TY - THES A1 - Constantino, Jennifer Anne T1 - Characterization of Novel Magnetic Materials: Ultra-Thin (Ga,Mn)As and Epitaxial-Growth MnSi Thin Films T1 - Charakterisierung von neuen magnetischen Materialien: ultradünne (Ga,Mn)As und epitaktisch gewachsene MnSi Dünnschichten N2 - The study of magnetic phases in spintronic materials is crucial to both our fundamental understanding of magnetic interactions and for finding new effects for future applications. In this thesis, we study the basic electrical and magnetic transport properties of both epitaxially-grown MnSi thin films, a helimagnetic metal only starting to be developed within our group, and parabolic-doped ultra-thin (Ga,Mn)As layers for future studies and applications. N2 - Um einerseits ein fundamentales Verständnis magnetischer Wechselwirkungen zu erhalten und andererseits neue Effekte für zukünftige Anwendungen zu finden, ist es entscheidend, magnetische Phasen spintronischer Materialien zu untersuchen. In dieser Arbeit fokussieren wir uns auf grundlegende elektrische und magnetische Transporteigenschaften zweier Materialsysteme. Das sind zum Ersten ultradünne (Ga,Mn)As Filme mit parabolischen Dotierprofilen, und zum Zweiten epitaktisch gewachsene Dünnschichten aus MnSi, einem helimagnetischen Metal, dessen Entwicklung seit Kurzem in unserer Gruppe vorangetrieben wird. KW - Galliumarsenid KW - Manganarsenide KW - Dünne Schicht KW - Magnetischer Halbleiter KW - Helimagnetism KW - Manganese Silicide KW - Gallium Manganese Arsenide KW - Ferromagnetic Semiconductors KW - Magnetic Anisotropy KW - Mangansilicide KW - Magnetotransport KW - Mangan KW - Halbleiter KW - Herstellung Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-90578 ER - TY - JOUR A1 - Kasprzak, J. A1 - Sivalertporn, K. A1 - Albert, F. A1 - Schneider, C. A1 - Höfling, S. A1 - Kamp, M. A1 - Forchel, A. A1 - Muljarov, E. A. A1 - Langbein, W. T1 - Coherence dynamics and quantum-to-classical crossover in an exciton-cavity system in the quantum strong coupling regime JF - New Journal of Physics N2 - Interaction between light and matter generates optical nonlinearities, which are particularly pronounced in the quantum strong coupling regime. When a single bosonic mode couples to a single fermionic mode, a Jaynes-Cummings (JC) ladder is formed, which we realize here using cavity photons and quantum dot excitons. We measure and model the coherent anharmonic response of this strongly coupled exciton-cavity system at resonance. Injecting two photons into the cavity, we demonstrate a \(\sqrt 2\) larger polariton splitting with respect to the vacuum Rabi splitting. This is achieved using coherent nonlinear spectroscopy, specifically four-wave mixing, where the coherence between the ground state and the first (second) rung of the JC ladder can be interrogated for positive (negative) delays. With increasing excitation intensity and thus rising average number of injected photons, we observe spectral signatures of the quantum-to-classical crossover of the strong coupling regime. KW - Jaynes-Cummings ladder KW - spectral interferometry KW - photon KW - dot KW - spectroscopy KW - oscillations KW - microcavity KW - resonance KW - light Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-123005 SN - 1367-2630 VL - 15 IS - 045013 ER - TY - JOUR A1 - Thierschmann, H. A1 - Henke, M. A1 - Knorr, J. A1 - Maier, L. A1 - Heyn, C. A1 - Hansen, W. A1 - Buhmann, H. A1 - Molenkamp, L. W. T1 - Diffusion thermopower of a serial double quantum dot JF - New Journal of Physics N2 - We have experimentally studied the diffusion thermopower of a serial double quantum dot, defined electrostatically in a GaAs/AlGaAs heterostructure. We present the thermopower stability diagram for a temperature difference 1T = (20±10)mK across the device and find a maximum thermovoltage signal of several μV in the vicinity of the triple points. Along a constant energy axis in this regime, the data show a characteristic pattern which is in agreement with Mott’s relation and