To see the other types of publications on this topic, follow the link: Strongly correlated systems.
Dissertations / Theses on the topic 'Strongly correlated systems'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Strongly correlated systems.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Shelton, David G. "Low dimensional strongly correlated systems." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320594.
Full text
2
Iqbal, Nabil. "Holography and strongly correlated systems." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/68873.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 221-231).
In this thesis we apply techniques arising from string theory - gauge-gravity/duality, or holography - to problems associated with strongly coupled quantum field theories under extreme conditions such as finite temperature or density. We first study a strongly coupled field theory at finite temperature. We demonstrate that its low frequency limit is determined by the horizon geometry of its gravity dual, i.e. by the "membrane paradigm" fluid of classical black hole mechanics. Thus generic boundary theory transport coefficients can be expressed in terms of geometric quantities evaluated at the horizon, providing a simple understanding of results such as the universality of the shear viscosity in theories with gravity duals. Away from the low frequency limit we find a nontrivial radial flow from the black hole horizon to the boundary of the spacetime; we derive equations governing this flow and demonstrate their use in the simple examples of charge and momentum diffusion. Next, we turn to the study of strongly coupled theories with a finite density of a U(1) charge. The near-horizon geometry of the gravity dual of such a state has an AdS 2 factor, indicating the existence of a nontrivial emergent conformal symmetry in the infrared with nontrivial scaling only in the time direction. We review earlier work indicating that fermionic perturbations of such a state reveal non-Fermi-liquid behavior, i.e. gapless fermionic excitations that are not those of Fermi liquid theory. We perform a one-loop calculation in the bulk to compute the contribution from these Fermi surfaces to the conductivity of the full system. Interestingly, within this class of non-Fermi liquids we find examples whose single-particle spectral function and transport behavior both resemble those of strange metals, i.e. the anomalous metallic state existing in the real-life high Tc cuprates above their superconducting transition temperature. In particular, for these examples the contribution to the conductivity is inversely proportional to temperature. In our treatment these properties can be understood as being controlled by the scaling dimension of the fermion operator in the emergent IR fixed point. We then turn to models of symmetry breaking in holographic models at finite density. We observe that the presence of the AdS₂ factor can result in the condensation of a neutral scalar operator. This can be used to model an "antiferromagnetic" phase in which a global SU(2) symmetry is broken down to U(1). We study the collective modes of the ordered phase and recover the expected spin waves from a gravitational treatment. We then note that the phase transition can be driven to zero temperature by tuning various bulk couplings, resulting in a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type. We study this transition in detail, revealing novel critical behavior, including locally quantum critical dynamics and the existence of an infinite tower of excited states related by a discrete subgroup of the original emergent conformal symmetry. Throughout this thesis we focus on how the novel viewpoint provided by holography can help us gain new insights into the physics of strongly correlated systems.
by Nabil Iqbal.
Ph.D.
3
Reja, Sahinur. "Strong electron-phonon interactions in some strongly correlated systems." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648367.
Full text
4
Hart, Ian. "Magnetostriction in strongly correlated electron systems." Thesis, University of Bristol, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259584.
Full text
5
Loh, Yen Lee. "Studies of strongly correlated electron systems." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615109.
Full text
6
Dordevic, Sasa V. "Electrodynamics of strongly correlated electron systems /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC IP addresses, 2002. http://wwwlib.umi.com/cr/ucsd/fullcit?p3044790.
Full text
7
Ramos, Igor Rochaid Oliveira. "Study of strongly correlated colloidal systems." reponame:Repositório Institucional da UFC, 2014. http://www.repositorio.ufc.br/handle/riufc/11286.
Full textAbstract:
RAMOS, Igor Rochaid Oliveira. Study of strongly correlated colloidal systems. 2014. 113 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014.Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-04-09T20:37:08Z No. of bitstreams: 1 2014_tese_iroramos.pdf: 4160505 bytes, checksum: 1c5b95406b1be080fece98a7bc0bf05a (MD5)
Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-04-10T20:54:10Z (GMT) No. of bitstreams: 1 2014_tese_iroramos.pdf: 4160505 bytes, checksum: 1c5b95406b1be080fece98a7bc0bf05a (MD5)
Made available in DSpace on 2015-04-10T20:54:10Z (GMT). No. of bitstreams: 1 2014_tese_iroramos.pdf: 4160505 bytes, checksum: 1c5b95406b1be080fece98a7bc0bf05a (MD5) Previous issue date: 2014
This thesis presents the study of the structural and dynamical properties, as well as, melting of colloidal systems. Initially, we study the structure and phonon spectrum of a system of charged magnetic dipoles, organized in a bilayer structure and oriented perpendicular to the plane of the layers. This system can be tuned through six different crystalline phases by changing parameters such as the interlayer separation and/or the charge and/or dipole moment of the particles. The presence of the electric charge on the dipole particles is responsible for the nucleation of five staggered phases and a disordered phase which are not found in the magnetic dipole bilayer system previously presented in the literature. These extra phases are a consequence of the competition between the repulsive Coulomb and the attractive dipole interlayer interaction. The minimum energy structures are summarized in a phase diagram associated to the separation between the layers and to the relative importance between the magnetic and electric interactions. We determine the order of the structural phase transitions. The phonon spectrum of the system was calculated within the harmonic approximation. A non-monotonic behavior of the phonon spectrum is found as a function of the effective strength of the inter-particle interaction. The thermodynamic stability of the different phases is determined. Then, we study the bilayer system of charged magnetic dipoles for nonzero temperatures, investigating the melting behavior of the system through the modified Lindemann criterion, as a function of the parameters: (i) the distance between the layers η and (ii) the relative intensity of the magnetic interaction with respect to the electric interaction λ. For large enough λ, one of the phases (the matching hexagonal phase) exhibits a re-entrant melting behavior as a function of η. Since the charges and the magnetic dipole moment of the colloidal particles can be altered, for example, by changing the pH of the solution in which they are immersed or an external magnetic field, respectively, this system can be in principle verified experimentally. Last, a two-dimensional (2D) binary colloidal system consisting of interacting dipoles is investigated. Within the harmonic approximation, we obtained the phonon spectrum of the system as a function of the composition, dipole moment ratio and mass ratio between the small and big particles. Through a systematic analysis of the phonon spectra, we are able to determine the stability region of the different lattice structures of colloidal alloys. The gaps in the phonon frequency spectrum, the optical frequencies in the long-wavelength limit and the sound velocity are discussed as well. Using the modified Lindemann criterion and within the harmonic approximation, we estimated the melting temperature of the sub-lattice generated by the big particles.
Nesta tese, estudamos as propriedades estruturais e dinâmicas, bem como, a fusão de sistemas coloidais. Inicialmente, abordamos o problema de determinar as estruturas de mínima energia e o espectro de fônons de um sistema de dipolos magnéticos carregados, organizados em uma estrutura de bicamadas e orientados perpendicularmente ao plano das camadas. Este sistema pode ser sintonizado através de seis diferentes fases cristalinas, através da variação de parâmetros tais como a separação entre as camadas e/ou a carga e/ou o momento de dipolo das partículas. A presença de carga elétrica nas partículas dipolares é responsável pela nucleação de cinco fases onde as camadas não estão alinhadas verticalmente e uma fase desordenada, que não são encontradas no sistema em bicamadas de dipolos magnéticos previamente apresentado na literatura. Estas fases extras são uma consequência da competição entre a repulsão coulombiana e a interação atrativa entre os dipolos em diferentes camadas. As estruturas de mínima energia são sumarizadas em um diagrama de fases associado à separação entre camadas e a importância relativa entre as interações elétrica e magnética. Determinamos, ainda, a ordem das transições estruturais entre as várias configurações de mínima energia. O espectro de fônons do sistema foi calculado usando a aproximação harmônica. Um comportamento não-monotônico do espectro de fônons é encontrado como função da interação efetiva entre as partículas. A estabilidade termodinâmica das diferentes fases é determinada. Em seguida, estudamos o sistema de bicamadas de dipolos magnéticos carregados para temperaturas diferentes de zero, investigando a fusão do sistema através do critério de Lindemann modificado, em função dos parâmetros: (i) a distância entre as camadas η e (ii) a intensidade relativa da interação magnética com respeito à interação elétrica λ. Para λ suficientemente grande, uma das fases (a fase hexagonal com alinhamento vertical) exibe um comportamento reentrante na temperatura de fusão em função de η. Uma vez que a carga e o momento de dipolo magnético das partículas coloidais pode ser alterado, por exemplo, pela variação do pH da solução na qual estão imersos e por um campo magnético externo, respectivamente, este sistema pode ser em princípio verificado experimentalmente. Por último, um sistema bidimensional (2D) coloidal binário consistindo de dipolos interagentes é investigado. Dentro da aproximação harmônica, calculamos o espectro de fônons do sistema em função da composição, da razão entre os momentos de dipolo e da razão entre as massas das partículas pequenas e grandes. Através de uma análise sistemática dos espectros de fônons, determinamos a região de estabilidade das diferentes estruturas das ligas coloidais. As lacunas no espectro de frequência dos fônons, as frequências óticas no limite de longos comprimentos de onda e a velocidade do som são também discutidos. Usando o critério de Lindemann modificado e dentro da aproximação harmônica, estimamos a temperatura de fusão da sub-rede gerada pelas partículas grandes.
