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1
Esteban, Puyuelo Raquel. "Electronic Properties of Correlated Systems." Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-287985.
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The aim of this project is to become familiar with the Hubbard-corrected energy functionals used in density functional theory, which are needed to describe the electronic properties of strongly correlated systems. This study focuses on two systems, gadolinium and nickel oxide, as examples of a lanthanide and a transition metal oxide, respectively, for which the conventional approaches to Density Functional Theory such as Local Density Approximationor Generalized Gradient Approximation fail.
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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.
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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.
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Taylor, Daniel J. "Correlated electronic structure theory for challenging systems." Thesis, Heriot-Watt University, 2015. http://hdl.handle.net/10399/3004.
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The photochemistry of molecules can be investigated computationally, and this provides great insight into the underlying chemistry and physics. Such computational approaches are challenging and can pose many difficulties compared to ground state methodologies. Care must be taken to accurately describe these systems, as some lowlevel approximate methods can fail. The geometrical and electronic structures (TiO2)n clusters (n=1-4) have been investigated. These are of enormous technological interest as wide band-gap semiconductors yet the nature of electronic transitions in nano-sized clusters has yet to be fully elucidated. Structures of the neutral closed-shell, radical cationic and radical anionic clusters at each size are described and rationalised in terms of the pseudo-Jahn- Teller effect. We have used high-level response theory to set benchmarks for such systems. The TiO2 monomer is the simplest of the clusters studied yet proves a stern test for many lower order ab-initio methods. It is shown that high-level methods are required to properly describe this simple molecule. The Monte Carlo Configuration Interaction method attempts to combine the power of Full CI with a scalability that allows it to be used to study much larger systems. It can be systematically improved and can approach the accuracy of the Full CI method. This method is applied here to investigate potential energy surfaces and multipole moments of a range of small but challenging systems.
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Pereira, Vítor Manuel. "Disorder and localization effects in correlated electronic systems." Tese, Porto : edição do autor, 2006. http://catalogo.up.pt/F?func=find-b&local_base=FCB01&find_code=SYS&request=000088367.
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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.
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Pereira, Vítor Manuel. "Disorder and localization effects in correlated electronic systems." Doctoral thesis, Porto : edição do autor, 2006. http://hdl.handle.net/10216/64278.
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Ueda, Suguru. "Theoretical study on electronic properties at interfaces of strongly correlated electron systems." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199081.
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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.
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Oakley, Gareth S. "Structural and magnetic studies of strongly correlated electronic systems." Thesis, University of Edinburgh, 2000. http://hdl.handle.net/1842/15548.
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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.
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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.
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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.
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Erten, Onur. "Electronic and Magnetic Properties of Double Perovskites and Oxide Interfaces." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1376496346.
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Pavosevic, Fabijan. "Explicitly Correlated Methods for Large Molecular Systems." Diss., Virginia Tech, 2018. http://hdl.handle.net/10919/82000.
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Wave function based electronic structure methods have became a robust and reliable tool for the prediction and interpretation of the results of chemical experiments. However, they suffer from very steep scaling behavior with respect to an increase in the size of the system as well as very slow convergence of the correlation energy with respect to the basis set size. Thus these methods are limited to small systems of up to a dozen atoms. The first of these issues can be efficiently resolved by exploiting the local nature of electron correlation effects while the second problem is alleviated by the use of explicitly correlated R12/F12 methods. Since R12/F12 methods are central to this work, we start by reviewing their modern formulation.
