Dissertations / Theses on the topic 'Low Dimensional Quantum Spin Systems'
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Heidrich-Meisner, Fabian. "Transport properties of low-dimensional quantum spin systems." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=974939242.
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Sugimoto, Takanori. "Dynamical Properties in Low-Dimensional Quantum Spin Systems." 京都大学 (Kyoto University), 2012. http://hdl.handle.net/2433/157746.
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Hofmann, Michael. "Anomalous heat transport in low dimensional quantum spin systems." [S.l.] : [s.n.], 2002. http://deposit.ddb.de/cgi-bin/dokserv?idn=964915626.
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Law, Joseph M. "Identification and investigation of new low-dimensional quantum spin systems." Thesis, Loughborough University, 2011. https://dspace.lboro.ac.uk/2134/8963.
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This thesis focuses on one area of modern condensed matter physics, namely low-dimensional magnetism, and more specifically one-dimensional linear chains. The work herein can be split into three parts. The first part provides a tool for the greater community. I herein propose a Pad´e approximation for the temperature dependent magnetic susceptibility of a S = 3/2 spin chain, that is more accurate than those already known. The approximation allows one to fit experimentally measured magnetic susceptibilities and ascertain values such as the near-neighbour spin exchange interaction and the g-factor. The second and third parts of this thesis are both concerned with experimentally and theoretically characterizing two isostructural linear S = 1/2 chain compounds on opposite ends of the 3d transition metal series. The compounds, CuCrO4 (3d9) and TiPO4 (3d1), are shown to have completely different ground states despite both being largely isostructural and S = 1/2 quantum spin systems. In this work and the resulting publications it is shown that CuCrO4 is a one-dimensional S = 1/2 spin chain with anti-ferromagnetic nearest- and next nearest-neighbour spin exchange interactions. The ratio of these spin exchange interactions is shown experimentally and theoretically to be approximately 2, putting CuCrO4 in the vicinity of the Majumdar-Ghosh point, for which the magnetic ground-state can ii be solved analytically. Small ferromagnetic inter-chain coupling leads to long-range ferromagnetic ordering between anti-ferromagnetic chains at 8.2(2) K. At this temperature a spontaneous electrical polarization is observed. This classifies CuCrO4 as a type-II multiferroic. Contrary to CuCrO4, TiPO4 has a non-magnetic ground state. At 111 and 74 K TiPO4 undergoes a two stage phase transition, which is interpreted as a spin-Peierls transition. There is evidence that below 74 K TiPO4 has a new crystal structure, in which there are alternating dimerised chains and two different PO4 tetrahedral units. Currently the new structure has not been identified and no super-structure reflections have been confirmed in either low-temperature neutron or x-ray diffraction. In summary this thesis presents some experimental and theoretical contributions to the field of low-dimensional magnetism.
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Mendoza, Arenas Juan José. "Spin and energy transport in boundary-driven low-dimensional open quantum systems." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:44b89c4d-e9eb-4136-a540-c80bcabeb6f6.
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In spite of being the subject of intense research, several key but complex questions on the nonequilibrium physics of correlated quantum systems remain controversial. For example, the nature of particle and energy transport in different interacting regimes, the relevance of integrability and the impact of environmental coupling are still under active debate. These problems can now be approached numerically, due to the development of powerful algorithms which allow the efficient simulation of the dynamics of correlated systems. In the present thesis we study numerically and analytically the transport properties of low-dimensional quantum systems. In particular, we consider the steady-state spin and energy conduction through XXZ boundary-driven spin-1/2 chains. In the first part, we analyse the transport through chains with only coherent processes in the bulk. For spin transport induced by a magnetisation imbalance between the boundaries, previously identified ballistic, diffusive and negative differential conductivity regimes are reproduced. We provide a comprehensive explanation of the latter. The energy conduction induced by this driving scheme features the same properties as spin transport. For thermally-driven chains, we discuss the nature of energy transport and the emergence of local thermal states when the integrability of the Hamiltonian is broken. In the second part of the thesis we analyse the effect of bulk incoherent effects on the transport properties previously discussed. First we find that for weak particle-particle interactions, pure dephasing degrades spin and energy conduction. In contrast, for strong interactions dephasing induces a significant transport enhancement. We identify the underlying mechanism and discuss its generality. Finally, motivated by the lattice structure of several organic conductors, we study the interplay between coherent and incoherent processes in systems of weakly-coupled chains. We find an enhancement effect due to incoherent interchain hopping, stronger than that by dephasing, which increases with the chain length and relates to superdiffusive transport.
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Rahnavard, Yousef [Verfasser], and Wolfram [Akademischer Betreuer] Brenig. "Transport and dynamics of low-dimensional quantum spin systems / Yousef Rahnavard ; Betreuer: Wolfram Brenig." Braunschweig : Technische Universität Braunschweig, 2014. http://d-nb.info/117582089X/34.
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Janson, Oleg. "DFT-based microscopic magnetic modeling for low-dimensional spin systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-91976.
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In the vast realm of inorganic materials, the Cu2+-containing cuprates form one of the richest classes. Due to the combined effect of crystal-field, covalency and strong correlations, all undoped cuprates are magnetic insulators with well-localized spins S=1/2, whereas the charge and orbital degrees of freedom are frozen out. The combination of the spin-only nature of their magnetism with the unique structural diversity renders cuprates as excellent model systems. The experimental studies, boosted by the discovery of high-temperature superconductivity in doped La2CuO4, revealed a fascinating variety of magnetic behaviors observed in cuprates. A digest of prominent examples should include the spin-Peierls transition in CuGeO3, the Bose-Einstein condensation of magnons in BaCuSi2O6, and the quantum critical behavior of Li2ZrCuO4. The magnetism of cuprates originates from short-range (typically, well below 1 nm) exchange interactions between pairs of spins Si and Sj, localized on Cu atoms i and j. Especially in low-dimensional compounds, these interactions are strongly anisotropic: even for similar interatomic distances |Rij|, the respective magnetic couplings Jij can vary by several orders of magnitude. On the other hand, there is an empirical evidence for the isotropic nature of this interaction in the spin space: different components of Si are coupled equally strong. Thus, the magnetism of cuprates is mostly described by a Heisenberg model, comprised of Jij(Si*Sj) terms. Although the applicability of this approach to cuprates is settled, the model parameters Jij are specific to a certain material, or more precisely, to a particular arrangement of the constituent atoms, i.e. the crystal structure. Typically, among the infinite number of Jij terms, only several are physically relevant. These leading exchange couplings constitute the (minimal) microscopic magnetic model. Already at the early stages of real material studies, it became gradually evident that the assignment of model parameters is a highly nontrivial task. In general, the problem can be solved experimentally, using elaborate measurements, such as inelastic neutron scattering on large single crystals, yielding the magnetic excitation spectrum. The measured dispersion is fitted using theoretical models, and in this way, the model parameters are refined.
Despite excellent accuracy of this method, the measurements require high-quality samples and can be carried out only at special large-scale facilities. Therefore, less demanding (especially, regarding the sample requirements), yet reliable and accurate procedures are desirable. An alternative way to conjecture a magnetic model is the empirical approach, which typically relies on the Goodenough-Kanamori rules. This approach links the magnetic exchange couplings to the relevant structural parameters, such as bond angles. Despite the unbeatable performance of this approach, it is not universally applicable. Moreover, in certain cases the resulting tentative models are erroneous. The recent developments of computational facilities and techniques, especially for strongly correlated systems, turned density-functional theory (DFT) band structure calculations into an appealing alternative, complementary to the experiment. At present, the state-of-the-art computational methods yield accurate numerical estimates for the leading microscopic exchange couplings Jij (error bars typically do not exceed 10-15%).
Although this computational approach is often regarded as ab initio, the actual procedure is not parameter-free. Moreover, the numerical results are dependent on the parameterization of the exchange and correlation potential, the type of the double-counting correction, the Hubbard repulsion U etc., thus an accurate choice of these crucial parameters is a prerequisite. In this work, the optimal parameters for cuprates are carefully evaluated based on extensive band structure calculations and subsequent model simulations.
Considering the diversity of crystal structures, and consequently, magnetic behaviors, the evaluation of a microscopic model should be carried out in a systematic way. To this end, a multi-step computational approach is developed. The starting point of this procedure is a consideration of the experimental structural data, used as an input for DFT calculations. Next, a minimal DFT-based microscopic magnetic model is evaluated. This part of the study comprises band structure calculations, the analysis of the relevant bands, supercell calculations, and finally, the evaluation of a microscopic magnetic model. The ground state and the magnetic excitation spectrum of the evaluated model are analyzed using various simulation techniques, such as quantum Monte Carlo, exact diagonalization and density-matrix renormalization groups, while the choice of a particular technique is governed by the dimensionality of the model, and the presence or absence of magnetic frustration.
To illustrate the performance of the approach and tune the free parameters, the computational scheme is applied to cuprates featuring rather simple, yet diverse magnetic behaviors: spin chains in CuSe2O5, [NO]Cu(NO3)3, and CaCu2(SeO3)2Cl2; quasi-two-dimensional lattices with dimer-like couplings in alpha-Cu2P2O7 and CdCu2(BO3)2, as well as the 3D magnetic model with pronounced 1D correlations in Cu6Si6O18*6H2O. Finally, the approach is applied to spin liquid candidates --- intricate materials featuring kagome-lattice arrangement of the constituent spins. Based on the DFT calculations, microscopic magnetic models are evaluated for herbertsmithite Cu3(Zn0.85Cu0.15)(OH)6Cl2, kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2, as well as for volborthite Cu3[V2O7](OH)2*2H2O. The results of the DFT calculations and model simulations are compared to and challenged with the available experimental data.
