, dynamic processes at the protonated amino group of the molecules after exciting the nitrogen atom have a strong influence on the emission spectra. Furthermore, it is shown that RIXS can be used to selectively excite the peptide nitrogen to probe the electronic structure around it. A simple building block approach for XES spectra is applied to separate the contribution of the emission attributed to transitions into core holes at the peptide and the amino nitrogen, respectively.
In the aqueous solution, the surrounding water molecules slightly change the electronic structure, probably via interactions with the charged functional groups. The effects on the x-ray emission spectra, however, are rather small. Much bigger changes are observed when manipulating the protonation state of the functional groups by adjusting the pH value of the solution. A protonation of the carboxyl group at low pH values, as well as a deprotonation of the amino group at high pH values lead to striking changes in the shape of the RIXS maps. In a comprehensive study of glycine’s XES spectra at varying pH values, changes in the local electronic structure are not only observed in the immediate surrounding of the manipulated functional groups but also in more distant moieties of the molecule.
Finally, the study is extended to mixed aqueous solutions of diglycine and a variety of different salts as examples for systems where Hofmeister effects are observed. To investigate the influence of different cations and anions on the electronic structure of diglycine, two series of chlorine and potassium salts are used. Ion-specific effects are identified for both cases. Some of the changes in the x-ray emission spectra of diglycine in the mixed solutions qualitatively follow the Hofmeister series as a function of the used salt. The observed trends thereby indicate an increased interaction between the electron density around the peptide oxygen with the cations, whereas anions seem to interact with the amino group of the peptide.
After the discovery of three-dimensional topological insulators (TIs), such as tetradymite chalcogenides Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ – a new class of quantum materials characterized by their unique surface electronic properties – the solid state community got focused on topological states that are driven by strong electronic correlations and magnetism. An important material class is the magnetic TI (MTI) exhibiting the quantum anomalous Hall (QAH) effect, i.e. a dissipationless quantized edge-state transport in the absence of external magnetic field, originating from the interplay between ferromagnetism and a topologically non-trivial band structure. The unprecedented opportunities offered by these new exotic materials open a new avenue for the development of low-dissipation electronics, spintronics, and quantum computation. However, the major concern with QAH effect is its extremely low onset temperature, limiting its practical application. To resolve this problem, a comprehensive understanding of the microscopic origin of the underlying ferromagnetism is necessary.
V- and Cr-doped (Bi,Sb)$_2$Te$_3$ are the two prototypical systems that have been widely studied as realizations of the QAH state. Finding microscopic differences between the strongly correlated V and Cr impurities would help finding a relevant model of ferromagnetic coupling and eventually provide better control of the QAH effect in these systems. Therefore, this thesis first focuses on the V- and Cr-doped (Bi,Sb)$_2$Te$_3$ systems, to better understand these differences. Exploiting the unique capabilities of x-ray absorption spectroscopy and magnetic circular dichroism (XAS/XMCD), combined with advanced modeling based on multiplet ligand-field theory (MLFT), we provide a detailed microscopic insight into the local electronic and magnetic properties of these systems and determine microscopic parameters crucial for the comparison with theoretical models, which include the $d$-shell filling, spin and orbital magnetic moments. We find a strongly covalent ground state, dominated by the superposition of one and two Te-ligand-hole configurations, with a negligible contribution from a purely ionic 3+ configuration. Our findings indicate the importance of the Te $5p$ states for the ferromagnetism in (Bi, Sb)$_2$Te$_3$ and favor magnetic coupling mechanisms involving $pd$-exchange. Using state-of-the-art density functional theory (DFT) calculations in combination with XMCD and resonant photoelectron spectroscopy (resPES), we reveal the important role of the $3d$ impurity states in mediating magnetic exchange coupling. Our calculations illustrate that the kind and strength of the exchange coupling varies with the impurity $3d$-shell occupation. We find a weakening of ferromagnetic properties upon the increase of doping concentration, as well as with the substitution of Bi at the Sb site. Finally, we qualitatively describe the origin of the induced magnetic moments at the Te and Sb sites in the host lattice and discuss their role in mediating a robust ferromagnetism based on a $pd$-exchange interaction scenario. Our findings reveal important clues to designing higher $T_{\text{C}}$ MTIs.
Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of TIs, which is favorable for the realization of the high-temperature QAH effect. Therefore, we have further focused on Eu-doped Bi$_2$Te$_3$ and scrutinized whether the conditions for formation of a substantial gap in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the $M_{4,5}$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu$^{2+}$ valence, unlike most other rare earth elements, and confirms a large magnetic moment. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic ordering. This is in good agreement with DFT results, which predict AFM interactions between the Eu impurities due to the direct overlap of the impurity wave functions. Our results support the notion of antiferromagnetism coexisting with topological surface states in rare-earth doped Bi$_2$Te$_3$ and corroborate the potential of such doping to result in an antiferromagnetic TI with exotic quantum properties.
The doping with impurities introduces disorder detrimental for the QAH effect, which may be avoided in stoichiometric, well-ordered magnetic compounds. In the last part of the thesis we have investigated the recently discovered intrinsic magnetic TI (IMTI) MnBi$_6$Te$_{10}$, where we have uncovered robust ferromagnetism with $T_{\text{C}} \approx 12$ K and connected its origin to the Mn/Bi intermixing. Our measurements reveal a magnetically intact surface with a large moment, and with FM properties similar to the bulk, which makes MnBi$_6$Te$_{10}$ a promising candidate for the QAH effect at elevated temperatures. Moreover, using an advanced ab initio MLFT approach we have determined the ground-state properties of Mn and revealed a predominant contribution of the $d^5$ configuration to the ground state, resulting in a $d$-shell electron occupation $n_d = 5.31$ and a large magnetic moment, in excellent agreement with our DFT calculations and the bulk magnetometry data. Our results together with first principle calculations based on the DFT-GGA$+U$, performed by our collaborators, suggest that carefully engineered intermixing plays a crucial role in achieving a robust long-range FM order and therefore could be the key for achieving enhanced QAH effect properties.
We expect our findings to aid better understanding of MTIs, which is essential to help increasing the temperature of the QAH effect, thus facilitating the realization of low-power electronics in the future.
This thesis focuses on the investigation of the electronic structure of amino acids and
salts in aqueous solution using X-ray spectroscopic methods. Both material groups are
of fundamental importance with regards to many physiological reactions, especially
for the Hofmeister effect which describes the solubility of proteins in salt solutions.
Hence, the investigation of the electronic structure of amino acids and the influence of
ions on the hydrogen bonding network of liquid water are important milestones to a
deeper understanding of the Hofmeister series.
Besides investigating the electronic structure of amino acids in aqueous solution,
the spectra were used to develop a building block model of the spectral fingerprints of
the functional groups and were compared to spectral signatures of suitable reference
molecules. In the framework of this thesis, it is shown that the building block approach
is a useful tool with allows the interpretation of spectral signatures of considerably
more complex molecules
In this work, the focus lies on the investigation of the occupied and unoccupied
electronic states of molecules in solid state, as well as in aqueous solution. Hereby,
different X-ray spectroscopic methods were applied. X-ray emission spectroscopy
(XES) was used to probe the occupied electronic structure of the solution, while the
unoccupied electronic structure was addressed by using X-ray absorption spectroscopy
(XAS). Finally, resonant inelastic X-ray scattering (RIXS) as a combination of XAS
and XES measurements provides the combined information about the unoccupied and
occupied molecular levels. The element specific character of the three measurement
methods is a feature which allows the investigation of the local electronic structure of
a single functional group. With RIXS, also non-equivalent atoms of the same element
can be addressed separately.