can be well understood within a model of sequential transport. KW - quantum dot Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129714 VL - 15 IS - 123010 ER - TY - THES A1 - Sperlich, Andreas T1 - Electron Paramagnetic Resonance Spectroscopy of Conjugated Polymers and Fullerenes for Organic Photovoltaics T1 - Elektron-Paramagnetische Resonanz-Spektroskopie von konjugierten Polymeren und Fullerenen für die organische Photovoltaik N2 - In the presented thesis, the various excited states encountered in conjugated organic semiconductors are investigated with respect to their utilization in organic thin-film solar cells. Most of these states are spin-baring and can therefore be addressed by means of magnetic resonance spectroscopy. The primary singlet excitation (spin 0), as well as positive and negative polaronic charge carriers (spin 1/2) are discussed. Additionally, triplet excitons (spin 1) and charge transfer complexes are examined, focussing on their differing spin-spin interaction strength. For the investigation of these spin-baring states especially methods of electron paramagnetic resonance (EPR) are best suited. Therefore according experimental methods were implemented in the course of this work to study conjugated polymers, fullerenes and their blends with continuous wave as well as time-resolved EPR and optically detected magnetic resonance. N2 - Die vorliegende Arbeit beschäftigt sich mit den vielfältigen Anregungszuständen in konjugierten organischen Halbleitern mit Hinblick auf deren Verwendung in organischen Dünnschicht-Solarzellen. Diese verschiedenen Zustände sind zumeist Spin-behaftet und daher mit Methoden der Magnetresonanz adressierbar. Es wird unterschieden zwischen Singulett-Exzitonen (Spin 0) als primärer Photoanregung, sowie positiven und negativen polaronischen Ladungsträgern (Spin 1/2). Des Weiteren werden Triplet-Exzitonen (Spin 1) und Ladungstransferkomplexe behandelt, die sich durch unterschiedlich starke Spin-Spin Wechselwirkung auszeichnen. Zur Untersuchung dieser Spin-behafteten Zustände bieten sich insbesondere Methoden der Elektron-Paramagnetischen Resonanz-Spektroskopie (EPR) an. Im Zuge dieser Arbeit wurden dafür entsprechede Messmethoden der Dauerstrich (cw) EPR, zeitaufgelösten, transienten EPR und der optisch detektierten Magnetresonanz (ODMR) implementiert und zur Erforschung von konjugierten Polymeren, Fullerenen und deren Mischungen eingesetzt. KW - Organische Solarzelle KW - Fotovoltaik KW - Organischer Halbleiter KW - organische Photovoltaik KW - organische Halbleiter KW - organic photovoltaic KW - organic solar cells KW - Elektronenspinresonanz KW - Fullerene KW - Konjugierte Polymere KW - Photovoltaik KW - Spektroskopie Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-81244 ER - TY - THES A1 - Scholz, Markus T1 - Energy-Dispersive NEXAFS: A Novel Tool for the Investigation of Intermolecular Interaction and Structural Phase Dynamics T1 - Energiedispersives NEXAFS: Eine neue Methode zur Untersuchung intermolekularer Wechselwirkung und Phasendynamik N2 - In the context of this thesis, the novel method soft X-ray energy-dispersive NEXAFS spectroscopy was explored and utilized to investigate intermolecular coupling and post-growth processes with a temporal resolution of seconds. 1,4,5,8- naphthalene tetracarboxylic acid dianhydride (NTCDA)multilayer films were the chosen model system for these investigations. The core hole-electron correlation in coherently coupled molecules was studied by means of energy-dispersive near-edge X-ray absorption fine-structure spectroscopy. A transient phase was found which exists during the transition between a disordered condensed phase and the bulk structure. This phase is characterized by distinct changes in the spectral line shape and energetic position of the X-ray absorption signal at the C K-edge. The findings were explained with the help of theoretical models based on the coupling of transition dipole moments, which are well established for optically excited