The advantages of the developed approach should be briefly discussed. First, it allows to distinguish between different microscopic models that yield similar macroscopic behavior. One of the most remarkable example is volborthite Cu3[V2O7](OH)2*2H2O, initially described as an anisotropic kagome lattice. The DFT calculations reveal that this compound features strongly coupled frustrated spin chains, thus a completely different type of magnetic frustration is realized.
Second, the developed approach is capable of providing accurate estimates for the leading magnetic couplings, and consequently, reliably parameterize the microscopic Hamiltonian. Dioptase Cu6Si6O18*6H2O is an instructive example showing that the microscopic theoretical approach eliminates possible ambiguity and reliably yields the correct parameterization.
Third, DFT calculations yield even better accuracy for the ratios of magnetic exchange couplings. This holds also for small interchain or interplane couplings that can be substantially smaller than the leading exchange. Hence, band structure calculations provide a unique possibility to address the interchain or interplane coupling regime, essential for the magnetic ground state, but hardly perceptible in the experiment due to the different energy scales.
Finally, an important advantage specific to magnetically frustrated systems should be mentioned. Numerous theoretical and numerical studies evidence that low-dimensionality and frustration effects are typically entwined, and their disentanglement in the experiment is at best challenging. In contrast, the computational procedure allows to distinguish between these two effects, as demonstrated by studying the long-range magnetic ordering transition in quasi-1D spin chain systems.
The computational approach presented in the thesis is a powerful tool that can be directly applied to numerous S=1/2 Heisenberg materials. Moreover, with minor modifications, it can be largely extended to other metallates with higher value of spin. Besides the excellent performance of the computational approach, its relevance should be underscored: for all the systems investigated in this work, the DFT-based studies not only reproduced the experimental data, but instead delivered new valuable information on the magnetic properties for each particular compound.
Beyond any doubt, further computational studies will yield new surprising results for known as well as for new, yet unexplored compounds. Such "surprising" outcomes can involve the ferromagnetic nature of the couplings that were previously considered antiferromagnetic, unexpected long-range couplings, or the subtle balance of antiferromagnetic and ferromagnetic contributions that "switches off" the respective magnetic exchange. In this way, dozens of potentially interesting systems can acquire quantitative microscopic magnetic models.
The results of this work evidence that elaborate experimental methods and the DFT-based modeling are of comparable reliability and complement each other. In this way, the advantageous combination of theory and experiment can largely advance the research in the field of low-dimensional quantum magnetism. For practical applications, the excellent predictive power of the computational approach can largely alleviate designing materials with specific properties.
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Lipps, Ferdinand. "Electron spins in reduced dimensions: ESR spectroscopy on semiconductor heterostructures and spin chain compounds." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-74470.
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Spatial confinement of electrons and their interactions as well as confinement of the spin dimensionality often yield drastic changes of the electronic and magnetic properties of solids. Novel quantum transport and optical phenomena, involving electronic spin degrees of freedom in semiconductor heterostructures, as well as a rich variety of exotic quantum ground states and magnetic excitations in complex transition metal oxides that arise upon such confinements, belong therefore to topical problems of contemporary condensed matter physics.
In this work electron spin systems in reduced dimensions are studied with Electron Spin Resonance (ESR) spectroscopy, a method which can provide important information on the energy spectrum of the spin states, spin dynamics, and magnetic correlations. The studied systems include quasi onedimensional spin chain materials based on transition metals Cu and Ni. Another class of materials are semiconductor heterostructures made of Si and Ge.
Part I deals with the theoretical background of ESR and the description of the experimental ESR setups used which have been optimized for the purposes of the present work. In particular, the development and implementation of axial and transverse cylindrical resonant cavities for high-field highfrequency ESR experiments is discussed. The high quality factors of these cavities allow for sensitive measurements on μm-sized samples. They are used for the investigations on the spin-chain materials. The implementation and characterization of a setup for electrical detected magnetic resonance is presented.
In Part II ESR studies and complementary results of other experimental techniques on two spin chain materials are presented. The Cu-based material Linarite is investigated in the paramagnetic regime above T > 2.8 K. This natural crystal constitutes a highly frustrated spin 1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearestneighbor interactions. The ESR data reveals that the significant magnetic anisotropy is due to anisotropy of the g-factor. Quantitative analysis of the critical broadening of the linewidth suggest appreciable interchain and interlayer spin correlations well above the ordering temperature. The Ni-based system is an organic-anorganic hybrid material where the Ni2+ ions possessing the integer spin S = 1 are magnetically coupled along one spatial direction. Indeed, the ESR study reveals an isotropic spin-1 Heisenberg chain in this system which unlike the Cu half integer spin-1/2 chain is expected to possess a qualitatively different non-magnetic singlet ground state separated from an excited magnetic state by a so-called Haldane gap. Surprisingly, in contrast to the expected Haldane behavior a competition between a magnetically ordered ground state and a potentially gapped state is revealed.
In Part III investigations on SiGe/Si quantum dot structures are presented. The ESR investigations reveal narrowlines close to the free electron g-factor associated with electrons on the quantum dots. Their dephasing and relaxation times are determined. Manipulations with sub-bandgap light allow to change the relative population between the observed states. On the basis of extensive characterizations, strain, electronic structure and confined states on the Si-based structures are modeled with the program nextnano3. A qualitative model, explaining the energy spectrum of the spin states is proposed.
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Carvalho, Julio Garcia. "Propriedades dinâmicas em sistemas quânticos de muitos corpos." [s.n.], 2006. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277848.
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Orientador: Guillermo Gerardo Cabrera OyarzunTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Abstract: Quantum spin systems are caracterized by huge spaces of states, whose dimensions grow exponentially with the particles number. If following the preparation of the initial state, the system is kept isolated from external variables, it will develop a unitary time evolution according to Schrödinger equation or to Liouville equation. The system is driven exclusively by quantum uctuations, whose origin is the Uncertainty Principle. The evolution of a quantum state or a physical observable or mathematical nonobservable operator mean values may involve all states of the whole space of states, or big or small fractions of the total number of states. The analysis of the relaxation of a spin system from an arbitrary initial state to the equilibrium has to cope in general with the difficulty of requiring an extraordinarily great number of eigenstates and eigenvalues. In this work the main interest is centered on the evolution of magnetization¿s Fourier components in low dimensional systems of spins 1/2, whose interactions be given by the exchange modeled by Heisenberg Hamiltonians with axial anisotopy, XXZ. Exact solutions, analitic or numeric, are obtained. This is the continuation of work done in our research group which dealt with XY Hamiltonian families. In the analysis of the systems with the Hamiltonian XXZ, it was specially analysed the subspace defined by null total magnetization and the subspace defined by one spin wave, where chains up to 14 and 1200 were treated, respectively. There are emergence of fast and slow relaxation processes, which depend on the interations and on the initial state, and which result from destructive or constructive quantum interferences. Connections between the presence of those processes and the energy spectrum structure is discussed. Finally, the time evolution of some measures of global entanglement from initial states in the subspace of one spin wave are analised: the considered dynamics creates global entanglement until each entanglement measure reaches a saturation
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Resumo: Os sistemas quânticos de spin são caracterizados por espaços de estados muito grandes, cujas dimensões crescem exponencialmente com o número de partículas. Se após a preparação do estado inicial, o sistema for mantido isolado de variáveis externas, desenvolve-se uma evolução temporal unitária prescrita pela equação de Schrödinger ou pela equação de Liouville. O sistema é movido exclusivamente por flutuações quânticas, as quais têm sua origem no Princípio da Incerteza. A evolução de um estado quântico ou de valores médios de observáveis físicos ou de operadores matemáticos não observáveis pode envolver todos os estados do espaço de estados, ou frações grandes ou pequenas do número total de estados. A análise da relaxação de um sistema de spins desde um estado inicial arbitrário até o equilíbrio apresenta a dificuldade de requerer em geral um número extraordinariamente grande de auto-estados e autovalores. Neste trabalho o maior interesse está na evolução das componentes de Fourier da magnetização em sistemas de baixa dimensão espacial, com spins 1/2 e cujas interações sejam dadas pela troca modelada por Hamiltonianos de Heisenberg com anisotropia axial, XXZ. Serão obtidas soluções exatas: numéricas ou analíticas. A motivação proveio de trabalhos anteriores realizados no grupo de pesquisa referentes a famílias do Hamiltoniano XY. Ao se considerar o Hamiltoniano XXZ, analisou-se especialmente o subespaço definido por magnetização total nula e o subespa¸ co de uma onda de spin, onde trataram-se cadeias com até 14 e 1200 sítios, respectivamente. Há emergência de processos rápidos e lentos de relaxação, os quais dependem das interações e do estado inicial, e resultam de interferência quântica destrutiva ou construtiva. Serão discutidas conexões entre a presença desses processos e a estrutura do espectro de energia. Finalmente serão analisadas as evoluções temporais de algumas medidas de emaranhamento global, a partir de estados contidos no subespaço de uma onda de spin: a dinâmica considerada cria emaranhamento global até cada medida atingir uma saturação
Doutorado
Física da Matéria Condensada
Doutor em Ciências
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Grijalva, Sebastian. "Boundary effects in quantum spin chains and Finite Size Effects in the Toroidal Correlated Percolation model." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP093.
