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Quantitative Electron Paramagnetic Resonance Studies of Charge Transfer in Organic Semiconductors
(2020)
In the present work we investigated various charge transfer processes, as they appear in the versatile world of organic semiconductors by probing the spin states of the corresponding charge carrier species via electron paramagnetic resonance (EPR) spectroscopy. All studied material systems are carbon-based compounds, either belonging to the group of polymers, fullerenes, or single-wall carbon nanotubes (SWNTs).
In the first instance, we addressed the change of the open circuit voltage (Voc) with the fullerene blend stoichiometry in fullerene-based solar cells for organic photovoltaics (OPV). The voltage depends strongly on the energy separation between the lowest unoccupied molecular orbital (LUMO) of the donor and the highest occupied molecular orbital (HOMO) of the acceptor. By exploiting the Gaussian distribution of the charge carriers in a two-level system, and thus also their spins in the EPR experiment, it could be shown that the LUMOs get closer by a few to a few hundred meV when going from pure fullerene materials to a fullerene mixture. The reason for this strong energetic effect is likely the formation of a fullerene alloy.
Further, we investigated the chemical doping mechanism of SWNTs with a (6,5)-chirality and their behaviour under optical excitation. In order to determine the unintentional (pre)-doping of SWNTs, EPR spectra of the raw material as well as after different purification steps were recorded. This facilitated the determination of nanotube defects and atmospheric p-doping as the causes of the measured EPR signals. In order to deliberately transfer additional charge carriers to the nanotubes, we added the redox-active substance AuCl3 where we determined an associated doping-yield of (1.5±0.2)%. In addition, a statistical occupation model was developed which can be used to simulate the distribution of EPR active, i.e. unpaired and localised charge carriers on the nanotubes.
Finally, we investigated the charge transfer behaviour of (6,5)-SWNTs together with the polymer P3HT and the fullerene PC60BM after optical excitation.
The control of quantum mechanical processes, especially the selective manipulation of photochemical reactions by shaped fs laser pulses was successfully demonstrated in many experiments in the fields of physics, chemistry and biology. In this work, attention is directed to the control of two systems that mark a bridge to real synthetic chemistry. In a liquid phase environment the outcome of the photo-induced Wolff rearrangement of an industrially relevant diazonaphthoquinone compound, normally used in photoresists (e.g. Novolak) was optimized using shaped fs laser pulses. In the second series of experiments chemical reactions on a catalyst metal surface which comprise laser induced molecular bond formation channels were selectively manipulated for the first time. The control of liquid phase reactions necessitates adequate spectroscopic signals that are characteristic for the formed product species. Therefore, a pump-probe setup for transient absorption spectroscopy in the mid-infrared for the purpose of investigating ultrafast structural changes of molecules during photoreactions was constructed. This versatile setup enables to monitor structural changes of molecules in the liquid phase and to find appropriate feedback signals for the control of these processes. Prior to quantum control experiments, the photoinduced Wolff-rearrangement reaction of 2-diazo-1-naphthoquinone (DNQ) dissolved in water and methanol was thoroughly investigated. Steady state absorption measurements in the mid-infrared in combination with quantum chemical density functional theory (DFT) calculations revealed the characteristic vibrational bands of DNQ and of possible products. A mid-infrared transient absorption study was performed, to illuminate the structural dynamics of the ultrafast rearrangement reaction of DNQ. The experimental observations indicate, that the Wolff rearrangement reaction of DNQ proceeds within 300 fs. A model for the relaxation dynamics of the ketene photoproduct and DNQ after photoexcitation can be deduced that fits the measured data very well. The object of the quantum control experiments on DNQ was the improvement of the ketene yield. It was shown that the ketene formation after Wolff rearrangement of DNQ is very sensitive to the shape of the applied excitation laser pulses. The variation of single parameters, like the linear chirp as well as the pulse separation of colored double pulses lead to the conclusion that the well known intrapulse dumping mechanism is responsible for the impact of the frequency ordering within the excitation pulse on the photoproduct yield. Adaptive optimizations using a closed learning loop basically lead to the same result. Adaptive fs quantum control was also applied to surface reactions on a catalyst metal surface for the first time. Therefore, the laser-induced catalytic reactions of carbon monoxide (CO) and hydrogen (H2) on a Pd(100) single crystal surface were studied. This photochemical reaction initiated with fs laser pulses has not been observed before. Several product molecules could be synthesized, among them also species (e.g. CH^3+) for whose formation three particles are involved. The systematic variation of different parameters showed that the reactions are sensitive to the catalyst surface, the composition of the adsorbate and to the laser properties. A pump-probe study revealed that they occur on an ultrafast time scale. These catalytic surface reactions were then investigated and improved with phaseshaped fs laser pulses. By applying a feedback optimal control scheme, the reaction outcome could be successfully manipulated and the ratio of different reaction channels could be selectively controlled. Evidence has been found that the underlying control mechanism is nontrivial and sensitive to the specific conditions on the surface. The experiments shown here represent the first successful experiment on adaptive fs quantum control of a chemical reaction between adsorbate molecules on a surface. In contrast to previous quantum control experiments, reaction channels comprising the formation of new molecular bonds rather than the cleavage of already existing bonds are controlled. This work successfully showed that quantum control can be extended to systems closer to situations encountered in synthetic chemistry as was demonstrated in the two examples of the optimization of a complicated rearrangement reaction and the selective formation of chemical bonds with shaped fs laser pulses.