systems. In consequence, the experimental results provides evidence for a core hole-electron pair delocalized over several molecules. Furthermore, the structure formation of NTCDA multilayer films on Ag(111) surfaces was investigated. With time-resolved and energy-dispersive NEXAFS experiments the intensity evolution in s- and p-polarization showed a very characteristic behavior. By combining these findings with the results of time-dependent photoemission measurements, several sub-processes were identified in the post- growth behavior. Upon annealing, the amorphous but preferentially flat-lying molecules flip into an upright orientation. After that follows a phase characterized by strong intermolecular coupling. Finally, three-dimensional islands are established. Employing the Kolmogorov-Johnson-Mehl-Avrami model, the activation energies of the sub-processes were determined. N2 - Im Rahmen dieser Arbeit wurden die Möglichkeiten der neuartigen Methode energiedispersive Röntgen-Nahkanten-Absorptions-Spektroskopie für die Untersuchung intermolekularer Wechselwirkungen und zeitabhängiger Prozesse während der Strukturbildung aufgezeigt. Als Modellsystem wurden hierbei 1,4,5,8- Naphthalin-Tetracarboxyls¨aure-Dianhydrid-(NTCDA-) Filme verwendet. Es wurde die Rumpfloch-Elektronen-Wechselwirkung kohärent gekoppelter Moleküle mittels energiedispersiver Röntgen-Nahkanten-Absorptions-Spektroskopie untersucht. Dabei wurde eine Übergangsphase gefunden, die während der Ausbildung einer langreichweitigen Ordnung zeitlich zwischen der ungeordneten und der Volumenstruktur auftritt. Diese Übergangsphase zeichnete sich durch eine charakteristische Änderung der spektralen Linienform und ihrer energetischen Position bei Messungen an der C K-Kante aus. Die experimentellen Befunde lassen sich mit Hilfe theoretischer Modelle erklären, welche die Kopplung von Übergangsdipolmomenten beschreiben. Diese theoretischen Konzepte sind bei optisch angeregten Systemen etabliert. Die experimentellen Ergebnisse zeigen den über mehrere Moleküle delokalisierten Charakter des Rumpfloch-Elektron-Paars. Zudem wurde die Strukturbildung von mehrlagigen NTCDA-Filmen auf Ag(111) untersucht. Zeitabhängige energiedispersive NEXAFS-Experimente mit s- und p-polarisiertem Licht zeigten ein charakteristisches Verhalten. In Kombination mit zeitabhängigen Photoemissionsmessungen wurden bei der Strukturbildung verschiedene Unterprozesse gefunden. Nach erwärmen der Probe richten sich die ursprünglich flach orientierte Moleküle zunächst auf. Im Anschluss folgt eine Aggregation der Moleküle in einer Phase mit starker intermolekularer Kopplung. Letztendlich bildet sich die bekannte dreidimensionale Filmstruktur aus. Anhand des Kolmogorov-Johnson-Mehl-Avrami Modells konnte die Aktivierungsenergie für die verschiedenen Unterprozesse ermittelt werden. KW - Organisches Molekül KW - NEXAFS KW - Zwischenmolekulare Kraft KW - Dünne Schicht KW - Energiedispersiv KW - Phasendynamik KW - Strukturbildung KW - NEXAFS KW - energy-dispersive KW - organic molecule KW - intermolecular interaction KW - phase dynamics Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-83839 ER - TY - JOUR A1 - Bentmann, Hendrik A1 - Reinert, Friedrich T1 - Enhancing and reducing the Rashba-splitting at surfaces by adsorbates: Na and Xe on Bi/Cu(111) JF - New Journal of Physics N2 - The surface alloy Bi/Cu(111) shows a paradigmatic free-electron-like surface state with a very large Rashba-type spin–orbit splitting. Using angle-resolved photoemission we investigate how adsorbates of different chemical nature influence the size of the spin splitting in this system. We find that the adsorption of small amounts of monovalent Na atoms leads to an enhancement of the spin splitting while an overlayer of the closed-shell rare gas Xe causes a reduction. The latter result is in contrast to the Au(111) surface for which an increased splitting size after Xe-adsorption was observed. We discuss these experimental findings in terms of the characteristic