Electron Energy-Loss Spectroscopy on Underdoped Cuprates and Transition-Metal Dichalcogenides

Die vorliegende Arbeit befasst sich mit Elektronenenergieverlustspektroskopie an unterdotierten Kupratsupraleitern und Übergangsmetalldichalcogeniden. Nach einem kurzen Abriss über die der experimentellen Methode zugrundeliegenden theoretischen Tatsachen folgen zwei experimentelle Kapitel. Für das prototypische Kupratsystem Ca2-xNaxCuO2Cl2 wird für verschiedene Dotierungskonzentrationen zunächst die Entwicklung der Ladungstransferanregungen untersucht. Man findet eine substanzielle Umverteilung des spektralen Gewichtes, verbunden mit einem starken Einbruch der Dispersion dieser Anregungen. Beides wird im Rahmen der Wechselwirkung mit Spinfreiheitsgraden innerhalb der Kupfer-Sauerstoff-Ebene diskutiert. Anschliessend erfolgt die Diskussion einer ausschließlich für zehnprozentige Dotierung auftretenden Symmetriebrechung der optischen Antwortfunktion, für die verschiedene mögliche Szenarien vorgeschlagen werden. Im Kapitel über die Dichalcogenide liegt der Fokus auf dem Verhalten des Ladungsträgerplasmons, das für alle Substanzen dieser Gruppe mit Ladungsordnung eine negative Dispersion aufweist. Dieses Verhalten läßt sich durch in-situ Interkalation zusätzlicher Ladungstraeger umkehren, so dass man eine dotierungsabhängige Plasmonendispersion erhält. Es werden verschiedene Szenarien für dieses Verhalten diskutiert.The present thesis describes electron energy-loss spectroscopy on underdoped cuprate superconductors and transition-metal dichalcogenides. After a brief introduction into the experimental method there are two experimental chapters. For the prototype cuprate system Ca2-xNaxCuO2Cl2 the behavior of the charge-transfer excitations is investigated as a function of doping. The observed substantial redistribution of spectral weight and the accompanying breakdown of their dispersion is discussed in terms of a coupling to the spin degrees of freedom within the copper-oxygen plane. For x=0.1 there is a pronounced symmetry breaking in the optical response function which is discussed in terms of different possible scenarios. The chapter on the dichalcogenides focuses on the properties of the charge-carrier plasmon which shows a negative dispersion for all representatives of this family exhibiting a charge-density wave instability. This behavior can be influenced by in-situ intercalation of additional charges, the result being a doping dependent plasmon dispersion. Several approaches to reconcile these findings are considered

Mapping of the energetically lowest exciton in bulk 1T1T-HfS2_2

By combining electron energy-loss spectroscopy and state-of-the-art computational methods, we were able to provide an extensive picture of the excitonic processes in $1T$-HfS$_2$. The results differ significantly from the properties of the more scrutinized group VI semiconducting transition metal dichalcogenides such as MoS$_2$ and WSe$_2$. The measurements revealed a parabolic exciton dispersion for finite momentum $\textbf{q}$ parallel to the $\Gamma$K direction which allowed the determination of the effective exciton mass. The dispersion decreases monotonically for momentum exchanges parallel to the $\Gamma$M high symmetry line. To gain further insight into the excitation mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The results matched the experimental loss spectra closely, thereby confirming the excitonic nature of the observed transitions, and produced the momentumdependent binding energies. The simulations also demonstrated that the excitonic transitions for $\textbf{q}$ || $\Gamma$M occur exactly along that particular high symmetry line. For $\textbf{q}$ || $\Gamma$K on the other hand, the excitations traverse the Brillouin zone crossing various high symmetry lines. A particular interesting aspect of our findings was that the calculation of the electron probability density revealed that the exciton assumes a six-pointed star-like shape along the real space crystal planes indicating a mixed Frenkel-Wannier character.Comment: 12 pages, 10 figure

Charakterisierung von nanokristallinen Metallen durch Röntgen-Linienprofilanalyse