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Cette thèse est divisée en deux parties : la première présente un modèle statistique en deux dimensions de percolation corrélée sur un réseau toroïdal. Nous présentons un protocole pour construire des surfaces corrélées à longue portée sur la base de surfaces gaussiennes fractionnaires, puis nous relions les ensembles de niveaux à une famille de modèles de percolation corrélés. Les clusters émergents sont ensuite étudiés numériquement, et nous testons leur symétrie conforme en vérifiant que les corrections de taille finie de connectivité à deux points suivent les prédictions de la théorie des champs conformes. Nous commentons également le comportement des fonctions à trois points et fournissons un code numérique pour reproduire les résultats. La deuxième partie de la thèse étudie la chaîne quantique XXZ intégrable de spin-1/2 avec des conditions aux bords ouvertes, pour un nombre pair et impair de sites. Dans régime antiferromagnétique, nous utilisons l'Ansatz de Bethe Algébrique pour déterminer les configurations possibles en termes des champs aux bords. On retrouve les conditions d'existence d'états fondamentaux quasi dégénérés séparés par un gap au reste du spectre. Nous calculons l'aimantation au bord à température nulle et constatons qu'elle dépend du champ sur le bord opposé même dans la limite de chaîne semi-infinie. Nous calculons enfin la fonction d'autocorrélation temporelle au bord et montrons que dans le cas de taille paire, elle est finie à la limite de temps long à cause de la quasi-dégénérescenceThis thesis is divided in two parts: The first one presents a 2D statistical model of correlated percolation on a toroidal lattice. We present a protocol to construct long-range correlated surfaces based on fractional Gaussian surfaces and then we relate the level sets to a family of correlated percolation models. The emerging clusters are then numerically studied, and we test their conformal symmetry by verifying that their planar-limit finite-size corrections follow the predictions of Conformal Field Theory. We comment also the behavior of three-point functions and provide a numerical code to reproduce the results.The second part of the thesis studies the quantum integrable XXZ spin-1/2 chain with open boundary conditions for even and odd number of sites. We concentrate in the anti-ferromagnetic regime and use the Algebraic Bethe Ansatz to determine the configurations that arise in terms of the boundary fields. We find the conditions of existence of quasi-degenerate ground states separated by a gap to the rest of the spectrum. We calculate the boundary magnetization at zero temperature and find that it depends on the field at the opposite edge even in the semi-infinite chain limit. We finally calculate the time autocorrelation function at the boundary and show that in the even-size case it is finite for the long-time limit as a result of the quasi-degeneracy
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Dutoit, Charles-Emmanuel. "Etude par résonance paramagnétique électronique des composés organiques (TMTTF)₂X (X=AsF₆,PF₆ et SbF₆)." Electronic Thesis or Diss., Aix-Marseille, 2016. http://www.theses.fr/2016AIXM4334.
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Ce travail de thèse porte sur l'étude par la résonance paramagnétique électronique (RPE) des sels à transfert de charge quasi-unidimensionnels (TMTTF)₂X (X=AsF₆, PF₆, SbF₆), matériaux modèles de chaînes de spins quantiques. Tout d'abord, nous avons examiné en onde continue et sur une large gamme de température et de fréquence, la phase d'ordre de charge déjà observée dans ces matériaux en dessous de la température TCO. Nous avons mis en évidence deux nouveaux phénomènes à T≺TCO: la rotation des axes principaux du facteur g et une modification structurale liée à un dédoublement de la maille cristallographique. Un calcul de chimie quantique a été réalisé à l'aide de la méthode DFT confirmant nos résultats expérimentaux. Dans la seconde partie de ces travaux de thèse, nous avons présenté les résultats obtenus par RPE en onde continue et en onde pulsée sur l'étude des défauts corrélés dans les systèmes à chaînes de spins. En onde continue, nous avons détecté pour la première fois une raie RPE fine à basse température, suggérant la présence de défauts corrélés ayant les caractéristiques de solitons. Les mesures par RPE pulsée nous ont permis d'observer les premières oscillations de Rabi de solitons piégés et de déterminer leur caractère robuste. Ces derniers résultats offrent une approche alternative aux qubits à base de spins pour le traitement de l’information quantiqueThis thesis focuses on the study by Electron Paramagnetic Resonance (EPR) of the quasi-one-dimensional charge transfer salts (TMTTF)₂X (X=AsF₆, PF₆, SbF₆), model materials of quantum spin chains. First, we have examined in continuous wave and on a wide range of temperature and frequency, the charge-ordered phase already observed in these materials below the temperature TCO. We have identified two new phenomena at T≺TCO: the rotation of the principal axes of the g factor and a structural change related to a doubling of the unit cell parameter. A quantum chemical calculation was carried out using DFT confirming our experimental results. In the second part of the thesis, we have presented the results obtained by EPR in continuous wave and pulsed wave on the correlated defects study in spin chain systems. In continuous wave, we have detected for the first time a narrow EPR line at low temperature, suggesting the presence of correlated defects having the characteristics of solitons. The pulsed EPR measurements allowed us to observe the first Rabi oscillations of trapped solitons and to determine their robust character. These latter results offer an alternative approach for spin qubits in quantum information processing
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Szyniszewski, Marcin. "Low-dimensional quantum systems." Thesis, Lancaster University, 2016. http://eprints.lancs.ac.uk/81787/.
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We study low-dimensional quantum systems with analytical and computational methods. Firstly, the one-dimensional extended t-V model of fermions with interactions of finite range is investigated. The model exhibits a phase transition between liquid and insulating regimes. We use various analytical approaches to generalise previous theoretical studies. We devise a strong coupling expansion to go beyond first-order perturbation theory. The method is insensitive to the presence or the lack of integrability of the system. We extract the ground state energy and critical parameters of the model near the Mott insulating commensurate density. A summary of the methods used is provided to give a broader view of their advantages and disadvantages. We also study the possible charge-density-wave phases that exist when the model is at the critical density. A complete description of phase diagrams of the model is provided: at low critical densities the phases are defined analytically, and at higher critical densities we tackle this problem computationally. We also provide a future outlook for determining the phases that occur at non-zero temperature. Secondly, we investigate Mott-Wannier complexes of two (excitons), three (trions) and four (biexcitons) charge carriers in two-dimensional semiconductors. The fermions interact through an effective interaction of a form introduced by Keldysh. Our study also includes impurity-bound complexes. We provide a classification of trions and biexcitons in transition-metal dichalcogenides, which incorporates the difference of spin polarisation between molybdenum- and tungsten-based materials. Using the diffusion Monte Carlo method, which is statistically exact for these systems, we extract binding energies of the complexes for a complete set of parameters of the model. Our results are compared with theoretical and experimental work on transition-metal dichalcogenides. Agreement is found for excitonic and trionic results, but we also observe a large discrepancy in the theoretical biexcitonic binding energies as compared to the experimental values. Possible reasons for this are outlined. Simple interpolation formulas for binding energies are provided, that can be used to easily determine the values within the accuracy of 5% for any two-dimensional semiconductor. We also calculate contact pair densities, which in the future can be used in the determination of the contact interaction.
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Göttel, Stefan [Verfasser], Herbert [Akademischer Betreuer] Schoeller, and Dirk [Akademischer Betreuer] Schuricht. "Renormalization group approaches to low dimensional systems : scrutinization of the spin functional RG for the 2D XXZ Model real time RG study of a generic 2-level quantum dot in the Coulomb blockade regime in nonequilibrium / Stefan Göttel ; Herbert Schoeller, Dirk Schuricht." Aachen : Universitätsbibliothek der RWTH Aachen, 2015. http://d-nb.info/1128316579/34.
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Kargl, Verena Sabine. "Magnetic properties of low dimensional spin systems." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=98016110X.
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Thomale, Ronny. "Fractional excitations in low-dimensional spin systems." Aachen Shaker, 2008. http://d-nb.info/992564492/04.
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Arango, Yulieth Cristina. "Electron spin resonance (ESR) spectroscopy of low-dimensional spin systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-69811.
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The research in low-dimensional (low-D) quantum spin systems has become an arduous challenge for the condensed matter physics community during the last years. In systems with low dimensional magnetic interactions the exchange coupling is restricted to dimensions lower than the full three-D exhibited by the bulk real material. The remarkable interest in this field is fueled by a continuous stream of striking discoveries like superconductivity, quantum liquid and spin gap states, chiral phases, etc, derived from the strong effect of quantum fluctuations on the macroscopic properties of the system and the competition between electronic and magnetic degrees of freedom. The main goal of the current studies is to reach a broad understanding of the mechanisms that participate in the formation of those novel ground states as well as the characteristic dependence with respect to relevant physical parameters. In this thesis we present the results of an Electron Spin Resonance (ESR)-based study on different quasi-1D spin systems, exemplifying the realization of 1D-magnetic spin-chains typically containing transition metal oxides such as Cu2+ or V4+.
The local sensitivity of the ESR technique has been considered useful in exploring magnetic excitation energies, dominant mechanisms of exchange interactions, spin fluctuations and the dimensionality of the electron spin system, among others. Aside from ESR other experimental results, e.g., magnetization and nuclear magnetic resonance besides some theoretical approaches were especially helpful in achieving a proper understanding and modeling of those low-D spin systems.
This thesis is organized into two parts: The first three chapters are devoted to the basic knowledge of the subject. The first chapter is about magnetic exchange interactions between spin moments and the effect of the crystal field potential and the external magnetic field. The second chapter is a short introduction on exchange interactions in a 1D-spin chain, and the third chapter is devoted to ESR basics and the elucidation of dynamic magnetic properties from the absorption spectrum parameters.
The second part deals with the experimental results. In the fourth chapter we start with the magnetization results from the zero-dimensional endohedral fullerene Dy3N@C80. This system is seemingly ESR “silent” at the frequency of X-band experiments. The fifth chapter shows an unexpected temperature dependence of the anisotropy in the homometallic ferrimagnet Na2Cu5Si4O14 containing alternating dimer-trimer units in the zig-zag Cu-O chains. In the sixth chapter different magnetic species in the layer structure of vanadium oxide nanotubes (VOx-NT) have been identified, confirming earlier magnetization measurements. Moreover the superparamagnetic-like nature of the Li-doped VOx-NT samples was found to justify its ferromagnetic character at particular Li concentration on the room temperature scale. In the seventh chapter the Li2ZrCuO4 system is presented as a unique model to study the influence of additional interactions on frustrated magnetism. The eighth chapter highlights the magnetic properties of the pyrocompound Cu2As2O7. The results suggest significant spin fluctuations below TN.