Next, we present the explicitly correlated second-order Mo ller-Plesset (MP2-F12) method in which all nontrivial post-mean-field steps are formulated with linear computational complexity in system size [Pavov{s}evi'c et al., {em J. Chem. Phys.} {bf 144}, 144109 (2016)]. The two key ideas are the use of pair-natural orbitals for compact representation of wave function amplitudes and the use of domain approximation to impose the block sparsity. This development utilizes the concepts for sparse representation of tensors described in the context of the DLPNO-MP2 method by Neese, Valeev and co-workers [Pinski et al., {em J. Chem. Phys.} {bf 143}, 034108 (2015)]. Novel developments reported here include the use of domains not only for the projected atomic orbitals, but also for the complementary auxiliary basis set (CABS) used to approximate the three- and four-electron integrals of the F12 theory, and a simplification of the standard B intermediate of the F12 theory that avoids computation of four-index two-electron integrals that involve two CABS indices. For quasi-1-dimensional systems (n-alkanes) the bigO{N} DLPNO-MP2-F12 method becomes less expensive than the conventional bigO{N^{5}} MP2-F12 for $n$ between 10 and 15, for double- and triple-zeta basis sets; for the largest alkane, C$_{200}$H$_{402}$, in def2-TZVP basis the observed computational complexity is $N^{sim1.6}$, largely due to the cubic cost of computing the mean-field operators. The method reproduces the canonical MP2-F12 energy with high precision: 99.9% of the canonical correlation energy is recovered with the default truncation parameters. Although its cost is significantly higher than that of DLPNO-MP2 method, the cost increase is compensated by the great reduction of the basis set error due to explicit correlation.
We extend this formalism to develop a linear-scaling coupled-cluster singles and doubles with perturbative inclusion of triples and explicitly correlated geminals [Pavov{s}evi'c et al., {em J. Chem. Phys.} {bf 146}, 174108 (2017)]. Even for conservative truncation levels, the method rapidly reaches near-linear complexity in realistic basis sets; e.g., an effective scaling exponent of 1.49 was obtained for n-alkanes with up to 200 carbon atoms in a def2-TZVP basis set. The robustness of the method is benchmarked against the massively parallel implementation of the conventional explicitly correlated coupled-cluster for a 20-water cluster; the total dissociation energy of the cluster ($sim$186 kcal/mol) is affected by the reduced-scaling approximations by only $sim$0.4 kcal/mol. The reduced-scaling explicitly correlated CCSD(T) method is used to examine the binding energies of several systems in the L7 benchmark data set of noncovalent interactions.
Additionally, we discuss a massively parallel implementation of the Laplace transform perturbative triple correction (T) to the DF-CCSD energy within density fitting framework. This work is closely related to the work by Scuseria and co-workers [Constans et al., {em J. Chem. Phys.} {bf 113}, 10451 (2000)]. The accuracy of quadrature with respect to the number of quadrature points has been investigated on systems of the 18-water cluster, uracil dimer and pentacene dimer. In the case of the 18-water cluster, the $mu text{E}_{text{h}}$ accuracy is achieved with only 3 quadrature points. For the uracil dimer and pentacene dimer, 6 or more quadrature points are required to achieve $mu text{E}_{text{h}}$ accuracy; however, binding energy of $<$1 kcal/mol is obtained with 4 quadrature points. We observe an excellent strong scaling behavior on distributed-memory commodity cluster for the 18-water cluster. Furthermore, the Laplace transform formulation of (T) performs faster than the canonical (T) in the case of studied systems. The efficiency of the method has been furthermore tested on a DNA base-pair, a system with more than one thousand basis functions.
Lastly, we discuss an explicitly correlated formalism for the second-order single-particle Green's function method (GF2-F12) that does not assume the popular diagonal approximation, and describes the energy dependence of the explicitly correlated terms [Pavov{s}evi'c et al., {em J. Chem. Phys.} {bf 147}, 121101 (2017)]. For small and medium organic molecules the basis set errors of ionization potentials of GF2-F12 are radically improved relative to GF2: the performance of GF2-F12/aug-cc-pVDZ is better than that of GF2/aug-cc-pVQZ, at a significantly lower cost.Ph. D.
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Chowdhury, Debanjan. "Interplay of Broken Symmetries and Quantum Criticality in Correlated Electronic Systems." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493455.