Quantum point contacts (QPCs) are one-dimensional constrictions in an otherwise extended two-dimensional electron or hole system. Since their first realization in GaAs based two-dimensional electron gases, QPCs have become basic building blocks of mesoscopic physics and are used in manifold experimental contexts. A so far unrealized goal however is the implementation of QPCs in the new material class of two-dimensional topological insulators, which host the emergence of the so-called quantum spin Hall (QSH) effect. The latter is characterized by the formation of conducting one-dimensional spin-polarized states at the device edges, while the bulk is insulating. Consequently, an implemented QPC technology can be utilized to bring the QSH edge channels in close spatial proximity, thus for example enabling the study of interaction effects between the edge states. The thesis at hand describes the technological realization as well as the subsequent experimental characterization and analysis of QPCs in a QSH system for the first time.
After an introduction is given in Chapter 1, the subsequent Chapter 2 starts with discussing the peculiar band structure of HgTe. The emergence of the QSH phase for HgTe quantum wells with an inverted band structure is explained. For the band inversion to occur, the quantum wells have to exhibit a well thickness d_QW above a critical value (d_QW > d_c = 6.3 nm). Subsequently, the concept of QPCs is explicated and the corresponding transport behaviour is analytically described. Following the discussion of relevant constraints when realizing a QPC technology in a QSH system, a newly developed lithography process utilizing a multi-step wet etching technique for fabricating QPC devices based on HgTe quantum wells is presented. Transport measurements of exemplary devices show the expected conductance quantization in steps of ΔG ≈ 2e^2/h within the conduction band for a topological as well as for a trivial (d_QW < d_c) QPC. For the topological case, the residual conductance within the bulk band gap saturates at G_QSH ≈ 2e^2/h due to presence of the QSH state, while it drops to G ≈ 0 for the trivial device. Moreover, bias voltage dependent measurements of the differential conductance of an inverted sample provide explicit proof of the unperturbed coexistence of topological and trivial transport modes.
In a next step, Chapter 3 describes the emergence of a QSH interferometer state in narrow QPC devices with a quantum well thickness of d_QW = 7 nm. Presented band structure calculations reveal that the spatial extension of the QSH edge states depends on the position of the Fermi energy within the bulk band gap. As a consequence, reservoir electrons with randomized spin couple to both edge channels with the same probability under certain conditions, thus causing the formation of a QSH ring. A straightforward model capturing and specifying the occurrence of such a QSH interferometer is provided as well as substantiated by two experimental plausibility checks. After relevant quantum phases are theoretically introduced, the discussion of the obtained data reveals the accumulation of an Aharonov-Bohm phase, of a dynamical Aharonov-Casher phase as well as of a spin-orbit Berry phase of π in appropriate QPC devices. These results are consistent with analytic model considerations.
The last part of this thesis, Chapter 4, covers the observation of an unexpected conductance pattern for QPC samples fabricated from quantum wells with d_QW = 10.5 nm. In these devices, an anomalous plateau at G ≈ e^2/h = 0.5 x G_QSH emerges in addition to the QSH phase entailed residual conductance of G_QSH ≈ 2e^2/h. This so-called 0.5 anomaly occurs only for a specific interval of QPC width values, while it starts to get lost for too large sample widths. Furthermore, presented temperature and bias voltage dependent measurements insinuate that the emergence of the 0.5 anomaly is related to a gapped topological state. Additional characterization of this peculiar transport regime is provided by the realization of a novel device concept, which integrates a QPC within a standard Hall bar geometry. The results of the experimental analysis of such a sample link the occurrence of the 0.5 anomaly to a backscattered QSH channel. Thus, following a single particle perspective argumentation, it is reasoned that only one edge channel is transmitted in the context of the 0.5 anomaly. Two theoretic models possibly explaining the emergence of the 0.5 anomaly -- based on electron-electron interactions -- are discussed.
