121 research outputs found

    A Heavy Graphene Analogue amongst the Bismuth Subiodides as Host for Unusual Physical Phenomena

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    This thesis was inspired by the discovery of Bi14Rh3I9, the first so-called weak three-dimensional topological insulator (3D-TI) and has been concerned with the topic of TIs in general. Two aspects were tackled to gain a deeper understanding of this new state of matter. On one hand, the expansion of the material’s basis and on the other hand developing a simple model of the structure and analysing it via density-functional theory (DFT) based methods. To discover new materials, a systematic investigation of the metal-rich parts of the bismuth–platinum-metal–iodine phase systems was conducted. It led to six new phases among the bismuth subiodides. Some of which, e.g. Bi14Rh3I9, share a honeycomb network of platinum-metal-centred bismuth-cubes and are the seed of a family of materials with this structural motive. The others show strand-like structures or layered structures with platinum-platinum bonds. The latter were so far unknown amongst bismuth subiodides. The honeycomb network was separately analysed and shown to host the TI properties. Structurally and electronically it can be seen as a “heavy graphene analogue”, which refers to the fact that graphene with hypothetical strong spin-orbit coupling (“heavy graphene”) was the first TI put forward by theoreticians. Apart from DFT-calculations, physical experiments confirmed the TI properties. Angle-resolved photoelectron spectroscopy (ARPES) was used to verify the electronic structure and scanning tunnelling microscopy and spectroscopy (STM and STS) to reveal the protected 1D edge states present at the cleaving surface of this material. As the arrangement of the honeycomb layer varies between the different known and newly discovered materials within this family of structures, this influence was also investigated. All further materials were also characterised by DFT-calculations and physical experiments, e.g. magnetisation and transport measurements. This thesis might give an experimental and theoretical basis for a deeper understanding of the TI state of matter. The 1D edge states on the surface of Bi14Rh3I9 could be a chance to handle spins and therefore propel spintronic research, or they could host Majorana fermions, which could be used as qubits in quantum computing

    A Heavy Graphene Analogue amongst the Bismuth Subiodides as Host for Unusual Physical Phenomena

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    This thesis was inspired by the discovery of Bi14Rh3I9, the first so-called weak three-dimensional topological insulator (3D-TI) and has been concerned with the topic of TIs in general. Two aspects were tackled to gain a deeper understanding of this new state of matter. On one hand, the expansion of the material’s basis and on the other hand developing a simple model of the structure and analysing it via density-functional theory (DFT) based methods. To discover new materials, a systematic investigation of the metal-rich parts of the bismuth–platinum-metal–iodine phase systems was conducted. It led to six new phases among the bismuth subiodides. Some of which, e.g. Bi14Rh3I9, share a honeycomb network of platinum-metal-centred bismuth-cubes and are the seed of a family of materials with this structural motive. The others show strand-like structures or layered structures with platinum-platinum bonds. The latter were so far unknown amongst bismuth subiodides. The honeycomb network was separately analysed and shown to host the TI properties. Structurally and electronically it can be seen as a “heavy graphene analogue”, which refers to the fact that graphene with hypothetical strong spin-orbit coupling (“heavy graphene”) was the first TI put forward by theoreticians. Apart from DFT-calculations, physical experiments confirmed the TI properties. Angle-resolved photoelectron spectroscopy (ARPES) was used to verify the electronic structure and scanning tunnelling microscopy and spectroscopy (STM and STS) to reveal the protected 1D edge states present at the cleaving surface of this material. As the arrangement of the honeycomb layer varies between the different known and newly discovered materials within this family of structures, this influence was also investigated. All further materials were also characterised by DFT-calculations and physical experiments, e.g. magnetisation and transport measurements. This thesis might give an experimental and theoretical basis for a deeper understanding of the TI state of matter. The 1D edge states on the surface of Bi14Rh3I9 could be a chance to handle spins and therefore propel spintronic research, or they could host Majorana fermions, which could be used as qubits in quantum computing