8
Sanchez, Lotero Adriana Mercedes. "Thermal transport in strongly correlated electron systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2005. http://nbn-resolving.de/urn:nbn:de:swb:14-1121946609637-03206.
Full text
9
Fehrmann, Henning. "Strongly correlated systems in ultracold quantum gases." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=981637442.
Full text
10
Shevchenko, Pavel Physics Faculty of Science UNSW. "Quantum Phenomena in Strongly Correlated Electrons Systems." Awarded by:University of New South Wales. Physics, 1999. http://handle.unsw.edu.au/1959.4/32669.
Full textAbstract:
Quantum phenomena in high-Tc superconductors and dimerized quantum Heisenberg antiferromagnets are studied analytically in this thesis. The implications of the Fermi surface consisting of the disjoint pieces, observed in cuprate superconductors, are considered. It is demonstrated that in this case the g-wave superconducting pairing is closely related to d-wave pairing. The superconductivity in this system can be described in terms of two almost degenerate superconducting condensates. As a result a new spatial scale lg, much larger than the superconducting correlation length x, arises and a new collective excitation corresponding to the relative phase oscillation between condensates, the phason, should exist. The Josephson tunneling for such a two-component system has very special properties. It is shown that the presence of g-wave pairing does not contradict the existing SQUID experimental data on tunneling in the ab-plane. Possible ways to experimentally reveal the g-wave component and the phason in a single tunnel junction, as well as in SQUID experiments, are discussed. The dimerized quantum spin models studied in this thesis include double-layer and alternating chain Heisenberg antiferromagnets. To account for strong correlations between the S=1 elementary excitations (triplets) in the dimerized phase; the analytic Brueckner diagram approach based on a description of the excitations as triplets above a strong-coupling singlet ground state; has been applied. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. Analytical calculations of physical observables are in excellent agreement with numerical data.Results obtained for double layer antiferromagnet near the (zero temperature) quantum critical point coincide with those previously obtained within the nonlinear s model approach Additional singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of the elementary triplets in the Heisenberg chain with frustration.
11
Reuter, Mortiz Emil. "Entanglement Properties of Strongly Correlated Spin Systems." Thesis, Imperial College London, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.499136.
Full text
12
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.
Full text
13
PUERTAS, LUIS ALBERTO PECHE. "PHYSICS OF STRONGLY CORRELATED AND DISORDERED SYSTEMS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2004. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=6580@1.
Full textAbstract:
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICONesta tese estudamos as propriedades físicas de materiais fortemente correlacionados e desordenados, usando Hamiltonianos modelos para descrevê-los. A tese está dividida em duas partes. Na primeira, estudamos o modelo de Anderson periódico para descrever as propriedades de um isolante Kondo. Em particular tomamos o composto de Ce3Bi4Pt3 como paradigma deste tipo de materiais caracterizados por apresentar um pequeno gap(da ordem dos meV ). Na presença de pequenas concentrações de impurezas metálicas como íons de La substituindo os de Ce, como é o caso da liga (Ce1-xLax)Bi4Pt3, sofre uma transição metal-isolante. O Hamiltoniano de Anderson periódico é resolvido a partir da solução de um único sítio atômico que logo é embebido numa rede de Bethe. Este modelo consegue explicar qualitativamente os resultados experimentais como a resistividade em função da temperatura para diferentes concentrações de íons de La, assim como as propriedades óticas do sistema puro. A influência da localização de Anderson nesta transição é analisada a partir do estudo da condutividade elétrica do sistema. A segunda parte está dedicada ao estudo das propriedades de sistemas descritos pelo Hamiltoniano de Falicov- Kimball, largamente utilizado para estudar fenômenos como a transição de valência e metal- isolante, também em compostos de Metais de Transição e Terras Raras. Neste modelo, o caráter destas transições ainda não está bem estabelecido já que o resultado é muito dependente da aproximação utilizada. Utilizamos o Hamiltoniano de Falicov-Kimball sem spin onde a banda de condução é tratada de forma exata já que mostramos a sua equivalência com o problema de uma liga. Os estados f são resolvidos em forma aproximada a partir da equação de movimento, aproximação que chamamos de Aproximação do Estreitamento Dinâmico(AED). Estudamos as propriedades eletrônicas como a ocupação dos estados localizados em função da energia local. Também neste caso, analisamos um sistema desordenado estudando o contraponto entre a correlação eletrônica e a desordem. As diferentes fases que aparecem no sistema como, metálica, isolante de Anderson e de Mott são investigadas em função dos parâmetros que definem o sistema.
In this thesis we study the properties of strongly correlated and disordered materials, using model Hamiltonians to describe them. The thesis is divided in two parts. The first one studies the periodic Anderson model used to describe the properties of a Kondo insulator. In particular we take Ce3Bi4Pt3 as a paradigmatic compound, characterized by a small gap(of the order of meV ). For small concentration of metallic impurities, ions of La substituting Ce, the alloy (Ce1-xLax)Bi4Pt3 suffers a metal- insulator transition. The periodic Anderson Hamiltonian is solved using the atomic solution that is embedded into a Bethe lattice. This model explains the experimental results as the resistivity as a function of temperature for different concentrations of ions of La, as well as, the optical properties of the pure system. The Anderson localization is analyzed studying the electric conductivity of the system. The second part of the thesis is dedicated to study the property of a system described by the Falicov- Kimball Hamiltonian. This Hamiltonian has been used to study the valence and metal-insulator transitions in Transitions Metal and Rare Earth compounds. In this model, the character of these transitions is still not well understood, since it is very dependent of the approximation used. We study the Falicov-Kimball Hamiltonian without spin. The conduction band is exactly described since we show its equivalence with the problem of an alloy. The f states are studied using the equation of motion for the Green functions, decoupling them in a way defined as the Dynamic Narrowing Approximation(DNA). We study the occupation of the local states as a function of energy and other electronic properties. For an alloy the interplay between the electronic correlation and disorder is analized. The different phases that appear in the system, as metallic and Anderson and Mott insulating, are investigated as a function of the parameters that define the system.
14
Carter, Edwin Christopher. "Anisotropic phenomena in strongly correlated electron systems." Thesis, University of Birmingham, 2005. http://etheses.bham.ac.uk//id/eprint/83/.
Full textAbstract:
This thesis is concerned with momentum anisotropy in strongly correlated electron systems, and explores its origin and its consequences through two contrasting projects. The first is a study of the temperature dependences of magnetotransport quantities in the normal state of the cuprate high-temperature superconductors. A phenomenological anisotropic small-angle scattering model is investigated; Hall effect measurements can be reproduced for parameters sufficiently close to particle-hole symmetry, but the experimentally observed magnetoresistance cannot be explained. The second project studies the phase diagram and quasiparticle properties of the square lattice Hubbard model within two-site cluster dynamical mean field theory (DMFT), at zero temperature. The "two-site" approach provides a drastically simplified but physically motivated self-consistency scheme for DMFT. This is combined for the first time with cluster DMFT, within which different magnetic orders and momentum anisotropy may be represented consistently. The extent of antiferromagnetism is determined; phases are discovered where the Fermi surface consists of small hole pockets, and the Mott transition happens as these pockets shrink to points. Anisotropic phenomena observed in the cuprates are reproduced by the theory; a pseudogap destroys the Fermi surface in some places, leaving behind Fermi arcs that closed into hole pockets by lines with very small quasiparticle residue.
15
Li, Chunhua. "Gutzwiller Approximation in Strongly Correlated Electron Systems." Thesis, Boston College, 2009. http://hdl.handle.net/2345/917.
Full textAbstract:
Thesis advisor: Ziqiang WangGutzwiller wave function is an important theoretical technique for treating local electron-electron correlations nonperturbatively in condensed matter and materials physics. It is concerned with calculating variationally the ground state wave function by projecting out multi-occupation configurations that are energetically costly. The projection can be carried out analytically in the Gutzwiller approximation that offers an approximate way of calculating expectation values in the Gutzwiller projected wave function. This approach has proven to be very successful in strongly correlated systems such as the high temperature cuprate superconductors, the sodium cobaltates, and the heavy fermion compounds. In recent years, it has become increasingly evident that strongly correlated systems have a strong propensity towards forming inhomogeneous electronic states with spatially periodic superstrutural modulations. A good example is the commonly observed stripes and checkerboard states in high-$T_\mathrm c$ superconductors under a variety of conditions where superconductivity is weakened. There exists currently a real challenge and demand for new theoretical ideas and approaches that treats strongly correlated inhomogeneous electronic states, which is the subject matter of this thesis. This thesis contains four parts. In the first part of the thesis, the Gutzwiller approach is formulated in the grand canonical ensemble where, for the first time, a spatially (and spin) unrestricted Gutzwiller approximation (SUGA) is developed for studying inhomogeneous (both ordered and disordered) quantum electronic states in strongly correlated electron systems. The second part of the thesis applies the SUGA to the $t$-$J$ model for doped Mott insulators which led to the discovery of checkerboard-like inhomogeneous electronic states competing with $d$-wave superconductivity, consistent with experimental observations made on several families of high-$T_{\mathrm c}$ superconductors. In the third part of the thesis, new concepts and techniques are developed to study the Mott transition in inhomogeneous electronic superstructures. The latter is termed ``SuperMottness'' which is shown to be a general framework that unifies the two paradigms in the physics of strong electronic correlation: Mott transition and Wigner crystallization. A cluster Gutzwiller approximation (CGA) approach is developed that treats the local ($U$) and extended Coulomb interactions ($V$) on equal footing. It is shown with explicit calculations that the Mott-Wigner metal-insulator transition can take place far away from half-filling. The mechanism by which a superlattice potential enhances the correlation effects and the tendency towards local moment formation is investigated and the results reveal a deeper connection among the strongly correlated inhomogeneous electronic states, the Wigner-Mott physics, and the multiorbital Mott physics that can all be united under the notion of SuperMottness. It is proposed that doping into a superMott insulator can lead to coexistence of local moment and itinerant carriers. The last part of the thesis studies the possible Kondo effect that couples the local moment and the itinerant carriers. In connection to the sodium rich phases of the cobaltates, a new Kondo lattice model is proposed where the itinerant carriers form a Stoner ferromagnet. The competition between the Kondo screening and the Stoner ferromagnetism is investigated when the conduction band is both at and away from half-filling
Thesis (PhD) — Boston College, 2009
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
16
Ye, Bing. "Unconventional Quantum Phases in Strongly Correlated Systems." Thesis, Boston College, 2016. http://hdl.handle.net/2345/bc-ir:106990.