The thesis closes with the summary and the list of references.
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Ozerov, Mykhaylo. "High-field electron spin resonance in low-dimensional spin systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-69892.
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Due to recent progress in theory and the growing number of physical realizations, low-dimensional quantum magnets continue to receive a considerable amount of attention. They serve as model systems for investigating numerous physical phenomena in spin systems with cooperative ground states, including the field-induced evolution of the ground-state properties and the corresponding rearrangement of their low-energy excitation spectra. This work is devoted to systematic studies of recently synthesized low-dimensional quantum spin systems by means of multi-frequency high-field electron spin resonance (ESR) investigations. In the spin- 1/2 chain compound (C6H9N2)CuCl3 [known as (6MAP)CuCl3] the striking incompatibility with a simple uniform S = 1/2 Heisenberg chain model employed previously is revealed. The observed ESR mode is explained in terms of a recently developed theory, revealing the important role of the alternation and next-nearest-neighbor interactions in this compound. The excitations spectrum in copper pyrimidine dinitrate [PM·Cu(NO3)2(H2O)2]n, an S = 1/2 antiferromagnetic chain material with alternating g-tensor and Dzyaloshinskii-Moriya interaction, is probed in magnetic fields up to 63 T. To study the high field behavior of the field-induced energy gap in this material, a multi-frequency pulsed-field ESR spectrometer is built. Pronounced changes in the frequency-field dependence of the magnetic excitations are observed in the vicinity of the saturation field, B ∼ Bs = 48.5 T. ESR results clearly indicate a transition from the soliton-breather to a spin-polarized state with magnons as elementary excitations. Experimental data are compared with results of density matrix renormalization group calculations; excellent agreement is found. ESR studies of the spin-ladder material (C5H12N)2CuBr4 (known as BPCB) completes the determination of the full spin Hamiltonian of this compound. ESR results provide a direct evidence for a pronounced anisotropy in this compound, that is in contrast to fully isotropic spin-ladder model employed previously for BPCB. Our observations can be of particular importance for describing the rich temperature-field phase diagram of this material. The frequency-field diagram of magnetic excitations in the quasi-two dimensional S = 1/2 compound [Cu(C4H4N2)2(HF2)]PF6 in the AFM-ordered state is studied. The AFM gap is observed directly. Using high-field magnetization and ESR results, parameters of the effective spin-Hamiltonian (exchange interaction, anisotropy and g-factor) are obtained and compared with those estimated from thermodynamic properties of this compound.
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Ozerov, Mykhaylo. "High-field electron spin resonance in low-dimensional spin systems." Doctoral thesis, Helmholtz-Zentrum Dresden-Rossendorf, 2010. https://tud.qucosa.de/id/qucosa%3A25598.
Full textAbstract:
Due to recent progress in theory and the growing number of physical realizations, low-dimensional quantum magnets continue to receive a considerable amount of attention. They serve as model systems for investigating numerous physical phenomena in spin systems with cooperative ground states, including the field-induced evolution of the ground-state properties and the corresponding rearrangement of their low-energy excitation spectra. This work is devoted to systematic studies of recently synthesized low-dimensional quantum spin systems by means of multi-frequency high-field electron spin resonance (ESR) investigations. In the spin- 1/2 chain compound (C6H9N2)CuCl3 [known as (6MAP)CuCl3] the striking incompatibility with a simple uniform S = 1/2 Heisenberg chain model employed previously is revealed. The observed ESR mode is explained in terms of a recently developed theory, revealing the important role of the alternation and next-nearest-neighbor interactions in this compound. The excitations spectrum in copper pyrimidine dinitrate [PM·Cu(NO3)2(H2O)2]n, an S = 1/2 antiferromagnetic chain material with alternating g-tensor and Dzyaloshinskii-Moriya interaction, is probed in magnetic fields up to 63 T. To study the high field behavior of the field-induced energy gap in this material, a multi-frequency pulsed-field ESR spectrometer is built. Pronounced changes in the frequency-field dependence of the magnetic excitations are observed in the vicinity of the saturation field, B ∼ Bs = 48.5 T. ESR results clearly indicate a transition from the soliton-breather to a spin-polarized state with magnons as elementary excitations. Experimental data are compared with results of density matrix renormalization group calculations; excellent agreement is found. ESR studies of the spin-ladder material (C5H12N)2CuBr4 (known as BPCB) completes the determination of the full spin Hamiltonian of this compound. ESR results provide a direct evidence for a pronounced anisotropy in this compound, that is in contrast to fully isotropic spin-ladder model employed previously for BPCB. Our observations can be of particular importance for describing the rich temperature-field phase diagram of this material. The frequency-field diagram of magnetic excitations in the quasi-two dimensional S = 1/2 compound [Cu(C4H4N2)2(HF2)]PF6 in the AFM-ordered state is studied. The AFM gap is observed directly. Using high-field magnetization and ESR results, parameters of the effective spin-Hamiltonian (exchange interaction, anisotropy and g-factor) are obtained and compared with those estimated from thermodynamic properties of this compound.
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Makivic, Miloje S. Cross Michael Clifford Cross Michael Clifford. "Monte Carlo studies of two dimensional quantum spin systems /." Diss., Pasadena, Calif. : California Institute of Technology, 1991. http://resolver.caltech.edu/CaltechETD:etd-07202007-094134.
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Dargel, Piet. "Spectral functions of low-dimensional quantum systems." Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2012. http://hdl.handle.net/11858/00-1735-0000-000D-F1A3-6.
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Lapilli, Cintia Mariela. "Properties of low-dimensional systems." Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/5862.
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Thesis (Ph. D.)--University of Missouri-Columbia, 2006.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (May 2, 2007) Vita. Includes bibliographical references.
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VITIELLO, ELISA. "Strain-dependent spin phenomena in Ge-based low dimensional structures." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2018. http://hdl.handle.net/10281/199103.
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L’ingegnerizzazione della struttura a bande attraverso l’applicazione dello strain è un approccio innovativo che permette di realizzare dispositivi che combinano funzionalità elettroniche, fotoniche e spintroniche.
Lo strain può essere infatti usato per rimuovere degenerazioni nella struttura a bande in punti a simmetria elevata dello spazio reciproco, sopprimendo meccanismi di rilassamento. Inoltre, gli effetti dello strain offrono un grande potenziale per migliorare le prestazioni dei dispositivi fotonici e optoelettronici. In optoelettronica, il Ge è già noto come materiale per fotorivelazione utilizzato nella distribuzione dei dati su chip grazie alla sua diretta compatibilità con la piattaforma Si microelettronica. Recentemente sono studiate ulteriori applicazioni nel campo emergente della fotonica del gruppo IV, inclusi emettitori di luce e laser a base di Ge. Sorprendentemente, nel Ge l’applicazione di strain comporta una transizione da gap indiretto a diretto. Questo porta a un cambiamento radicale delle sue proprietà ottiche, con l'aumento dell'efficienza radiativa interbanda e la possibilità di avere inversione di popolazione e guadagno ottico: vari LED e fotodiodi basati su Ge sono stati fabbricati con successo su Si, ottenendo infine emissione laser. Nonostante tali progressi, molte domande rimangono senza risposta, come ad esempio l'impatto della ricombinazione non radiativa sulla dinamica dei portatori e il calcolo del guadagno ottico. Oltre alle sue applicazioni nel campo della fotonica, il Ge ha un potenziale unico anche nel campo della spintronica. La spintronica ha come obiettivo principale la codifica dei bit d’informazione nello spin dell'elettrone. Con questo obiettivo, lo sforzo della ricerca scientifica è stato focalizzato su materiali che possiedono caratteristiche altamente desiderate, come tempi di vita e diffusione dello spin lunghi. Tra i semiconduttori, gli elementi del IV gruppo presentano proprietà dello spin di ampio interesse. Il comportamento a gap pseudo-diretto del Ge offre inoltre la possibilità unica di orientare gli spin dei portatori attraverso l'assorbimento di luce polarizzata circolarmente. Sfruttando l'accoppiamento tra il momento angolare dei fotoni polarizzati circolarmente e lo stato di spin elettronico, la polarizzazione della luminescenza emessa può essere utilizzata per raccogliere informazioni sui meccanismi fisici che regolano la dinamica dello spin. La banda di conduzione a multivalle e la forte interazione spin-orbita offrono nel germanio una fisica dello spin più complessa e interessante rispetto ai semiconduttori a gap diretto di composti III-V. Il lavoro presentato è uno studio fondamentale sulle proprietà di spin degli elettroni in eterostrutture di Ge sotto strain, con l'obiettivo di comprendere l'effetto di quest’ultimo sulla cinetica dei portatori e dello spin. Questo è di fondamentale interesse e di importanza centrale per le applicazioni in spintronica e fotonica.
