Дисертації: "Correlated fermions"

1

Schofield, Andrew John. "Flux phases for correlated fermions." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.282101.

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2

De, Lia Anthony Frances. "Functional-integral studies of correlated fermions." Honors in the Major Thesis, University of Central Florida, 1993. http://digital.library.ucf.edu/cdm/ref/collection/ETH/id/113.

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This item is only available in print in the UCF Libraries. If this is your Honors Thesis, you can help us make it available online for use by researchers around the world by following the instructions on the distribution consent form at http://library.ucf.edu/Systems/DigitalInitiatives/DigitalCollections/InternetDistributionConsentAgreementForm.pdf You may also contact the project coordinator, Kerri Bottorff, at kerri.bottorff@ucf.edu for more information.
Bachelors
Arts and Sciences
Physics

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3

Shelton, David G. "Low dimensional strongly correlated systems." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320594.

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4

Corboz, Philippe Roger. "Simulations of strongly correlated fermions and bosons /." Zürich : ETH, 2008. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17994.

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5

Cheuk, Lawrence W. "Quantum gas microscopy of strongly correlated fermions." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/112078.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 239-251).
This thesis describes experiments on ultracold fermionic atoms, and can be divided into two areas. The first concerns spin-orbit coupling; the second concerns quantum gas microscopy. With the use of Raman transitions, ID spin-orbit coupling of ultracold 6Li was realized. Using a novel type of spectroscopy, spin-injection spectroscopy, where the spin, energy, and momentum are all resolved, we directly observed the spinful dispersions of the spin-orbit bands. In addition, we demonstrated selective adiabatic loading of the spin-orbit bands, which can be used to create a spinless Fermi gas with effective p-wave interactions. Spin-injection spectroscopy was further applied to a novel spinful lattice system created using Raman and radio-frequency coupling, which allowed for state tomography of spinful bands. The second part of this thesis describes quantum gas microscopy of ultracold fermions. This enables one to simulate the Fermi-Hubbard model, a prototypical strongly correlated model, with site-resolved detectioi and control capablities. A new apparatus that can detect fermionic 40K in a square lattice with single-site resolution was constructed. High-fidelity site-resolved imaging was achieved using Raman imaging, which allowed for the direct observation of the band-insulating, the metallic, and the Mott-insulating states of the Hubbard model. The interactiondriven Mott insulator, where doubly occupied sites are highly suppressed, illustrates the strongly correlated nature of the Hubbard model. Harnessing the capability to measure the occupations of individual lattice sites with the microscope, we explored spatial correlations of both spin and charge in the Hubbard model as a function of doping. For the spin correlations, we observed weakening of antiferromagnetic correlations away from half-filling. However, in the charge correlations between local magnetic moments, non-monotonic behavior was observed. This can be understood as arising from competition between Pauli-blocking, dominant at low fillings, and doublon-holon bunching, which arises from superexchange and is strongest at half-filling. The anti-bunching correlations at low filling can be interpreted as the first direct real-space observation of the interaction-enhanced Pauli hole.
by Lawrence W. Cheuk.
Ph. D.

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6

Del, Re Lorenzo. "Multicomponent strongly correlated fermions in optical lattices." Doctoral thesis, SISSA, 2016. http://hdl.handle.net/20.500.11767/4907.

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The present thesis is devoted to the study of physical phenomena emerging from strong correlations in strongly interacting quantum many-body systems with several components. Hubbard models are widely used as minimal models which take into account the interactions between particles and they have been studied in relation to phenomena such as Mott localization, unconventional superconductivity, quantum magnetism and many others. All of these striking phenomena share their origin from the strong correlations among fermions induced by their mutual interactions. Furthermore, condensed matter models are usually realized only in an approximate fashion in actual solid-state systems, making the situation all the more puzzling and hard to be treated analytically or numerically. Therefore, a great effort has been performed to simulate Hubbard models in a system of atoms cooled down to ultra low temperatures and trapped in optical lattices. The most peculiar feature of cold atoms experiments consists in the possibility of tuning relevant physical parameters of the systems, as the density or the interactions among atoms, using laser and/or magnetic fields. This paved the way to the observation of fundamental quantum states of matter as the weakly interacting Bose-Einstein condensate, the super fluid to Mott insulator transition, the super fluid BEC-BCS crossover, the Mott transition in systems of composite fermions and so on. Hence, it is considered of great interest establishing connections between the quantum simulations cold atomic toolbox and systems realized in solid-state physics...

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7

Sandri, Matteo. "The Gutzwiller Approach to out-of-equilibrium correlated fermions." Doctoral thesis, SISSA, 2014. http://hdl.handle.net/20.500.11767/3900.

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Correlated electron systems represent a wide class of materials which at equilibrium display fascinating properties. Several recent experimental breakthroughs in the field of femtosecond spectroscopy and cold atomic gases allow nowadays to investigate the real time dynamics of these many-body quantum systems. Since strongly correlated systems usually escape single particle approaches, the theoretical study of their dynamics constitutes a formidable problem which necessitates the development of novel techniques. In this Thesis we investigate the out-of-equilibrium physics of simple paradigmatic models that are believed to capture some essential physics of interacting fermions by means of the time dependent extension of the Gutzwiller Variational Approach. After an introductory Chapter on the recent results in this field, in Chapter 2 we present the Gutzwiller Approach in-and-out of equilibrium. In Chapter 3 we investigate the dynamics for the single band Hubbard model after a linear ramp of the Coulomb interaction. We will show that a dynamical transition appears for any duration of the ramp; this dynamical point is adiabatically connected to the zero temperature Metal-to-Insulator transition. We will then consider the role of quantum fluctuations beyond mean field. In Chapter 4 we consider the dynamics of an initial antiferromagnetic state under a quench of the interaction in the single band fermionic Hubbard model. We will show that non-thermal ordered states survive more than expected and that two different nonequilibrium antiferromagnets can be distinguished. Finally in Chapter 5 we will consider a two-band Hubbard model which we believe captures the main physics of the paradigmatic compound vanadium sesquioxide, V2 O3 . After an investigation of the equilibrium properties for this model, we will provide evidences that non-thermal metallic phases can emerge upon an excitation of a Mott insulator.

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8

Liu, Tianhan. "Strongly Correlated Topological Phases." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066403.

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Cette thèse porte principalement sur l'étude de modèles de fermions en interactions contenant un couplage spin-orbite. Ces modèles (i) peuvent décrire une classe de matériaux composés d'iridates sur le réseau en nid d'abeille ou (ii) pourraient être réalisés artificiellement dans des systèmes d’atomes froids. Nous avons étudié, dans un premier temps, le système à demi-remplissage avec l'interaction de Hubbard et un couplage spin-orbite anisotrope. Nous avons trouvé plusieurs phases: la phase isolant topologique pour de faibles corrélations, et deux phases avec des ordres magnétiques frustrés, l'ordre de Néel et l'ordre spiral, dans la limite de très fortes corrélations. La transition entre les régimes de faibles et de fortes corrélations est une transition de Mott dans laquelle les excitations électroniques se fractionnent en excitations de charge et de spin. Les charges sont localisées par l'interaction. Le secteur de spin présente de fortes fluctuations qui sont modélisées par un gaz d’instantons. Nous avons ensuite exploré la physique d'un système régi au demi-remplissage par le modèle de Kitaev-Heisenberg, qui présente une phase magnétique de type zig-zag. En dopant le système, autour du quart remplissage, la structure de bande présente de nouveaux centres de symétrie en plus de la symétrie d'inversion. Le couplage de spin de Kitaev-Heisenberg favorise alors la formation de paires de Cooper dans un état triplet autour de ces centres de symétrie. La condensation de ces paires de Cooper autour de ces vecteurs d'onde non triviaux se manifeste par une modulation spatiale du paramètre d'ordre supraconducteur, comme dans la supraconductivité de Fulde–Ferrell–Larkin–Ovchinnikov (FFLO). La dernière partie de la thèse propose et étudie une implémentation des phases topologiques dite de Haldane et de Kane-Mele dans un système avec deux espèces de fermions sur le réseau en nid d'abeille, stabilisée grâce à l’interaction RKKY médiée par l’espèce rapide et qui agit sur l’espèce lente
This thesis is dedicated largely to the study of theoretical models describing interacting fermions with a spin-orbit coupling. These models (i) can describe a class of 2D iridate materials on the honeycomb lattice or (ii) could be realized artificially in ultra-cold gases in optical lattices. We have studied, in the first part, the half-filled honeycomb lattice model with on-site Hubbard interaction and anisotropic spin-orbit coupling. We find several different phases: the topological insulator phase at weak coupling, and two frustrated magnetic phases, the Néel order and spiral order, in the limit of strong correlations. The transition between the weak and strong correlation regimes is a Mott transition, through which electrons are fractionalized into spins and charges. Charges are localized by the interactions. The spin sector exhibits strong fluctuations which are modeled by an instanton gas. Then, we have explored a system described by the Kitaev-Heisenberg spin Hamiltonian at half-filling, which exhibits a zig-zag magnetic order. While doping the system around the quarter filling, the band structure presents novel symmetry centers apart from the inversion symmetry point. The Kitaev-Heisenberg coupling favors the formation of triplet Cooper pairs around these new symmetry centers. The condensation of these pairs around these non-trivial wave vectors is manifested by the spatial modulation of the superconducting order parameter, by analogy to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconductivity. The last part of the thesis is dedicated to an implementation of the Haldane and Kane-Mele topological phases in a system composed of two fermionic species on the honeycomb lattice. The driving mechanism is the RKKY interaction induced by the fast fermion species on the slower one

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9

Soni, Medha. "Investigation of exotic correlated states of matter in low dimension." Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30381/document.