To conclude, the implementation of a working QPC technology in a QSH system represents a paramount development in the context of researching two-dimensional topological insulators and enables a multitude of future experiments. QPC devices realized in a QSH system are for example envisaged to allow for the detection of Majorana fermions and parafermions. Furthermore, the reported formation of a QSH interferometer state in appropriate QPC devices is of high interest. The observed dynamical Aharonov-Casher phase in the QSH regime enables a controllable modulation of the topological conductance, thus providing the conceptual basis for a topological transistor. Moreover, due to the resilience of geometric phases against dephasing, the presence of a spin-orbit Berry phase of π represents a promising perspective with regard to possible quantum computation concepts. Besides that, the transmission of only one QSH edge channel due to the emergence of the 0.5 anomaly is equivalent to 100 % spin polarization, which is an essential ingredient for realizing spintronic applications. Hence, the thesis at hand covers the experimental detection of three effects of fundamental importance in the context of developing new generations of logic devices -- based on QPCs fabricated from topological HgTe quantum wells.
In the present work, the energetic structure and coherence properties of the silicon vacancy point defect in the technologically important material silicon carbide are extensively studied by the optically detected magnetic resonance (ODMR) technique in order to verify its high potential for various quantum applications. In the spin vacancy, unique attributes are arising from the C3v symmetry and the spin-3/2 state, which are not fully described by the standard Hamiltonian of the uniaxial model. Therefore, an advanced Hamiltonian, describing well the appearing phenomena is established and the relevant parameters are experimentally determined. Utilizing these new accomplishments, several quantum metrology techniques are proposed.
First, a vector magnetometry scheme, utilizing the appearance of four ODMR lines, allows for simultaneous detection of the magnetic field strength and the tilting angle of the magnetic field from the symmetry axis of the crystal.
The second magnetometry protocol utilizes the appearance of energetic level anticrossings (LAC) in the ground state (GS) energy levels. Relying only on the change in photoluminescence in the vicinity of this GSLACs, this all-optical method does not require any radio waves and hence provides a much easier operation with less error sources as for the common magnetometry schemes utilizing quantum points.
A similar all-optical method is applied for temperature sensing, utilizing the thermal shift of the zero field splitting and consequently the anticrossing in the excited state (ES). Since the GSLACs show no dependence on temperature, the all-optical magnetometry and thermometry (utilizing the ESLACs) can be conducted subsequently on the same defect.
In order to quantify the achievable sensitivity of quantum metrology, as well as to prove the potential of the Si-vacancy in SiC for quantum processing, the coherence properties are investigated by the pulsed ODMR technique. The spin-lattice relaxation time T1 and the spin-spin relaxation time T2 are thoroughly analyzed for their dependence on the external magnetic field and temperature.
For actual sensing implementations, it is crucial to obtain the best signal-to-noise ratio without loss in coherence time. Therefore, the irradiation process, by which the defects are created in the crystal, plays a decisive role in the device performance. In the present work, samples irradiated with electrons or neutrons with different fluences and energies, producing different defect densities, are analyzed in regard to their T1 and T2 times at room temperature.
Last but not least, a scheme to substantially prolong the T2 coherence time by locking the spin polarization with the dynamic decoupling Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence is applied.
The presented thesis summarizes the results from four and a half years of intense lithography development on (Cd,Hg)Te/HgTe/(Cd,Hg)Te quantum well structures. The effort was motivated by the unique properties of this topological insulator. Previous work from Molenkamp at al.\ has proven that the transport through such a 2D TI is carried by electrons with opposite spin, counter-propagating in 1D channels along the sample edge. However, up to this thesis, the length of quantized spin Hall channels has never been reported to exceed 4 µm. Therefore, the main focus was put on a reproducible and easy-to-handle fabrication process that reveals the intrinsic material parameters.
Every single lithography step in macro as well as microscopic sample fabrication has been re-evaluated. In the Development, the process changes have been presented along SEM pictures, microgaphs and, whenever possible, measurement responses.
We have proven the conventional ion milling etch method to damage the remaining mesa and result in drastically lower electron mobilities in samples of microscopic size.
The novel KI:I2:HBr wet etch method for macro and microstructure mesa fabrication has been shown to leave the crystalline structure intact and result in unprecedented mobilities, as high as in macroscopic characterization Hall bars. Difficulties, such as an irregular etch start and slower etching of the conductive QW have been overcome by concentration, design and etch flow adaptations. In consideration of the diffusive regime, a frame around the EBL write field electrically decouples the structure mesa from the outside wafer. As the smallest structure, the frame is etched first and guarantees a non-different etching of the conductive layer during the redox reaction. A tube-pump method assures reproducible etch results with mesa heights below 300 nm. The PMMA etch mask is easy to strip and leaves a clean mesa with no redeposition. From the very first attempts, to the final etch process, the reader has been provided with the characteristics and design requirements necessary to enable the fabrication of nearly any mesa shape within an EBL write field of 200 µm.