    Quantum walks: background geometry and gauge invariance

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    Ciertos tipos de problemas no pueden resolverse usando los actuales ordenadores clásicos. Una forma de encontrar una solución, es mediante el uso de ordenadores cuánticos. Sin embargo, construir un ordenador cuántico es realmente complicado actualmente, debido a las limitaciones tecnológicas. Mientras tanto, los simuladores cuánticos han sido capaces de resolver algunos de estos problemas, ya que los simuladores cuánticos son más accesibles experimentalmente. Las llamadas caminatas cuánticas, en su versión discreta, son una herramienta muy útil para simular ciertos sistemas físicos. En el límite al continuo, se puede obtener una serie de ecuaciones diferenciales, particularmente, la ecuación de Dirac entre ellas. En la presente tesis, se seguirán estudiando las propiedades de las caminatas cuánticas, como posibles simuladores cuánticos. Podemos resumir los resultados en: i) Se introduce un modelo de caminata cuántica, en el que se simula, en el continuo, la dinámica de fermiones en una teoría de branas. Eso abre la posibilidad de estudiar diferentes modelos de teorías de Kaluza-Klein; ii) Se discute la invariancia gauge en caminatas cuánticas, acopladas a campos electromagnéticos, donde se exhiben similitudes y diferencias con modelos previos. Este modelo presenta conexiones con la invariancia gauge realizada en "lattice gauge theories"; iii) Se introducen caminatas cuánticas sobre redes no rectangulares, como la red triangular o hexagonal, con el propósito de simular la ecuación de Dirac en el límite al continuo. Estos modelos se pueden extender, por medio de operadores locales unitarios, que permiten reproducir la dinámica de fermiones en espacio tiempo curvo.There are many problems that cannot be solved using current \textit{classical} computers. One manner to approach a solution of these systems is by using \textit{quantum} computers. However, building a quantum computer is really challenging from the experimental side. Quantum simulators have been capable to solve some of these problems, as they are realizable experimentally. Discrete Time Quantum Walks (DTQWs) have been proved to be an useful tool to quantum simulate physical systems. In the continuous limit, a family of differential equations can be achieved, in particular, the Dirac equation can be recovered. In this thesis we study QWs as possible schemes for quantum simulation. Specifically, we can summarize our results in: i) We introduce a QW-based model in which a brane theory can be simulated in the continuum, opening the possibility to study more general theories with extra dimensions; ii) Electromagnetic gauge invariance in QWs is discussed, presenting some similarities and differences to previous models. This QW model also makes a connection to gauge invariance in lattice gauge theories (LGT); iii) We introduce QWs over non-rectangular lattices, such a triangular or honeycomb structures, for the purpose of simulating the Dirac equation in the continuum. We also extent these models, by introducing local coin operators, that allow us to reproduce the dynamics of quantum particles under a curved space time

    Caminhadas quânticas por modelo de espalhamento em redes hexagonais e triangulares : uma investigação no espaço de posição e de momento