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La physique statistique quantique formule les règles permettant de classifier les différentes particules. Dans cette thèse nous avons étudié deux projets, l'un portant sur les anyons dits de "Fibonacci" et l'autre sur les fermions sur réseau optique. Ici, nous avons naturellement étendu cette étude aux cas pertinent d'anyons itinérants en interaction sur des échelles. Notre but a été de construire le modèle 2D le simple possible d'anyons itinérants en interaction, analogue direct des systèmes fermioniques et inspiré par les études précédentes. En particulier, nous nous sommes demandé si la séparation spin-charge, bien connu à 1D, pouvait subsister dans le cas d'anyons sur une échelle. De plus, dans l'étude de ce modèle, nous avons découvert une nouvelle phase incompressible pouvant présenter un caractère topologique. Dans le cas des fermions confinés sur un réseau optique unidimensionnel, nous avons étudié les effets d'un chargement non-adiabatique et proposé des protocoles visant à minimiser le réchauffement du gaz quantique. Les atomes ultra-froids sur réseau optique constituent une réalisation idéale pour étudier les systèmes fortement corrélés soumis à un potentiel périodique. Le refroidissement évaporatif d'un nuage d'atomes confiné, c.a.d. sans le potentiel du réseau, s'est avéré être un processus très efficace. Les protocoles courants permettent d'obtenir(pour des fermions) des températures aussi basses que T/TF ≈ 0.08, impossible à réaliser en présence du réseau optique. Notre étude concerne les effets de redistribution de densité pour un système 1D de fermions. Notre but était de voir si des défauts causés par la mauvaise répartition des particules lors du chargement du réseau optique pouvaient empêcher les atomes de se refroidir jusqu'à la température voulue. Nous avons conçu des scenario améliorés où certains paramètres sont modifiés de façon dynamique afin de réduire la densité de défauts créés
Quantum statistics is an important aspect of quantum mechanics and it lays down the rules for identifying dfferent classes of particles. In this thesis, we study two projects, one that surveys models of Fibonacci anyons and another that delves into fermions in optical lattices. We analyse the physics of mobile non-Abelian anyons beyond one-dimension by constructing the simplest possible model of 2D itinerant interacting anyons in close analogy to fermionic systems and inspired by the previous anyonic studies. In particular, we ask the question if spin-charge separation survives in the ladder model for non-Abelian anyons. Furthermore, in the study of this model, we have found a novel physical effective model that possibly hosts a topological gapped state. For fermions in one dimensional optical lattices, we survey the effects of non-adiabatic lattice loading on four different target states, and propose protocols to minimise heating of quantum gases. The evaporative cooling of a trapped atomic cloud, i.e. without the optical lattice potential, has been proven to be a very effective process. Current protocols are able to achieve temperatures as low as T/TF ≈ 0.08, which are lost in the presence of the optical lattice. We aim to understand if defects caused by poor distribution of particles during lattice loading are important for the fermionic case, forbidding the atoms to cool down to the desired level. We device improved ramp up schemes where we dynamically change one or more parameters of the system in order to reduce density defects

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10

de, Woul Jonas. "Fermions in two dimensions and exactly solvable models." Doctoral thesis, KTH, Matematisk fysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-50471.

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This Ph.D. thesis in mathematical physics concerns systems of interacting fermions with strong correlations. For these systems the physical properties can only be described in terms of the collective behavior of the fermions. Moreover, they are often characterized by a close competition between fermion localization versus delocalization, which can result in complex and exotic physical phenomena. Strongly correlated fermion systems are usually modelled by many-body Hamiltonians for which the kinetic- and interaction energy have the same order of magnitude. This makes them challenging to study as the application of conventional computational methods, like mean field- or perturbation theory, often gives unreliable results. Of particular interest are Hubbard-type models, which provide minimal descriptions of strongly correlated fermions. The research of this thesis focuses on such models defined on two-dimensional square lattices. One motivation for this is the so-called high-Tc problem of the cuprate superconductors. A main hypothesis is that there exists an underlying Fermi surface with nearly flat parts, i.e. regions where the surface is straight. It is shown that a particular continuum limit of the lattice system leads to an effective model amenable to computations. This limit is partial in that it only involves fermion degrees of freedom near the flat parts. The result is an effective quantum field theory that is analyzed using constructive bosonization methods. Various exactly solvable models of interacting fermions in two spatial dimensions are also derived and studied.
QC 20111207

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11

Moosavi, Per. "An Exactly Solvable Gauge Theory Model for Correlated Fermions in 3+1 Dimensions." Thesis, KTH, Teoretisk fysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-145032.

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12

Lacroix-A-Chez-Toine, Bertrand. "Extreme value statistics of strongly correlated systems : fermions, random matrices and random walks." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS122/document.

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La prévision d'événements extrêmes est une question cruciale dans des domaines divers allant de la météorologie à la finance. Trois classes d'universalité (Gumbel, Fréchet et Weibull) ont été identifiées pour des variables aléatoires indépendantes et de distribution identique (i.i.d.).La modélisation par des variables aléatoires i.i.d., notamment avec le modèle d'énergie aléatoire de Derrida, a permis d'améliorer la compréhension des systèmes désordonnés. Cette hypothèse n'est toutefois pas valide pour de nombreux systèmes physiques qui présentent de fortes corrélations. Dans cette thèse, nous étudions trois modèles physiques de variables aléatoires fortement corrélées : des fermions piégés,des matrices aléatoires et des marches aléatoires. Dans la première partie, nous montrons plusieurs correspondances exactes entre l'état fondamental d'un gaz de Fermi piégé et des ensembles de matrices aléatoires. Le gaz Fermi est inhomogène dans le potentiel de piégeage et sa densité présente un bord fini au-delà duquel elle devient essentiellement nulle. Nous développons une description précise des statistiques spatiales à proximité de ce bord, qui va au-delà des approximations semi-classiques standards (telle que l'approximation de la densité locale). Nous appliquons ces résultats afin de calculer les statistiques de la position du fermion le plus éloigné du centre du piège, le nombre de fermions dans un domaine donné (statistiques de comptage) et l'entropie d'intrication correspondante. Notre analyse fournit également des solutions à des problèmes ouverts de valeurs extrêmes dans la théorie des matrices aléatoires. Nous obtenons par exemple une description complète des fluctuations de la plus grande valeur propre de l'ensemble complexe de Ginibre.Dans la deuxième partie de la thèse, nous étudions les questions de valeurs extrêmes pour des marches aléatoires. Nous considérons les statistiques d'écarts entre positions maximales consécutives (gaps), ce qui nécessite de prendre en compte explicitement le caractère discret du processus. Cette question ne peut être résolue en utilisant la convergence du processus avec son pendant continu, le mouvement Brownien. Nous obtenons des résultats analytiques explicites pour ces statistiques de gaps lorsque la distribution de sauts est donnée par la loi de Laplace et réalisons des simulations numériques suggérant l'universalité de ces résultats
Predicting the occurrence of extreme events is a crucial issue in many contexts, ranging from meteorology to finance. For independent and identically distributed (i.i.d.) random variables, three universality classes were identified (Gumbel, Fréchet and Weibull) for the distribution of the maximum. While modelling disordered systems by i.i.d. random variables has been successful with Derrida's random energy model, this hypothesis fail for many physical systems which display strong correlations. In this thesis, we study three physically relevant models of strongly correlated random variables: trapped fermions, random matrices and random walks.In the first part, we show several exact mappings between the ground state of a trapped Fermi gas and ensembles of random matrix theory. The Fermi gas is inhomogeneous in the trapping potential and in particular there is a finite edge beyond which its density vanishes. Going beyond standard semi-classical techniques (such as local density approximation), we develop a precise description of the spatial statistics close to the edge. This description holds for a large universality class of hard edge potentials. We apply these results to compute the statistics of the position of the fermion the farthest away from the centre of the trap, the number of fermions in a given domain (full counting statistics) and the related bipartite entanglement entropy. Our analysis also provides solutions to open problems of extreme value statistics in random matrix theory. We obtain for instance a complete description of the fluctuations of the largest eigenvalue in the complex Ginibre ensemble.In the second part of the thesis, we study extreme value questions for random walks. We consider the gap statistics, which requires to take explicitly into account the discreteness of the process. This question cannot be solved using the convergence of the process to its continuous counterpart, the Brownian motion. We obtain explicit analytical results for the gap statistics of the walk with a Laplace distribution of jumps and provide numerical evidence suggesting the universality of these results

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13

Chan, Chun Fai [Verfasser]. "Quantum simulation of strongly-correlated two-dimensional fermions in optical lattices / Chun Fai Chan." Bonn : Universitäts- und Landesbibliothek Bonn, 2020. http://d-nb.info/1218301503/34.

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14

Sugawa, Seiji. "Strongly Correlated Quantum Phases in Ultracold Mixtures of Bosons and Fermions in an Optical Lattice." 京都大学 (Kyoto University), 2012. http://hdl.handle.net/2433/157745.

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15

Angelone, Adriano. "Strongly correlated systems of bosons and fermions : a diagrammatic, variational and path integral Monte Carlo study." Thesis, Strasbourg, 2017. http://www.theses.fr/2017STRAF028/document.

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Mon travail de thèse se concentre sur l'étude, à l'aide de techniques numériques, de systèmes de fermions et bosons fortement corrélés. J'étudie Hamiltoniens de bosons sur réseau avec interactions à portée étendue, avant un intérêt pour expériences concernant atomes en états Rydberg-dressed, par moyen de simulations Path Integral Monte Carlo. Mon résultat principal est la démonstration d'un état de superverre en absence de sources de frustration dans le système.J'étudie également la modèle t-J fermionique avec deux trous par moyen de simulationsVariational Monte Carlo avec l’ansatz Entangled Plaquette States (EPS). Mon étude est fondamental en la perspective d'appliquer l'ansatz EPS à autres systèmes fermioniques, d’intérêt pour la supraconductivité à haute temperature, dont le comportement n'a pas encore été déterminé. Finalement, je présente mon travail sur une implémentation de l'algorithme Diagrammatic Monte Carlo
The focus of my thesis is the investigation, via numerical approaches, of strongly correlated models of bosons and fermions. I study bosonic lattice Hamiltonians with extended--range interactions, of interest for experiments with cold Rydberg-dressed atoms, via Path Integral MonteCarlo simulations. My main result is the demonstration of a superglass in the absence of frustration sources in the system. I also study the fermionic $t-J$ model in the presence of two holes via Variational Monte Carlo with the Entangled Plaquette States Ansatz. My study is foundational to the extension of this approach to other fermionic systems, of interest for high temperature superconductivity, where the physical picture is still under debate (such as, e.g., the $t-J$ model in the case of finite hole concentration). Finally, I discuss my work on an implementation of the Diagrammatic Monte Carlo algorithm

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16

Markhof, Lisa [Verfasser], Volker [Akademischer Betreuer] Meden, and Carsten [Akademischer Betreuer] Honerkamp. "Novel approaches to the description of one-dimensional correlated fermions / Lisa Carola Markhof ; Volker Meden, Carsten Honerkamp." Aachen : Universitätsbibliothek der RWTH Aachen, 2019. http://d-nb.info/1193734827/34.