Magneto resistance measurement of feed-back samples have been presented along the development chronology of wet etch method and subsequent lithography steps. With increasing feature quality, more and more physics has been revealed enabling detailed evaluation of smallest disturbances. The following lithography improvements have been implemented. They represent a tool-box for high quality macro and microstructure fabrication on (CdHg)Te/HgTe of almost any kind.
The optical positive resist ECI 3027 can be used as wet and as dry etch mask for structure sizes larger than 1 µm. It serves to etch mesa structures larger than the EBL write field.
The double layer PMMA is used for ohmic contact fabrication within the EBL write field. Its thickness allows to first dry etch the (Cd,Hg)Te cap layer and then evaporate the AuGe contact, in situ and self-aligned. Because of an undercut, up to 300 nm can be metalized without any sidewalls after the lift-off. An edge channel mismatch within the contact leads can be avoided, if the ohmic contacts are designed to reach close to the sample and beneath the later gate electrode.
The MIBK cleaning step prior to the gate application removes PMMA residuals and thereby improves gate and potential homogeneity.
The novel low HfO2-ALD process enables insulator growth into optical and EBL lift-off masks of any resolvable shape. Directly metalized after the insulator growth, the self-aligned method results in thin and homogeneous gate electrode reproducibly withholding gate voltages to +-10 V.
The optical negative resist ARN 4340 exhibits an undercut when developed. Usable as dry etch mask and lift-off resist, it enables an in-situ application of ohmic contacts first etching close to the QW, then metalizing AuGe. Up to 500 nm thickness, the undercut guarantees an a clean lift-off with no sidewalls.
The undertaken efforts have led to micro Hall bar measurements with Hall plateaus and SdH-oszillations in up to now unseen levels of detail.
The gap resistance of several micro Hall bars with a clear QSH signal have been presented in Quantum Spin Hall. The first to exhibit longitudinal resistances close to the expected h/2e2 since years, they reveal unprecedented details in features and characteristics. It has been shown that their protection against backscattering through time reversal symmetry is not as rigid as previously claimed. Values below and above 12.9 kΩ been explained, introducing backscattering within the Landauer-Büttiker formalism of edge channel transport. Possible reasons have been discussed. Kondo, interaction and Rashba-backscattering arising from density inhomogeneities close to the edge are most plausible to explain features on and deviations from a quantized value. Interaction, tunneling and dephasing mechanisms as well as puddle size, density of states and Rashba Fields are gate voltage dependent. Therefore, features in the QSH signal are fingerprints of the characteristic potential landscape.
Stable up to 11 K, two distinct but clear power laws have been found in the higher temperature dependence of the QSH in two samples. However, with ΔR = Tα, α = ¼ in one (QC0285) and α = 2 in the other (Q2745), none of the predicted dependencies could be confirmed. Whereas, the gap resistances of QC0285 remains QSH channel dominated up to 3.9 T and thereby confirmed the calculated lifting of the band inversion in magnetic field. The gate-dependent oscillating features in the QSH signal of Q2745 immediately increase in magnetic field. The distinct field dependencies allowed the assumption of two different dominant backscattering mechanisms.
Resulting in undisturbed magneto transport and unprecedented QSH measurements The Novel Micro Hall Bar Process has proven to enable the fabrication of a new generation of microstructures.
Ziel dieser Arbeit war die Herstellung individuell formbarer Strukturen mittels des 3D-Pulverdrucks auf Basis von bei Raumtemperatur hydraulisch abbindenden Knochenzementpulvern. Neben der Entwicklung neuartiger Zementformulierungen auf Basis von Magnesiumphosphaten war vor allem die gleichzeitige Ausstattung der Werkstoffe mit temperaturlabilen und bioaktiven Verbindungen ein wichtiger Entwicklungsschritt. Die Lokalisation der Wirkstoffe korreliert dabei mit entsprechenden Farbinformationen im Design der Konstrukte, die durch einen Mehrfarbendrucker physikalisch abgebildet werden. Das auf Calciumphosphat basierende System hat den Nachteil, dass die Abbindereaktion bei stark sauren pH-Werten abläuft, was negative Auswirkungen auf die gleichzeitige Ausstattung mit sensitiven Wirkstoffen hat. Zur