Full textAbstract:
Thesis advisor: Ying RanIn this thesis, I investigated and implemented various numerical and simulation methods, including mean field theory, functional renormalization group method (fRG), density matrix renormalization group (DMRG) method etc., to find different quantum phases and quantum phase diagrams on models of correlated electronic systems. I found different phase diagrams with phases such as magnetism, superconductivity. By summarizing the strength and limitations of these methods, I investigated the projected entangled paired states (PEPS) with symmetry quantum number to sharply distinguish phases into crude classes and applied a variation of fast full update (FFU) prototype[58] to simulate different phases numerically. This method provides a promising, powerful and efficient way to simulate unconventional quantum phases and quantum phase diagrams in correlated electronic systems
Thesis (PhD) — Boston College, 2016
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
17
Derry, Philip. "Quasiparticle interference in strongly correlated electronic systems." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:f487c821-dbbb-4ebe-8b05-c13807379c2c.
Full textAbstract:
We investigate the manifestation of strong electronic correlations in the quasiparticle interference (QPI), arising from the scattering of conduction electrons from defects and impurities in an otherwise translationally-invariant host. The QPI may be measured experimentally as the Fourier transform of the spatial modulations in the host surface density of states that result, which are mapped using a scanning tunnelling microscope. We calculate the QPI for a range of physically relevant models, demonstrating the effect of strong local electronic correlations arising in systems of magnetic impurities adsorbed on the surface of non-interacting host systems. In the first instance the effect of these magnetic impurities is modelled via the single Anderson impurity model, treated via numerical renormalization group (NRG) calculations. The scattering of conduction electrons, and hence the QPI, demonstrate an array of characteristic signatures of the many-body state formed by the impurity, for example due to the Kondo effect. The effect of multiple impurities on the QPI is also investigated, with a numerically-exact treatment of the system of two Anderson impurities via state-of-the-art NRG calculations. Inter-impurity interactions are found to result in additional scattering channels and additional features in the QPI. The QPI is then investigated for the layered transition metal oxide Sr2RuO4, for which strong interactions in the host conduction electrons give rise to an unconventional triplet superconducting state at Tc ∼ 1.5K. The detailed mechanism for this superconductivity is still unknown, but electron-electron or electron-phonon interactions are believed to play a central role. We simulate the QPI in Sr2RuO4, employing an effective parametrized model consisting of three conduction bands derived from the Ru 4d t2g orbitals that takes into account spin orbit coupling and the anisotropy of the Ru t2g orbitals. Signatures of such interactions in the normal state are investigated by comparing these model calculations to experimental results. We also calculate the QPI in the superconducting state, and propose how experimental measurements may provide direct evidence of the anisotropy and symmetry of the superconducting gap, and thus offer insight into the pairing mechanism and the superconducting state.
18
Raum, Peter Thomas. "Exact Diagonalization Studies of Strongly Correlated Systems." Diss., Virginia Tech, 2020. http://hdl.handle.net/10919/96440.
Full textAbstract:
In this dissertation, we use exact diagonalization to study a few strongly correlated systems, ranging from the Fermi-Hubbard model to the fractional quantum Hall effect (FQHE). The discussion starts with an overview of strongly correlated systems and what is meant by strongly correlated. Then, we extend cluster perturbation theory (CPT), an economic method for computing the momentum and energy resolved Green's function for Hubbard models to higher order correlation functions, specifically the spin susceptibility. We benchmark our results for the one-dimensional Fermi-Hubbard model at half-filling. In addition we study the FQHE at fillings $nu = 5/2$ for fermions and $nu = 1/2$ for bosons. For the $nu = 5/2$ system we investigate a two-body model that effectively captures the three-body model that generates the Moore-Read Pfaffian state. The Moore-Read Pfaffian wave function pairs composite fermions and is believed to cause the FQHE at $nu = 5/2$. For the $nu = 1/2$ system we estimate the entropy needed to observe Laughlin correlations with cold atoms via an ansatz partition function. We find entropies achieved with conventional cooling techniques are adequate.Doctor of Philosophy
Strongly correlated quantum many-body physics is a rich field that hosts a variety of exotic phenomena. By quantum many-body we mean physics that is concerned with the behavior of interacting particles, such as electrons, where the quantum behavior cannot be ignored. By strongly correlated, we mean when the interactions between particles are sufficiently strong such that they cannot be treated as a small perturbation. In contrast to weakly correlated systems, strongly correlated systems are much more difficult to solve. That is because methods that reduce the many-body problem to a single independent body problem do not work well. In this dissertation we use exact diagonalization, a method to computationally solve quantum many-body systems, to study two strongly correlated systems: the Hubbard model and the fractional quantum Hall effect.The Hubbard model captures the physics of many interesting materials and is the standard toy model. Originally developed with magnetic properties in mind, it has been extended to study superconductivity, topological phases, cold atoms, and much more. The fractional quantum Hall effect is a novel phase of matter that hosts exotic excitations, some of which may have applications to quantum computing.
19
ARYANPOUR, KARAN. "APPROXIMATION TECHNIQUES IN STRONGLY CORRELATED ELECTRON SYSTEMS." University of Cincinnati / OhioLINK, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1069788785.
Full text
20
Manmana, Salvatore Rosario. "Nonequilibrium dynamics of strongly correlated quantum systems." [S.l. : s.n.], 2006. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-29095.
Full text
21
Gomes, Niladri, and Niladri Gomes. "Superconductivity in Strongly Correlated Quarter Filled Systems." Diss., The University of Arizona, 2017. http://hdl.handle.net/10150/625678.
Full textAbstract:
The objective of this thesis is to reach theoretical understanding of the unusual relationship between charge-ordering and superconductivity in correlated-electron systems. The competition between these broken symmetries and magnetism in the cuprate high temperature superconductors has been extensively discussed, but exists also in many other correlated-electron superconductors, including quasi-two-dimensional organic charge-transfer solids. It has been suggested that the same attractive interaction is responsible for both charge-order and superconductivity. We propose that the specific interaction is the
tendency in correlated-electron systems to form spin-singlet bonds, which is strongly enhanced at the commensurate carrier density p of ½ a charge carrier per site, characteristic of all superconducting charge-transfer solids. To probe superconductivity driven by electron correlations, a necessary condition is that electron-electron interactions enhance superconducting pair-pair correlations, relative to the non-interacting limit. We have performed state of the art numerical calculations on the two-dimensional Hubbard model on different triangular lattices, as well as other lattices corresponding to K-BEDT-TTF based organic charge transfer solids, for the complete range of carrier densities per site p (0 ≤ p ≤ 1). We have shown that pair-pair correlation for each cluster is enhanced by electron-electron interaction only for p ≃ 0.5, far away from the density range thought to be important for superconductivity. Although initial focus is on charge-transfer solids, the results of the research will impact the field of correlated electrons as a whole. We believe our calculations will provide fundamental and fresh insight to the theory of superconductivity in strongly correlated systems.
22
Guarnaccia, Giuseppe. "Phase transitions in strongly correlated electronic systems." Doctoral thesis, Universita degli studi di Salerno, 2014. http://hdl.handle.net/10556/1844.