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17

Scheerer, Gernot. "Étude à Fort Champ Magnétique du Système à Fermions Lourds URu2Si2." Phd thesis, Université Paul Sabatier - Toulouse III, 2013. http://tel.archives-ouvertes.fr/tel-00952430.

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Les composés à fermions lourds, qui sont à base de terres rares comme le cérium et l'ytterbium ou d'actinides comme l'uranium, sont connus pour leurs propriétés extraordinaires à basse température. Leur physique est gouvernée par l'hybridation des électrons f avec des électrons de conduction, ce qui mène à la formation de quasi-particules avec de très grandes masses effectives. URu2Si2 occupe une place particulière dans la famille des fermions lourds. Une transition de phase du second ordre à la température T0 = 17.5 K a été observée par de nombreuses techniques expérimentales. Malgré des propositions théoriques multiples, aucun consensus n'existe concernant le paramètre d'ordre de la phase - dite à ordre caché - qui se développe sous T0. Lorsqu'on le soumet à des champs magnétiques intenses, URu2Si2 a par ailleurs un comportement unique : une cascade de trois transitions du premier ordre entre 35 et 39 T mène le système de son état paramagnétique à un état polarisé paramagnétique à fort champ. Ce travail a consisté en l'investigation systématique des propriétés magnétiques et électroniques d'échantillons monocristallins de très haute qualité d' URu2Si2 dans des champs magnétiques intenses allant jusqu'à 80 T, et des températures descendant jusqu'à 100 mK. Des expériences d'aimantation et de magnétorésistivité ont été faites en champ magnétique pulsé non destructif au Laboratoire National des Champs Magnétiques Intenses de Toulouse (LNCMI-T). Le diagramme de phase champ magnétique-température de URu2Si2 a été étudié la première fois sur les gammes étendues de champs magnétiques H||c allant jusqu'à 60 T et de températures allant jusqu'à 80 K. Il indique que la domaine critique [35 T-39 T] est initié par la destabilisation d'un " crossover ", dont la température caractéristique atteint 40-50 K à champ nul. Il est démontré que ce crossover, qui résulte probablement des corrélations inter-site, est aussi un précurseur de la phase à ordre caché. Une étude de la magnétorésistivité pour différentes orientations du champ magnétique dans les plans (a,a) and (a,c) a permis d'établir la dépendance en angle du diagramme de phase. Des mesures de l'aimantation du composé dopé en rhodium U(Ru0.96Rh0.04)2Si2 révèlent un diagramme de phase " simplifié ", où la phase à ordre caché a disparu et le domaine critique a été remplace par une phase intermédiaire entre 26 et 37 T. La magnetoresistivité à très basse température se révèle être fortement dépendente de la qualité des échantillons et est la signature des propriétés orbitales d'URu2Si2. Une dépendance exceptionnellement intense de la magnétorésistivité en fonction de la température confirme que la surface de Fermi est reconstruite à T0. Des anomalies dans la magnetoresistivité à fort champ magnétique H||c suggèrent que la surface de Fermi est modifiée à l'intérieur de la phase à ordre caché. Des oscillations quantiques - effet Shubnikov-de Haas - sont observées dans la magnétorésistivité à très basse température pour une multitude d'orientations des échantillons dans le champ magnétique. Elles confirment qu'un champ magnétique H||c induit des reconstructions de la surface de Fermi dans la phase à ordre caché. Dans un champ magnétique H||a, des oscillations quantiques sont observées pour la première fois jusqu'à 80 T. Leur analyse a révélé une nouvelle branche de fréquence  avec une faible masse effective. La dépendance en angle des fréquences Shubnikov-de Haas a été étudiée dans un champ magnétique allant jusqu'à 60 T, pour des champs appliqués dans les plans (a,a) et (a,c). Ce travail expérimental indique que le couplage entre le magnétisme des électrons f et les propriétés de la surface de Fermi joue un rôle important pour la physique du système à ordre caché URu2Si2.

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18

Bastien, Gaël. "Interplay between magnetic quantum criticality, Fermi surface and unconventional superconductivity in UCoGe, URhGe and URu2Si2." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY005/document.

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Cette thèse montre de nouveaux résultats sur les supraconducteurs ferromagnétiques UCoGe et URhGe et sur l’ordre caché dans URu2Si2. Le diagramme de phase pression température d’UCoGe a été étudié jusqu’à 10.5 GPa. L’ordre ferromagnétique subsiste jusqu’à la pression critique pc≈1 GPa et la supraconductivité non conventionnelle jusqu’à p = 4 GPa. Les fluctuations magnétiques responsables de la supraconductivité peuvent être réduites par l’application d’un champ magnétique. Les surfaces de Fermi d’UCoGe et d’URhGe ont été mesurées grace aux oscillations quantiques. Quatre poches ont été détectées dans UCoGe, elles subissent une succession de transition de Lifshitz de la surface de Fermi sous champ magnétique. Les poches détectés évoluent continument avec la pression jusqu’à 2.5 GPa, sans montrer de reconstruction de la surface de Fermi à la pression critique pc. Dans URhGe, trois poches lourdes de la surface de Fermi ont aussi été découvertes. Enfin dans la phase d’ordre caché d’URu2Si2, les oscillations quantiques ont révélé une forte anisotropie du facteur gyromagnétique g pour deux poches de la surface de Fermi, qui est comparable à l’anisotropie macroscopique. Cette dernière a été étudiée à partir du champcritique supérieur de la supraconductivité
This thesis is concentrated on the ferromagnetic superconductors UCoGe and URhGe andon the hidden order state in URu2Si2. In the first part the pressure temperature phase diagram of UCoGe was studied up to 10.5 GPa. Ferromagnetism vanishes at the critical pressure pc≈1 GPa. Unconventional superconductivity and non Fermi liquid behavior can be observed in a broad pressure range around pc. The superconducting upper critical field properties were explained by the suppression of the magnetic fluctuations under field. In the second part the Fermi surfaces of UCoGe and URhGe were investigated by quantum oscillations. In UCoGe four Fermi surface pockets were observed. Under magnetic field successive Lifshitz transitions of the Fermi surface have been detected. The observed Fermi surface pockets in UCoGe evolve smoothly with pressure up to 2.5 GPa and do not show any Fermi surface reconstruction at the critical pressure pc. In URhGe, three heavy Fermi surface pockets were detected by quantum oscillations. In the last part the quantum oscillation study in the hidden order state of URu2Si2 shows a strong g factor anisotropy for two Fermi surface pockets, which is compared to the macroscopic g factor anisotropy extractedfrom the upper critical field study

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19

Combier, Tristan. "Criticalité quantique ferromagnétique dans les composés ternaires à base d'uranium URhSi, URhAl et UCoAl." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY062/document.

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Dans cette thèse, on étudie la criticalité quantique ferromagnétique dans trois composés ternaires à base d'uranium, par des mesures thermodynamiques et de transport sur des échantillons monocristallins, à basse température et sous haute pression. URhSi et URhAl sont des systèmes ferromagnétiques itinérants, tandis que UCoAl est un système paramagnétique étant proche d'une instabilité ferromagnétique. Tous ont une phase ordonnée de type Ising. Dans le composé orthorhombique URhSi, on montre que la température de Curie diminue lorsqu'un champ magnétique est appliqué perpendiculairement à l'axe facile d'aimantation, et une transition de phase quantique est attendue autour de 40~T. Dans le système hexagonal URhAl, on établit le diagramme de phase pression--température pour la première fois, lequel indique une transition de phase quantique vers 5~GPa. Dans le composé isostructural UCoAl, on étudie la transition métamagnétique par des mesures d'aimantation, d'effet Hall, de résistivité et de dichroïsme circulaire magnétique des rayons X. On observe des phénomènes de relaxation magnétique intrigants, avec des sortes de marches. L'effet Hall et la résistivité ont été mesurés à des températures de réfrigérateur à dilution, sous pression hydrostatique jusqu'à 2,2~GPa, et sous champ magnétique jusqu'à 16~T. La transition métamagnétique se termine sous pression et champ magnétique au niveau d'un point critique quantique terminal. Dans cette région, il se produit une forte augmentation de la masse effective, et une différence intrigante entre champ montant et descendant apparaît dans la résistivité transverse. Ce pourrait être la signature d'une nouvelle phase, éventuellement reliée aux phénomènes de relaxation observés dans les mesures d'aimantation, et résultant de frustrations au sein du réseau quasi-Kagomé que forment les atomes d'uranium dans cette structure cristalline
In this thesis we explore the ferromagnetic quantum criticality in three uranium-based ternary compounds, by means of thermodynamical and transport measurements on single crystal samples, at low temperature and high pressure. URhSi and URhAl are itinerant ferromagnets, while UCoAl is a paramagnet being close to a ferromagnetic instability. All of them have Ising-type magnetic ordering. In the orthorhombic compound URhSi, we show that the Curie temperature decreases upon applying a magnetic field perpendicular to the easy magnetization axis, and a quantum phase transition is expected around 40~T. In the hexagonal system URhAl, we establish the pressure--temperature phase diagram for the first time, indicating a quantum phase transition around 5~GPa. In the isostructural compound UCoAl, we investigate the metamagnetic transition with measurements of magnetization, Hall effect, resistivity and X-ray magnetic circular dichroism. Some intriguing magnetic relaxation phenomena are observed, with step-like features. Hall effect and resistivity have been measured at dilution temperatures, under hydrostatic pressure up to 2.2 GPa and magnetic field up to 16~T. The metamagnetic transition terminates under pressure and magnetic field at a quantum critical endpoint. In this region, a strong effective mass enhancement occurs, and an intriguing difference between up and down field sweeps appears in transverse resistivity. This may be the signature of a new phase, supposedly linked to the relaxation phenomena observed in magnetic measurements, arising from frustration on the quasi-Kagome lattice of uranium atoms in this crystal structure

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20

Villar, Arribi Pablo. "Fermions lourds et métaux de Hund dans les supraconducteurs à base de fer." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY070/document.