In particolare, abbiamo impiegato la fotoluminescenza (PL) risolta in polarizzazione per studiare la fisica degli spin dei sistemi basati su Ge. In questa tesi viene presentata la generazione ottica di portatori spin-polarizzati e la polarizzazione della PL di due eterostrutture di importanza strategica dal punto di vista applicativo. In primo luogo, sfrutteremo gli effetti dello strain sulla polarizzazione della PL del gap diretto in epistrati di Ge su substrati di silicio. Quindi ci concentreremo sul decadimento della polarizzazione PL dell'emissione indiretta in quantum well di Ge in barriere di SiGe. In questo modo forniremo una misurazione diretta del tempo di vita dello spin in queste eterostrutture.Strain offers an effective degree of freedom for tailoring the band structure of semiconducting materials, opening up interesting physical phenomena and advanced application perspectives. Strain can be used to lift degeneracies in the band structure at high symmetry points of reciprocal space, thereby suppressing intervalley and interband scattering. Indeed, the modification of the energy bands curvature near their extrema change therefore the effective masses which are relevant to electronic transport. These ideas have already been widely applied to group-IV semiconductors (including Si as well as Ge and SiGe) to produce significant carrier mobility enhancements, leading to improve performances in MOSFET devices. Moreover, strain effects on the band structure offer great potential for enhancing the performance of photonic and optoelectronic devices. In optoelectronics, Ge is already a well-established photodetector material used for on-chip data distribution, thanks to its direct compatibility with the Si microelectronics platform. Additional applications within the emerging field of group-IV photonics, including light emitters and lasers are also under investigation. Particularly remarkable for Ge is that strain can also be used to modify the nature of its fundamental energy band gap. This modification leads to a dramatic change in its optical radiative properties, with increasing the interband radiative efficiency and allowing population inversion and optical gain: various Ge-based LEDs and photodiodes were successfully fabricated on Si, eventually achieving lasing action. Despite such advances, several questions remain unanswered, such as the impact of nonradiative recombination on carrier dynamics and optical gain. Beside its applications in photonics, Ge have a unique potential also in the field of spintronics. The main goal of spintronics is to encode digital data in the electron spin. With this aim, research effort have been focus on materials which possess highly desired features, such as long spin lifetimes and large diffusion lengths. As opposed to Si, the pseudo-direct-gap behaviour of Ge offers also the unique possibility to efficiently orient spin polarized carriers by the absorption of circularly polarized light through the direct-gap transition. By exploiting the coupling between the angular momentum of circularly polarized photons and the electronic spin state, the polarization of emitted luminescence can be used to gather information about the physical mechanisms governing nonequilibrium spin dynamics. The multivalley conduction band (CB) of Ge and the sizeable spin-orbit interaction thus offer a nontrivial and intriguing spin physics. These properties, together with the feasible monolithic integration in the microelectronics technology, recently put forward the potential of Ge in bridging the gap between spintronic concepts and semiconductor device physics. The present work is a fundamental study about spin properties of electrons in strained Ge heterostructures, with the aim of understand the strain effect on carrier and spin kinetics. This is of both fundamental interest and central importance for applications in spintronics and photonics. In particular, we have employed polarization-resolved photoluminescence (PL) to investigate the spin physics of Ge-based systems. In this thesis we will focus on the optical generation of spin polarized carriers and on the polarization of the PL of of two notable systems. First, we will exploit strain effects on the PL polarization of the direct-gap emission in tensile strained Ge layers epitaxially grown on Si substrates. Then we will focus on the PL polarization decay of the indirect emission in compressively strained Ge quantum wells embedded in SiGe barriers. By doing so we will provide a direct measurement of the spin lifetime in this heterostructure.
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Stone, Matthew B. "Quantum critical behavior of low-dimensional spin 1/2 Heisenberg antiferromagnets." Available to US Hopkins community, 2002. http://wwwlib.umi.com/dissertations/dlnow/3068783.
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王筠 and Yun Wang. "Some quantum effects in confined low-dimensional systems." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B31214708.
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Lee, Junhyun. "Novel Quantum Phase Transitions in Low-Dimensional Systems." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493318.
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We study a number of quantum phase transitions, which are exotic in their nature and separates non-trivial phases of matter. Since quantum fluctuations, which drive these phase transitions, are stronger in low-dimensions, we concentrate on low-dimensional systems. We consider two different two-dimensional systems in this thesis and study their phase transition.
First, we investigate a phase transition in graphene, one of the most famous two-dimensional systems in condensed matter. For a suspended bilayer graphene in ν = 0 quantum Hall regime, the conductivity data and mean-field analysis suggests a phase transition from an antiferromagnetic (AF) state to a valence bond solid (VBS) state, when perpendicular electric field is increased. This AF to VBS phase transition is reminiscent of deconfined criticality, which is a novel phase transition that cannot be explained by Landau’s theory of symmetry breaking. We show that in the strong coupling regime of bilayer graphene, the AF state is destabilized by the transverse electric field, likely resulting in a VBS state. We also consider monolayer and bilayer graphene in the large cyclotron gap limit and show that the effective action for the AF and VBS order parameters have a topological Wess-Zumino-Witten term, supporting that the phase transition observed in experiments is in the deconfined criticality class.
Second, we study the model systems of cuprate superconductor, which is effectively a two-dimensionalal system in the CuO_2 plane. The proposal that the pseudogap metal is a fractionalized Fermi liquid described by a quantum dimer model is extended using the density matrix renormalization group. Measuring the Friedel oscillations in the open boundaries reveals that the fermionic dimers have dispersion minima near (π/2,π/2), which is compatible with the Fermi arcs in photoemission. Moreover, investigating the entanglement entropy suggests that the dimer model with low fermion density is similar to the free fermion system above the Lifshitz transition. We also study the phase transition from a metal with SU(2) spin symmetry to an AF metal. By applying the functional renormalization group to the two-band spin-fermion model, we establish the existence of a strongly coupled fixed point and calculate critical exponents of the fixed point.Physics
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Wang, Yun. "Some quantum effects in confined low-dimensional systems /." Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B18598444.
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Saideh, Ibrahim. "Entanglement in high dimensional quantum systems." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS198/document.
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La détection de l’intrication est une étape indispensable dans le contexte de l’information et du calcul quantique. Cette tâche importante s’est avérée difficile pour les systèmes quantiques de grandes dimensions supérieures à 2 × 3, auquel cas il existe des conditions nécessaires et suffisantes bien établies.Notre approche consiste à réduire la dimensionalité du problème. Pour ce faire, on transforme, localement, chaque sous-système en un qubit sans créer de l’intrication. Le mapping est exprimé en fonction des valeurs moyennes de trois opérateurs arbitraires dans l’état original. Nous donnons des conditions nécessaires et suffisantes pour que cette transformation soit valide d'un point de vue physique. Nous exploitons ce formalisme pour dériver des critères d’intrication pour des systèmes bipartites ou multipartites sur la base des critères existants pour les qubits.En transformant localement chaque sous-système, l’application de critères d’intrication connus pour les qubits à l’état résultant induit automatiquement des critères d’intrication en fonction d’opérateurs utilisés pour réaliser le mapping.Pour le cas multipartite, on s’intéresse aux inégalités de compression de spin. Cependant, lorsqu’on applique notre formalisme à ce cas, il est possible d’obtenir une superposition cohérente d’états avec un nombre de particules différent. Par conséquent, pour obtenir de meilleurs critères, nous avons dû prendre en compte les fluctuations quantiques et/ou classiques que l’opérateur du nombre de particules peut présenter. Nous avons dérivé une forme généralisée des inégalités de spin squeezing pour un nombre de particules fluctuant et opérateurs collectifs arbitraires. Nous avons appliqué nos résultats à un système d’atomes de chrome ultrafroids piégés dans un réseau optique, en collaboration avec l’équipe Gazes Dipolaires Quantiques du Laboratoire LPL de l’Université Paris Nord 13. Nous avons montré, dans une simulation numérique, que nos inégalités généralisées sont capables de détecter l’intrication à l’aide d’opérateurs collectifs mesurables en utilisant des techniques accessibles dans dans ce type de dispositifEntanglement detection is crucial and a necessity in the context of quantum information and quantum computation. This important task has proved to be quite hard for quantum systems of dimensions higher than 2×3, in which case, there exists well established necessary and sufficient conditions like Peres-Horodecki criterion.To tackle this challenge for bipartite systems, we introduce a mathematical framework to reduce the problem to entanglement in a two qubit system. This is done by mapping each subsystem locally into a qubit without increasing entanglement. The mapping is expressed in terms of expectation values of three arbitrary operators in the original state. We give necessary and sufficient conditions for such mapping to be valid from physical point of view, providing thence a versatile tool for dimension reduction in various applications.Our main use of this formalism is as a gate way to derive entanglement criteria for bipartite or multi-partite systemas based on existing ones derived for qubit systems. By mapping each subsystem locally into a qubit, applying entanglement criteria known for qubits on the resulting state automatically gives us entanglement criteria in terms of the chosen operators used to implement the mapping.For the multi-partite case, we focus on spin squeezing inequalities for qubits to derive entanglement criteria for general systems. However, when applying our formalism to this case, an interesting situation arises where one is able to obtain coherent superposition of multi-partite qubit states with different particle number. Hence, to derive better entanglement criteria, we had to consider quantum and/or classical fluctuationsthat may be exhibited by the particle number operator. We derive generalized form of Sørensen-Mølmer’s criterion and of spin squeezing inequalities for fluctuating particle number in terms of arbitrary collective operators. We applied our results to study entanglement in a system of ultra-cold Chromium atoms with spin s = 3 trapped in a bi-dimensional optical lattice incollaboration with Quantum Dipolar Gazes team in Laboratoire de Physique de Laser at Paris Nord 13 university. We showed, in a numerical simulation, that our generalized inequalities are able to detect entanglement in their system using collective operators. Moreover, we show that such observables can be measured using available techniques
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Bao, Yunjuan, and 暴云娟. "Theoretical study of spin transport in low-dimensional systems." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2008. http://hub.hku.hk/bib/B40687570.
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Thomale, Ronny [Verfasser]. "Fractional Excitations in low–dimensional spin systems / Ronny Thomale." Aachen : Shaker, 2009. http://d-nb.info/1161309616/34.
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Bao, Yunjuan. "Theoretical study of spin transport in low-dimensional systems." Click to view the E-thesis via HKUTO, 2008. http://sunzi.lib.hku.hk/hkuto/record/B40687570.
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Scopa, Stefano. "Non-equilibrium dynamics of driven low-dimensional quantum systems." Thesis, Université de Lorraine, 2019. http://www.theses.fr/2019LORR0084/document.