Full textAbstract:
2012 - 2013We studied the some type of phase transitions in Strongly Correlated Electronic Systems. In particular we rigorously established some exact properties of a multi-orbital Hubbard model, here formulated to describe a nematic phase transition. In the first step, using Bogoliubov’s inequality, we rigorously showed that the multiorbital Hubbard model with narrow bands, eventually in the presence of the spin-orbit coupling, does not exhibit long-range nematic order, in the low dimensions. This result holds at any finite temperature for both repulsive and attractive on-site Coulomb interactions, with and without spin-orbit coupling. In the following step, using the reflection positivity method, we showed that this model supports a staggered nematic order if repulsive or attractive on-site inter-orbital and intra-orbital interactions and off-site repulsive inter-orbital interaction are considered. Depending on the dimensions of the lattice where the model is defined, the order may or not may exist. Indeed, in three dimensions the order may exist at finite temperature, and we get the condition for its existence finding out an upper bound for the critical temperature. On the other hand, for two dimensional lattices, the order may exist at least in the ground state, if the hopping amplitude is small enough. Furthermore, in the final step, we studied the symmetry properties of the non-degenerate Hubbard model with spin-orbit interactions of Rashba and Dresselhaus type. These interactions break the rotational symmetry in spin space, so that the magnetic order cannot be excluded by using the Bogoliubov inequality method. Nevertheless, we rigorously show that the existence of the magnetic long-range orders may be ruled out when the Rashba and Dresselhaus coupling constants are equal in modulus, whereas the -pairing can be always ruled out, regardless of the microscopic parameters of the model. These results are obtained by imposing locally the SU(2) gauge symmetry on the lattice, and rewriting the spin-orbit interactions in such a way that they are included in the path ordered of the gauge field on lattice. [edited by author]
XII n.s.
23
Mazza, Giacomo. "Non-Equilibrium Phenomena in Strongly Correlated Systems." Doctoral thesis, SISSA, 2015. http://hdl.handle.net/20.500.11767/4843.
Full textAbstract:
Correlated systems are a wide class of materials in which the strong electron-electron repulsion is the origin of very fascinating and unusual properties, among which metal-to-insulator transitions and high temperature superconductivity are the most striking examples. In the recent years, the fast development of non-adiabatic probing techniques opened new interesting perspectives for the investigation of such materials in non-equilibrium conditions. In this thesis, we discuss the theoretical description of few relevant cases which represent different examples of non-equilibrium phenomena in correlated materials. In particular, we will focus on the dynamics following a sudden excitation and the coupling to an external driving field. As a first example we consider the dynamics across a phase transition, namely we explore the possibility of driving a phase transition as the result of a sudden excitation, as e.g. the coupling with a short light pulse. We consider systems showing different equilibrium phases and study the conditions under which the off-equilibrium dynamics may lead to non-trivial dynamical phase transitions. A different case is represented by the dynamics induced by a driving electric field. This problem is particularly relevant for the possible applications of correlated materials in electronic devices. Here we consider the paradigmatic case of a correlated material coupled to external sources which impose a finite bias across the system. We analyze the formation and the properties of the non-equilibrium stationary states in which a finite current flows through the system. This allows us to study the non-linear response properties of a correlated system. In this context, a particularly relevant aspect is the problem of the dielectric breakdown of a Mott insulator, namely the formation of conducting states in the Mott insulating phase. In this thesis we explore different mechanisms leading to such possibility. First we discuss a quantum tunneling mechanism of carriers driven across the insulating gap by the effect of strong electric-fields. Eventually, we discuss the possibility of a resistive transition from an insulating to a metallic state induced by the application of an external electric-field.
24
Controzzi, Davide. "Non perturbative aspects of strongly correlated electron systems." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.343661.
Full text
25
Varadarajan, Vijayalakshmi. "SPECIFIC HEAT MEASUREMENTS ON STRONGLY CORRELATED ELECTRON SYSTEMS." UKnowledge, 2009. http://uknowledge.uky.edu/gradschool_diss/805.
Full textAbstract:
Studies on strongly correlated electron systems over decades have allowed physicists to discover unusual properties such as spin density waves, ferromagnetic and antiferromagnetic states with unusual ordering of spins and orbitals, and Mott insulating states, to name a few.
In this thesis, the focus will be on the specific heat property of these materials exhibiting novel electronic ground states in the presence and absence of a field. The purpose of these measurements is to characterize the phase transitions into these states and the low energy excitations in these states. From measurements at the phase transitions, one can learn about the amount of order involved [i.e. entropy: ΔS = ∫Δc p/T dT], while measurements at low temperatures illuminate the excitation spectrum. In order to study the thermodynamic properties of the materials at their phase transitions, a high sensitive technique, ac-calorimetry was used. The ac-calorimeter, workhorse of our low dimensional materials lab, is based on modulating the power that heats the sample and measuring the temperature oscillations of the sample around its mean value. The in-house ac-calorimetry set up in our lab has the capability to produce a quasi-continuous readout of heat capacity as a function of temperature. A variety of single crystals were investigated using this technique and a few among them are discussed in my thesis.
Since many of the crystals that are studied by our group are magnetically active, it becomes useful for us to also study them in the presence of a moderate to high magnetic field. This motivated me to design, develop, and build a heat capacity probe that would enable us to study the crystals in the presence of non-zero magnetic fields and at low temperatures. The probe helped us not only to revisit some of the studied materials and to draw firm conclusions on the previous results but also is vital in exploring the untouched territory of novel materials at high magnetic fields (~ 14 T).
26
Chamon, Cláudio de Carvalho. "Electronic conduction and noise in strongly correlated systems." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/38772.
Full text
27
Sordi, Giovanni. "Mott-Hubbard transition in strongly correlated electron systems." Paris 11, 2008. http://www.theses.fr/2008PA112160.
Full textAbstract:
J’ai étudié la transition méta-isolant avec la théorie du champ moyen dynamique appliquée à deux Hamiltoniens largement employés pour décrire les systèmes d’électrons fortement corrélés : le modèle de Hubbard et le modèle d’Anderson périodique. Le scénario pour la transition dans le modèle de Hubbard a été passé en revue et l’analyse du spectre de photoémission près de la transition a été présentée en détail. La transition de Mott induite par le dopage dans le modèle d’Anderson périodique a été discutée par rapport à celle réalisée dans le modèle de Hubbard. Le résultat principal nous conduit à établir un scénario qualitativement différent pour les transitions induites par dopage avec des électrons ou avec des trous. Dans le premier cas, la transition est, comme attendue, similaire à la transition du premier ordre du modèle de Hubbard. Toutefois, dans le dernier cas, une transition du deuxième ordre a été trouvée. J’ai donc démontré que le scénario pour la transition de Mott du modèle de Hubbard n’est pas générique pour le modèle d’Anderson périodiqueI study the Mott metal-insulator transition within the dynamical mean-field theory in two schematic Hamiltonians widely used to describe the strongly correlated electron systems : the Hubbard model and the periodic Anderson model. The scenario for the transition in the Hubbard model is reviewed and the analysis of the photoemission spectra near the transition is presented in detail. The doping driven Mott transition in the periodic Anderson model is discussed with respect to the one realized in the Hubbard model. The main finding is a qualitatively different scenario for electron or hole driven transitions. In the former case the transition is expectedly similar to the first order transition of the Hubbard model. However, in the latter case, a second order transition is found. Thus I demonstrate that the transition scenario of the Hubbard model is not generic for the periodic Anderson model
28
Carleo, Giuseppe. "Spectral and dynamical properties of strongly correlated systems." Doctoral thesis, SISSA, 2011. http://hdl.handle.net/20.500.11767/4289.
Full textAbstract:
In the first part of the Thesis we mostly concentrate on spectral properties of strongly correlated systems and on their equilibrium properties. This is accomplished by the general concept of imaginary-time dynamics which we apply to a number of different problems in which different strengths of this approach emerge.
In Chapter 1 we introduce the formalism that allows for a connection between the quantum and the classical worlds. The connection is established by means of the imaginary-time quantum evolution which, under certain circumstances, is shown to be equivalent to a classical stochastic process. It is further shown that exact static and spectral properties of correlated systems can be obtained when this mapping is feasible. The relationship between the imaginary-time dynamics in different frameworks such as the path-integral and the
perturbative one is also underlined.
In Chapter 2 we present a specific implementation of the general ideas previously presented. In particular we introduced an extension to lattice systems of the Reptation Monte Carlo algorithm [30] which benefits of a sampling scheme based on directed updates. Specific improvements over the existing methodologies consist in the unbiased evaluation of the imaginary-time path integrals for bosons and a systematic scheme to improve over the Fixed-node approximation for fermions. Applications to the Hubbard and the Heisenberg models are presented.
In Chapter 3 we demonstrate the application of the imaginary-time dynamics to the exact study of spectral properties. Subject of our attention is a highly anharmonic and correlated quantum crystal such as Helium 4 at zero temperature.[33] Concerning this system, we have obtained the first ab-initio complete phonon dispersion in good agreement with neutron spectroscopy experiments. Moreover, we have also studied the density excitations of solid
helium in a region of wave-vectors in between the collective (phonon) and the single-particle regimes, where the presence of residual coherence in the dynamics shows analogies between the highly anharmonic crystal and the superfluid phase.
In Chapter 4 we introduce a novel method, based on the imaginary-time dynamics, to obtain unbiased estimates of fermionic properties.[34] By means of this method and of a very accurate variational state, we provide strong evidence for the stability of a saturated ferromagnetic phase in the high-density regime of the two-dimensional infinite-U Hubbard model. By decreasing the electron density, we observe a discontinuous transition to a paramagnetic phase, accompanied by a divergence of the susceptibility on the paramagnetic side. This behavior, resulting from a high degeneracy among different spin sectors, is consistent with an infinite-order phase transition scenario.