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Matériaux dans lesquels les électrons responsables des propriétés de basse énergie son soumis à fortes corrélations sont aujourd'hui très étudiés à la recherche de nouvelles phases émergentes aux propriétés surprenantes et/ou utiles.Les supraconducteurs à base de fer (IBSC) sont maintenant considérés dans cette classe de composés. En utilissant des techniques multi-corps nécessaires pour le traitement théorique de ces corrélations (théorie du champ moyen de spin esclave - SSMFT et théorie du champ moyen dynamique - DMFT - en conjonction avec la théorie du fonctionnelle de la densité, DFT), dans cette thèse, j'etudie plusieurs propriétés d'IBSC.D’abord, j'analyse les composés très dopés de la famille de IBSC, qui montrent expérimentalement certains comportements typiques des ``fermions lourds'', des composés typiquement des terres rares ou des actinides, où des électrons extrêmement corrélés coexistent avec des électrons moins corrélés. En particulier je me concentre sur la chaleur spécifique et le pouvoir thermoélectrique et je montre comment ces propriétés peuvent être comprises dans le paradigme récemment développé ``métaux de Hund''. En effet, l’échange intra-atomique (le ``couplage de Hund'') est responsable de ces matériaux à éléments métal de transition en montrant la physique des fermions lourds. Je montre aussi que les caractéristiques typiquement fermions-lourds du spectre d’excitation, connues car les singularités de Van Hove sont bien capturées par notre modélisation au sein de DFT+SSMFT. J'utilise ensuite DMFT dans un modèle afin d'étudier l'impact direct des singularités de Van Hove sur la force des corrélations.Dans une seconde partie, je montre comment FeSe, le IBSC actuellement le plus étudié, se trouve également dans une phase métal de Hund, mais il est amené à la frontière de cette phase par la pression. Cette frontière est liée à une augmentation de la compressibilité électronique qui est positivement corrélée à l’augmentation de la supraconductivité trouvée dans les expériences.Je réalise une étude analogue sur le détenteur du record pour la température supraconductrice critique la plus élevée, la monocouche FeSe où je trouve également une compressibilité augmentée. Cela appuie la récente proposition selon laquelle la frontière du métal de Hund favorise la supraconductivité à haute température.Enfin, j'étudie la nature du magnétisme dans une autre famille de IBSC, les germanides de fer. J'explore différents ordres magnétiques possibles avec des simulations DFT et leur concurrence (ce qui peut en principe favoriser la supraconductivité) dans plusieurs composés où différents substitutions sont appliquées au composé parent YFe2Ge2. J'étudie également l'effet de la pression chimique sur ce composé
Materials where the electrons responsible for the low-energy properties experience strong correlations are today very investigated in search of emerging new phases with surprising and/or useful properties. Iron-based superconductors (IBSC) are now considered in this class of compounds. Using the many-body techniques necessary for the theoretical treatment of these correlations (slave-spin mean field theory - SSMFT- and dynamical mean field theory - DMFT- in conjunction with density functional theory, DFT), in this thesis I address several properties of IBSC.First I analyze the very hole-doped compounds in the IBSC family, that show experimentally some behaviors typical of the so-called “heavy fermions”, compounds typically of rare earth or actinides, where extremely correlated electrons coexist with others less correlated. In particular I focus on the specific heat and the thermoelectric power and show how these properties can be understood in the recently developed paradigm of “Hund’s metals”. Indeed the intra-atomic exchange (the “Hund’s coupling”) is responsible for these materials of transition metal elements showing heavy-fermionic physics. I show also that typical heavy-fermionic features of the excitation spectrum, known as Van Hove singularities are well captured by our modelization within DFT+SSMFT. I then use DMFT in a model in order to study the direct impact of the Van Hove singularities on the strength of correlations.In a second part I show how FeSe, the presently most studied IBSC, is also in a Hund’s metal phase, but it is brought to the frontier of this phase by pressure. This frontier is connected to an enhancement of the electronic compressibility which correlates positively then with the enhancement of superconductivity found in experiments. I perform an analogous study on the record holder for the highest critical superconducting temperature, the monolayer FeSe where I also find an enhanced compressibility. This supports the recent proposal that the frontier of a Hund's metal favors high-temperature superconductivity.Finally I study the nature of magnetism in another family of IBSC, the iron-germanides. I explore different possible magnetic orders with DFT simulations and study their competition (which can in principle favor superconductivity) in several compounds where different chemical substitutions are applied to the parent compound YFe2Ge2. I also study the effect of chemical pressure on this compound

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21

Lidmar, Jack. "Phase transitions in high-temperature superconductors." Doctoral thesis, Stockholm : Tekniska högsk, 1998. http://www.lib.kth.se/abs99/lidm0111.pdf.

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22

Heath, Joshuah. "Novel metallic behavior in topologically non-trivial, quantum critical, and low-dimensional matter:." Thesis, Boston College, 2021. http://hdl.handle.net/2345/bc-ir:109180.

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Thesis advisor: Kevin S. Bedell
We present several results based upon non-trivial extensions of Landau-Fermi liquid theory. First proposed in the mid-20th century, the Fermi liquid approach assumes an adiabatic “switching-on” of the interaction, which allows one to describe the collective excitations of the many-body system in terms of weakly-interacting quasiparticles and quasiholes. At its core, Landau-Fermi liquid theory is often considered a perturbative approach to study the equilibrium thermodynamics and out-of-equilibrium response of weakly-correlated itinerant fermions, and therefore non-trivial extensions and consequences are usually overlooked in the contemporary literature. Instead, more emphasis is often placed on the breakdown of Fermi liquid theory, either due to strong correlations, quantum critical fluctuations, or dimensional constraints. After a brief introduction to the theory of a Fermi liquid, I will first apply the Landau quasiparticle paradigm to the theory of itinerant Majorana-like fermions. Defined as fermionic particles which are their own anti-particle, traditional Majorana zero modes found in topological materials lack a coherent number operator, and therefore do not support a Fermi liquid-like ground state. To remedy this, we will apply a combinatorical approach to build a statistical theory of self-conjugate particles, explicitly showing that, under this definition, a filled Fermi surface exists at zero temperature. Landau-Fermi liquid theory is then used to describe the interacting phase of these Majorana particles, from which we find unique signatures of zero sound in addition to exotic, non-analytic contributions to the specific heat. The latter is then exploited as a “smoking-gun” signature for Majorana-like excitations in the candidate Kitaev material Ag3LiIr2O6, where experimental measurements show good agreement with a sharply-defined, “Majorana-Fermi surface” predicted in the underlying combinatorial treatment. I will then depart from Fermi liquid theory proper to tackle the necessary conditions for the applicability of Luttinger’s theorem. In a nutshell, Luttinger’s theorem is a powerful theorem which states that the volume of phase space contained in the Fermi surface is invariant with respect to interaction strength. In this way, whereas Fermi liquid only describes fermionic excitations near the Fermi surface, Luttinger’s theorem describes the fermionic degrees of freedom throughout the entire Fermi sphere. We will show that Luttinger’s theorem remains valid only for certain frequency and momentum-dependencies of the self-energy, which correlate to the exis- tence of a generalized Fermi surface. In addition, we will show that the existence of a power-law Green’s function (a unique feature of “un-particle” systems and a proposed characteristic of the pseudo-gap phase of the cuprate superconductors) forces Luttinger’s theorem and Fermi liquid theory to be mutually exclusive for any non-trivial power of the Feynman propagator. Finally, we will return to Landau-Fermi liquid theory, and close with novel out-of-equilibrium behavior and stability in unconventional Fermi liquids. First, we will consider a perfectly two- dimensional Fermi liquid. Due to the reduction in dimension, the traditional mode expansion in terms of Legendre polynomials is modified to an expansion in terms of Chebyshev polynomials. The resulting orthogonality conditions greatly modifies the stability and collective modes in the 2D system. Second, we will look at a Fermi liquid in the presence of a non-trivial gauge field. The existence of a gauge field will effectively shift the Fermi surface in momentum space, resulting in, once again, a modified stability condition for the underlying Fermi liquid. Supplemented with a modernized version of Mermin’s condition for the propagation of zero sound, we outline the full effects a spin symmetric or anti-symmetric gauge would have on a Fermi liquid ground state
Thesis (PhD) — Boston College, 2021
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

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23

Dionicio, Gabriel Alejandro. "Electrical resistivity of YbRh2Si2 and EuT2Ge2 (T = Co, Cu) at extreme conditions of pressure and temperature." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1170255664129-64812.