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This thesis delves into a study of phases of strongly correlated quantum matter confined to two spatial dimensions. The thesis can broadly be divided into three parts. In the first part, comprising of chapters 2 and 3, we investigate some interesting aspects of symmetry breaking and quantum criticality in the superconducting phase of the iron-based superconductors. In particular, motivated by tunneling microscopy measurements on FeSe, in chapter 2 we study the effect of spontaneously broken rotational symmetry on the structure of the superconducting vortex. In chapter 3, we study the critical singularities associated with the superfluid-density at a wide class of symmetry-breaking and topological phase transitions in a clean superconductor. Inspired by experiments on BaFe$_2$(As$_{1-x}$P$_x$)$_2$, we also analyze the effect of quenched disorder on the superfluid-density in the vicinity of magnetic quantum critical points.
The second part of this thesis, consisting of chapters 4 and 5, is devoted to a study of the pseudogap phase in the underdoped cuprates. In chapter 4 we study the effect of thermal fluctuations of various competing order parameters, including preformed superconductivity and short-ranged charge-density wave, on the electronic excitations. In chapter 5 we analyze the feedback of pairing fluctuations on the landscape of various competing charge-density wave order parameters within the framework of fermi-liquid theory.
In the final part of the thesis, consisting of chapters 6 and 7, we propose an alternative picture for describing the pseudogap metal. In chapter 6, we study a quantum-disordered phase of matter---the fractionalized fermi-liquid (FL*)---where the electrons are coupled to the fractionalized excitations of a strongly fluctuating antiferromagnet and propose it to be a candidate state for the pseudogap. We investigate instabilities of the FL* to density-wave order and compare with experiments. In chapter 7, we describe a framework for describing a novel quantum phase transition without any broken-symmetries---a Higgs transition---that describes a transition from a conventional fermi-liquid to a parent phase of the FL* state via an intermediate non-fermi liquid. We discuss its possible connection to the optimal doping critical point in the cuprates.Physics
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Tchaplyguine, Igor. "Electronic structure of strongly correlated low-dimensional spin ½ systems: cuprates and vanadates." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2003. http://nbn-resolving.de/urn:nbn:de:swb:14-1052218731218-09287.
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In the first two chapters we presented the basics of density functional theory and semiempirical LSD+U approximation, which was implemented in the full-potential local-orbital (FPLO) minimal-basis calculation scheme. In the third chapter we tested the implemented version of LSDA+U on 3d transitional metal monoxides. Essential improvement of the spectroscopic properties was obtained. A simple model describing the value and direction of the magnetic moment of a transition metal ion was presented. The model visualizes the interplay of the spin-orbit coupling and crystal field splitting. In the fourth chapter we calculated the electronic spectrum of the single Zn impurity in CuO2 plane considered as a vacancy in Cu 3d states. The analytic solution for the states of different symmetry was obtained. Depending on the strength of perturbation induced by the impurity on the neighboring Cu ions, the states are either resonant or localized. The critical values of the perturbation were computed. In the fifth chapter we presented the calculations for three novel vanadates: MgVO3, Sb2O2VO3 and VOMoO4. The tight-binding parameters and the exchange integrals were computed. The magnesium and antimony vanadates appeared to be spin-½ one-dimensional systems, the latter having much stronger one-dimensional character and being probably the best realization of inorganic spin-Peierls system. The molybdenum vanadate was found to be two-dimensional spin-½ system. The Mo 4d orbitals play an important role in the electronic transfer.
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Hackenberger, Christian. "Development of models to implement low dimensional structures in correlated electronic systems." kostenfrei, 2006. http://d-nb.info/992037174/34.
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Tchaplyguine, Igor. "Electronic structure of strongly correlated low-dimensional spin ½ systems: cuprates and vanadates." Doctoral thesis, Technische Universität Dresden, 2002. https://tud.qucosa.de/id/qucosa%3A24217.