Gerade bei nanokristallinen Materialien hat die Mikrostruktur große Bedeutung für ihre makroskopischen Eigenschaften. Zu den bedeutendsten Verfahren zur Herstellung von massiven nanokristallinen Materialien gehören die Methoden der "Severe-Plastic-Deformation" (SPD). Unter diesen ist "High-Pressure-Torsion" (HPT), also Torsionsverformung unter hohem hydrostatischen Druck, die am besten geeignete Methode, um grundlegende systematische Untersuchungen durchzuführen. In dieser Arbeit wurde mittels Linienprofilanalyse von Röntgenbeugungsmessungen - eine der wichtigsten Methoden zur Quantifizierung von Kristallitgrößen und der Natur und Dichte von Gitterdefekten - HPT-verformtes Kupfer, Nickel und Silber sowie gesintertes Ruthenium untersucht. Für HPT-verformtes Kupfer und Nickel wird gezeigt, dass die Entlastung vom hydrostatischen Druck nach der HPT-Verformung erheblichen Einfluß auf die verbleibende Mikrostruktur hat. Dabei kommt es zu massiver Versetzungsannihilation durch Mobilisierung von Leerstellen, während die Fragmentierung des Materials schon im Laufe des HPT-Verformungsprozesses unter hydrostatischem Druck weitgehend abgeschlossen ist. In HPT-verformtem Silber kommt es aufgrund der niedrigen Stapelfehlerenergie neben hohen Versetzungsdichten auch zu erheblicher Produktion von Verformungszwillingen. Außerdem können spannungsinduzierte Erholungs- bzw. Rekristallisationseffekte bei hohen Verformungsgraden und hohem hydrostatischen Druck festgestellt werden. Die mikrostrukturelle Analyse von kompaktiertem Ruthenium-Nanopulver ergibt aufschlußreiche Unterschiede in Abhängigkeit von den angewandten Sintermethoden und -temperaturen. Ebenso werden wichtige Erkenntnisse über die optimale Anwendung verschiedener Auswerteverfahren der Profilanalyse gewonnen.The microstructure is very important for the macroscopic behaviour of materials, especially in the case of nanocrystalline materials. The methods of severe plastic deformation (SPD) belong to the most important techniques for the production of bulk nanocrystalline materials. Among these, high pressure torsion (HPT), which is torsional deformation under high hydrostatic pressure, is the most convenient method to conduct fundamental and systematic research. In this work X-ray line profile analysis, which is one of the most important methods to quantify crystallite sizes and the nature and density of lattice defects, was used to study HPT-deformed copper, nickel and silver as well as sintered ruthenium. For HPT-deformed copper and nickel it is shown that the unloading of the hydrostatic pressure after the HPT-deformation strongly influences the remaining microstructure. The unloading leads to a massive annihilation of dislocations due to the higher mobility of vacancies, whereas the fragmentation of the material is -for the most part- completed already during the HPT-deformation under hydrostatic pressure. Due to the low stacking fault energy of silver twins in addition to high dislocation densities are produced during HPT-deformation. Furthermore, stress induced recovery and recrystallization effects are observed at high deformations and high hydrostatic pressures. The analysis of the microstructure of compacted ruthenium nano-powder yields interesting characteristics due to different sintering methods and sintering temperatures. Moreover, important insights into the ideal application of different evaluation methods of line profile analysis are achieved

Signatures of electronic polarons in La1−x_{1-x}Sr1+x_{1+x}MnO4_4 observed by electron energy-loss spectroscopy

The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can propagate within the ab-plane, as revealed by a clear upward dispersion of the corresponding loss peak with increasing momentum transfer. The hole-induced excitation also shifts to higher energies with the onset of magnetic correlations for x = 0.5, implying a strong coupling of charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a signature of electronic polarons, which are created around doped holes, and that (ii) this low-energy excitation involves the charge transfer between manganese and oxygen. The finite dispersion of these excitations further indicates significant polaron-polaron interactions.Comment: 7 pages, 4 figure

Dissipationless conductance in a topological coaxial cable

We present a dynamical mechanism leading to dissipationless conductance, whose quantized value is controllable in a (3+1)-dimensional electronic system. The mechanism is exemplified by a theory of Weyl fermions coupled to a Higgs field, also known as an axion insulator. We show that the insertion of an axial gauge flux can induce vortex lines in the Higgs field, similar to the development of vortices in a superconductor upon the insertion of magnetic flux. We further show that the necessary axial gauge flux can be generated using Rashba spin-orbit coupling or a magnetic field. Vortex lines in the Higgs field are known to bind chiral fermionic modes, each of which serves as a one-way channel for electric charge with conductance e²/h. Combining these elements, we present a physical picture, the “topological coaxial cable,” illustrating how the value of the quantized conductance could be controlled in such an axion insulator.National Science Foundation (U.S.) (DGE-1247312)United States. Department of Energy (DEF-06ER46316

Fourier factorization with complex polarization bases in the plane-wave expansion method applied to two-dimensional photonic crystals

We demonstrate an enhancement of the plane wave expansion method treating two-dimensional photonic crystals by applying Fourier factorization with generally elliptic polarization bases. By studying three examples of periodically arranged cylindrical elements, we compare our approach to the classical Ho method in which the permittivity function is simply expanded without changing coordinates, and to the normal vector method using a normal-tangential polarization transform. The compared calculations clearly show that our approach yields the best convergence properties owing to the complete continuity of our distribution of polarization bases. The presented methodology enables us to study more general systems such as periodic elements with an arbitrary cross-section or devices such as photonic crystal waveguides

Momentum dependence of the excitons in pentacene

We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.Comment: 13 pages, 4 figure