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This investigation address the effect that pressure, p, and temperature, T, have on 4f-states of the rare-earth elements in the isostructural YbRh2Si2, EuCo2Ge2, and EuCu2Ge2 compounds. Upon applying pressure, the volume of the unit cell reduces, enforcing either the enhancement of the hybridization of the 4f-localized electrons with the ligand or a change in the valence state of the rare-earth ions. Here, we probe the effect of a pressure-induced lattice contraction on these system by means of electrical resistivity, from room temperature down to 100 mK. At ambient pressure, the electrical resistivity of YbRh2Si2 shows a broad peak at 130 K related to the incoherent scattering on the ground state and the excited crystalline electrical field (CEF) levels. At T_N = 70 mK, YbRh2Si2 undergoes a magnetic phase transition. Upon applying pressure up to p_1 = 4 GPa , T_N increases monotonously while the peak in the electrical resistivity is shifted to lower temperatures. For p < p_1 a different behavior is observed; namely, T_N depends weakly on the applied pressure and a decomposition of the single peak in the electrical resistivity into several shoulders and peaks occurs. Above p_2 = 9 GPa, the electrical resistivity is significantly reduced for T < 50 K and this process is accompanied by a sudden enhancement of T_N. Thus, our results confirm the unexpected behavior of the magnetization as function of pressure reported by Plessel et al. The small value of the magnetic ordering temperature for p < p_2 and the strength of the mechanism that leads to the peaks and shoulders in the electrical resistivity suggest that the f-electrons are still screened by the conduction electrons. Therefore, the observed behavior for pressures lower than p_2 might be a consequence of the competition of two different types of magnetic fluctuations (seemingly AFM and FM). Furthermore, the results suggest that a sudden change of the CEF scheme occurs at pressures higher than p_1, which would have an influence on the ground state. Additionally, a comparison of the pressure dependent features in the electrical resistivity of YbRh2Si2 with similar maxima in other isostructural YbT2X2 (T = transition metal; X = Si or Ge) compounds was performed. For the comparison, a simple relation that considers the Coqblin-Schrieffer model and the hypothesis of Lavagna et al. is proposed. A systematic behavior is observed depending on the transition metal; namely, it seems that the higher the atomic radii of the T-atom the smallest the pressure dependence of the maximum in the electrical resistivity, suggesting a weaker coupling of localized- and conduction-electrons. It is also observed that an increase in the density of conduction electrons reduces the pressure dependence of the characteristic Kondo temperature. The mechanism responsible for the sudden enhancement of T_N in YbRh2Si2 at about p_2 is still unknown. However two plausible scenarios are discussed. The Eu-ions in EuCo2Ge2 and EuCu2Ge2 have a divalent character in the range 100 mK < T < 300 K. Therefore, these systems order magnetically at T_N = 23 K and T_N = 12 K, respectively. The studies performed on EuCo2Ge2 and EuCu2Ge2 as a function of pressure suggest that a change to a non-magnetic trivalent state of the Eu-ions might occur at zero temperature for pressures higher than 3 GPa and 7 GPa, respectively. A common and characteristic feature on EuCo2Ge2 and EuCu2Ge2 is the absence of a clear first order transition from the divalent to the trivalent state of the Eu-ions at finite temperature for p > 3 GPa and for p > 7 GPa, respectively. In other isostructural Eu-based compounds, a discontinuous and abrupt change in the thermodynamic and transport properties associated to the valence transition of the Eu-ions is typically observed at finite temperatures. In contrast, the electrical resistivity of EuCo2Ge2 and EuCu2Ge2 changes smoothly as a function of pressure and temperature. The analysis of the the electrical resistivity of EuCo2Ge2 suggest that a classical critical point might be close to the AFM-ordered phase, being a hallmark of this compound. The overall temperature dependence of the the electrical resistivity of EuCo2Ge2 changes significantly at 3 GPa; therefore, it seems that the system suddenly enters to a T-dependent valence-fluctuating regime. Additionally, the pressure-dependent electrical-resistivity isotherms show a step-like behavior. Thus, it is concluded that discontinuous change of the ground state might occur at 3 GPa. The electrical resistivity of EuCu2Ge2 at high pressure is characterized by a negative logarithmic T-dependence in the pressure range 5 GPa < p < 7 GPa for T > T_N and by a broad peak in the pressure dependent residual resistivity, whose maximum is located at 7.3 GPa. The first behavior resembles the incoherent scattering process typical for an exchange coupling mechanism between the localized electrons and the ligand. This and the peak effect in the local 4f susceptibility observed in NMR measurements are consistent with such a coupling mechanism. Thus, it would be for the first time that a dense Eu-based compound like EuCu2Ge2 show such a behavior. Combining the results of the experiment performed at high pressures on EuCu2Ge2 with the studies performed in the EuCu2(Ge1-xSix)2 series, a crossover from an antiferromagnetically ordered state into a Fermi-liquid state for pressures higher than 7.3 GPa may be inferred from the analysis. Therefore, it may be possible that the sudden depopulation of 4f-level occur mediated by quantum fluctuation of the charge due to a strong Coulomb repulsion between the localized-electrons and the ligand. This phenomenon would explain the broad peak in the residual resistivity. To our knowledge, this would be the first Eu-based compound, isostructural to ThCr2Si2, that show such a transition as function of pressure at very low temperatures.

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24

Yang, Kang. "Geometric Aspects in the Hamiltonian Theory of the Fractional Quantum Hall Effect." Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS425.

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Les propriétés topologiques des systèmes quantiques de Hall ont bien été étudiées au cours des trente dernières années. En contraste, les aspects géométriques des systèmes quantiques de Hall sont loin d’être pleinement compris. Dans cette thèse, je vais étudier les aspects géométriques de la vue de la théorie Hamiltonienne de fermions composites et tester la réponse de les états quantiques de Hall sous perturbation anisotrope. Je trouve dans le présence d'anisotropie, les fermions composites reçoivent des effets de mélange entre les différents niveaux de Landau de fermions composites. Une métrique variationnelle peut être définié aux fermions composites afin de minimiser un tel effet. Les gaps d'activation et les gaps collectifs neutres sont calculé pour un système quantique à effet Hall avec champ magnétique incliné. Le premier se présente une robustesse tandis que le dernier est susceptible de perturbation anisotrope. Les états d'onde de densité de charge sous masse anisotropes sont également étudiés. La phase de bulle se révèle être fortement supprimé par l'anisotropie de masse. Toute les transitions de phase du premier ordre dans le cas isotrope sont remplacées par des transitions de phase continues dans le cas anisotrope
The topological properties in quantum Hall systems are thoroughly studied in the past thirty years. In constrast, the geometric aspects of quantum Hall systems are far from being fully understood. In this thesis, I am going to investigate the geometric aspects from the view of the composite fermion Hamiltonian theory and test the response of quantum Hall states under anisotropic perturbation. I find in the presence of anisotropy, composite fermions receive mixing effects between different composite fermion Landau levels. A variational metric can be combined to the composite fermions in order to minimize such an effect. The activation gaps and neutral collective gaps are calculated for a quantum Hall system with tilted magnetic field. The former exhibits a robustness while the latter is susceptible to anisotropic perturbation. The charge density wave states under mass anisotropy are also studied. The bubble phase is found to be strongly suppressed by the mass anisotropy. All the first-order phase transitions present in the isotropic case are replaced by continuous phase transitions in the anisotropic case

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25

Raymond, Stéphane. "Excitations de basse énergie dans les fermions lourds par diffusion inélastique des neutrons." Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10103.

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La technique de diffusion inelastique des neutrons a ete utilisee pour caracteriser le spectre des excitations magnetiques de basse energie dans les composes a electrons fortement correles cenisn, ce#1#-#xla#xru#2si#2 et ceru#2ge#2. Cenisn est un compose kondo semimetallique dont le spectre des excitations magnetiques est compose de deux modes d'energie 2 et 4 mev et respectivement piques au centre et bord de zone de brillouin. L'application d'un champ magnetique met en evidence la nature differente de ces modes : le premier se deplace vers les plus hautes energies et la reponse du second s'elargit. Les modeles de gap d'hybridation particularises pour ce compose permettent de relier le mode a 4 mev aux proprietes macroscopiques observees. La dynamique des moments magnetiques pres d'une instabilite magnetique a ete etudiee dans le compose ce#1##xla#xru#2si#2 avec x = 0. 075. De facon surprenante, un ordre magnetique non conventionnel a ete mis en evidence (m = 0. 02 #b et t#n = 1. 8 k). La dynamique de spin par rapport au compose pur a ete reliee, grace a la theorie des fluctuations de spin de moriya, aux mesures macroscopiques. L'application d'un champ magnetique sur le compose ceru#2si#2 met en evidence le changement de la reponse dynamique du systeme de part et d'autre de la transition pseudometamagnetique. La reponse quasielastique et a courte portee a bas champ est remplacee par un ordre statique a longue distance en champ fort (h = 12 t). Dans ce contexte, la phase antiferromagnetique du compose ferromagnetique ceru#2ge#2 a ete caracterisee. Il existe alors une analogie entre ce compose sous sa temperature de curie et ceru#2si#2 au dessus du champ metamagnetique.

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26

Taupin, Mathieu. "Etude des fermions lourds magnétiques UCoGe et YbRh2Si2 par mesures de transport." Phd thesis, Grenoble, 2013. http://tel.archives-ouvertes.fr/tel-01026388.

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Анотація:

Les mesures de conduction thermique ont été effectuées à basses températures dans le supraconducteur ferromagnétique UCoGe et dans le composé faiblement antiferromagnétique YbRh2Si2. Les fluctuations magnétiques sont un élément important dans les propriétés de ces deux composés, et sont responsables d'un canal de chaleur à basses températures. Dans UCoGe, la contribution supplémentaire causée par les fluctuations magnétiques ont la même dépendance en champ magnétique que celles vues par RMN. Étonnamment, un nouveau canal de chaleur apparaît à très basses températures. Les mesures dans l'état supraconducteur ont confirmé le caractère multigap de UCoGe. Des mesures de XMCD ont également faites dans UCoGe. Dans YbRh2Si2, les fluctuations magnétiques sont suspectées d'être responsables d'un canal de chaleur visible à très basses températures, empêchant de pouvoir conclure sur la violation ou la validité de la loi de Wiedemann-Franz au niveau du point critique quantique. Cependant, les résultats peuvent être interprétés sans avoir recours à sa violation.

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27

Dionicio, Gabriel Alejandro. "Electrical resistivity of YbRh2Si2 and EuT2Ge2 (T = Co, Cu) at extreme conditions of pressure and temperature." Doctoral thesis, Technische Universität Dresden, 2006. https://tud.qucosa.de/id/qucosa%3A25011.

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Анотація:

This investigation address the effect that pressure, p, and temperature, T, have on 4f-states of the rare-earth elements in the isostructural YbRh2Si2, EuCo2Ge2, and EuCu2Ge2 compounds. Upon applying pressure, the volume of the unit cell reduces, enforcing either the enhancement of the hybridization of the 4f-localized electrons with the ligand or a change in the valence state of the rare-earth ions. Here, we probe the effect of a pressure-induced lattice contraction on these system by means of electrical resistivity, from room temperature down to 100 mK. At ambient pressure, the electrical resistivity of YbRh2Si2 shows a broad peak at 130 K related to the incoherent scattering on the ground state and the excited crystalline electrical field (CEF) levels. At T_N = 70 mK, YbRh2Si2 undergoes a magnetic phase transition. Upon applying pressure up to p_1 = 4 GPa , T_N increases monotonously while the peak in the electrical resistivity is shifted to lower temperatures. For p < p_1 a different behavior is observed; namely, T_N depends weakly on the applied pressure and a decomposition of the single peak in the electrical resistivity into several shoulders and peaks occurs. Above p_2 = 9 GPa, the electrical resistivity is significantly reduced for T < 50 K and this process is accompanied by a sudden enhancement of T_N. Thus, our results confirm the unexpected behavior of the magnetization as function of pressure reported by Plessel et al. The small value of the magnetic ordering temperature for p < p_2 and the strength of the mechanism that leads to the peaks and shoulders in the electrical resistivity suggest that the f-electrons are still screened by the conduction electrons. Therefore, the observed behavior for pressures lower than p_2 might be a consequence of the competition of two different types of magnetic fluctuations (seemingly AFM and FM). Furthermore, the results suggest that a sudden change of the CEF scheme occurs at pressures higher than p_1, which would have an influence on the ground state. Additionally, a comparison of the pressure dependent features in the electrical resistivity of YbRh2Si2 with similar maxima in other isostructural YbT2X2 (T = transition metal; X = Si or Ge) compounds was performed. For the comparison, a simple relation that considers the Coqblin-Schrieffer model and the hypothesis of Lavagna et al. is proposed. A systematic behavior is observed depending on the transition metal; namely, it seems that the higher the atomic radii of the T-atom the smallest the pressure dependence of the maximum in the electrical resistivity, suggesting a weaker coupling of localized- and conduction-electrons. It is also observed that an increase in the density of conduction electrons reduces the pressure dependence of the characteristic Kondo temperature. The mechanism responsible for the sudden enhancement of T_N in YbRh2Si2 at about p_2 is still unknown. However two plausible scenarios are discussed. The Eu-ions in EuCo2Ge2 and EuCu2Ge2 have a divalent character in the range 100 mK < T < 300 K. Therefore, these systems order magnetically at T_N = 23 K and T_N = 12 K, respectively. The studies performed on EuCo2Ge2 and EuCu2Ge2 as a function of pressure suggest that a change to a non-magnetic trivalent state of the Eu-ions might occur at zero temperature for pressures higher than 3 GPa and 7 GPa, respectively. A common and characteristic feature on EuCo2Ge2 and EuCu2Ge2 is the absence of a clear first order transition from the divalent to the trivalent state of the Eu-ions at finite temperature for p > 3 GPa and for p > 7 GPa, respectively. In other isostructural Eu-based compounds, a discontinuous and abrupt change in the thermodynamic and transport properties associated to the valence transition of the Eu-ions is typically observed at finite temperatures. In contrast, the electrical resistivity of EuCo2Ge2 and EuCu2Ge2 changes smoothly as a function of pressure and temperature. The analysis of the the electrical resistivity of EuCo2Ge2 suggest that a classical critical point might be close to the AFM-ordered phase, being a hallmark of this compound. The overall temperature dependence of the the electrical resistivity of EuCo2Ge2 changes significantly at 3 GPa; therefore, it seems that the system suddenly enters to a T-dependent valence-fluctuating regime. Additionally, the pressure-dependent electrical-resistivity isotherms show a step-like behavior. Thus, it is concluded that discontinuous change of the ground state might occur at 3 GPa. The electrical resistivity of EuCu2Ge2 at high pressure is characterized by a negative logarithmic T-dependence in the pressure range 5 GPa < p < 7 GPa for T > T_N and by a broad peak in the pressure dependent residual resistivity, whose maximum is located at 7.3 GPa. The first behavior resembles the incoherent scattering process typical for an exchange coupling mechanism between the localized electrons and the ligand. This and the peak effect in the local 4f susceptibility observed in NMR measurements are consistent with such a coupling mechanism. Thus, it would be for the first time that a dense Eu-based compound like EuCu2Ge2 show such a behavior. Combining the results of the experiment performed at high pressures on EuCu2Ge2 with the studies performed in the EuCu2(Ge1-xSix)2 series, a crossover from an antiferromagnetically ordered state into a Fermi-liquid state for pressures higher than 7.3 GPa may be inferred from the analysis. Therefore, it may be possible that the sudden depopulation of 4f-level occur mediated by quantum fluctuation of the charge due to a strong Coulomb repulsion between the localized-electrons and the ligand. This phenomenon would explain the broad peak in the residual resistivity. To our knowledge, this would be the first Eu-based compound, isostructural to ThCr2Si2, that show such a transition as function of pressure at very low temperatures.

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28

Gray, Ian R. "Fermion quasiparticles in strongly correlated electron systems." Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316668.

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29

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 clivage
Materials 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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30

Green, Alexander Christopher Maurice. "Correlated electrons in heavy fermion and double exchange systems." Thesis, Imperial College London, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.312479.

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31

Harrington, A. P. "Enhanced masses and superconductivity in highly correlated fermionic systems." Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233549.

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32

Scheffler, Marc. "Broadband microwave spectroscopy on correlated electrons." [S.l. : s.n.], 2004. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB11612028.

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33

Dmytruk, Olesia. "Quantum transport in a correlated nanostructure coupled to a microwave cavity." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS335/document.

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Dans cette thèse, nous étudions d’un point de vue théorique les propriétés physiques de nanostructures couplées à des cavités micro-ondes. L’électrodynamique quantique (QED) en cavité en présence d’une boîte quantique s’est révélée être une technique expérimentale puissante, permettant d'étudier cette dernière par des mesures photoniques en plus des mesures de transport électronique conventionnelles. Dans cette thèse, nous proposons d'utiliser le champ micro-ondes de la cavité afin d’extraire des informations supplémentaires sur les propriétés des conducteurs quantiques : le coefficient de transmission optique est directement lié à la susceptibilité électronique de ces conducteurs quantiques. Nous appliquons ce cadre général à différents systèmes mésoscopiques couplés à une cavité supraconductrice micro-ondes comme une jonction tunnel, une boîte quantique couplée à des réservoirs, un fil topologique et un anneau supraconducteur. La QED en cavité peut être utilisée pour sonder, par l'intermédiaire de mesures photoniques, la dépendance en fréquence de l’admittance du puits quantique couplé à la cavité micro-ondes. En ce qui concerne le fil topologique, nous avons montré que la cavité permet de caractériser la transition de phase topologique, l'émergence de fermions de Majorana, ainsi que la parité de l'état fondamental. Pour l'anneau supraconducteur, nous étudions par l'intermédiaire de la réponse optique de la cavité l’effet Josephson et le passage à l'effet Josephson fractionnaire, qui est associé à l'apparition de fermions de Majorana dans le système. Le cadre théorique proposé dans cette permet de sonder de manière non-invasive un large éventail de nanostructures, des boîtes quantiques aux supraconducteurs topologiques. En outre, il donne de nouvelles informations sur les propriétés de ces conducteurs quantiques, informations non accessibles via des expériences de transport
In this thesis, we study theoretically various physical properties of nanostructures that are coupledto microwave cavities. Cavity quantum electrodynamics (QED) with a quantum dot has been proven to be a powerful experimental technique that allows to study the latter by photonic measurements in addition to electronic transport measurements. In this thesis, we propose to use the cavity microwave field to extract additional information on the properties of quantum conductors: optical transmission coefficient gives direct access to electronic susceptibilities of these quantum conductors. We apply this general framework to different mesoscopic systems coupled to a superconducting microwave cavity, such as a tunnel junction, a quantum dot coupled to the leads, a topological wire and a superconducting ring. Cavity QED can be used to probe the finite frequency admittance of the quantum dot coupled to the microwave cavity via photonic measurements. Concerning the topological wire, we found that the cavity allows for determining the topological phase transition, the emergence of Majorana fermions, and also the parity of the ground state. For the superconducting ring, we propose to study the Josephson effect and the transition from the latter to the fractional Josephson effect, which is associated with the emergence of the Majorana fermions in the system, via the optical response of the cavity. The proposed framework allows to probe a broad range of nanostructures, including quantum dots and topological superconductors, in a non-invasive manner. Furthermore, it gives new information on the properties of these quantum conductors, which was not available in transport experiments

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34

Hu, Tao. "The Interplay Between Magnetism and Superconductivity in Strongly Correlated Materials." Kent State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=kent1254297944.

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35

Dandrea, Lucia. "Quantum Monte Carlo Methods applied to strongly correlated and highly inhomogeneous many-Fermion systems." Doctoral thesis, Università degli studi di Trento, 2009. https://hdl.handle.net/11572/369209.

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36

Dandrea, Lucia. "Quantum Monte Carlo Methods applied to strongly correlated and highly inhomogeneous many-Fermion systems." Doctoral thesis, University of Trento, 2009. http://eprints-phd.biblio.unitn.it/84/1/tesidottorato.pdf.

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37

Winograd, Emilio. "Orbital-selectivity in strongly correlated fermionic systems. From materials to cold-atoms." Thesis, Paris 11, 2013. http://www.theses.fr/2013PA112031.