differences of the surface state wave functions and their spatial deformation in the presence of different types of adsorbates. Our results provide insight into the complex interplay of atomic and interface potential gradients governing the Rashba effect. KW - Rashba-Splitting Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129722 VL - 15 IS - 115011 ER - TY - JOUR A1 - Okada, Michio A1 - Rotenberg, Eli A1 - Kevan, S. D. A1 - Schäfer, J. A1 - Ujfalussy, Balazs A1 - Stocks, G. Malcolm A1 - Genatempo, B. A1 - Bruno, E. A1 - Plummer, E. W. T1 - Evolution of the electronic structure in \(Mo_{1-x}Re_x\) alloys JF - New Journal of Physics N2 - We report a detailed experimental and theoretical study of the electronic structure of \(Mo_{1-x}Re_x\) random alloys. We have measured electronic band dispersions for clean and hydrogen-covered \(Mo_{1-x}Re_x\) ( 110) with x = 0-0.25 using angle-resolved photoemission spectroscopy. Our results suggest that the bulk and most surface electronic bands shift relative to the Fermi level systematically and approximately rigidly with Re concentration. We distinguish and quantify two contributions to these shifts: a raise of the Fermi energy and an increase of the overall bandwidth. Alloy bands calculated using the first-principles Korringa-Kohn-Rostoker coherent-potential-approximation method accurately predict both of these effects. As derived from the rigid band model, the Fermi energy shift is inversely related to the bulk density of states in this energy region. Using our results, we also characterize an electronic topological transition of the bulk Fermi surface and relate this to bulk transport properties. Finally, we distinguish effects beyond the rigid band approximation: a highly surface-localized state and a composition-dependent impact of the spin-orbit interaction. KW - topological transitions KW - surface state KW - metals KW - total energy KW - W(110) KW - hydrogen KW - mo KW - superconductivity KW - spectra KW - coherent potential approximation Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-122993 SN - 1367-2630 VL - 15 IS - 093010 ER - TY - JOUR A1 - Kim, N. Y. A1 - Kusudo, K. A1 - Löffler, A. A1 - Höfling, S. A1 - Forchel, A. A1 - Yamamoto, Y. T1 - Exciton-polariton condensates near the Dirac point in a triangular lattice JF - New Journal of Physics N2 - Dirac particles, massless relativistic entities, obey linear energy dispersions and hold important implications in particle physics. The recent discovery of Dirac fermions in condensed matter systems including graphene and topological insulators has generated a great deal of interest in exploring the relativistic properties associated with Dirac physics in solid-state materials. In addition, there are stimulating research activities to engineer Dirac particles, elucidating their exotic physical properties in a controllable setting. One of the successful platforms is the ultracold atom-optical lattice system, whose dynamics can be manipulated and probed in a clean environment. A microcavity exciton-polariton-lattice system offers the advantage of forming high-orbital condensation in non-equilibrium conditions, which enables one to explore novel quantum orbital order in two dimensions. In this paper, we experimentally construct the band structures near Dirac points, the vertices of the first hexagonal Brillouin zone with exciton-polariton condensates trapped in a triangular lattice. Due to the finite spectral linewidth, the direct map of band structures at Dirac points is elusive; however, we identify the linear part above Dirac points and its associated velocity value is similar to ~0.9-2 x \(10^8 cm s^{-1}\), consistent with the theoretical estimate \(1 x 10^8 cm s^{-1}\) with a \(2 \mu m\) lattice constant. We envision that the exciton-polariton condensates in lattices would be a promising solid-state platform, where the system order parameter can be accessed in both real and momentum spaces. KW - Bose-Einstein condensation KW - carbon nanotubes