Lösung dieser Problematik wurde ein neues Knochenzementpulver auf Magnesiumphosphatbasis entwickelt, welches unter neutralen pH-Bedingungen mit ammoniumhaltigem Binder zu dem Mineral Struvit abbindet. Das Zementpulver aus Trimagnesiumphosphat wurde bezüglich der pulvertechnologischen Eigenschaften, wie Partikelgröße, Partikelgrößenverteilung, Glättungseigenschaften und Schüttdichte sowie hinsichtlich des Abbindeverhaltens charakterisiert und für den Druckprozess optimiert. Die hohe Strukturgenauigkeit ermöglichte die Darstellung von makroporösen Strukturen mit einem minimalen Porendurchmesser von ca. 200 µm. Gute mechanische Kennwerte der gedruckten Strukturen, sowie eine hohe Umsetzungsrate zur gewünschten Phase Struvit wurden durch eine Nachhärtung in Ammoniumphosphatlösung erhalten. Die Druckfestigkeit betrug > 20 MPa und der Phasenanteil von Struvit konnte auf insgesamt 54 % gesteigert werden. Die Darstellung von wirkstoffmodifizierten Calciumphosphat- und Magnesiumphosphatstrukturen durch Verwendung eines Mehrfarbendruckers wurde beginnend vom Design der Strukturen bis hin zur experimentellen Bestimmung der Korrelation von Farbinformation und Binderapplikation etabliert. Zur Sicherstellung einer hohen Druckqualität und der Ortsständigkeit gedruckter Wirkstoffe erwies sich eine zusätzliche Modifikation des Tricalciumphosphatpulvers mit quellfähigen Polymeren (Hydroxypropylmethyl-cellulose (HPMC) bzw. Chitosan) als erfolgreich. Eine maximale Auflösung von ca. 400 µm konnte für eine HPMC/Chitosan/Calciumphosphat-Variante erreicht werden, während das hochreaktive Magnesiumphosphat/Magnesiumoxid-System eine Auflösung von 480 µm aufwies. Die Ortsständigkeit eingebrachter Lösungen war Voraussetzung für die Steuerung der Freisetzungskinetik. Das Freisetzungsverhalten in vitro wurde in Abhängigkeit von der Wirkstofflokalisation (homogen, Depot, Gradient) innerhalb der Matrix und unter Einbringung zusätzlicher polymerer Diffusionsbarrieren für den Wirkstoff Vancomycin untersucht. Dabei zeigte sich, dass die Modifikation der Matrices mit Polymeren zu einer verzögerten Freisetzung führte. Die lokale Wirkstoffmodifikation der Matrices in Form eines Depots oder Gradienten hatte Einfluss auf die Freisetzungskinetik, wobei eine lineare Freisetzung mit der Zeit (Kinetik 0. Ordnung) erreicht werden konnte. Die applizierten Wirkstoffe umfassten sowohl niedermolekulare Verbindungen, wie etwa das Antibiotikum Vancomycin oder das Polysaccharid Heparin, als auch proteinbasierte Faktoren wie den Knochenwachstumsfaktor rhBMP-2. Beurteilt wurde die pharmakologische Wirksamkeit der Verbindungen nach dem Druck, sowie nach der Freisetzung aus einer Calciumphosphatmatrix für den Wirkstoff Vancomycin. Es konnte belegt werden, dass die biologische Aktivität nach dem Druckprozess zu über 80 % erhalten blieb. Limitierend war der stark saure pH-Wert bei bruschitbasierten Systemen, der zu einer Inaktivierung des Proteins führte. Diesem Problem könnte durch die Nutzung des neutral abbindenden Magnesiumphosphatsystems entgegengewirkt werden. Abschließend erfolgten eine mikrostrukturelle Charakterisierung der Calciumphosphat- und Magnesiumphosphatmatrices mittels µ-CT-Analyse und Heliumpyknometrie, sowie eine quantitative Phasenanalyse nach Rietveld. Experimentell konnte nachgewiesen werden, dass mit Hilfe des 3D-Pulverdruck die Darstellung von Makroporen > 200 µm möglich ist. Die Analyse der Phasenzusammensetzung ergab, dass die Umsetzungsrate von Tricalciumphosphat und Trimagnesiumphosphat zu den gewünschten Phasen Bruschit und Struvit infolge des Nachhärtungsprozesses signifikant gesteigert werden konnte. Im Zuge dessen nahm die Porosität der gedruckten Matrices der Phase Struvit von 58 % auf 26 % und der Phase Bruschit von 47 % auf 38 % ab.
In the framework of this thesis, the structural and electronic properties of bismuth and lead deposited on Ag(111) have been investigated by means of low-temperature scanning tunneling microscopy (LT-STM) and spectroscopy (STS).
Prior to spectroscopic investigations the growth characteristics have been investigated by means of STM and low energy electron diffraction (LEED) measurements. Submonolayer coverages as well as thick films have been investigated for both systems.
Subsequently the quantum well characteristics of thick Pb films on Ag(111) have been analyzed and the quantum well character could be proved up to layer thicknesses of N ≈ 100 ML. The observed characteristics in STS spectra were explained by a simple cosine Taylor expansion and an in-plane energy dispersion could be detected by means of quasi-particle interferences.