Within this thesis firstly, a library of the XE spectra of all 20 proteinogenic amino
acids in zwitterionic form is presented. From this sample-set XES fingerprints of
the protonated alpha-amino group NH3+ and the deprotonated carboxylic group COO- were evaluated and used to identify the XES fingerprints of the nitrogen and oxygen
containing functional groups of the side chains of the amino acids. The data is discussed
based on a building block approach. Furthermore, the XE spectra of the functional
groups of lysine and histidine, namely the NH2 group and the C3N2H4 ring structure,
are both compared to XE spectra of suitable reference molecules (imidazole, ammonia
and methylamine). It is found that the XE and RIXS spectra of the side chains of lysine
and histidine show large similarities to the XE spectra of the reference molecules. This
agreement in the XE and RIXS spectra allows a qualitative investigation of XE and
RIXS spectra of more complex amino acids using the XE and RIXS spectra of suitable
reference molecules.
The chemical structure of histidine and proline is quite different from the structures
of the other proteinogenic amino acids. Due to the unique chemical structure of
the side chain which in both cases consists of a heterocyclic ring structure, these two
amino acids were investigated in more detail. Zubavichus et al. [1] have shown that
amino acids are decomposing while exposed to X-ray radiation of the experiment. The
damage is irreversible and molecular fragments can adsorb on the membrane of the
experimental setup. This contamination can also create a spectral signature which
then overlaps with the signal of the solution and which complicates the interpretation
of the data. To record spectra which are free from contributions of adsorbed molecular
fragments on the membrane, the adsorption behavior was investigated.
In contrast to the solid phase in which the amino acids are present as salts in one
electronic conformation, the charge state of the amino acids can be manipulated in
aqueous solution by tuning the pH-value. By doing this, all possible charge states are
accessible (cation, anion, zwitterion). In this work it is shown that also the spectra
of the different charge states can be modeled by the spectra of suitable reference
molecules using the building block approach. The spectral changes occurring upon
protonation and deprotonation of the functional groups are explored and verified by
comparing them to theoretical calculations.
The comparison with measurements of pyrrolidine show that the electronic structure
which surrounds the nitrogen atom of proline is strongly influenced by the
ring structure of the side chain. Furthermore, the proline, pyrrolidine, and histidine
molecules are also degrading during the liquid sample measurements. This can be
observed by the detection of a new spectral component which increases with the
measurement time originating from the window membrane. In all cases, the speed of
the agglomeration of molecular fragments at the membrane was observed to be highly
sensitive to the pH value of the solution.
To understand the Hofmeister series, also the impact of the salt ions have to be
investigated. In this study the influence of potassium chloride (KCl) on the hydrogen
bond network of water was studied by using non-resonantly excited XES as well as
RIXS. A decreased dissociation of hydrogen molecules and changes in the molecular
vibrations could be detected. These changes were interpreted with a molecular
reorganization of the water molecules and a decreased number of hydrogen bonds.
Optical quantum information science and technologies require the capability to generate, control, and detect single or multiple quanta of light. The need to detect individual photons has motivated the development of a variety of novel and refined single-photon detectors (SPDs) with enhanced detector performance. Superconducting nanowire single-photon detectors (SNSPDs) and single-photon avalanche diodes (SPADs) are the top-performer in this field, but alternative promising and innovative devices are emerging. In this review article, we discuss the current state-of-the-art of one such alternative device capable of single-photon counting: the resonant tunneling diode (RTD) single-photon detector. Due to their peculiar photodetection mechanism and current-voltage characteristic with a region of negative differential conductance, RTD single-photon detectors provide, theoretically, several advantages over conventional SPDs, such as an inherently deadtime-free photon-number resolution at elevated temperatures, while offering low dark counts, a low timing jitter, and multiple photon detection modes. This review article brings together our previous studies and current experimental results. We focus on the current limitations of RTD-SPDs and provide detailed design and parameter variations to be potentially employed in next-generation RTD-SPD to improve the figure of merits of these alternative single-photon counting devices. The single-photon detection capability of RTDs without quantum dots is shown.