KW - graphene KW - electron KW - dynamics KW - fermions KW - trap KW - gas Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-123103 SN - 1367-2630 VL - 15 IS - 035032 ER - TY - JOUR A1 - Winter, Patrick A1 - Kampf, Thomas A1 - Helluy, Xavier A1 - Gutjahr, Fabian T. A1 - Meyer, Cord B. A1 - Rommel, Eberhard A1 - Bauer, Wolfgang R. A1 - Jakob, Peter M. A1 - Herold, Volker T1 - Fast retrospectively triggered local pulse-wave velocity measurements in mice with CMR-microscopy using a radial trajectory JF - Journal of Cardiovascular Magnetic Resonance N2 - Background The aortic pulse-wave velocity (PWV) is an important indicator of cardiovascular risk. In recent studies MRI methods have been developed to measure this parameter noninvasively in mice. Present techniques require additional hardware for cardiac and respiratory gating. In this work a robust self-gated measurement of the local PWV in mice without the need of triggering probes is proposed. Methods The local PWV of 6-months-old wild-type C57BL/6J mice (n=6) was measured in the abdominal aorta with a retrospectively triggered radial Phase Contrast (PC) MR sequence using the flow-area (QA) method. A navigator signal was extracted from the CMR data of highly asymmetric radial projections with short repetition time (TR=3 ms) and post-processed with high-pass and low-pass filters for retrospective cardiac and respiratory gating. The self-gating signal was used for a reconstruction of high-resolution Cine frames of the aortic motion. To assess the local PWV the volume flow Q and the cross-sectional area A of the aorta were determined. The results were compared with the values measured with a triggered Cartesian and an undersampled triggered radial PC-Cine sequence. Results In all examined animals a self-gating signal could be extracted and used for retrospective breath-gating and PC-Cine reconstruction. With the non-triggered measurement PWV values of 2.3±0.2 m/s were determined. These values are in agreement with those measured with the triggered Cartesian (2.4±0.2 m/s) and the triggered radial (2.3±0.2 m/s) measurement. Due to the strong robustness of the radial trajectory against undersampling an acceleration of more than two relative to the prospectively triggered Cartesian sampling could be achieved with the retrospective method. Conclusion With the radial flow-encoding sequence the extraction of a self-gating signal is feasible. The retrospective method enables a robust and fast measurement of the local PWV without the need of additional trigger hardware. KW - pulse-wave velocity KW - mouse KW - self-gating KW - phase-contrast CMR KW - non-triggered KW - retrospective KW - radial KW - aorta Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-96602 UR - http://jcmr-online.com/content/15/1/88 ER - TY - THES A1 - Kern, Julia T1 - Field Dependence of Charge Carrier Generation in Organic Bulk Heterojunction Solar Cells T1 - Feldabhängige Ladungsträgergenerierung in organischen Bulk Heterojunction-Solarzellen N2 - In the field of organic photovoltaics, one of the most intensely researched topics to date is the charge carrier photogeneration in organic bulk heterojunction solar cells whose thorough understanding is crucial for achieving higher power conversion efficiencies. In particular, the mechanism of singlet exciton dissociation at the polymer–fullerene interface is still controversially debated. This work addresses the dissociation pathway via relaxed charge transfer states (CTS) by investigating its field dependence for reference material systems consisting of MDMO-PPV and one of the fullerene derivatives PC61BM, bisPCBM and PC71BM. Field dependent photoluminescence (PL(F)) and transient absorption (TA(F)) measurements give insight into the recombination of charge transfer excitons (CTE) and the generation of polarons, respectively. Optically detected magnetic resonance and atomic force microscopy are used to characterize the morphology of the samples. The comparison of the experimental