1,721,044 research outputs found
Elektronenkorrelationen und unkonventionelle Supraleitung in realistischen Festkörpermaterialien und Heterostrukturen
No full textThe interplay of electron correlations and unconventional superconductivity is a pivotal area of condensed matter research, driven by the discovery of novel superconducting materials and the development of theoretical techniques. This thesis contributes to this expanding field by examining different aspects of superconductivity in strongly correlated electron systems. A core focus is the development and application of computational tools that enhance the efficiency and scope of material-realistic studies of superconductors, thereby enabling the investigation of previously inaccessible parameter regimes and properties of correlated superconducting matter. The findings of this thesis are contextualized within a broader overview of recent advancements in superconductivity research, along with a review of the theoretical methods and models employed.
For advancing the characterization of superconducting materials, we follow two complementary paths. One direction leverages low-rank representations of many-body correlation functions to address computational challenges posed by the intrinsic complexity of real materials and their low-temperature behavior. Specifically, we employ the intermediate representation basis for compact and efficient data handling to study spin-fluctuation-mediated superconductivity across a variety of multi-orbital materials. This includes a characterization of water-intercalated sodium cobalt oxides, where the numerical improvement enables the study of superconductivity and possible pairing symmetries at the experimentally relevant temperature scales on the order of a few Kelvin. For the recently discovered bilayer nickelate, we uncover the crucial role of inter-layer correlations in the formation of high-temperature superconductivity. Shifting focus to moiré materials as highly tunable quantum materials, we investigate possible superconductivity in twisted transition metal dichalcogenides. Our analysis reveals a strong charge carrier density dependence of the critical temperature for different pairing mechanisms, facilitating simple experimental scrutiny between them. In addition, we thoroughly evaluate the possibility of room-temperature superconductivity in copper-doped lead apatite, for which we do not find sustainable evidence. In this context, we discuss the general scientific challenges involved in achieving superconductivity at ambient conditions.
Furthermore, this thesis advances the microscopic understanding of superconductors by developing a Green’s function-based method to access intrinsic superconducting length scales, which were previously inaccessible in strongly correlated materials. These length scales, namely the coherence length and magnetic penetration depth, dictate many of the key properties of superconductors, including excitation energies, critical fields, and condensate stiffness. We validate our approach through application to alkali-doped fullerides. Here, our extended characterization reveals enhanced superconductivity with resilient phase coherence in the strong coupling regime of pairing interactions. The robust superconducting state is facilitated by multi-orbital physics. It stands in contrast to conventional limitations seen in single-band systems, where superconductivity is usually suppressed in the strong coupling regime due to the high effective masses of tightly-bound pairs. Our results pinpoint towards strategies for optimizing superconducting materials by surpassing traditional constraints with multi-orbital physics. The methodological advancements presented in this thesis broaden the scope of theoretically accessible parameters, enhancing the characterization of superconducting properties and laying a foundation for future innovations in superconductor design.Die Erforschung des Zusammenspiels von Elektronenkorrelationen und unkonventioneller Supraleitung ist ein zentrales Gebiet der Festkörperforschung, welches durch die Entdeckung neuartiger supraleitender Materialien und die Entwicklung theoretischer Methoden stetig vorangetrieben wird. Die vorliegende Arbeit trägt zu diesem wachsenden Forschungsbereich bei, indem sie verschiedene Aspekte der Supraleitung in stark korrelierten Elektronensystemen untersucht. Ein zentraler Schwerpunkt liegt auf der Entwicklung und Anwendung von computergestützten Rechenmethoden, welche die Effizienz und den Umfang materialrealistischer Simulationen von Supraleitern verbessern. Dadurch wird die Untersuchung bisher unzugänglicher Parameterbereiche und Eigenschaften korrelierter supraleitender Materie ermöglicht. Die Ergebnisse dieser Arbeit sind in eine umfassende Übersicht der aktuellen Fortschritte in der Supraleitungsforschung eingebettet und werden durch eine Darstellung der verwendeten theoretischen Methoden und Modelle ergänzt.