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    Orientador: Prof. Dr. Marcos Gomes Eleutério da LuzTese (doutorado) - Universidade Federal do Paraná, Setor de Ciências Exatas, Programa de Pós-Graduação em Física. Defesa : Curitiba, 31/05/2023Inclui referênciasResumo: Neste trabalho abordamos como implementar Caminhada Quantica por Modelo de Espalhamento (CQME) em dois tipos distintos de rede: hexagonal e na rede triangular infinita. No caso da rede hexagonal, apresentamos tanto o formalismo para CQME's no espaco de posicao, no qual abordamos redes infinitas e com condicoes de contorno, como para CQME's no espaco de momento. No caso da rede triangular infinita, apresentamos apenas o formalismo para CQME's no espaco de posicao. Em um primeiro momento, apresentamos os principais conceitos e o formalismo de uma Caminhada Aleatoria Classica (CAC) e de seus analogos quanticos, a Caminhada Quantica por Modelo de Moeda (CQMM) e a CQME, estes com tratamento unidimensional. Em seguida apresentamos o formalismo geral para CQME's em redes hexagonais infinitas, e, propomos uma parametrizacao AB para as matrizes de espalhamento desta rede. Nos tambem apresentamos os principais resultados para CQME's com matriz AB, isto e, distribuicao de probabilidades e o Deslocamento Quadratico Medio Radial (DQMR), obtido de maneira numerica e, por um modelo apresentado. Dentro deste contexto, apresentamos entao o formalismo e o resultados obtidos para CQME's imparciais em nanofitas quadradas, alem de propor um modelo que relaciona a distribuicao de probabilidades nas bordas das nanofitas com o tamanho da nanofita. Na sequencia, apresentamos o formalismo generico para CQME's no espaco de momento de redes hexagonais, e, em seguida, propomos um metodo para obter os autovalores do sistema com dependencia da parametrizacao das matrizes espalhamento para os vertices nao equivalentes da rede. Dentro deste contexto, apresentamos os resultados para as bandas de energia, velocidade de Fermi e velocidade de grupo, todos obtidos para CQME's no espaco de momento com matrizes AB. Alem disso, mostramos que para regioes proximas aos pontos de alta simetria K e K' na primeira Zona de Brillouin, os resultados obtidos via CQME convergem com os via Tight Binding First Nearest Neighbors (TB-1NN). Ademais, utilizando a equacao generica de autovalores para CQME no espaco de momento e, tomando uma parametrizacao geradora do SU(3) para as matrizes de espalhamento, nos apresentamos os resultados obtidos por meio de de um metodo numerico computacional para as bandas ? e ?? de tres materiais de Dirac 2D: grafeno, germaneno e siliceno. Apresentamos tambem o formalismo para CQME's na rede triangular infinita e, uma abordagem para analisar a simetria dessa rede utilizando permutacao de rotulos entre os estados topologicamente equivalentes. Alem disso, apresentamos todas as permutacoes possiveis e as simetrias presente entre elas. Por fim, nos apresentamos as conclusoes associadas aos topicos de pesquisa deste trabalho.Abstract: In this thesis we show how to implement Scattering Quantum Walk (SQW) in two types of networks: honeycomb and infinite triangular network. In the case of the hexagonal lattice, we present the SQW formalism in position space with and without boundary conditions, as well as in momentum space. In the case of the triangular lattice, we present the formalism for SQW in position space. First of all, we present the main concepts and the formalism of a Classical Random Walk (CRW) and its quantum analogs, the Coined Quantum Walk (CQW), and the SQW, all of which have a one-dimensional treatment. We then present the formalism for SQW in the infinite honeycomb lattice in position space, and we propose an AB parameterization for the scattering matrices of the system. We also present several key quantities related to SQW using this type of matrix, including probability distributions, radial mean square displacement, and over. We then extend our analysis to SQW with boundary, i.e., the honeycomb square nanoribbons. Within this context, we then present the formalism and the results obtained for unbiased SQW's in square nanoribbons, in addition to proposing a model that relates the probability distribution at the edges of the nanoribbons with the size of the nanoribbon. Next, we present the generic formalism for SQW's in the momentum space of hexagonal lattices, and then propose a method to obtain the eigenvalues of the system with dependence on the parameterization of the scattering matrices for the non-equivalent vertices of the lattice. Within this context, we present results for the energy bands, Fermi velocity and group velocity, all obtained for SQW's in momentum space with AB matrices. Furthermore, we show that for regions close to the high symmetry points K and K' in the first Brillouin Zone, the results obtained via SQW's converge with those via Tight Binding First Nearest Neighbors (TB-1NN). Moreover, using the generic eigenvalues equation for SQW in momentum space and taking a SU(3) generating parameterization for the scattering matrices, we present the results obtained through a numerical computational method for the ? and ?? bands of three 2D Dirac materials: graphene, germanene and silicene. We also present the formalism for SQW's on the infinite triangular lattice and, an approach to analyze the symmetry of this lattice using label permutation between the topologically equivalent states. Moreover, we present all possible permutations, and the symmetries present among them. Finally, we present the conclusions associated with the research topics of this thesis

    Effective field theories for strongly correlated fermions - Insights from the functional renormalization group