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Cette thèse analyse certains aspects de la dynamique hors équilibre de systèmes quantiques unidimensionnels lorsqu’ils sont soumis à des champs externes dépendant du temps. Nous considérons plus particulièrement le cas des forçages périodiques, et le cas d’une variation temporelle lente d’un paramètre de l’Hamiltonien qui permet de traverser une transition de phase quantique. La première partie contient une présentation des notions, des modèles et des outils nécessaires pour comprendre la suite de la thèse, avec notamment des rappels sur les modèles quantiques critiques (en particulier sur les chaines de spin et sur le modèle de Bose-Hubbard), le mécanisme de Kibble-Zurek, et la théorie de Floquet. Ensuite, nous étudions la dynamique hors équilibre des gaz de Tonks-Girardeau dans un potentiel harmonique dépendant du temps par différentes techniques : développements perturbatifs, diagonalisation numérique exacte et solutions analytiques exactes basées sur la théorie des invariants dynamiques d’Ermakov-Lewis. Enfin, nous analysons la dynamique hors équilibre des systèmes quantiques ouverts markoviens soumis à des variations périodiques des paramètres du système et de l’environnement. Nous formulons une théorie de Floquet afin d’obtenir des solutions exactes des équations de Lindblad périodiques. Ce formalisme de Lindblad-Floquet est utilisé pour obtenir une caractérisation exacte du fonctionnement en temps fini des machines thermiques quantiquesThis thesis analyzes some aspects regarding the dynamics of one-dimensional quantum systems which are driven out-of-equilibrium by the presence of time- dependent external fields. Among the possible kinds of driven systems, our focus is dedicated to the slow variation of a Hamiltonian’s parameter across a quantum phase transition and to the case of a time-periodic forcing. To begin with, we prepare the background and the tools needed in the following. This includes a brief introduction to quantum critical models (in particular to the xy spin chain and to the Bose-Hubbard model), the Kibble-Zurek mechanism and Floquet theory. Next, we consider the non-equilibrium dynamics of Tonks-Girardeau gases in time-dependent harmonic trap potentials. The analysis is made with different techniques: perturbative expansions, numerical exact diagonalization and exact methods based on the theory of Ermakov-Lewis dynamical invariants. The last part of the thesis deals instead with the non-equilibrium dynamics of markovian open quantum systems subject to time-periodic perturbations of the system parameters and of the environment. This has led to an exact formulation of Floquet theory for a Lindblad dynamics. Moreover, within the Lindblad-Floquet framework it is possible to have an exact characterization ofthe finite-time operation of quantum heat-engines
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Wheeler, Elisa Maria da Silva. "Neutron scattering from low-dimensional quantum magnets." Thesis, University of Oxford, 2007. http://ora.ox.ac.uk/objects/uuid:a8411774-4a3e-4fc3-80a1-d7e8612cba71.
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Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO2. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb2O6, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb2O6. It has a square lattice of Cu2+ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.
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Sandoildo, Freitas Tenório Antônio. "Phase transitions and thermodynamics of quasione- dimensional quantum rotor and spin systems." Universidade Federal de Pernambuco, 2009. https://repositorio.ufpe.br/handle/123456789/6664.
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Previous issue date: 2009Sandoildo Freitas Tenório, Antônio; Domingues Coutinho Filho, Maurício. Phase transitions and thermodynamics of quasione- dimensional quantum rotor and spin systems. 2009. Tese (Doutorado). Programa de Pós-Graduação em Física, Universidade Federal de Pernambuco, Recife, 2009.
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Grosse, Harald, Karl-Georg Schlesinger, and grosse@doppler thp univie ac at. "A Suggestion for an Integrability Notion for Two Dimensional Spin." ESI preprints, 2001. ftp://ftp.esi.ac.at/pub/Preprints/esi1015.ps.
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Knetter, Christian. "Perturbative continuous unitary transformations: spectral properties of low dimensional spin systems." [S.l. : s.n.], 2003. http://deposit.ddb.de/cgi-bin/dokserv?idn=968587712.
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Kaul, Enrique Eduardo. "Experimental Investigation of New Low-Dimensional Spin Systems in Vanadium Oxides." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2005. http://nbn-resolving.de/urn:nbn:de:swb:14-1131439690937-49243.
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In this dissertation we reported our experimental investigation of the magnetic properties of nine low-dimensional vanadium compounds. Two of these materials are completely new (Pb2V5O12 and Pb2VO(PO4)2) and were found during our search for new low-dimensional vanadium oxides. Among the other seven vanadium compounds studied, three were physically investigated for the first time (Sr2VO(PO4)2, BaZnVO(PO4)2 and SrZnVO(PO4)2). Two had hitherto only preliminary, and wrongly interpreted, susceptibility measurements reported in the literature (Sr2V3O9 and Ba2V3O9) while the remaining two (Li2VOSiO4 and Li2VOGeO4) were previously investigated in some detail but the interpretation of the data was controversial. We investigated the magnetic properties of these materials by means of magnetic susceptibility and specific heat (Cp(T)) measurements (as well as single crystal ESR measurements in the case of Sr2V3O9). We synthesized the samples necessary for our physical studies. That required a search of the optimal synthesis conditions for obtaining pure, high quality, polycrystalline samples. Single crystals of Sr2V3O9 and Pb2VO(PO4)2 were also successfully grown. Pb2VO(PO4)2, BaZnVO(PO4)2, SrZnVO(PO4)2, Li2VOSiO4 and Li2VOGeO4 were found to be experimental examples of frustrated square-lattice systems which are described by the J1-J2 model. We found that Li2VOSiO4 and Li2VOGeO4 posses a weakly frustrated antiferromagnetic square lattice while Pb2VO(PO4)2, BaZnVO(PO4)2 and SrZnVO(PO4)2 form a more strongly frustrated ferromagnetic square lattice. Pb2V5O12 is structurally and compositionally related to the two dimensional A2+V4+nO2n+1 vanadates. Its structure consists of layers formed by edge- and corner-shared square VO5 pyramids. The basic structural units are plaquettes consisting of six corner-shared pyramids pointing in the same direction, which form a spin lattice of novel geometryIn dieser Dissertation berichteten wir über unsere experimentelle Untersuchung der magnetischen Eigenschaften von neun Niedrigdimensionalen vanadiumverbindungen. Zwei dieser Materialien sind vollständig neu (Pb2VO12 und Pb2VO(PO4)2) und wurden während unserer Suche nach neuen Niedrigdimensionalen Vanadiumoxiden gefunden. Unter den anderen sieben studierten Vanadiumverbindungen, wurden drei physikalisch zum ersten Mal nachgeforscht (Sr2VO(PO4)2, BaZnVO(PO4)2 und SrZnVO(PO4)2). Zwei hatten bisher nur einleitendes, und falsch gedeutet, magnetische Susceptibilitaet Messungen, die in der Literatur berichtet wurden (Sr2V3O9 und Ba2V3O9) während die restlichen zwei (Li2VOSiO4 und Li2VOGeO4) vorher in irgendeinem Detail aber in der Deutung der Daten waren umstritten nachgeforscht wurden. Wir forschten die magnetischen Eigenschaften dieser Materialien mittels der magnetischen Susceptibilitaet und der spezifischen Waerme (Cp(T)) nach (sowie ESR-Messungen des einzelnen Kristalles im Fall von Sr2V3O9). Wir synthetisierten die Proben, die für unsere körperlichen Studien notwendig sind. Das erforderte eine Suche der optimalen Synthesezustände für das Erreichen der reinen, hohen Qualität, polykristalline Proben. Einzelne Kristalle von Sr2V3O9 und von Pb2VO(PO4)2 wurden auch erfolgreich gewachsen. Pb2VO(PO4)2, BaZnVO(PO4)2, SrZnVO(PO4)2, Li2VOSiO4 und Li2VOGeO4 werden gefunden, um experimentelle Beispiele der frustrierten Quadrat-Gittersysteme zu sein, die durch das J1-j2 model. Wir fanden daß posses Li2VOSiO4 und Li2VOGeO4 ein schwach frustriertes antiferromagnetische quadratisches Gitter, während Pb2VO(PO4)2, BaZnVO(PO4)2 und SrZnVO(PO4)2 ein stärker frustriertes ferromagnetisches quadratisches Gitter bilden. Pb2V5O12 strukturell und zusammenhängt kreativ mit den zweidimensionalen vanadates A2+V4+nO2n+1 beschrieben werden. Seine Struktur besteht aus den Schichten, die durch Rand und Ecke-geteilte quadratische Pyramiden VO5 gebildet werden. Die grundlegenden strukturellen Maßeinheiten sind die plaquettes, die aus sechs Ecke-geteilten Pyramiden bestehen, die in die gleiche Richtung zeigen, die ein Drehbeschleunigunggitter von Romangeometrie bilden
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Huberman, Tom. "Neutron scattering studies and classical simulations on low-dimensional spin systems." Thesis, University of Oxford, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409798.
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Kennes, Dante Marvin [Verfasser]. "Dynamics in low-dimensional correlated quantum systems / Dante Marvin Kennes." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2014. http://d-nb.info/1065416458/34.
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Nakpathomkun, Natthapon 1973. "Thermoelectric properties of quantum dots and other low-dimensional systems." Thesis, University of Oregon, 2010. http://hdl.handle.net/1794/11060.