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Cette thèse se concentre sur des aspects multiorbitales des systèmes fermioniques fortement corrélés. En particulier, sur l'existence d'une différentiation orbitale dans laquelle la coexistence de caractère itinérant et localisé peut être associée à différentes orbitales. Cette problématique est examinée dans le contexte des atomes froids et des matériaux, offrant un pont entre les deux communautés.Dans la première partie de la thèse, nous donnons un aperçu du problème des corrélations fortes dans les matériaux, et nous introduisons le concept de 'transition de Mott sélective en orbitales'. Nous fournissons également les principaux outils pour comprendre comment les matériaux peuvent être simulés avec des atomes froids, et nous présentons des résultats importants liés à la transition métal-isolant de Mott. Les aspects techniques, basées sur la théorie du champ moyen dynamique sont également discutés, et la solution de deux principaux modèles de systèmes fermioniques fortement corrélés, à savoir le modèle d'Hubbard (HM) et le modèle de Falicov-Kimball (FKM), sont passés en revue.Ensuite, nous étudions en détail la physique de deux espèces fermioniques en interaction forte avec des masses différentes dans un réseau optique. Nous établissons les différentes phases (avec et sans ordre à longue portée) en termes de la force des interactions (U), du rapport des masses et de la température (T), et aussi nous discutons les variables thermodynamiques, qui sont pertinentes pour les expériences d'atomes froids. Nous montrons que dans la phase métallique (U inférieure à une valeur critique) et avec un certain degré de différence de masses, un 'crossover' apparaît entre un état métallique du type de liquide de Fermi à basse T, et un état avec différentiation orbital à haute T, où les fermions lourds se localisent tandis que les fermions légers restent itinérant. Par conséquent, nous proposons ce modèle minimal pour étudier la physique des systèmes qui présentent une différentiation orbitale avec des expériences d'atomes froids.Basé sur les propriétés du modèle étudié, nous proposons la 'chromatographie entropique' comme une nouvelle méthode pour refroidir des atomes fermioniques dans les réseaux optiques. Nous discutons son efficacité et ses limites, et fournissons quelques idées afin de les surmonter.Dans la dernière partie de la thèse, nous généralisons le modèle précédent aux matériaux corrélés à plusieurs bandes qui permet d'afficher la différentiation orbitale. Nous montrons que l'état de Mott sélectif en orbital peut être stable sous les distorsions du réseau, modélisées par une hybridation locale entre les orbitales. Cependant, l'état de Mott est caractérisé par un pseudo-gap, où les fluctuations de charge sont brusquement réduites, mais l'état reste compressible. En relation au modèle précédent, nous discutons le 'crossover' entre l'état métallique et l'état sélectif induit par des effets température, nous comparons nos résultats avec les expériences de photoémission, et nous prédisons ce qui se passerait dans les matériaux qui présentent une hybridation locale entre les bandes
This thesis focuses on multiorbital aspects of strongly correlated fermionic systems. In particular, it focuses on the existence of orbital differentiation in which coexistence of itinerant and localized character can be associated to different orbitals. This subject is discussed in the context of cold atoms and materials, providing a bridge between both communities.In the first part of the thesis, we give an insight into the problem of strong correlations in materials, and we introduce the concept of 'orbital-selective Mott transition'. We also provide the main tools to understand how materials can be simulated with cold atoms experiments, and we present important related results in the context of the metal-Mott insulator transition. The technical aspects, based on dynamical mean-field theory are also discussed, and the solution of two key models of strongly correlated fermionic systems, i.e., the Hubbard model (HM) and the Falicov-Kimball model (FKM), are reviewed.Then we study in detail the physics of two interacting fermionic species with different masses in an optical lattice. We establish the different phases (with and without long-range order) in terms of the interactions strength (U), mass ratio and temperature (T), and also discuss the thermodynamic variables, which are relevant in cold atoms experiments. We show that in the metallic phase (U below a critical value) and for some degree of mass imbalance, a crossover appears between a Fermi-liquid metallic state at low T, and an 'orbital-selective' state at higher T, where the heavy fermions effectively localize while the light species remain itinerant. Hence, we propose this minimal model for addressing orbital-selective physics with cold atoms experiments.Based on the properties of the studied model, we propose the 'entropic chromatography' as a new method for cooling fermionic atoms in optical lattices. We discuss its efficiency and limitations, and provide some ideas in order to overcome them.In the last part of the thesis we generalize the previous model to a model relevant for multiband correlated materials that can display orbital differentiation. We show that the orbital-selective Mott state can be stable under lattice distortions modeled by local hybridization between the orbitals. However, the Mott state is characterized by a pseudogap, where charge fluctuations abruptly reduce, but the state remains compressible. In connection with the previous model, we discuss the temperature-induced orbital-selective crossover in this problem, we compare our results with photoemission experiments, and predict what would happen in materials that display local hybridization between the bands

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38

Binz, Benedikt. "Weak-coupling instabilities of two-dimensional lattice electrons." Phd thesis, Université Paris-Diderot - Paris VII, 2002. http://tel.archives-ouvertes.fr/tel-00001981.

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Les systèmes électroniques bidimensionnels sont d'une grande actualité tout particulièrement depuis la découverte de la supraconductivité à haute température. Ici, on se restreint à l'étude d'un modèle de Hubbard étendu, à la limite d'un couplage faible. En général, le gaz électronique subit une instabilité supraconductrice même sans phonons. Cependant, dans le cas spécial d'une bande demi-remplie, la surface de Fermi est emboîtée et se trouve à une singularité de Van Hove. Cette situation conduit à une compétition entre six instabilités différentes. Outre la supraconductivité en onde $s$ et $d$, on trouve des ondes de densités de spin et de charge ainsi que deux phases qui sont caractérisées par des courants circulaires de charge et de spin respectivement. Le formalisme du groupe de renormalisation est présenté en reliant l'idée de la "< sommation parquet "> au concept plus moderne de l'action effective de Wilson. Comme résultat on obtient un diagramme de phases riche en fonction de l'interaction du modèle. Ce diagramme de phase est exact dans la limite d'une interaction infiniment faible, puisque dans ce cas les lignes de transitions sont fixées par des symétries du modèle. Les comportements à basse température de la susceptibilité de spin ainsi que de la compressibilité de charge complètent l'image physique de ces instabilités. Il s'avère que la surface de Fermi à une tendence générale de se déformer spontanément, mais l'emboîtement n'est pas détruit. En résumé, le modèle de Hubbard à couplage faible reproduit deux propriétés essentielles des cuprates: une phase antiferromagnetique à demi remplissage et la supraconductivité en onde $d$ dans le cas dopé. Mais elle n'éxplique pas les propriétés inhabituelles de l'état métallique dans le régime sous-dopé. Une extension systématique de l'approche perturbative pourrait aider à mieux comprendre ces propriétés, mais reste difficile puisque les techniques nécessaires ne sont pas encore complètement développées.

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39

Lu, Yuan-Ming. "Exotic phases of correlated electrons in two dimensions." Thesis, Boston College, 2011. http://hdl.handle.net/2345/2363.

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Thesis advisor: Ziqiang Wang
Exotic phases and associated phase transitions in low dimensions have been a fascinating frontier and a driving force in modern condensed matter physics since the 80s. Due to strong correlation effect, they are beyond the description of mean-field theory based on a single-particle picture and Landau's symmetry-breaking theory of phase transitions. These new phases of matter require new physical quantities to characterize them and new languages to describe them. This thesis is devoted to the study on exotic phases of correlated electrons in two spatial dimensions. We present the following efforts in understanding two-dimensional exotic phases: (1) Using Zn vertex algebra, we give a complete classification and characterization of different one-component fractional quantum Hall (FQH) states, including their ground state properties and quasiparticles. (2) In terms of a non-unitary transformation, we obtain the exact form of statistical interactions between composite fermions in the lowest Landau level (LLL) with v=1/(2m), m=1,2... By studying the pairing instability of composite fermions we theoretically explains recently observed FQHE in LLL with v=1/2,1/4. (3) We classify different Z2 spin liquids (SLs) on kagome lattice in Schwinger-fermion representation using projective symmetry group (PSG). We propose one most promising candidate for the numerically discovered SL state in nearest-neighbor Heisenberg model on kagome lattice}. (4) By analyzing different Z2 spin liquids on honeycomb lattice within PSG classification, we find out the nature of the gapped SL phase in honeycomb lattice Hubbard model, labeled sublattice pairing state (SPS) in Schwinger-fermion representation. We also identify the neighboring magnetic phase of SPS as a chiral-antiferromagnetic (CAF) phase and analyze the continuous phase transition between SPS and CAF phase. For the first time we identify a SL called 0-flux state in Schwinger-boson representation with one (SPS) in Schwinger-fermion representation by a duality transformation. (5) We show that when certain non-collinear magnetic order coexists in a singlet nodal superconductor, there will be Majorana bound states in vortex cores/on the edges of the superconductor. This proposal opens a window for discovering Majorana fermions in strongly correlated electrons. (6) Motivated by recent numerical discovery of fractionalized phases in topological flat bands, we construct wavefunctions for spin-polarized fractional Chern insulators (FCI) and time reversal symmetric fractional topological insulators (FTI) by parton approach. We show that lattice symmetries give rise to different FCI/FTI states even with the same filling fraction. For the first time we construct FTI wavefunctions in the absence of spin conservation which preserve all lattice symmetries. The constructed wavefunctions also set up the framework for future variational Monte Carlo simulations
Thesis (PhD) — Boston College, 2011
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

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40

Bercx, Martin Helmut [Verfasser], and Fakher [Gutachter] Assaad. "Numerical studies of heavy-fermion systems and correlated topological insulators / Martin Helmut Bercx. Gutachter: Fakher Assaad." Würzburg : Universität Würzburg, 2015. http://d-nb.info/1111508771/34.

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41

Hackl, Andreas. "Quantum criticality and non-equilibrium dynamics in correlated electron systems." Aachen Shaker, 2009. http://d-nb.info/1001216466/04.

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42

Radmanesh, Seyed Mohammad Ali. "Ultra-low Temperature Properties of Correlated Materials." ScholarWorks@UNO, 2018. https://scholarworks.uno.edu/td/2511.

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Abstract After the discovery of topological insulators (TIs), it has come to be widely recognized that topological states of matter can actually be widespread. In this sense, TIs have established a new paradigm about topological materials. Recent years have seen a surge of interest in topological semimetals, which embody two different ways of generalizing the effectively massless electrons to bulk materials. Dirac and, particularly, Weyl semimetals should support several transport and optical phenomena that are still being sought in experiments. A number of promising experimental results indicate superconductivity in members of half-Hesuler semimetals which realize the mixing singlet and triplet pairing symmetry. We now turn to results we got through the work on topological semimetals. This work presents quantum high field transports on Dirac and Weyl topological semimetals including Sr1-yMn1-zSb2 (y, z < 0.1), YbMnBi2 and TaP. In case of Sr1-yMn1-zSb2 (y, z < 0.1), massless relativistic fermion was reported with m* = 0.04-0.05m0. This material presented a ferromagnetic order for in 304 K < T < 565 K, but a canted antiferromagnetic order with a net ferromagnetic component for T < 304 K. These are considered striking features of Dirac fermions For YbMnBi2, we reported the unusual interlayer quantum transport behavior in magnetoresistivity, resulting from the zeroth LL mode observed in this time reversal symmetry breaking type II Weyl semimetal. Also, for Weyl semimetal TaP the measurements probed multiple Fermi pockets, from which nontrivial π Berry phase and Zeeman splitting were extracted. Our ultra-low penetration depth measurements on half-Heuslers YPdBi and TbPdBi revealed a power- law behavior with n= 2.76 ± 0.04 for YPdBi samples and n=2.6 ± 0.3 for TbPdBi sample. We may conclude the exponent n > 2 implies nodless superconducting gap in our samples. Also, we found that despite the increase in magnetic correlations from YPdBi to TbPdBi, superconductivity remains robust in both systems which indicates that AF fluctuations do not play a major role in superconducting mechanism.

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43

Hilker, Timon [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Spin-resolved microscopy of strongly correlated fermionic many-body states / Timon Hilker ; Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2017. http://d-nb.info/115015943X/34.

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44

Vieyra, Villegas Hugo Abdiel. "Resistivity and thermal conductivity measurements on heavy-fermion superconductors in rotating magnetic fields." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-107550.