Zur umfassenden Charakterisierung supraleitender Materialien werden zwei sich ergänzende Ansätze verfolgt. Eine Richtung fokussiert sich auf die Nutzung von Niedrigrang-Darstellungen für Vielteilchenkorrelationsfunktionen, um rechnerische Hürden zu bewältigen, die aus der Komplexität realer Materialstrukturen und der Herausforderung ihrer Beschreibung bei tiefen Temperaturen resultieren. Insbesondere wird die sogenannte „Intermediate Representation Basis“ verwendet, um eine kompakte und effiziente Datenverarbeitung während der numerischen Simulation zu ermöglichen. Als Anwendung dieser wird die durch Spinfluktuationen vermittelte Supraleitung in ausgewählten Materialien mit multiorbitaler Elektronstruktur untersucht. Hierzu zählt die Charakterisierung der Supraleitung und der möglichen Paarsymmetrien des supraleitenden Ordnungsparameters in wasserinterkaliertem Natrium-Kobaltoxid. Die Verbesserung der numerischen Effizienz ermöglicht Simulationen bei experimentell relevanten Temperaturen im Bereich von wenigen Kelvin. Ein weiteres untersuchtes Material ist die Bilagen-Nickeloxidstruktur, bei der kürzlich Hochtemperatursupraleitung entdeckt wurde. Unsere Berechnungen zeigen dabei die tragende Rolle von Interlagenkorrelationen bei der Ausbildung der Supraleitung auf. Zudem werden im Rahmen dieser Arbeit Moiré-Materialien untersucht, deren Eigenschaften experimentell vielseitig einstellbar und präzise kontrollierbar sind. Im Speziellen untersuchen wir mögliche Supraleitung in verdrehten Übergangsmetalldichalkogeniden. Dabei wird eine starke Ladungsträgerdichteabhängigkeit der kritischen Temperatur für verschiedene Paarungsmechanismen gefunden, was eine einfache experimentelle Differenzierung zwischen diesen Mechanismen ermöglicht. Darüber hinaus untersuchen und falsifizieren wir die Möglichkeit von Raumtemperatursupraleitung in kupferdotiertem Bleiapatit und diskutieren die allgemeinen Herausforderungen, die mit Supraleitung unter Umgebungsbedingungen verbunden sind.
Als zweites zentrales Ergebnis verbessert die vorliegende Arbeit das mikroskopische Verständnis von Supraleitern, indem eine modellunabhängige Methode basierend auf Greenschen Funktionen entwickelt wird, um intrinsische Längenskalen von Supraleitern zu berechnen. Diese Längenskalen, die durch die Kohärenzlänge und die magnetische Eindringtiefe gegeben sind, lassen sich insbesondere in stark korrelierten Materialien nur schwer bestimmen. Dennoch sind sie von hoher Relevanz für eine Vielzahl an Eigenschaften supraleitender Materialien, darunter Anregungsenergien, kritische Felder und die Kondensatsteifigkeit. Wir validieren unseren Ansatz durch Anwendung auf alkalidotierte Fulleride. Durch die erweiterte Charakterisierung des supraleitenden Phasendiagramms wird im Fall starker Paarwechselwirkung eine verbesserte Supraleitung mit stabiler Phasenkohärenz entdeckt. Dieser robuste supraleitende Zustand wird durch Multiorbitalwechselwirkungen ermöglicht und steht im Gegensatz zu den herkömmlichen Einschränkungen, die in Einbandsystemen auftreten. In diesen wird Supraleitung im Grenzfall starker Paarwechselwirkungen durch hohe effektive Massen stark gebundener Paare unterdrückt. Unsere Ergebnisse zeigen Strategien zur Optimierung supraleitender Materialien auf, die durch gezielte Nutzung von Multiorbitalphysik ermöglicht werden. Die methodischen Fortschritte erweitern zudem den numerisch zugänglichen Parameterraum, verbessern die Charakterisierung der supraleitenden Eigenschaften und legen den Grundstein für zukünftige Innovationen im Design von Supraleitern
Emergent properties and trends of a new class of carbon nanocomposites: Graphene nanoribbons encapsulated in a carbon nanotube
No full textUsing density functional theory calculations, we show that recently synthesized carbon nanocomposites of graphene nanoribbons encapsulated in a carbon nanotube (GNR@CNT) possess rich emergent electronic and magnetic properties that offer new functionality and tunability and display systematic trends that are sensitive to the matchup of constitutive GNRs and CNTs. The encapsulation of H-passivated GNRs in metallic armchair CNTs always leads to a metallic complex while those in semiconducting zigzag CNTs can be either metallic or semiconducting depending on the chirality of GNRs. In particular, the complex of armchair GNRs in a zigzag CNT exhibits an oscillating electronic band gap with changing GNR width and a well-separated spatial distribution of electrons and holes localized in the CNT and GNR components, respectively. When bare large zigzag GNRs are encapsulated in an armchair CNT, the resulting complex shows strong structural stability and enhanced magnetism and, most interestingly, such GNR@CNT configurations can be tuned to be metallic or semiconducting depending on relative bond position and magnetic order. These results offer key insights for understanding and predicting emergent properties of GNR@CNT, which establish a roadmap for guiding design and synthesis of specific nanocomposite configurations with tailor-made properties for nanoelectronic, photovoltaic and spintronic applications
Variationsansätze für nichtlokale Coulomb-Wechselwirkungen in Mott-Hubbard-Systemen