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In the first two chapters we presented the basics of density functional theory and semiempirical LSD+U approximation, which was implemented in the full-potential local-orbital (FPLO) minimal-basis calculation scheme. In the third chapter we tested the implemented version of LSDA+U on 3d transitional metal monoxides. Essential improvement of the spectroscopic properties was obtained. A simple model describing the value and direction of the magnetic moment of a transition metal ion was presented. The model visualizes the interplay of the spin-orbit coupling and crystal field splitting. In the fourth chapter we calculated the electronic spectrum of the single Zn impurity in CuO2 plane considered as a vacancy in Cu 3d states. The analytic solution for the states of different symmetry was obtained. Depending on the strength of perturbation induced by the impurity on the neighboring Cu ions, the states are either resonant or localized. The critical values of the perturbation were computed. In the fifth chapter we presented the calculations for three novel vanadates: MgVO3, Sb2O2VO3 and VOMoO4. The tight-binding parameters and the exchange integrals were computed. The magnesium and antimony vanadates appeared to be spin-½ one-dimensional systems, the latter having much stronger one-dimensional character and being probably the best realization of inorganic spin-Peierls system. The molybdenum vanadate was found to be two-dimensional spin-½ system. The Mo 4d orbitals play an important role in the electronic transfer.
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Hunter, Emily Claire. "An exploration of novel correlated electronic states in 5d transition metal oxides." Thesis, University of Edinburgh, 2016. http://hdl.handle.net/1842/22008.
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The crystal growth conditions of compounds of the series Srn+1IrnO3n+1 (n=1, 2 and ∞) are investigated. It was found that the ratio of IrO2:SrCO3 in the starting mixture is the most important variable in determining the phase formed. Good quality samples of Sr3Ir2O7 were found to have a sharp change in gradient at the Néel temperature of 287.5 K and no secondary T* transition between 230 K and 260 K. All crystals of Sr3Ir2O7 grown were found to be heavily oxygen deficient by EPMA regardless of the crystal growth conditions used with an average stoichiometry of Sr2:87Ir2O6:27. Adding more electrons via replacing strontium with lanthanum causes (Sr(1-x)Lax)3Ir2O7 to become metallic by x=0.072, which also fully quenches the long-range antiferromagnetic order. Heat capacity and resistivity measurements show that metallic (Sr(1-x)Lax)3Ir2O7 is a weakly correlated Fermi-liquid metal. Given that there are only subtle changes to the structure upon lanthanum doping, the metal-insulator transition is a result of electron doping rather than structural distortions. No structural phase transitions were found up to a temperature of 800°C and no additional evidence was found to support the Bbcb space group model of the structure of Sr3Ir2O7. Using crystals five times better in quality than those reported in the literature, SrIrO3 was found to be a Fermi-liquid metal, rather than a non-Fermi liquid metal as previously reported, and no superconductivity was found down to temperatures of 20 mK. A known Pt(III) compound, CaPt2O4, was found to be a weakly correlated metal down to 2 K and a novel Pt(III) based compound, K2CaPt3-δ O6 (δ ≈ 0.4), was discovered. K2CaPt3δ-O6 has a structure consisting of monolayers of edge-sharing PtO6 octahedra separated by layers of ordered K+ and Ca2+ ions in a 2:1 ratio. The structure of K2CaPt3-δO6 was found to be flexible to doping with copper, causing the magnetic properties to change from temperature independent to paramagnetic.
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Khatami, Ehsan. "Criticality and Superconductivity in the Two-dimensional Hubbard Model of Strongly Correlated Electronic Systems." University of Cincinnati / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1250711006.
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Krempasky, Juraj. "Angle- and spin-resolved photoemission on La²/3Sr1/3MnO3." Cergy-Pontoise, 2008. http://biblioweb.u-cergy.fr/theses/08CERG0398.pdf.