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    'There are very few things that can be proved rigorously in condensed matter physics.' These famous words, brought to us by Nobel laureate Anthony James Leggett in 2003, summarize very well the challenging nature of problems researchers find themselves confronted with when entering the fascinating field of condensed matter physics. The former roots in the inherent many-body character of several quantum mechanical particles with modest to strong interactions between them: their individual properties might be easy to understand, while their collective behavior can be utterly complex. Strongly correlated electron systems, for example, exhibit several captivating phenomena such as superconductivity or spin-charge separation at temperatures far below the energy scale set by their mutual couplings. Moreover, the dimension of the respective Hilbert space grows exponentially, which impedes the exact diagonalization of their Hamiltonians in the thermodynamic limit. For this reason, renormalization group (RG) methods have become one of the most powerful tools of condensed matter research - scales are separated and dealt with iteratively by advancing an RG flow from the microscopic theory into the low-energy regime. In this thesis, we report on two complementary implementations of the functional renormalization group (fRG) for strongly correlated electrons. Functional RG is based on an exact hierarchy of coupled differential equations, which describe the evolution of one-particle irreducible vertices in terms of an infrared cutoff Lambda. To become amenable to numerical solutions, however, this hierarchy needs to be truncated. For sufficiently weak interactions, three-particle and higher-order vertices are irrelevant at the infrared fixed point, justifying their neglect. This one-loop approximation lays the foundation for the N-patch fRG scheme employed within the scope of this work. As an example, we study competing orders of spinless fermions on the triangular lattice, mapping out a rich phase diagram with several charge and pairing instabilities. In the strong-coupling limit, a cutting-edge implementation of the multiloop pseudofermion functional renormalization group (pffRG) for quantum spin systems at zero temperature is presented. Despite the lack of a kinetic term in the microscopic theory, we provide evidence for self-consistency of the method by demonstrating loop convergence of pseudofermion vertices, as well as robustness of susceptibility flows with respect to occupation number fluctuations around half-filling. Finally, an extension of pffRG to Hamiltonians with coupled spin and orbital degrees of freedom is discussed and results for exemplary model studies on strongly correlated electron systems are presented

    Robust recognition and exploratory analysis of crystal structures using machine learning

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    In den Materialwissenschaften läuten Künstliche-Intelligenz Methoden einen Paradigmenwechsel in Richtung Big-data zentrierter Forschung ein. Datenbanken mit Millionen von Einträgen, sowie hochauflösende Experimente, z.B. Elektronenmikroskopie, enthalten eine Fülle wachsender Information. Um diese ungenützten, wertvollen Daten für die Entdeckung verborgener Muster und Physik zu nutzen, müssen automatische analytische Methoden entwickelt werden. Die Kristallstruktur-Klassifizierung ist essentiell für die Charakterisierung eines Materials. Vorhandene Daten bieten vielfältige atomare Strukturen, enthalten jedoch oft Defekte und sind unvollständig. Eine geeignete Methode sollte diesbezüglich robust sein und gleichzeitig viele Systeme klassifizieren können, was für verfügbare Methoden nicht zutrifft. In dieser Arbeit entwickeln wir ARISE, eine Methode, die auf Bayesian deep learning basiert und mehr als 100 Strukturklassen robust und ohne festzulegende Schwellwerte klassifiziert. Die einfach erweiterbare Strukturauswahl ist breit gefächert und umfasst nicht nur Bulk-, sondern auch zwei- und ein-dimensionale Systeme. Für die lokale Untersuchung von großen, polykristallinen Systemen, führen wir die strided pattern matching Methode ein. Obwohl nur auf perfekte Strukturen trainiert, kann ARISE stark gestörte mono- und polykristalline Systeme synthetischen als auch experimentellen Ursprungs charakterisieren. Das Model basiert auf Bayesian deep learning und ist somit probabilistisch, was die systematische Berechnung von Unsicherheiten erlaubt, welche mit der Kristallordnung von metallischen Nanopartikeln in Elektronentomographie-Experimenten korrelieren. Die Anwendung von unüberwachtem Lernen auf interne Darstellungen des neuronalen Netzes enthüllt Korngrenzen und nicht ersichtliche Regionen, die über interpretierbare geometrische Eigenschaften verknüpft sind. Diese Arbeit ermöglicht die Analyse atomarer Strukturen mit starken Rauschquellen auf bisher nicht mögliche Weise.In materials science, artificial-intelligence tools are driving a paradigm shift towards big data-centric research. Large computational databases with millions of entries and high-resolution experiments such as electron microscopy contain large and growing amount of information. To leverage this under-utilized - yet very valuable - data, automatic analytical methods need to be developed. The classification of the crystal structure of a material is essential for its characterization. The available data is structurally diverse but often defective and incomplete. A suitable method should therefore be robust with respect to sources of inaccuracy, while being able to treat multiple systems. Available methods do not fulfill both criteria at the same time. In this work, we introduce ARISE, a Bayesian-deep-learning based framework that can treat more than 100 structural classes in robust fashion, without any predefined threshold. The selection of structural classes, which can be easily extended on demand, encompasses a wide range of materials, in particular, not only bulk but also two- and one-dimensional systems. For the local study of large, polycrystalline samples, we extend ARISE by introducing so-called strided pattern matching. While being trained on ideal structures only, ARISE correctly characterizes strongly perturbed single- and polycrystalline systems, from both synthetic and experimental resources. The probabilistic nature of the Bayesian-deep-learning model allows to obtain principled uncertainty estimates which are found to be correlated with crystalline order of metallic nanoparticles in electron-tomography experiments. Applying unsupervised learning to the internal neural-network representations reveals grain boundaries and (unapparent) structural regions sharing easily interpretable geometrical properties. This work enables the hitherto hindered analysis of noisy atomic structural data