The main part of this work investigates the giant Rashba-type spin-split surface alloys of
(√3 × √3)Pb/Ag(111)R30◦ and (√3 × √3)Bi/Ag(111)R30◦. With STS experiments the band positions and splitting strengths of the unoccupied (√3 × √3)Pb/Ag(111)R30◦ band
dispersions could be resolved, which were unclear so far. The investigation by means of quasi-particle interferences resulted in the observation of several scattering events, which could be assigned as intra- and inter-band transitions.
The analysis of scattering channels within a simple spin-conservation–approach turned out to be incomplete and led to contradictions between experiment and theory. In this framework more sophisticated DFT calculations could resolve the apparent deviations by a complete treatment of scattering in spin-orbit–coupled materials, which allows for
constructive interferences in spin-flip scattering processes as long as the total momentum J_
is conserved.
In a similar way the band dispersion of (√3 × √3)Bi/Ag(111)R30◦ was investigated. The
STS spectra confirmed a hybridization gap opening between both Rashba-split bands and several intra- and inter-band scattering events could be observed in the complete energy range. The analysis within a spin-conservation–approach again turned out to be insufficient for explaining the observed scattering events in spin-orbit–coupled materials, which was confi by DFT calculations. Within these calculations an inter-band scattering event that has been identified as spin-conserving in the simple model could be assigned as a spin-flip scattering channel. This illustrates evidently how an incomplete description can lead to completely different indications.
The present work shows that different spectroscopic STM modes are able to shed light on Rashba-split surface states. Whereas STS allowed to determine band onsets and splitting strengths, quasi-particle interferences could shed light on the band dispersions. A very important finding of this work is that spin-flip scattering events may result in constructive interferences, an eff which has so far been overlooked in related publications. Additionally it has been found that STM measurements can not distinguish between spin-conserving scattering events or spin-flip scattering events, which prevents to give a definite conclusion on the spin polarization for systems with mixed orbital symmetries just from the observed scattering events.
Quantencomputer können manche Probleme deutlich effizienter lösen als klassische Rechner. Bisherige Umsetzungen leiden jedoch an einer zu geringen Dekohärenzzeit, weshalb die Lebenszeit der Quantenzustände einen limitierenden Faktor darstellt. Topologisch geschützte Anregungen, wie Majorana-Fermionen, könnten hingegen dieses Hindernis überwinden. Diese lassen sich beispielsweise in topologischen Supraleitern realisieren. Bis zum jetzigen Zeitpunkt existieren nur wenige Materialien, die dieses Phänomen aufweisen. Daher ist das Verständnis der elektronischen Eigenschaften für solche Verbindungen von großer Bedeutung.
In dieser Dissertation wird die Koexistenz von Supraleitung an der Probenoberfläche und topologischem Oberflächenzustand (engl. topological surface state, TSS) auf potentiellen topologischen Supraleitern überprüft. Diese beiden Bedingungen sind essentiell zur Ausbildung von topologischer Supraleitung in zeitumkehrgeschützten Systemen. Hierzu wird mittels Landaulevelspektroskopie und Quasiteilcheninterferenz das Vorhandensein des TSS am Ferminiveau auf Tl$_{x}$Bi$_{2}$Te$_{3}$ und Nb$_{x}$Bi$_{2}$Se$_{3}$ verifiziert, die mittels Transportmessungen als supraleitend identifiziert wurden. Anschließend folgen hochaufgelöste Spektroskopien an der Fermienergie, um die supraleitenden Eigenschaften zu analysieren.
Zur Interpretation der analysierten Eigenschaften wird zu Beginn der Ni-haltige Schwere-Fermion-Supraleiter TlNi$_{2}$Se$_{2}$ untersucht, der eine vergleichbare Übergangstemperatur besitzt. Anhand diesem werden die gängigen Messmethoden der Rastertunnelmikroskopie und -spektroskopie für supraleitende Proben vorgestellt und die Leistungsfähigkeit der Messapparatur demonstriert. Im Einklang mit der Literatur zeigt sich ein $s$-Wellencharakter des Paarungsmechanismus sowie die Formation eines für Typ~II-Supraleiter typischen Abrikosov-Gitters in schwachen externen Magnetfeldern.
Im folgenden Teil werden die potentiellen topologischen Supraleiter Tl$_{x}$Bi$_{2}$Te$_{3}$ und Nb$_{x}$Bi$_{2}$Se$_{3}$ begutachtet, für die eindeutig ein TSS bestätigt wird. Allerdings weisen beide Materialien keine Oberflächensupraleitung auf, was vermutlich durch eine Entkopplung der Oberfläche vom Volumen durch Bandverbiegung zu erklären ist. Unbeabsichtigte Kollisionen der Spitze mit der Probe führen jedoch zu supraleitenden Spitzen, die wesentlich erhöhte Werte für die kritische Temperatur und das kritische Feld zeigen.