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xii, 106 p. : ill. (some col.)Quantum dots are systems in which all three spatial sizes are comparable to the Fermi wavelength. The strong confinement leads to a discrete energy spectrum. A goal of thermoelectric research is to find a system with a high thermoelectric figure of merit, which is related to the efficiency of solid-state heat engines. The delta-like density of states of quantum dots has been predicted to boost this figure of merit. This dissertation addresses some thermoelectric properties relevant to the thermal-to-electric energy conversion using InAs/InP quantum dots embedded in nanowires. In thermoelectric experiments, a temperature difference must be established and its value needs to be determined. A novel technique for measuring electron temperature across the dot is presented. A strong nonlinearity of the thermocurrent as a function of temperature difference is observed at a small ratio of temperature gradient and cryostat temperature. At large heating currents, a sign reversal is observed. Numerical calculations explore the contribution of the energy dependence of the transmission function to this effect. Depending on the relative contributions from sequential tunneling and co-tunneling, thermovoltages of quantum dots generally have one of two different lineshapes: a sawtooth shape or a shape similar to the derivative of the conductance peak. Here a simple picture is presented that shows that thermovoltage lineshape is accurately predicted from the energy level spacing inside the dot and the width of the transmission function. An important figure of merit of all heat engines is the efficiency at maximum power. Here the thermoelectric efficiency at maximum power of quantum dots is numerically compared to that of two other low-dimensional systems: an ideal one-dimensional conductor (1D) and a thermionic power generator (TI). The numerical calculations show that either 1D or TI systems can produce the highest maximum power depending on the operating temperature, the effective mass of the electron, and the effective area of the TI system. In spite of this, 1D systems yield the highest efficiency at maximum power.
Committee in charge: Dr. Richard Taylor, Chair; Dr. Heiner Linke, Research Advisor; Dr. Dietrich Belitz; Dr. David Johnson; Dr. David Strom
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Jestadt, Thomas. "Muon studies of low-dimensional solid state systems." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.301173.
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Dixit, Mehul. "Topics in Low-Dimensional Systems and a Problem in Magnetoelectricity." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1351878616.
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Hadley, Christopher Andrew. "Entanglement and quantum information theory in the context of higher dimensional spin systems." Thesis, University College London (University of London), 2008. http://discovery.ucl.ac.uk/1444185/.
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Quantum information theory is an exciting, inter-disciplinary field, combining elements of condensed matter theory, quantum mechanics and information theory. In this thesis, I shall make a modest contribution to this field by examining entanglement in many-body systems with more than two levels. In the first section, I consider the dynamics of a system of qutrits three-level quantum systems which are coupled through an SU(3)-invariant permutation Hamiltonian. Each term in this Hamil- tonian is a nearest-neighbour permutation operator, and thus this Hamiltonian may be considered a generalisation of the standard SU (2)-invariant Heisenberg Hamiltonian, in which every term (up to the addition of the identity operator) is a nearest-neighbour permutation operator for two-level system. The system considered has the topology of a cross, and thus may be considered (to a limited extent) analogous to a beam-splitter. The aim of the study is to establish a Bell singlet state between two distant parties. Building on this work, I shall go on to consider the ground state of a system made up of many-level systems coupled by the same Hamiltonian I shall show that this state is a generalisation of the two-level singlet to many levels and many systems. It thus has a high degree of symmetry. I will consider its application in entanglement distribution through measurements (localisable entanglement), and discuss how it may be physically implemented in systems of ultracold atoms, through the Hubbard model. I shall also show that in the famous valence bond solid (the ground state of the Affleck-Kennedy- Lieb-Tasaki spin chain), all the entanglement present in the state may be extracted from a single copy of the chain this is in contrast to gapless, critical chains, in which only half the total entanglement is extractable from a single copy.
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Calvanese, Strinati Marcello. "Topological effects in one-dimensional quantum systems." Doctoral thesis, Scuola Normale Superiore, 2018. http://hdl.handle.net/11384/85903.
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Tchaplyguine, Igor. "Electronic structure of strongly correlated low-dimensional spin ½ systems: cuprates and vanadates." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2003. http://nbn-resolving.de/urn:nbn:de:swb:14-1052218731218-09287.
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In the first two chapters we presented the basics of density functional theory and semiempirical LSD+U approximation, which was implemented in the full-potential local-orbital (FPLO) minimal-basis calculation scheme. In the third chapter we tested the implemented version of LSDA+U on 3d transitional metal monoxides. Essential improvement of the spectroscopic properties was obtained. A simple model describing the value and direction of the magnetic moment of a transition metal ion was presented. The model visualizes the interplay of the spin-orbit coupling and crystal field splitting. In the fourth chapter we calculated the electronic spectrum of the single Zn impurity in CuO2 plane considered as a vacancy in Cu 3d states. The analytic solution for the states of different symmetry was obtained. Depending on the strength of perturbation induced by the impurity on the neighboring Cu ions, the states are either resonant or localized. The critical values of the perturbation were computed. In the fifth chapter we presented the calculations for three novel vanadates: MgVO3, Sb2O2VO3 and VOMoO4. The tight-binding parameters and the exchange integrals were computed. The magnesium and antimony vanadates appeared to be spin-½ one-dimensional systems, the latter having much stronger one-dimensional character and being probably the best realization of inorganic spin-Peierls system. The molybdenum vanadate was found to be two-dimensional spin-½ system. The Mo 4d orbitals play an important role in the electronic transfer.
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Tchaplyguine, Igor. "Electronic structure of strongly correlated low-dimensional spin ½ systems: cuprates and vanadates." Doctoral thesis, Technische Universität Dresden, 2002. https://tud.qucosa.de/id/qucosa%3A24217.
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In the first two chapters we presented the basics of density functional theory and semiempirical LSD+U approximation, which was implemented in the full-potential local-orbital (FPLO) minimal-basis calculation scheme. In the third chapter we tested the implemented version of LSDA+U on 3d transitional metal monoxides. Essential improvement of the spectroscopic properties was obtained. A simple model describing the value and direction of the magnetic moment of a transition metal ion was presented. The model visualizes the interplay of the spin-orbit coupling and crystal field splitting. In the fourth chapter we calculated the electronic spectrum of the single Zn impurity in CuO2 plane considered as a vacancy in Cu 3d states. The analytic solution for the states of different symmetry was obtained. Depending on the strength of perturbation induced by the impurity on the neighboring Cu ions, the states are either resonant or localized. The critical values of the perturbation were computed. In the fifth chapter we presented the calculations for three novel vanadates: MgVO3, Sb2O2VO3 and VOMoO4. The tight-binding parameters and the exchange integrals were computed. The magnesium and antimony vanadates appeared to be spin-½ one-dimensional systems, the latter having much stronger one-dimensional character and being probably the best realization of inorganic spin-Peierls system. The molybdenum vanadate was found to be two-dimensional spin-½ system. The Mo 4d orbitals play an important role in the electronic transfer.
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Dai, Ji. "Low-dimensional electron systems studied by angle- and spin-resolved photoemission spectroscopy." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS345.
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Les matériaux dans lesquels des interactions à plusieurs particules, un confinement de faible dimension et/ou un fort couplage spin-orbite sont présents témoignent d’une grande variété de phénomènes, mais sont encore mal compris. Des informations essentielles sur l’origine de tels phénomènes peuvent être obtenues en mesurant leur structure électronique. Cette thèse présente une étude expérimentale de la structure électronique de matériaux de faible dimension et/ou fortement corrélés présentant un intérêt fondamental actuel, en utilisant la spectroscopie par photoémission résolue en angle et en spin (ARPES et SARPES).Dans la partie introductive, je présente mon travail sur deux exemples de type "livre de texte", mais innovants, montrant comment les interactions affectent la structure de bande d'un matériau: le couplage des électrons avec des phonons dans une distribution de Debye dans un système électronique à deux dimensions (2DES) dans ZnO, semi-conducteur à oxyde à bande interdite large utilisé dans les applications photovoltaïques, et le dédoublement induit par un fort couplage spin-orbite (SOC) dans la bande de valence du ZnTe, un autre semi-conducteur important utilisé dans les dispositifs optoélectroniques. Ensuite, dans la suite de cette thèse, je discute de mes résultats originaux dans trois systèmes différents de basse dimensionnalité et d'intérêt actuel en recherche : 1.La réalisation d'un 2DES à la surface (110) de SnO₂, le premier du genre dans une structure rutile. L'ajustabilité de la densité de ses porteurs au moyen de la température ou du dépôt d'Eu, et la robustesse vis-à-vis les reconstructions de surface et l'exposition aux conditions ambiantes rendent ce 2DES prometteur pour les applications. Au moyen d'une simple réaction redox à la surface, ces travaux ont prouvé que les lacunes en oxygène pouvaient doper la bande de conduction à la surface de SnO₂, résolvant ainsi un problème longtemps débattu concernant le rôle desdites lacunes dans le dopage de type n dans SnO₂. 2.L'étude des états de surface topologiques dans M₂Te₂X (avec M = Hf, Zr ou Ti; et X = P ou As), une nouvelle famille de métaux topologiques en trois dimensions, provenant du SOC et étant protégés par la symétrie du renversement du temps. Leur structure électronique et leur texture de spin, étudiées par ARPES et SARPES, révèlent la présence de fermions de Dirac sans masse donnant naissance à des arcs de nœuds de Dirac. 3.L'étude du matériau YbNi₄P₂ à fermions lourds quasi unidimensionnel, qui présente une transition de phase quantique de second ordre d’une phase ferromagnétique à une phase paramagnétique de liquide de Fermi lors de la substitution partielle du phosphore par l'arséniure. Une telle transition ne devrait se produire que dans les systèmes zéro ou unidimensionnels, mais la mesure directe de la structure électronique des matériaux ferromagnétiques quantiques critiques faisait jusqu'à présent défaut. Grâce à une préparation et nettoyage méticuleux in situ de la surface des monocristaux YbNi₄P₂, qui sont impossibles à cliver, leur structure électronique a été mesurée avec succès au moyen de l'ARPES, dévoilant ainsi le caractère quasi-1D, nécessaire à la compréhension de la criticité quantique ferromagnétique, dans YbNi₄P₂. Le protocole utilisé pour rendre ce matériau accessible à l'ARPES peut être facilement généralisé à d'autres matériaux exotiques dépourvus de plan de clivageMaterials in which many-body interactions, low-dimensional confinement, and/or strong spin-orbit coupling are present show a rich variety of phenomena, but are still poorly understood. Essential information about the origin of such phenomena can be obtained by measuring their electronic structure. This thesis presents an experimental study of the electronic structure of some low-dimensional and/or strongly correlated materials of current fundamental interest, using angle- and spin-resolved photoemission spectroscopy (ARPES and SARPES). In the introductory part, I present my work on two innovative textbook examples showing how interactions affect the band structure of a material: the coupling of electrons with phonons in a Debye distribution in a two-dimensional electron system (2DES) in ZnO, a wide-band-gap oxide semiconductor used in photovoltaic applications, and the splitting induced by strong spin-orbit coupling (SOC) in the bulk valence band of ZnTe, another important semiconductor used in optoelectronic devices. Then, in the rest of this thesis, I discuss my original results in three different low-dimensional systems of current interest: 1.The realisation of a 2DES at the (110) surface of SnO₂, the first of its kind in a rutile structure. Tunability of its carrier density by means of temperature or Eu deposition and robustness against surface reconstructions and exposure to ambient conditions make this 2DES promising for applications. By means of a simple redox reaction on the surface, this work has proven that oxygen vacancies can dope the conduction band minimum at the surface of SnO₂, solving a long-debated issue about their role in n-type doping in SnO₂. 2.The study of topological surface states in M₂Te₂X (with M = Hf, Zr, or Ti; and X = P or As), a new family of three-dimensional topological metals, originating from SOC and being protected by time-reversal symmetry. Their electronic structure and spin texture, studied by ARPES and SARPES, reveal the presence of massless Dirac fermions giving rise to Dirac-node arcs. 3.The investigation of the quasi-one-dimensional heavy-fermion material YbNi₄P₂, which presents a second-order quantum phase transition from a ferromagnetic to a paramagnetic phase upon partial substitution of phosphorous by arsenide. Such a transition is expected to occur only in zero- or one-dimensional systems, but a direct measurement of the electronic structure of ferromagnetic quantum-critical materials was missing so far. By careful in-situ preparation and cleaning of the surface of YbNi₄P₂ single crystals, which are impossible to cleave, their electronic structure has been successfully measured by ARPES, thus effectively unveiling the quasi-one-dimensionality of YbNi₄P₂. Moreover, the protocol used to make this material accessible to ARPES can be readily generalised to other exotic materials lacking a cleavage plane
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Robinson, Neil Joe. "Pairing, paramagnetism and prethermalization in strongly correlated low-dimensional quantum systems." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:167d164c-e318-49b3-83ea-69b54ec531e0.