In Chapter 5 the use of imaginary-time dynamics in the context of finite-temperature response functions is highlighted. As an application, we study an intriguing quantum phase featuring both glassy order and Bose-Einstein condensation. [35] We introduce and validate a model for the role of geometrical frustration in the coexistence of off-diagonal long range
order with an amorphous density profile. The exact characterization of the response of the system to an external density perturbation is what allows here to establish the existence of a spin-glass phase. The differences between such a phase and the otherwise insulating "Bose glasses" are further elucidated in the Chapter.
In the second part of the Thesis we focus our attention on the dynamics of closed systems out of equilibrium. This is accomplished by both non-stochastic exact methods for the dynamics and the introduction of a novel time-dependent Variational Monte Carlo scheme.
In Chapter 6 exact diagonalization schemes and renormalization-based methods for one-dimensional systems are introduced. We identify key phenomenological traits resulting from the many-body correlation in closed systems driven sufficiently away from equilibrium.[31]
We provide evidences that the dynamics of interacting lattice bosons away from equilibrium can be trapped into extremely long-lived inhomogeneous metastable states. The slowing down of incoherent density excitations above a threshold energy, much reminiscent of a dynamical arrest on the verge of a glass transition, is identified as the key feature of this phenomenon.
In Chapter 7 we present an extension to dynamical properties of the Variational Quantum Monte Carlo method.[32] This is accomplished by introducing a general class of time-dependent variational states which is based on the mapping of the many-body dynamics onto an instantaneous ground-state problem. The application of the method to the experimentally relevant quantum quenches of interacting bosons reveals the accuracy and the
reliability of the introduced numerical scheme. We indeed obtain for the first time a consistent variational description of the approach to the equilibrium of local observables and underline the origin of the metastability and glassy behavior previously identified.
In the very last part we draw our conclusions and show some possible paths for stimulating future research.
29
Majidi, Muhammad Aziz. "Computational Studies of Ferromagnetism in Strongly Correlated Electronic Systems." University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1148320220.
Full text
30
Liu, Jun. "Quantum phases in frustrated strongly correlated 2-D systems." [Ames, Iowa : Iowa State University], 2007.
Find full text
31
Depenbrock, Stefan. "Tensor networks for the simulation of strongly correlated systems." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-159631.
Full textAbstract:
This thesis treats the classical simulation of strongly-interacting many-body quantum-mechanical systems in more than one dimension using matrix product states and the more general tensor product states.
Contrary to classical systems, quantum many-body systems possess an exponentially larger number of degrees of freedom, thereby significantly complicating their numerical treatment on a classical computer.
For this thesis two different representations of quantum many-body states were employed.
The first, the so-called matrix product states (MPS) form the basis for the extremely successful density matrix renormalization group (DMRG) algorithm.
While originally conceived for one-dimensional systems, MPS are in principle capable of describing arbitrary quantum many-body states.
Using concepts from quantum information theory it is possible to show that MPS provide a representation of one-dimensional quantum systems that scales polynomially in the number of particles, therefore allowing an efficient simulation of one-dimensional systems on a classical computer.
One of the key results of this thesis is that MPS representations are indeed efficient enough to describe even large systems in two dimensions, thereby enabling the simulation of such systems using DMRG.
As a demonstration of the power of the DMRG algorithm, it is applied to the Heisenberg antiferromagnet with spin $S = 1/2$ on the kagome lattice.
This model's ground state has long been under debate, with proposals ranging from static spin configurations to so-called quantum spin liquids, states where quantum fluctuations destroy conventional order and give rise to exotic quantum orders.
Using a fully $SU(2)$-symmetric implementation allowed us to handle the exponential growth of entanglement and to perform a large-scale study of this system, finding the ground state for cylinders of up to 700 sites.
Despite employing a one-dimensional algorithm for a two-dimensional system, we were able to compute the spin gap (i.e. the energy gap to the first spinful excitation) and study the ground state properties, such as the decay of correlation functions, the static spin structure factors, and the structure and distribution of the nearest-neighbor spin-spin correlations.
Additionally, by applying a new tool from quantum information theory, the topological entanglement entropy, we could also with high confidence demonstrate the ground state of this model to be the elusive gapped $Z_2$ quantum spin liquid with topological order.
To complement this study, we also considered the extension of MPS to higher dimensions, known as tensor product states (TPS).
We implemented an optimization algorithm exploiting symmetries for this class of states and applied it to the bilinear-biquadratic-bicubic Heisenberg model with spin $S=3/2$ on the $z=3$ Bethe lattice.
By carefully analyzing the simulation data we were able to determine the presence of both conventional and symmetry-protected topological order in this model, thereby demonstrating the analytically predicted existence of the Haldane phase in higher dimensions within an extended region of the phase diagram.
Key properties of this symmetry-protected topological order include a doubling of the levels in the entanglement spectrum and the presence of edge spins, both of which were confirmed in our simulations.
This finding simultaneously validated the applicability of the novel TPS algorithms to the search for exotic order.
32
Ghaemi, Mohammadi Pouyan. "Phases and phase transitions of strongly correlated electron systems." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45456.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (leaves 169-174).
Different experiments on strongly correlated materials have shown phenomena which are not consistent with our conventional understandings. We still do not have a general framework to explain these properties. Developing such a general framework is much beyond the scope of this thesis, but here we try to address some of challenges in simpler models that are more tractable. In correlated metals it appears as strong correlations have different effect on different parts of fermi surface. Perhaps most striking example of this is normal state of optimally doped cuprates; the quasiparticle peaks on the nominal fermi surface do not appear uniformly. We try to track such phenomena in heavy fermion systems, which are correlated fermi liquids. In these systems, a lattice of localized electrons in f or d orbitals is coupled to the conduction electrons through an antiferromagnetic coupling. Singlets are formed between localized and conduction electrons. This singlet naturally have non-zero internal angular momentum. This nontrivial structure leads to anisotropic effect of strong correlations. Internal structure of Kondo singlet can also lead to quantum Hall effect in Kondo insulator, and formation of isolated points on the fermi surface with fractionalized quasiparticles. In the second part we study a phase transition in Heisenberg model between two insulating phases, Neel ordered and certain spin liquid state, popular in theories of the cuprates. The existence of such a transition has a number of interesting implications for spin liquid based approaches to the underdoped cuprates and clarifies existing ideas for incorporating antiferromagnetic long range order into such a spin liquid based approach. This transition might also be enlightening, despite fundamental differences, for the heavy fermion critical points where a second order transition between the heavy fermion phase and a metallic phase with magnetic antiferromagnetic order is observed.
by Pouyan Ghaemi Mohammadi.
Ph.D.
33
Sahebsara, Peyman. "Competing phases in strongly correlated electron systems with frustration." Thèse, Université de Sherbrooke, 2008. http://savoirs.usherbrooke.ca/handle/11143/5104.
Full textAbstract:
In this thesis we use the Variational Cluster Approximation (VCA) in the investigation of broken symmetry states of strongly correlated systems with frustration. Layered organic compounds, in which dimers of organic molecules form an anisotropic triangular lattice, are among materials that show this frustration. We discuss the two-dimensional one-band Hubbard model used for studying these compounds. Then we introduce VCA, which allows to study ordered phases by a variational principle based on the electron self-energy. We explain the computational methods that we used in conjunction with VCA. A comparison of the normal state and Néel antiferromagnetic ordered phase energies enables us to conclude that this order is dominant at large values of U, below some critical value of frustration (t'/t ). By observing the saturation of the order parameter, we argue that U [greater or approximately equal to] 8 is already in the strong coupling limit. d -wave superconductivity is discussed in relation with cluster and lattice point group symmetries. The two different pairings, d[subscript x[superscript 2]]-[subscript y[superscript 2]] and d[subscript xy] , are studied separately. A comparison of the energies of the antiferromagnetic and superconducting phases shows that while d -wave superconductivity dominates the antiferromagnetic phase, the d[subscript x[superscript 2]]-[subscript y[superscript 2]] order exists at intermediate U and d[subscript xy] is dominant at low values of U. We found no evidence of homogeneous coexistence of antiferromagnetic and d -wave superconducting phases. In addition, we investigate a spiral magnetic order on the isotropic triangular lattice, where no Néel antiferromagnetic order is found. By looking at the density of states, we see that the system is metallic at weak coupling. For U [greater or approximately equal to] 6 until a value in the range [8,12), we find an insulating phase, without long-range order, which we conjecture to be a spin liquid phase. This spiral order is found at stronger coupling.
34
Oakley, Gareth S. "Structural and magnetic studies of strongly correlated electronic systems." Thesis, University of Edinburgh, 2000. http://hdl.handle.net/1842/15548.
Full textAbstract:
Understanding of strongly correlated systems is of great importance in our understanding of fundamental solid-state science, and in the design and improvement of many technologically useful magnetic systems. In this thesis studies of two such systems are presented. The first system is the jarosite mineral family AM3(SO4)2(OH)6 (where A = H3O, K; M = Fe, Cr) which is an experimental manifestation of a kagome lattice antiferromagnet. Such a lattice displays unusual magnetic behaviour which may be of direct relevance to high temperature superconducting materials. A variety of neutron experiments have been performed to investigate the nature of the spin dynamics in the case of the hydronium iron salt, which is unique to the iron series in not exhibiting long range magnetic order. Single crystal studies have been used to probe the nature of the ground state of the potassium salt, and the first unambiguous determination of the magnetic structure is presented. Neutron diffraction studies and muon measurements have been performed on the hydronium chromium salt, the behaviour of which appears to contrast with that of the iron analogue. The second system of study is the series of compounds Lal-xMxMnO3 (where M = Ca,Pb) which are of interest due to their potential application in read-write head devices. A combination of both dc susceptibility measurements and neutron diffraction studies have been used to investigate the magnetic behaviour of both these systems in key areas of the temperature-composition phase diagrams. The electronic fluctuations in the calcium system have been studied using muon spin relaxation techniques.