No full textFrom the high-temperature superconducting cuprates to novel materials such as twisted bilayer graphene, adequate modelling of two-dimensional, interacting systems is of tremendous interest. At the center of interacting electrons on a lattice stands the Hubbard model which, despite its simplicity, remains unsolved for many important cases. The three works and ideas presented in this thesis share the goal of improving computational possibilities and broaden the understanding of otherwise inaccessible parameter regimes. To that end, within the first two projects, a variational scheme has been developed which describes extended Hubbard models with both nonlocal repulsive and exchange interactions through an effective, purely local Hubbard system. Lastly, an implementation of a strong-coupling perturbation theory is presented, which has the aim of circumventing the famous, fermionic Monte Carlo sign problem. While we treat the important square lattice in all cases, the hope is that with further developments, these novel approaches may be applied to much more complex problems
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Density Functional Tight Binding für neue Nanomaterialien : Entwicklung von Parametern für Bor und Anwendung von DFTB auf Adsorbate auf Graphen
No full textNanomaterials with specially tuned properties are an active field of research for many application purposes. These materials are of interest due to their large surface area in catalysis or because of the combination of their dimensions and electrical properties in the field of electronics. A tool to determine these properties and the stability of new nanomaterials is the computational modeling at the atomic scale. With the increase of computational power and the development of approximate methods it is possible to handle systems composed of several hundred atoms with high accuracy. One of the methods used is the Density Functional-basd Tight-Binding (DFTB), which combines computational speed of the semi-empirical methods with the accuracy of more sophisticated methods like Density Functional Theory (DFT). Therefore the method relies on pre-determined parameters, which are needed for every combination of chemical elements present in the problem at hand. The scope of this thesis is two-folded. One point is to provide an extension to an existing parameter set in order to enlarge the application range of said set. Therefore new parameters for the element boron and its combination with hydrogen, carbon and nitrogen have been developed. The performance of the parameters is evaluated by comparison to other computational methods like full DFT for molecular and periodic systems. Computed properties like bond lengths, bond angles, and vibrational frequencies are close to DFT predictions. Hence, the proposed parameterization provides a transferable and balanced description of both finite and periodic systems. The second point is the application of existing parameters to one of the most interesting new materials, graphene, in order to answer the question whether the sublattice symmetry of that system can be broken if atoms adsorb on one of its surfaces. For this purpose different adsorption patterns, adatom concentrations and types, and various degrees of electron doping have been considered. The results show that symmetry breaking should be possible if a specific ratio of adatoms to charge doping is exceeded
Correlation effects in the presence of excited charge carriers in semiconductor nanostructures taking the example of InGaAs quantum dots and atomic monolayers of MoS2
No full textSemiconductor nanostructures are applied in various electronic and optoelectronic devices. As miniaturization of these devices progresses, a microscopic treatment of correlations between excited carriers is essential for understanding and describing the governing physics. We investigate two different types of semiconductor nanostructures, which have each received considerable attention over the last years. These are self-assembled InGaAs quantum dots (QDs) on the one hand and atomic monolayers of MoS2 on the other hand. Self-assembled semiconductor QDs are used as active material in conventional lasers and as efficient non-classical light sources with applications in quantum information. As they can confine a small number of carriers in localized stats with discrete energies, it is questionable to neglect correlations between the carriers when describing their dynamics. We analyze the influence of carrier correlations in a single QD on Coulomb scattering processes, which are due to the contact with a quasi-continuum of wetting-layer (WL) states. Results obtained from a Boltzmann equation are compared with the fully correlated dynamics governed by a von-Neumann-Lindblad equation. In a first step, we take into account correlations generated by the exact treatment of Pauli blocking due to the contributing QD carrier configurations. Subsequently, we include correlations generated by energy renormalizations due to Coulomb interaction between the QD carriers. It is shown that at low WL carrier densities, neither Pauli correlations nor Coulomb correlations can be safely neglected, if the dynamics of