Der letzte Abschnitt widmet sich dem supraleitenden Substrat Nb(110), für den der Reinigungsprozess erläutert wird. Hierbei sind kurze Heizschritte bis nahe des Schmelzpunktes nötig, um die bei Umgebungsbedingungen entstehende Sauerstoffrekonstruktion effektiv zu entfernen. Des Weiteren werden die elektronischen Eigenschaften untersucht, die eine Oberflächenresonanz zum Vorschein bringen. Hochaufgelöste Messungen lassen eine durch die BCS-Theorie gut repräsentierte Struktur der supraleitenden Energielücke erkennen. Magnetfeldabhängige Experimente offenbaren zudem eine mit der Kristallstruktur vereinbare Anisotropie des Paarungspotentials. Mit diesen Erkenntnissen kann Nb(110) zukünftig als Ausgang für das Wachstum von topologischen Supraleitern herangezogen werden.
[...] Bei dem hier untersuchten multinären System CIGSe stellt sich ob seiner polykristallinen Struktur zudem die Frage nach der lateralen Homogenität der elektrischen Eigenschaften. Mit der verwendeten Meßmethode, einer photounterstützten Rastertunnelspektroskopie, können Inhomogenitäten in der Oberflächenphotospannung (SPV) und im Photoinduzierten Tunnelstrom (PITC) nachgewiesen werden. Die Messung von PITC und SPV ist dann schnell durchzuführen und damit für Reihenuntersuchungen geeignet, wenn Modulationsverfahren verwendet werden. Modulationen der Biasspannung und/oder der Beleuchtung wurden in der Tunnelspektroskopie bereits auf eine ganze Anzahl von Materialsystemen angewendet. Dabei auftretende, über die Kapazität zwischen Tunnelspitze und Probe einkoppelnde störende Signalbeiträge sind ein bekanntes Problem. Eine mögliche Lösung bietet die elektronische Kompensation durch eine entsprechende Schaltung. Wie in dieser Arbeit gezeigt wird, ist der Ansatz sehr gut geeignet, die durch Biasmodulation erzeugte Streukomponente zu unterdrücken. Wird dagegen die einfallende Beleuchtung moduliert, erfolgt die Kompensation nur unvollständig. Ein besonderes Problem bereitet dies, wenn beide Modulationen kombiniert werden. Der Unterschied zwischen beiden Modulationen liegt darin, daß sich das Spitze-Probe-System im Fall der Spannungsmodulation wie ein klassischer Kondensator verhält und das Streusignal daher unabhängig von der Art der Probe ist. Bei Lichtmodulation ist im Ersatzschaltbild dagegen die unter der Probenoberfläche befindliche Stromquelle zu berücksichtigen. Sie führt dazu, daß sich das Streusignal von Probe zu Probe, und sogar von einem Präparationszustand zum nächsten, deutlich unterscheidet. Daher ist es angebracht, das Streusignal separat zu messen und anschließend analytisch zu kompensieren. Wie aus der vorliegenden Arbeit hervorgeht, ist dabei die Abhängigkeit des Streusignals vom Spitze-Probe-Abstand unbedingt zu berücksichtigen. Nach der Etablierung und eingehenden Analyse des Verfahrens im ersten Teil folgt im zweiten Teil der Arbeit dessen Anwendung auf eine Reihe von unterschiedlichen CIGS-Proben. Dabei wird deutlich, daß die bereits angesprochenen Inhomogenitäten im PITC-Signal eine immanente Eigenschaft dieser (und vermutlich aller) polykristallinen Halbleitersysteme sind. Neben den lateralen Unterschieden in der Stromamplitude lassen sich auch Inhomogenitäten in der komplexen Phase des Photostroms nachweisen. Wie sich herausstellt, sind daraus aber wegen der dominierenden Admittanz der Tunnellücke keine Rückschlüsse auf die beteiligte Kapazität der RLZ zu ziehen. Dagegen ist es möglich, durch die Untersuchung einer größeren Zahl von Stellen auf einer Probe eine Statistik der Flächenhäufigkeit des PITC zu erstellen. Wird diese Verteilung durch eine exponentiell abfallende Häufigkeit beschrieben, weist dies auf eine übergroße Dichte an "schwachen" Dioden hin; bei einer kleinen Zahl schwacher Dioden zeigt die Verteilung ein deutliches Maximum bei höheren Photoströmen. Korngrenzen sind für die elektronischen Eigenschaften polykristalliner Systeme wichtig, ihre Struktur allerdings unbekannt. Aus dem Forschungsgebiet der ebenfalls polykristallinen CdS/CdTe-Solarzellen kommt die Vorstellung, daß die Korngrenzen bevorzugte Transportpfade der Ladungsträger darstellen; sie wird inzwischen auch für CIGS-Zellen diskutiert. Hunderte von untersuchten Probenstellen können diese Theorie jedoch nicht unterstützen. Nur in einer äußerst geringen Zahl von Fällen zeigen Korngrenzen einen deutlich höheren Photostrom im Vergleich zu den umgebenden Kornflächen. Desweiteren werden die abrupten lateralen Änderungen im PITC-Signal als nicht passivierte Korngrenzen interpretiert, die Transportbarrieren für die Minoritätsladungsträger bilden. Umgekehrt begünstigen passivierte Korngrenzen das Angleichen der elektronischen Eigenschaften benachbarter Körner. Verfolgt man die PITC-Werte über einen längeren Zeitraum hinweg, lassen sich metastabile Effekte beobachten. Das Abklingen des Photostroms wird durch den Einfang von Minoritätsladungsträgern in tiefen Störstellen erklärt. Vergleicht man die erhaltenen PITC-Werte mit dem makroskopischen Kurzschlußstrom der Zellen, kann man die erhoffte Korrelation nicht nachweisen. Wie sich herausstellt, haben die zur Vorbereitung für die STM-Messungen nötigen Präparationsschritte starke Auswirkung auf die Meßergebnisse. Aus dieser Sicht wäre eine in-situ-Messung wünschenswert. Daher schließen einige Gedanken hinsichtlich der Realisierung der Meßmethode zur in-situ-Qualitätskontrolle in der Solarzellenherstellung die Arbeit ab.