35
Mikelsons, Karlis. "Extensions of Numerical Methods for Strongly Correlated Electron Systems." University of Cincinnati / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1256909270.
Full text
36
Tanaka, Kazunori. "Theoretical study on superconductivity in strongly-correlated electron systems." 京都大学 (Kyoto University), 2006. http://hdl.handle.net/2433/144161.
Full textAbstract:
Kyoto University (京都大学)0048
新制・課程博士
博士(理学)
甲第12077号
理博第2971号
新制||理||1444(附属図書館)
23913
UT51-2006-J72
京都大学大学院理学研究科物理学・宇宙物理学専攻
(主査)教授 山田 耕作, 教授 松田 祐司, 教授 前野 悦輝
学位規則第4条第1項該当
37
SANGIOVANNI, GIORGIO. "The electron-phonon interaction in strongly correlated electron systems." Doctoral thesis, La Sapienza, 2004. http://hdl.handle.net/11573/917137.
Full text
38
Linnér, Erik. "Interplay of collective fluctuations in strongly correlated fermionic systems." Electronic Thesis or Diss., Institut polytechnique de Paris, 2023. http://www.theses.fr/2023IPPAX090.
Full textAbstract:
Les systèmes fortement corrélés présentent souvent des diagrammes de phase riches avec différentes phases ordonnées impliquant des degrés de liberté de spin, de charge, d'appariement ou d'orbitale. La description théorique de la compétition entre les différentes instabilités dans les systèmes fortement corrélés, qui donne lieu à cette phénoménologie, reste l'un des Saint-Graal de la théorie moderne de la matière condensée. Elle pose un énorme défi de complexité à la fois conceptuelle et numérique, et l'interaction des fluctuations électroniques concurrentes constitue donc un obstacle à la compréhension des diagrammes de phase complexes d'une large gamme de matériaux quantiques corrélés. Cela motive la recherche de méthodes simplifiées pour étudier l'interaction des fluctuations collectives.Nous présentons une extension multicanal de l'approche du champ fluctuant récemment développée pour les fluctuations collectives concurrentes dans les systèmes électroniques corrélés. La méthode est basée sur une optimisation variationnelle d'une action d'essai qui contient explicitement les paramètres d'ordre des principaux canaux de fluctuation. Elle donne un accès direct à l'énergie libre du système, facilitant la distinction entre les phases stables et métastables du système. Nous appliquons notre approche au modèle de Hubbard étendu, un modèle de fermions sur réseau paradigmatique, qui occupe une place de choix dans la théorie de la matière condensée en raison de la pertinence potentielle de ses versions répulsives et attractives pour les matériaux électroniques et les systèmes artificiels. En utilisant notre technique pour étudier le régime de couplage faible à intermédiaire de l'interaction répulsive, nous constatons qu'elle capture la compétition entre les fluctuations d'onde de densité de charge et des fluctuations antiferromagnétiques en accord qualitatif avec des méthodes numériquement plus coûteuses. En outre, cette méthode permet d'accéder aux propriétés des états excités et aux effets de corrélation à plusieurs corps, directement sur l'axe des fréquences réelles sans utiliser de techniques de continuation analytique numériques. L'approche du champ fluctuant multicanal offre donc une voie prometteuse pour un traitement numériquement peu coûteux de l'interaction entre les fluctuations collectives dans les systèmes de petite et grande taille.En utilisant l'approche introduite du champ fluctuant multicanal, nous explorons le diagramme de phase du modèle de Hubbard étendu dans les régimes répulsif et attractif, en abordant l'interaction des fluctuations dans les canaux antiferromagnétiques, de l'onde de densité de charge, de l'onde s supraconductrice et de la séparation de phases. Bien que ce modèle ait été étudié de manière intensive depuis des décennies, notre nouvelle approche nous permet d'identifier une nouvelle phase caractérisée par la coexistence de fluctuations collectives de l'onde s supraconductrice et de la séparation de phases. Ces résultats sont en accord avec les observations précédentes de phases supraconductrices et de séparation de phases dans les systèmes électroniques, notamment dans les supraconducteurs à haute température critique. En outre, la méthode des champs fluctuants multicanaux permet de mettre en évidence la quintessence du modèle de Hubbard étendu grâce à la grande variété de types de compétitions qui émergent des différentes instabilités. La nature générale de la théorie proposée, qui permet d'incorporer une grande variété de modes collectifs, en fait un outil prometteur pour l'étude de l'interaction des fluctuations collectives dans les systèmes fermioniques fortement corrélésStrongly correlated systems often display rich phase diagrams exhibiting different ordered phases involving spin, charge, pairing, or orbital degrees of freedom. The theoretical description of the competition between different instabilities in strongly correlated systems giving rise to this phenomenology, remains one of the holy grails of modern condensed matter theory. It poses a tremendous challenge of both conceptual and computational complexity, and thus the interplay of competing electronic fluctuations constitutes a roadblock to the understanding of the complex phase diagrams of a wide range of correlated quantum materials. This motivates the search for constructing simplified methods to study interplaying collective fluctuations.We introduce a multichannel extension of the recently developed fluctuating field approach to competing collective fluctuations in correlated electron systems. The method is based on a variational optimization of a trial action that explicitly contains the order parameters of the leading fluctuation channels. It gives direct access to the free energy of the system, facilitating the distinction between stable and metastable phases of the system.We apply our approach to the extended Hubbard model, a paradigmatic fermionic lattice model, occupying a prime place in condensed matter theory due to the potential relevance of its repulsive and attractive versions for both electronic materials and artificial systems.Utilising the technique to study the weak to intermediate coupling regime of the repulsive interaction, we find it to capture the interplay of competing charge density wave and antiferromagnetic fluctuations with qualitative agreement with more computationally expensive methods. In addition, the method allows access to excited-state properties, through the one-particle excitation spectrum, and many-body correlation effects, through the self-energy, directly on the real-frequency axis without using numerical analytic continuation techniques. The multichannel fluctuating field approach thus offers a promising route for a numerically low-cost treatment of the interplay between collective fluctuations in small to large systems.Using the introduced multichannel fluctuating field approach, we explore the phase diagram of the extended Hubbard model in both repulsive and attractive regimes, addressing the interplay of fluctuations in the antiferromagnetic, charge density wave, s-wave superconducting, and phase separation channels. Despite the fact that this model has been intensively studied for decades, our novel approach allows us to identify a novel phase that is characterised by the coexistence of collective s-wave superconducting and phase separation fluctuations. These findings resonate with previous observations of interplaying phase separation and superconducting phases in electronic systems, most importantly in high-temperature superconductors. In addition, the multichannel fluctuating field method allows to display the quintessential nature of the extended Hubbard model through the large variety of types of competitions which emerges from the interplaying instabilities. The general nature of the proposed theory, allowing to incorporate a variety of collective modes, makes it a promising tool for studying the interplay of collective fluctuations in strongly correlated fermionic systems
39
Kragset, Steinar. "Phase transitions in effective lattice models for strongly correlated systems." Doctoral thesis, Norwegian University of Science and Technology, Department of Physics, 2006. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-1718.
Full textAbstract:
In three research articles we have studied the critical properties of effective lattice models for strongly correlated electron systems by Monte Carlo simulations. A similar model is used in a fourth article for investigating thermal fluctuations of vortices in a rotating Bose–Einstein condensate. In the first part of this thesis we review the necessary background and introduce the models one by one. The last part is a collection of the papers.
Paper I [1]: We consider the scaling of the mean square dipole moment in a plasma with logarithmic interactions in a two- and three-dimensional system. In both cases, we establish the existence of a low-temperature regime where the mean square dipole moment does not scale with system size and a hightemperature regime does scale with system size. Thus, there is a nonanalytic change in the polarizability of the system as a function of temperature, and hence a metal-insulator transition in both cases. The relevance of this transition in three dimensions to quantum phase transitions in 2 + 1-dimensional systems is briefly discussed.
Paper II [2]: The existence of a discontinuity in the inverse dielectric constant of the two-dimensional Coulomb gas is demonstrated on purely numerical grounds. This is done by expanding the free energy in an applied twist and performing a finite-size scaling analysis of the coefficients of higher-order terms. The phase transition, driven by unbinding of dipoles, corresponds to the Kosterlitz-Thouless transition in the 2D XY model. The method developed is also used for investigating the possibility of a Kosterlitz-Thouless phase transition in a threedimensional system of point charges interacting with a logarithmic pair-potential, a system related to effective theories of low-dimensional strongly correlated systems. We also contrast the finite-size scaling of the fluctuations of the dipole moments of the two-dimensional Coulomb gas and the three-dimensional logarithmic system to those of the three-dimensional Coulomb gas.