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CeCu_2Si_2 was the first heavy-fermion compound showing signatures of bulk superconductivity (T_c = 0.5 K). Further observations have put in evidence the correlations between superconductivity, magnetic order, Kondo physics, and quantum critical phenomena. In spite of the interest generated, a systematic study of such correlations was hampered by strong sample dependences. Fortunately, the inherent complexity associated to the stoichiometric composition has been recently understood. The availability of single-crystals with well-defined properties has thus reignited the interest in CeCu_2Si_2 as a window to novel phenomena, such as unconventional superconductivity. The present work summarizes the results of my doctoral research. It exemplifies the importance not only of high-quality materials, but also of suitable experimental techniques. A first step in this project involved the design of angle-dependent techniques in the milli-kelvin range, namely: electrical resistivity and thermal conductivity. It comprised the development of a rotational stage, the construction of sample holders, and the implementation of controlling and measuring components. In the second part of the project, electrical- and thermal-transport measurements on CeCu_2Si_2 were performed. Power-law behavior below T_c in the thermal conductivity suggests the presence of lines of nodes in the gap function. Also, the non-vanishing extrapolated residual terms (k_00/T ) support the presence of a residual density of states. The nodes are broadened by potential scattering, which appears to be significant in CeCu_2Si_2. The scattering hinders the determination of the symmetry of the order parameter and might be responsible for the observed isotropic angle dependence of the thermal conductivity. In contrast, angle-dependent measurements of the upper critical field exhibit a four-folded behavior, which also points towards the presence of nodes. By comparing with a weak-coupling model including the effects of Pauli limiting and anisotropic Fermi velocity, the results point towards a d_xy-wave symmetry of the order parameter. Such results represent the first angle-dependent measurements supporting a d-wave symmetry in CeCu_2Si_2.

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45

Haines, Charles Robert Sebastian. "Pressure tuned magnetism in d- and f-electron materials." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/282990.

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Quantum phase transitions (QPT) on the border of magnetism have provided a fertile hunting ground for the discovery of new states of matter, for example; the marginal Fermi Liquid and non Fermi Liquid states as well high T$_C$ cuprate and magnetically mediated superconductivity. In this thesis I present work on three materials in which it may be possible to tune the system through a magnetic QPT with the application of hydrostatic pressure. Although the details of the underlying physics are different in each of the materials, they are linked by the possibility of finding new states on the border of magnetism. Applying hydrostatic pressure, we have suppressed the ferromagnetic (FM) transition in metallic Fe$_2$P to very low temperature and to a potential QPT. Counter-intuitive broadening of the magnetic hysteresis leading up to the FM-AFM QPT may well be a crucial clue as to the nature of the model needed to understand this phase transition. A sharp increase in the quasi-particle scattering cross-section as well as the residual resistivity accompany a departure from the quadratic temperature dependence of the resistivity. This possible deviation from Fermi liquid behaviour is stable over a significant range of temperature. The unexplained upturn in the resistivity of CeGe that accompanies the AFM transition was studied under pressure. Pressure increased the residual resistivity as well as decreasing the relative size of the upturn, but had a moderate effect on the Neel temperature. The insensitivity of the N$\acute e$el temperature to pressure has been compared to its relative sensitivity to applied feld. The existence of the upturn and its evolution with pressure and applied feld can reasonably be argued to be due to the details of the electron band structure in the system. By applying pressure we have drastically reduced the resistivity of the insulating antiferromagnet NiPS$_3$. Concurrent work on FePS$_3$ has shown metallisation under pressure. It seems reasonable to speculate that NiPS$_3$ may also metallise at higher pressure. The energy gap is narrowed in both materials as pressure is increased. Magnetisation measurements have revealed a low temperature upturn indicating some possible ferromagnetic component or proximity to another magnetic state. A peak in the magnetisation is also seen at 45K in zero-feld cooled measurements. Both of these features point to a system with a complex magnetic ground state.

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46

Haase, Patrick [Verfasser], Thomas [Akademischer Betreuer] Pruschke, Reiner [Akademischer Betreuer] Kree, and Fakher [Akademischer Betreuer] Assaad. "Dual Fermion Approach to Disordered Correlated Systems / Patrick Haase. Betreuer: Thomas Pruschke. Gutachter: Reiner Kree ; Thomas Pruschke ; Fakher Assaad." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2015. http://d-nb.info/1078420238/34.

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47

Takasan, Kazuaki. "Nonequilibrium phenomena and dynamical controls in strongly correlated quantum systems driven by AC and DC electric fields." Kyoto University, 2019. http://hdl.handle.net/2433/242586.

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48

O'Brien, Aroon. "Charge degrees of freedom on the kagome lattice." Doctoral thesis, Universitätsbibliothek Chemnitz, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-71860.

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Within condensed matter physics, systems with strong electronic correlations give rise to fascinating phenomena which characteristically require a physical description beyond a one-electron theory, such as high temperature superconductivity, or Mott metal-insulator transitions. In this thesis, a class of strongly correlated electron systems is considered. These systems exhibit fractionally charged excitations with charge +e/2 or -e/2 in two dimensions (2D) and three dimensions (3D), a consequence of both strong correlations and the geometrical frustration of the interactions on the underlying lattices. Such geometrically frustrated systems are typically characterized by a high density of low-lying excitations, leading to various interesting physical effects. This thesis constitutes a study of a model of spinless fermions on the geometrically frustrated kagome lattice. Focus is given in particular to the regime in which nearest-neighbour repulsions V are large in comparison with hopping t between neighbouring sites, the regime in which excitations with fractional charge occur. In the classical limit t = 0, the geometric frustration results in a macroscopically large ground-state degeneracy. This degeneracy is lifted by quantum fluctuations. A low-energy effective Hamiltonian is derived for the spinless fermion model for the case of 1/3 filling in the regime where |t| << V . In this limit, the effective Hamiltonian is given by ring-exchange of order ~ t^3/V^2, lifting the degeneracy. The effective model is shown to be equivalent to a corresponding hard-core bosonic model due to a gauge invariance which removes the fermionic sign problem. The model is furthermore mapped directly to a Quantum Dimer model on the hexagonal lattice. Through the mapping it is determined that the kagome lattice model exhibits plaquette order in the ground state and also that fractional charges within the model are linearly confined. Subsequently a doped version of the effective model is studied, for the case where exactly one spinless fermion is added or subtracted from the system at 1/3 filling. The sign of the newly introduced hopping term is shown to be removable due to a gauge invariance for the case of hole doping. This gauge invariance is a direct result of the bipartite nature of the hole hopping and is confirmed numerically in spectral density calculations. For further understanding of the low-energy physics, a derivation of the model gauge field theory is presented and discussed in relation to the confining quantum electrodynamic in two dimensions. Exact diagonalization calculations illustrate the nature of the fractional charge confinement in terms of the string tension between a bound pair of defects. The calculations employ topological symmetries that exist for the manifold of ground-state configurations. Dynamical calculations of the spectral densities are considered for the full spinless fermion Hamiltonian and compared in the strongly correlated regime with the doped effective Hamiltonian. Calculations for the effective Hamiltonian are then presented for the strongly correlated regime where |t| << V . In the limit g << |t|, the fractional charges are shown to be effectively free in the context of the finite clusters studied. Prominent features of the spectral densities at the Gamma point for the hole and particle contributions are attributed to approximate eigenfunctions of the spinless fermion Hamiltonian in this limit. This is confirmed through an analytical derivation. The case of g ~ t is then considered, as in this case the confinement of the fractional charges is observable in the spectral densities calculated for finite clusters. The bound states for the effectively confined defect pair are qualitatively estimated through the solution of the time-independent Schroedinger equation for a potential which scales linearly with g. The double-peaked feature of spectral density calculations over a range of g values can thus be interpreted as a signature of the confinement of the fractionally charged defect pair. Furthermore, the metal-insulator transition for the effective Hamiltonian is studied for both t > 0 and t < 0. Exact diagonalization calculations are found to be consistent with the predictions of the effective model. Further calculations confirm that the sign of t is rendered inconsequential due to the gauge invariance for g in the regime |t| << V . The charge-order melting metal-insulator transition is studied through density-matrix renormalization group calculations. The opening of the energy gap is found to differ for the two signs of t, reflecting the difference in the band structure at the Fermi level in each case. The qualitative nature of transition in each case is discussed. As a step towards a realization of the model in experiment, density-density correlation functions are introduced and such a calculation is shown for the plaquette phase for the effective model Hamiltonian at 1/3 filling in the absence of defects. Finally, the open problem of statistics of the fractional charges is discussed.

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49

Peotta, Sebastiano. "Nonequilibrium dynamics of strongly correlated one-dimensional ultracold quantum gases." Doctoral thesis, Scuola Normale Superiore, 2013. http://hdl.handle.net/11384/85863.

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In the original work of this Thesis we use Time Dependent Density Matrix Renormalization Group (TDMRG) to follow and study the unitary dynamics of 1D strongly interacting quantum systems. In the rest part we present our work on the collision of spin polarized fermionic clouds. We study spin drag e ects immediately after the collision. This work is relevant to current experiments where pure spin currents have been realized with ultracold atomic gases. Several of our predictions can be veri ed in future experiments on strongly interacting few-fermion systems. In the second part the highly imbalanced case of an impurity immersed in a bath of bosonic atoms is considered. The interaction of the impurity with the bath manifests in the mass renormalization and in the damping of the oscillations of the breathing mode of the impurity in a harmonic potential. We compare the TDMRG results with an analytically tractable model in which the bath is treated as a Luttinger liquid and pinpoint striking deviations from this picture due to the nonlinear nature of the Lieb-Liniger gas. This results are relevant to current and future experiments on impurities coupled to one-dimensional ultracold gases. Finally, we employ DMRG to study spin-orbit coupled bosons in 1D optical lattices, following recent remarkable experimental advances on arti cially engineered gauge elds and spin-orbit coupling in ultracold atoms. We concentrate in the Mott insulator region of the phase diagram of pseudospin-1/2 bosons with spin-orbit coupling and anisotropic interaction terms that fully break spin rotational symmetry.

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50

Hofmann, Felix [Verfasser], and Michael [Akademischer Betreuer] Potthoff. "Nonequilibrium self-energy functional theory : Accessing the real-time dynamics of strongly correlated fermionic lattice systems / Felix Hofmann. Betreuer: Michael Potthoff." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2016. http://d-nb.info/1111778248/34.

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