Nuclear spins in motion is an intrinsic component of any dynamic process when studied using magnetic resonance imaging (MRI). Moving spins define many functional characteristics of the human body such as diffusion, perfusion and blood flow. Quantitative MRI of moving spins
can provide valuable information about the human physiology or of a technical system. In particular, phase-contrast MRI, which is based on two images with and without a flow-encoding gradient, has emerged as an important diagnostic tool in medicine to quantify human blood flow. Unfortunately, however, its clinical usage is hampered by long acquisition times which only provide mean data averaged across multiple cardiac cycles and therefore preclude Monitoring the immediate physiological responses to stress or exercise. These limitations are expected to be overcome by real-time imaging which constitutes a primary aim of this thesis.
Short image acquisition times, as the core for real-time phase-contrast MRI, can be mainly realized through undersampling of the acquired data. Therefore the development focused on related technical aspects such as pulse sequence design, k-space encoding schemes and image
reconstruction. A radial encoding scheme was experimentally found to be robust to motion and
less sensitive to undersampling than Cartesian encoding. Radial encoding was combined with a FLASH acquisition technique for building an efficient real-time phase-contrast MRI sequence.
The sequence was further optimized through overlapping of gradients to achieve the shortest possible echo time. Regularized nonlinear inverse reconstruction (NLINV), a technique which jointly estimates the image content and its corresponding coil sensitivities, was used for image
reconstruction. NLINV was adapted specifically for phase-contrast MRI to produce both Magnitude images and phase-contrast maps. Real-time phase-contrast MRI therefore combined two highly undersampled (up to a factor of 30) radial gradient-echo acquisitions with and without a
flow-encoding gradient with modified NLINV reconstructions. The developed method achieved
real-time phase-contrast MRI at both high spatial (1.3 mm) and temporal resolution (40 ms).
Applications to healthy human subjects as well as preliminary studies of patients demonstrated
real-time phase-contrast MRI to offer improved patient compliance (e.g., free breathing) and immediate access to physiological variations of flow parameters (e.g., response to enhanced intrathoracic pressure). In most cases, quantitative blood flow was measured in the ascending aorta as an important blood vessel of the cardiovascular circulation system commonly studied
in the clinic. The performance of real-time phase-contrast MRI was validated in comparison to standard Cine phase-contrast MRI using studies of flow phantoms as well as under in vivo conditions. The evaluations confirmed good agreement for comparable results.
As a further extension to real-time phase-contrast MRI, this thesis implemented and explored a dual-echo phase-contrast MRI method which employs two sequential gradient echoes with and without flow encoding. The introduction of a flow-encoding gradient in between the two echoes
aids in the further reduction of acquisition time. Although this technique was efficient under in vitro conditions, in vivo studies showed the influence of additional motion-induced Phase contributions. Due to these additional temporal phase information, the approach showed Little promise for quantitative flow MRI.
As a further method three-dimensional real-time phase-contrast MRI was developed in this thesis to visualize and quantify multi-directional flow at about twice the measuring time of the standard real-time MRI method, i.e. at about 100 ms temporal resolution. This was achieved
through velocity mapping along all three physical gradient directions. Although the method is still too slow to adequately cover cardiovascular blood flow, the preliminary results were found to be promising for future applications in tissues and organ systems outside the heart. Finally, future developments are expected to benefit from the adaptation of model-based reconstruction
techniques to real-time phase-contrast MRI.