Paper III [3]: We perform large-scale Monte Carlo simulations on an effective gauge theory for an easy plane quantum anti-ferromagnet, including a Berry phase term that projects out the S = 1/2 sector. Without a Berry phase term, the model exhibits a phase transition in the 3DXY universality class associated with proliferation of gauge-charge neutral U(1) vortices. The instantons that eliminate the phase transition in the gauge-charged sector are cancelled by the Berry phases. The result is a first order phase transition. This gauge theory therefore does not exhibit deconfined criticality.
Paper IV [4]: We perform Monte Carlo studies of vortices in three dimensions in a cylindrical confinement, with uniform and nonuniform density. The former is relevant to rotating 4He, the latter is relevant to a rotating trapped Bose condensate. In the former case we find dominant angular thermal vortex fluctuations close to the cylinder wall. For the latter case, a novel effect is that at low temperatures the vortex solid close to the center of the trap crosses directly over to a tension-less vortex tangle near the edge of the trap. At higher temperatures an intermediate tensionful vortex liquid located between the vortex solid and the vortex tangle, may exist.
40
Narduzzo, Alessandro. "GHz magnetoconductivity and spin resonance studies of strongly correlated systems." Thesis, University of Oxford, 2005. https://ora.ox.ac.uk/objects/uuid:6bda6233-3853-4967-a3b3-c0447e620518.
Full textAbstract:
This thesis describes a series of experiments probing the millimetre-wave response of the low-dimensional organic superconductors β"-(BEDT-TTF)4[(H3O)-M(C2O4)3]S, κ-(BEDT-TTF)2Cu(NCS)2 and (TMTSF)2 PF6 in applied magnetic fields. Chapter 1 introduces the physics of charge carriers in the presence of a mag- netic field and field-induced resonant phenomena in the conductivity of metals. The basic concept of Fermi liquid theory, that of the existence of quasiparticles with effective mass, is presented, as well as the principles of electron spin resonance. Chapter 2 outlines the experimental technique of millimetre-wave spectroscopy, the principle of guided radiation and the use of metallic resonators and for perturbative measurements. Some results on novel dielectric whispering-gallery resonators and hybrid "tunnel-gap" resonators for potential use in pulsed and destructive field experiments are also presented. Chapter 3 is a detailed angle-, temperature- and frequency- dependent investigation of electron spin resonance in the family of quasi-two-dimensional charge transfer salts β"-(BEDT-TTF)4[(H3O)M(C2O4)3]S, where M = Cr3+ and Fe3+. The effective spin Hamiltonians for the Cr3+ and Fe3+ ions are identified, and the presence of weak antiferromagnetic correlations established; insight is gained into the relaxation mechanism and the effect of the superconducting state on the localised spins. Chapter 4 reports the angle-dependent magneto-optical response of isotopically substituted β"-(BEDT-TTF)2Cu(NCS)2. Both normal and superconducting states are investigated, and Josephson plasma resonances and Fermi-surface traversal resonances are identified. Chapter 5 presents some preliminary results from the investigation of the quasi- one-dimensional Bechgaard salt (TMTSF)2PF6 under hydrostatic pressure and in applied magnetic field. The millimetre-wave response is investigated as a function of frequency as the field-induced spin-density-wave cascade is crossed. Possible evidence for resonant absorptions due to collective spin excitations is reported.
41
Verbockhaven, Gilles Claude. "Study of strongly correlated systems by means of B-splines." [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 2000. http://dare.uva.nl/document/57240.
Full text
42
Dittes, Andrew. "A hybrid-parallel implementation of a strongly-correlated systems simulation." Cincinnati, Ohio : University of Cincinnati, 2008. http://rave.ohiolink.edu/etdc/view.cgi?acc_num=ucin1212174983.
Full textAbstract:
Thesis (M.S.)--University of Cincinnati, 2008.Advisor: Karen Tomko. Title from electronic thesis title page (viewed Sep. 8, 2008). Includes abstract. Keywords: parallell computing. Includes bibliographical references.
43
Ouellette, Daniel Gerald. "Dynamical conductivity of strongly correlated electron systems at oxide interfaces." Thesis, University of California, Santa Barbara, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3602181.
Full textAbstract:
The Mott metal-insulator transition (MIT) in transition-metal complex oxides results from strong electron-electron interactions and is accompanied by a rich spectrum of phenomena, including magnetic, charge, and orbital ordering, superconductivity, structural distortions, polarons, and very high-density 2-dimensional interface electron liquids. Recent advances in oxide heteroepitaxy allow interface control as a promising new approach to tuning the exotic properties of materials near the quantum critical point, with potential application to technologies including phase-change electronics, high power transistors, and sensors. The dynamical conductivity of oxide heterostructures is measured using a combination of terahertz time-domain spectroscopy, Fourier transform infrared spectroscopy, and dc magnetotransport. The rare-earth nickelates RNiO3 (R = La, Nd...) exhibit a temperature and bandwidth controlled MIT in bulk. Measurements of the Drude response in epitaxial thin films provide quantification of the strain-dependent mass enhancement in the metallic phase due to strong correlations. Reduction of LaNiO 3 film thickness leads to additional mass renormalization attributed to structural distortions at the heteroepitaxial interface, and an MIT is observed depending on the interfacing materials in coherent perovskite heterostructures. The rare-earth titanates RTiO3 exhibit a bandwidth and band filling controlled Mott MIT. Furthermore, the heterointerface between Mott insulating GdTiO3 and band insulating SrTiO3 exhibits a 2-dimensional itinerant electron liquid, with extremely high sheet densities of 3 × 1014 cm-2. The dynamical conductivity of the interface electrons is analyzed in terms of subband-dependent electron mobility and the established large polaron dynamics in bulk SrTiO3. Additional confinement of the electron liquids is achieved by decreasing the SrTiO3 layer thickness, with attendant increase in the dynamical mass. Taking the confinement to its extreme limit, a single (GdO) + plane in Mott insulating GdTiO3 is replaced with a (SrO) 0 plane. This is equivalent to "delta-doping" the Mott insulator with an extremely high density sheet of holes. The transport and absorption in the resulting two-dimensional insulator are consistent with a simple model of small polaron hopping. A comparison is made to similar features in the conductivity of randomly doped Sr1-xGdxTiO3 films.
44
Carr, Sam T. "Non-perturbative solutions to quasi-one-dimensional strongly correlated systems." Thesis, University of Oxford, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496837.
Full text
45
Exeter, Jason Wesley. "A study of rigorous techniques in strongly correlated electron systems." Thesis, King's College London (University of London), 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294910.
Full text
46
SILVA, GUILLERMO ANTONIO MAXIMILIANO GOMEZ. "ELECTRONIC TRANSPORT AND THERMOELECTRIC PROPERTIES OF STRONGLY CORRELATED NANOSCOPIC SYSTEMS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2018. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=36047@1.
Full textAbstract:
PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCOORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
FUNDAÇÃO DE APOIO À PESQUISA DO ESTADO DO RIO DE JANEIRO
PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTS. DE ENSINO
BOLSA NOTA 10
Nesta tese foram estudados três sistemas nanoscópicos compostos de pontos quânticos (PQs). No primeiro deles foi analisada a denominada nuvem Kondo, ou a extensão da blindagem que os spins da banda de condução fazem do spin de uma impureza magnética embebida em uma matriz metálica e representada, no nosso caso, por um PQ. As propriedades da nuvem Kondo foram obtidas através da manifestação da ressonância Kondo na densidade de estados local nos sítios da matriz metálica e também através das correlações de spin entre o spin do elétron no PQ e os spins da banda de condução. Foi possível encontrar uma concordância entre as extensões da nuvem Kondo obtidas com ambos métodos. O segundo sistema estudado consiste em uma estrutura de três PQs alinhados e com o PQ central acoplado a dois contatos metálicos. Foi analisada a operação deste sistema como uma porta lógica quântica cujo funcionamento depende do estado de carga do PQ central. Foi feito um estudo dependente do tempo das propriedades do sistema e, em particular, da correlação dos spins dos PQs laterais. Mostramos que o efeito Kondo, refletido na condutância do sistema, pode ser uma ferramenta fundamental para conhecer o estado da porta quântica. Os primeiros dois sistemas foram tratados usando o método dos Bósons Escravos na aproximação de campo médio. Finalmente, foi estudado o transporte termoelétrico em um sistema de dois PQs quando um deles está acoplado a contatos metálicos unidimensionais. O sistema foi analisado no regime de resposta linear e não linear a um potencial externo no regime de bloqueio de Coulomb. Mostramos que a presença de ressonâncias Fano e de uma singularidade de Van-Hove na densidade de estados dos contatos unidimensionais perto do nível de Fermi são ingredientes fundamentais para o aumento da eficiência termoelétrica do dispositivo. O problema de muitos corpos foi resolvido na aproximação de Hubbard III que permite um estudo correto das propriedades de transporte deste sistema para T maior que TK, onde TK é a temperatura Kondo.