    Investigations of topological phases for quasi-1D systems

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    For a long time, quantum states of matter have been successfully characterized by the Ginzburg-Landau formalism that was able to classify all different types of phase transitions. This view changed with the discovery of the quantum Hall effect and topological insulators. The latter are materials that host metallic edge states in an insulating bulk, some of which are protected by the existing symmetries. Complementary to the search of topological phases in condensed matter, great efforts have been made in quantum simulations based on cold atomic gases. Sophisticated laser schemes provide optical lattices with different geometries and allow to tune interactions and the realization of artificial gauge fields. At the same time, new concepts coming from quantum information, based on entanglement, are pushing the frontier of our understanding of quantum phases as a whole. The concept of entanglement has revolutionized the description of quantum many-body states by describing wave functions with tensor networks (TN) that are exploited for numerical simulations based on the variational principle. This thesis falls within the framework of the studies in condensed matter physics: it focuses indeed on the so-called synthetic realization of quantum states of matter, more specifically, of topological ones, which may have on the long-run outfalls towards robust quantum computers. We propose a theoretical investigation of cold atoms in optical lattice pierced by effective (magnetic) gauge fields and subjected to experimentally relevant interactions, by adding a modern numerical approach based on TN algorithms. More specifically, this work will focus on (i) interacting topological phases in quasi-1D systems and, in particular, the Creutz-Hubbard model, (ii) the connection between condensed matter and high energy physics studying the Gross-Neveu model and the discretization of Wilson-Hubbard model, (iii) implementing tensor network-based algorithms.Durante mucho tiempo, los estados cuánticos de la materia se han caracterizado con éxito por el formalismo de Ginzburg-Landau que permitió de clasificar todos los diferentes tipos de transiciones de fase. Esta visión cambió con el descubrimiento del efecto Hall cuántico y los aislantes topológicos. Estos últimos son materiales que albergan estados de borde metálicos en una masa aislante, algunos de los cuales están protegidos por las simetrías existentes. Conjuntamente a la búsqueda de fases topológicas en materia condensada, se han hecho grandes esfuerzos en simulaciones cuánticas basadas en gases atómicos fríos. Los sofisticados esquemas láser proporcionan redes ópticas con diferentes geometrías y permiten ajustar las interacciones y la realización de campos de gauge artificial. Al mismo tiempo, los nuevos conceptos que provienen de la información cuántica, basados en el entanglement, están empujando la frontera de nuestra comprensión de las fases cuánticas en su conjunto. El concepto de entanglement ha revolucionado la descripción de los estados cuánticos de muchos cuerpos al describir las funciones de onda con redes tensoras (TN) que se explotan para simulaciones numéricas basadas en el principio de variación. Esta tesis se enmarca en los estudios de física de la materia condensada: en particular, se centra en la llamada realización sintética de los estados cuánticos de la materia, más específicamente, de los topológicos, que pueden tener en las salidas a largo plazo hacia computadoras cuánticas robustas. Se propone una investigación teórica de los átomos fríos en la red óptica con campos de gauge efectivos y sometidos a interacciones relevantes experimentalmente, agregando un enfoque numérico moderno basado en algoritmos TN. Más específicamente, este trabajo se centrará en (i) fases topológicas en los sistemas cuasi-1D y, en particular, el modelo Creutz-Hubbard, (ii) la conexión entre la materia condensada y la física de alta energía estudiando el modelo Gross-Neveu y el discretización del modelo Wilson-Hubbard, (iii) implementación de algoritmos basados en redes tensoras
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