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Quasi-one-dimensional quantum models are ideal for theoretically exploring the physical phenomena associated with strong correlations. In this thesis we study three examples where strong correlations play an important role in the static or dynamic properties of the system. Firstly, we examine the behaviour of a doped fermionic two-leg ladder in which umklapp interactions are present. Such interactions arise at special band fillings and can be induced by the formation of charge density wave order in an array of two-leg ladders with long-range (three-dimensional) interactions. For the umklapp which arises from the half-filling of one of the bands, we show that the low-energy theory has a number of phases, including a strong coupling regime in which the dominant fluctuations are superconducting in nature. These superconducting fluctuations carry a finite wave vector – they are the one-dimensional analogue of Fulde-Ferrell-Larkin-Ovchinnikov superconductivity. In a second example, we consider a quantum spin model which captures the essential one-dimensional physics of CoNb2O6, a quasi-one-dimensional Ising ferromagnet. Motivated by high-resolution inelastic neutron scattering experiments, we calculate the dynamical structure in the paramagnetic phase and show that a small misalignment of the transverse field can lead to quasi-particle breakdown – a surprising broadening in the single particle mode observed in experiment. Finally, we study the out-of-equilibrium dynamics of a model with tuneable integrability breaking. When integrability is broken by the presence of weak interactions, we show that the system relaxes to a non-thermal state on intermediate time scales, the so-called “prethermalization plateau”. We describe the approximately stationary behaviour in this regime by constructing a generalised Gibbs ensemble with charges deformed to leading order in perturbation theory. Expectation values of these charges are time-independent, but interestingly the charges do not commute with the Hamiltonian to leading order in perturbation theory. Increasing the strength of the integrability breaking interactions leads to behaviour compatible with thermalisation. In each case we use a combination of perturbative analytical calculations and non-perturbative numerical computations to study the problem at hand.
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Coak, Matthew. "Quantum tuning and emergent phases in charge and spin ordered materials." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/280284.
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A major area of interest in condensed matter physics over the past decades has been the emergence of new states of matter from strongly correlated electron systems. A few limited examples would be the emergence of unconventional superconductivity in the high-T$_c$ superconductors and heavy-fermion systems, the appearance of the skyrmion magnetic vortex state in MnSi and magnetically mediated superconductivity in UGe$_2$. While detailed studies of many of the emergent phases have been made, there are still many gaps in understanding of the underlying states and mechanisms that allow them to form. This work aims to add to knowledge of the basic physics behind such states, and the changes within them as they are tuned to approach new phases. The cubic perovskite material SrTiO$_3$ has been studied for many decades and is well-documented to be an incipient ferroelectric, theorised to exist in the absence of any tuning in the proximity of a ferroelectric quantum critical point. This work presents the first high-precision dielectric measurements under hydrostatic pressure carried out on a quantum critical ferroelectric, leading to a full pressure-temperature phase diagram for SrTiO$_3$. The influence of quantum critical fluctuations is seen to diminish as the system is tuned away from the quantum critical point and a novel low temperature phase is shown to be emergent from it. The Néel Temperature of the two-dimensional antiferromagnet FePS$_3$ was found to increase linearly with applied hydrostatic pressure. Evidence of an insulator-metal transition is also presented, and an unexplained upturn in the resistivity at low temperatures in the metallic phase.
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Ford, Richard Anthony. "Optical studies of low-dimensional electron systems at high magnetic fields." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320157.
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Bleu, Olivier. "Physics of quantum fluids in two-dimensional topological systems." Thesis, Université Clermont Auvergne (2017-2020), 2018. http://www.theses.fr/2018CLFAC044/document.
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Cette thèse est consacrée à la description de la physique à une particule ainsi qu'à celle de fluides quantiques bosoniques dans des systèmes topologiques. Les deux premiers chapitres sont introductifs. Dans le premier, nous introduisons des éléments de théorie des bandes et les quantités géométriques et topologiques associées : tenseur métrique quantique, courbure de Berry, nombre de Chern. Nous discutons différents modèles et réalisations expérimentales donnant lieu à des effets topologiques. Dans le second chapitre, nous introduisons les condensats de Bose-Einstein ainsi que les excitons-polaritons de cavité.La première partie des résultats originaux discute des phénomènes topologiques à une particule dans des réseaux en nid d'abeilles. Cela permet de comparer deux modèles théoriques qui mènent à l'effet Hall quantique anormal pour les électrons et les photons dû à la présence d'un couplage spin-orbite et d'un champ Zeeman. Nous étudions aussi l'effet Hall quantique de vallée photonique à l'interface entre deux réseaux de cavités avec potentiels alternés opposés.Dans une seconde partie, nous discutons de nouveaux effets qui émergent due à la présence d'un fluide quantique interagissant décrit par l’équation de Gross-Pitaevskii dans ces systèmes. Premièrement, il est montré que les interactions spin anisotropes donnent lieu à des transitions topologiques gouvernées par la densité de particules pour les excitations élémentaires d’un condensat spineur d’exciton-polaritons.Ensuite, nous montrons que les tourbillons quantifiés d'un condensat scalaire dans un système avec effet Hall quantique de vallée, manifestent une propagation chirale le long de l'interface contrairement aux paquets d'ondes linéaires. La direction de propagation de ces derniers est donnée par leur sens de rotation donnant lieu à un transport de pseudospin de vallée protégé topologiquement, analogue à l’effet Hall quantique de spin.Enfin, revenant aux effets géométriques linéaires, nous nous sommes concentrés sur l’effet Hall anormal. Dans ce contexte, nous présentons une correction non-adiabatique aux équations semi-classiques décrivant le mouvement d’un paquet d’ondes qui s’exprime en termes du tenseur géométrique quantique. Nous proposons un protocole expérimental pour mesurer cette quantité dans des systèmes photonique radiatifsThis thesis is dedicated to the description of both single-particle and bosonic quantum fluid Physics in topological systems. After introductory chapters on these subjects, I first discuss single-particle topological phenomena in honeycomb lattices. This allows to compare two theoretical models leading to quantum anomalous Hall effect for electrons and photons and to discuss the photonic quantum valley Hall effect at the interface between opposite staggered cavity lattices.In a second part, I present some phenomena which emerge due to the interplay of the linear topological effects with the presence of interacting bosonic quantum fluid described by mean-field Gross-Pitaevskii equation. First, I show that the spin-anisotropic interactions lead to density-driven topological transitions for elementary excitations of a condensate loaded in the polariton quantum anomalous Hall model (thermal equilibrium and out-of-equilibrium quasi-resonant excitation configurations). Then, I show that the vortex excitations of a scalar condensate in a quantum valley Hall system, contrary to linear wavepackets, can exhibit a robust chiral propagation along the interface, with direction given by their winding in real space, leading to an analog of quantum spin Hall effect for these non-linear excitations. Finally, coming back to linear geometrical effects, I will focus on the anomalous Hall effect exhibited by an accelerated wavepacket in a two-band system. In this context, I present a non-adiabatic correction to the known semiclassical equations of motion which can be expressed in terms of the quantum geometric tensor elements. We also propose a protocol to directly measure the tensor components in radiative photonic systems