single-particle states in the QD are to be predicted qualitatively and quantitatively. In the high-density regime, both types of correlations play a lesser role and thus a description of carrier dynamics by a Boltzmann equation becomes reliable. Furthermore, the efficiency of WL-assisted scattering processes as well as scattering-induced dephasing rates depending on the WL carrier density are discussed. Subsequently, experimental results for the carrier capture and relaxation dynamics in QDs are analyzed using a microscopic theory including also carrier-LO-phonon interaction. Time-resolved differential transmission changes of the QD transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by QD polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of a configuration-picture and single-particle picture description, both with non-perturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence. Monolayer MoS2 is the most prominent member of the class of two-dimensional transition-metal dichalcogenides and exhibits a direct band gap, making it a promising candidate for optical applications. We study the ground-state and finite-density optical response of MoS2 by solving the semiconductor Bloch equations, using ab-initio band structures and Coulomb interaction matrix elements. Spectra for excited carrier densities up to 10^13/cm^2 reveal a redshift of the excitonic ground-state absorption, whereas higher excitonic lines are found to disappear successively due to Coulomb-induced band-gap shrinkage of more than 500 meV and binding-energy reduction. Strain-induced band variations lead to a redshift of the lowest exciton line by approx. 110 meV/% and change the direct transition to indirect while maintaining the magnitude of the optical response
Elektronische Struktur von neuartigen zwei-dimensionalen Materialien und von Graphen Heterostrukturen
No full textToday a well-equipped library of two-dimensional materials can be synthesized or exfoliated, ranging from insulating hexagonal boron nitride, to semi-metallic graphene, and metallic as well as superconducting transition metal dichalcogenides and many others. Due to strong intra-layer covalent bondings, but weak inter-layer Van-der-Waals interactions, these layered materials can be stacked in a Lego-like fashion to artificial heterostructures which do not occur in nature. Thereby, these novel systems offer the possibility to combine specific properties of each of its constituents to tailor the heterostructure's properties on demand which might allow for completely new device classes. In fact, these kind of systems are already constructed and studied in labs around the world. In order to guide these efforts, we need an in-depth understanding of these complex heterostructures starting with its smallest components, namely the different two-dimensional materials and their mutual interactions. To this end, we study electronic and optical properties of novel two-dimensional materials in this thesis. In more detail, we here aim to investigate functionalized graphene, graphene heterostructures and doped or optically excited molybdenum disulfide (MoS) monolayers for which we combine \abinitio based models with many-body or multi-scale approaches. The first part is devoted to functionalized graphene and is subdivided into the investigation of disorder-induced optical effects of fluorographene and into a detailed study of the Coulomb interaction in graphene heterostructures in form of multilayer graphene, intercalated graphite and few-layer graphene within a dielectric environment. In the case of fluorographene we use a multi-scale approach to study the effects of realistic disorder patterns to the optical conductivity. Thereby, we provide important insights into the role of non-perfect fluorination of graphene. Regarding the graphene heterostructures we present a novel approach to easily and reliably derive Coulomb-interaction matrix elements in these structures. This method is used to study the robustness of bilayer graphene's ground state to changes in its dielectric surrounding. In the second part of the thesis we study a variety of many-body effects that arise in doped and optically excited MoS monolayers. Once again, by deriving simplified yet accurate models from first-principles we are able to investigate many-body excitations like plasmons or excitons as well as many-body instabilities like superconductivity or charge-density wave phases. Regarding the latter, we are able to extend the electron-doping phase diagram of MoS by the formation of a charge-density-wave phase and reveal its potential coexistence with the superconducting state. In the field of many-body excitations we study in detail excitonic line shifts upon optical excitations and we precisely describe different types of plasmonic excitations under electron or hole doping in MoS. Finally, we make use of the fundamental properties of the many-body interactions in layered materials in order to externally induce heterojunctions within homogeneous semiconducting monolayers by non-local manipulations of the Coulomb interaction
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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