In this thesis, were studied three nanoscopic quantum dot (QD) systems. First, the so-called Kondo cloud was analyzed, the extension of the conduction band spin screening of a magnetic impurity embedded in a metallic matrix and represented, in our case, by a QD. The Kondo cloud properties were obtained studying the way in which the local density of states of the metallic matrix sites reflects the Kondo resonance and also through the spin-spin correlations between the QD and the conduction band spins. It was possible to find a good agreement between the Kondo cloud extensions obtained using both methods. The second system consists of three aligned QDs with the central QD connected to two metallic leads. The operation of this system as a quantum gate was studied, which depends on the central QD charge. A time dependent study of the system properties and, in particular, of the lateral QDs spin correlation was developed. We showed that the Kondo effect, reflected in the conductance, could be a fundamental tool to measure the information contained in the quantum gate state. The first two systems were treated using the Slave Bosons Mean Field Approximation method. Finally, we studied the thermoelectric transport of a two QD system when one of them is connected to two onedimensional leads. The system was analyzed in the linear and nonlinear response to an external applied potential, always in the Coulomb blockade regime. It was found that the presence of Fano resonances and a Van-Hove singularity in the one-dimensional lead density of states near the Fermi level are fundamental ingredients to enhance thermoelectric efficiency. The many-body problem was treated in the Hubbard III approximation, which is a correct approach to study the transport properties for T greater than TK, where TK is the Kondo temperature.
47
Sandeman, Karl George. "Anisotropy in strongly correlated electron systems : transport, magnetism and superconductivity." Thesis, University of Cambridge, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.619568.
Full text
48
Sica, G. "Electron-electron and electron-phonon interactions in strongly correlated systems." Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/12194.
Full textAbstract:
In this work we investigate some aspects of the physics of strongly correlated systems by taking into account both electron-electron and electron-phonon interactions as basic mechanisms for reproducing electronic correlations in real materials. The relevance of the electron-electron interactions is discussed in the first part of this thesis in the framework of a self-consistent theoretical approach, named Composite Operator Method (COM), which accounts for the relevant quasi-particle excitations in terms of a set of composite operators that appear as a result of the modification imposed by the interactions on the canonical electronic fields. We show that the COM allows the calculation of all the relevant Green s and correlation functions in terms of a number of unknown internal parameters to be determined self-consistently. Therefore, depending on the balance between unknown parameters and self-consistent equations, exact and approximate solutions can be obtained. By way of example, we discuss the application of the COM to the extended t-U-J-h model in the atomic limit, and to the two-dimensional single-band Hubbard model. In the former case, we show that the COM provides the exact solution of the model in one dimension. We study the effects of electronic correlations as responsible for the formation of a plethora of different charge and/or spin orderings. We report the phase diagram of the model, as well as a detailed analysis of both zero and finite temperature single-particle and thermodynamic properties. As far as the single-band Hubbard model is concerned, we illustrate an approximated self-consistent scheme based on the choice of a two-field basis. We report a detailed analysis of many unconventional features that arise in single-particle properties, thermodynamics and system's response functions. We emphasize that the accuracy of the COM in describing the effects of electronic correlations strongly relies on the choice of the basis, paving the way for possible multi-pole extensions to the two-field theory. To this purpose, we also study a three-field approach to the single-band Hubbard model, showing a significant step forward in the agreements with numerical data with respect to the two-pole results. The role of the electron-phonon interaction in the physics of strongly correlated systems is discussed in the second part of this thesis. We show that in highly polarizable lattices the competition between unscreened Coulomb and Fröhlich interactions results in a short-range polaronic exchange term Jp that favours the formation of local and light pairs of bosonic nature, named bipolarons, which condense with a critical temperature well in excess of hundred kelvins. These findings, discussed in the framework of the so-called polaronic t-Jp model, are further investigated in the presence of a finite on-site potential U, coming from the competition between on-site Coulomb and Fröhlich interactions. We discuss the role of U as the driving parameter for a small-to-large bipolaron transition, providing a possible explanation of the BEC-BCS crossover in terms of the properties of the bipolaronic ground state. Finally, we show that a hard-core bipolarons gas, studied as a charged Bose-Fermi mixture, allows for the description of many non Fermi liquid behaviours, allowing also for a microscopic explanation of pseudogap features in terms of a thermal-induced recombination of polarons and bipolarons, without any assumption on preexisting order or broken symmetries.
49
Romero, Bermudez Aurelio. "Aspects of transport in strongly correlated systems with gravity duals." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/262902.
Full textAbstract:
In this thesis we consider various applications the gauge/gravity duality to study transport in strongly coupled systems. The main content is organized in three parts. In the first part we investigate the interrelation between dimensionality and strength of interactions. It is known that the dynamics of systems in Condensed Matter and General Relativity simplify for high dimensionality. Therefore, in this limit of large dimensionality, analytic results are usually possible. We study the dependence of the conductivity and the entanglement entropy on the space-time dimensionality in two different models of holographic superconductors: one dual to a quantum critical point with spontaneous symmetry breaking, and the other modelled by a charged scalar that condenses at a sufficiently low temperature in the presence of a Maxwell field. In the large dimensionality limit we obtain explicit analytical results for the conductivity at zero temperature and the entanglement entropy. Our results suggest that, as dimensionality increases, the condensate interactions become weaker. In the second part we first investigate the Drude weight and the related Mazur-Suzuki (MS) bound in a broad variety of strongly coupled field theories with a gravity dual at nonzero temperature and chemical potential. We show that the MS bound, which in the context of Condensed Matter provides information on the integrability of the theory, is saturated in Einstein-Maxwell-dilaton (EMd) and R-charged backgrounds. We then explore EMd theories with U(1) spontaneous symmetry breaking, and gravity duals of non-relativistic field theories, in which the MS bound is not saturated. Finally, we study the effect of a weak breaking of translational symmetry and we show that the MS bound sets a lower bound on the DC conductivity for a given scattering time. In the last part, we study asymptotically anti de Sitter Brans-Dicke (BD) backgrounds as effective models of metals with a varying coupling constant. We show that, for translational invariant backgrounds, the zero-frequency conductivity (dc conductivity) deviates from the universal result of EMd models. Once translational symmetry is broken, the shear viscosity to entropy ratio is always lower than the Kovtun-Son-Starinets bound, in line with other gravity backgrounds with momentum relaxation. In the BD models studied, we observed insulating like features in the dc conductivity. However, the module and argument of the optical conductivity at intermediate frequencies are not consistent with cuprates experimental results, even assuming several channel of momentum relaxation. We have also included the research carried out in the first year of the PhD as appendices. The topics studied in these appendices lie outside the main framework of this thesis.
50
Moliner, Marion. "Effects of lattice distortions on low-dimensional strongly correlated systems." Strasbourg, 2009. https://publication-theses.unistra.fr/public/theses_doctorat/2009/MOLINER_Marion_2009.pdf.
Full textAbstract:
Cette thèse porte sur l’effet de phonons, dans la limite adiabatique, sur des systèmes de basse dimensionalité fortement corrélés. Dans une première partie, nous considérons uniquement des systèmes de spins classiques (limite grand S). Nous étudions la chaîne de spins frustrés J1 − J2 couplée avec des distorsions du réseau et en présence d’un champ magnétique. Par des méthodes analytiques et numériques (Monte-Carlo), nous montrons que pour une large gamme de couplage spin-réseau un plateau d’aimantation est stabilisé à 1/3 de l’aimantation de saturation. Nous étudions ensuite un réseau frustré à deux dimensions, le réseau de Shastry-Sutherland. Nous trouvons un pseudo-plateau d’aimantation à 1/3 de l’aimantation de saturation à température non-nulle. Nous montrons, via des ondes de spins classiques et des simulations Monte-Carlo, que ce pseudo-plateau est dû à une sélection entropique d’une configuration colinéaire à travers l’effet d’Ordre par le Désordre. Nous obtenons par des simulations Monte-Carlo un diagramme de phase en fonction de la température et du champ appliqué. La seconde partie passe en revue le travail effectué sur la chaîne de Hubbard quart-remplie. Nous considérons des distorsions de Holstein (sur site) et de Peierls. Nous dérivons un Hamiltonien bosonique dans la limite basse énergie en incluant davantage d’harmoniques aux champs fermioniques et montrons que le couplage avec le réseau couple les degrés de liberté de charge et de spin. Nous retrouvons qualitativement les phases tétra et dimérisées qui ont été obtenues dans des travaux numériques antérieursThis thesis deals with the effects of phonons, in the adiabatic limit, on low-dimensional strongly-correlated systems. In a first part, we focus on spin systems in the classical limit (large S). We study the frustrated J1 − J2 chain coupled with lattice distortions and under an applied magnetic field. By means of analytical and numerical (Monte-Carlo) methods, we show that, for a wide range of the spin-lattice coupling, a magnetization plateau at 1/3 of the saturated magnetization is stabilized. We then study the two-dimensional frustrated Shastry-Sutherland lattice. Amagnetization pseudo-plateau is found at 1/3 of the saturatedmagnetization for nonzero temperature. Classical spin-waves and Monte-Carlo simulations show that this pseudoplateau is due to the entropic selection of a particular collinear configuration through the Order by Disorder effect. By means of Monte-Carlo simulations, we obtain a phase diagram in the planemagnetic field versus temperature. The second part is dedicated to the quarter-filled Hubbard chain. Both Holstein (on-site) and Peierls distortions are considered. We derive a bosonic Hamiltonian in the low-energy limit by means of the bosonization technique. We includemore harmonics in the bosonic expansion of the fermionic fields and showthat the lattice coupling couples the spin and charge degrees of freedom. We qualitatively recover tetramerized and dimerized phases that were obtained in previous numerical works