field dependent exciton recombination recorded by PL(F) and the theoretical exciton dissociation probability given by the Onsager–Braun model yields the exciton binding energy as one of the key parameters determining the dissociation efficiency. The binding energies of both the singlet exciton in neat MDMO-PPV and the CTE in MDMO-PPV:PC61BM 1:1 are extracted, the latter turning out to be significantly reduced with respect to the one of the singlet exciton. Based on these results, the field dependence of CTE dissociation is evaluated for MDMO-PPV:PC61BM blends with varying fullerene loads by PL(F) and TA(F). For higher PC61BM contents, the CTE binding energies decrease notably. This behavior is ascribed to a larger effective dielectric constant for well-intermixed blends and to an interplay between dielectric constant and CTE delocalization length for phase separated morphologies, emphasizing the importance of high dielectric constants for the charge carrier photogeneration process. Finally, the CTE binding energies are determined for MDMO-PPV blends with different fullerene derivatives, focusing on the influence of the acceptor LUMO energy. Here, the experimental results suggest the latter having no or at least no significant impact on the binding energy of the CTE. Variations of this binding energy are rather related to different trap levels in the acceptors which seem to be involved in CTS formation. N2 - Einer der aktuellen Forschungsschwerpunkte im Bereich der organischen Photovoltaik ist die Ladungsträgergenerierung in „Bulk Heterojunction-Solarzellen“, deren Verständnis für das Erreichen höherer Wirkungsgrade essentiell ist. In diesem Zusammenhang wird derzeit vor allem der Dissoziationsmechanismus der Singulett-Exzitonen an der Donator–Akzeptor-Grenzfläche kontrovers diskutiert. Die vorliegende Arbeit adressiert die Dissoziation über relaxierte Ladungstransferzustände (CTS) durch die Untersuchung der Feldabhängigkeit des Prozesses für Referenzsysteme aus MDMO-PPV und den Fullerenderivaten PC61BM, bisPCBM sowie PC71BM. Feldabhängige Photolumineszenz (PL(F)) und transiente Absorption (TA(F)) geben Aufschluss über Rekombination der Ladungstransfer-Exzitonen (CTE) bzw. Polaronengenerierung, während die Morphologie der Proben durch optisch detektierte Magnetresonanz und Rasterkraftmikroskopie charakterisiert wird. Durch den Vergleich der experimentellen feldabhängigen Exzitonenrekombination mit der theoretischen Dissoziationswahrscheinlichkeit nach dem Onsager–Braun-Modell lässt sich die Bindungsenergie der Exzitonen ermitteln, welche die Dissoziationseffizienz entscheidend beeinflusst. Diese Bindungsenergie wird sowohl für das Singulett-Exziton in reinem MDMO-PPV als auch für das CTE in MDMO-PPV:PC61BM 1:1 bestimmt, wobei letztere deutlich geringer als die des Singulett-Exzitons ist. Ausgehend von diesen Ergebnissen wird die Feldabhängigkeit der CTE-Dissoziation für MDMO-PPV:PC61BM-Gemische mit unterschiedlichen Fullerenanteilen durch PL(F) und TA(F) untersucht. Für höhere PC61BM-Konzentrationen nimmt die CTE-Bindungsenergie merklich ab. Dieses Verhalten ist für gut durchmischte Systeme einer höheren dielektrischen Konstante und für phasenseparierte Systeme dem Zusammenspiel zwischen Dielektrizitätskonstante und Delokalisation der CTE zuzuschreiben. Schließlich werden die CTE-Bindungsenergien für Gemische aus MDMO-PPV und unterschiedlichen Fullerenderivaten bestimmt, wobei der Einfluss des LUMO-Niveaus der Akzeptoren im Fokus steht. Dieses scheint jedoch keine oder nur eine geringe Bedeutung für die CTE-Bindungsenergie zu besitzen. Die beobachteten Variationen der Bindungsenergie sind vielmehr auf die Fallenzustände der Akzeptoren zurückzuführen, welche offenbar an der Ausbildung der CTS beteiligt sind. KW - Organische Solarzelle KW - organic photovoltaics KW - charge transfer state KW - bulk heterojunction KW - binding energy KW - charge carrier generation KW - Bindungsenergie Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-91963 ER -