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La2/3Sr1/3MnO3 est un des oxydes de manganèse qui suscite un vif intérêt car on s’attend à ce qu’il soit un demi-métal. La spectroscopie de photoélectrons résolue en angle est une technique puissante de caractérisation de la structure électronique des systèmes complexes, dans lesquels les degrés de liberté de charge, d’orbitale et de spin déterminent des propriétés inattendues, comme la supraconductivité non conventionnelle ou la magnétorésistance colossale. Pour développer de nouveaux types de matériaux magnétiques adaptés à la spintronique, il est important d’accéder au degré de liberté de spin. Cette thése porte sur la spectroscopie haute résolution des photoélectrons, résolue en spin et en angle, de La2/3Sr1/3MnO3. Des échantillons sous forme de couches minces (< 100 nm) ont été préparés par ablation laser. Les spectres de photoélectrons ont été simulés à partir de calculs de structure de bandes en prenant en compte les effets de durées de vie du phototrou et l’élargissement de la composante perpendiculaire du vecteur d’onde. Les simulations reproduisent l’existence d’un gap au niveau de Fermi pour les électrons minoritaires et confirment que La2/3Sr1/3MnO3 a bien un caractère demi-métallique, en dépit du fait que la polarisation de spin mesurée est inférieure à 100%La2/3Sr1/3MnO3 is one of the manganese oxides that attracted a lot of interest because it is expected to be a half metal. Angle-resolved photoemission spectroscopy is a powerful technique for characterizing the electronic structure of complex systems, where charge, orbital, and spin degrees of freedom determine surprising properties, such as non-conventional superconductivity and colossal magnetoresistance. For the development of new types of magnetic materials suitable for spintronics, it is important to access to the spin degree of freedom. This thesis deals with high-resolution spin- and angle-resolved photoemission spectroscopy on La2/3Sr1/3MnO3. Thin film (< 100 nm) samples were prepared by laser ablation. The photoemission spectra were simulated from band structure calculations considering the photohole lifetime effects and the broadening of the perpendicular component of the wavevector. The simulations reproduce a gap between minority electrons at the Fermi level and confirm the half-metallic nature of La2/3Sr1/3MnO3, despite the fact that the measured spin polarization is lower than 100%
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Locht, Inka L. M. "Theoretical methods for the electronic structure and magnetism of strongly correlated materials." Doctoral thesis, Uppsala universitet, Materialteori, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-308699.
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In this work we study the interesting physics of the rare earths, and the microscopic state after ultrafast magnetization dynamics in iron. Moreover, this work covers the development, examination and application of several methods used in solid state physics. The first and the last part are related to strongly correlated electrons. The second part is related to the field of ultrafast magnetization dynamics. In the first part we apply density functional theory plus dynamical mean field theory within the Hubbard I approximation to describe the interesting physics of the rare-earth metals. These elements are characterized by the localized nature of the 4f electrons and the itinerant character of the other valence electrons. We calculate a wide range of properties of the rare-earth metals and find a good correspondence with experimental data. We argue that this theory can be the basis of future investigations addressing rare-earth based materials in general. In the second part of this thesis we develop a model, based on statistical arguments, to predict the microscopic state after ultrafast magnetization dynamics in iron. We predict that the microscopic state after ultrafast demagnetization is qualitatively different from the state after ultrafast increase of magnetization. This prediction is supported by previously published spectra obtained in magneto-optical experiments. Our model makes it possible to compare the measured data to results that are calculated from microscopic properties. We also investigate the relation between the magnetic asymmetry and the magnetization. In the last part of this work we examine several methods of analytic continuation that are used in many-body physics to obtain physical quantities on real energies from either imaginary time or Matsubara frequency data. In particular, we improve the Padé approximant method of analytic continuation. We compare the reliability and performance of this and other methods for both one and two-particle Green's functions. We also investigate the advantages of implementing a method of analytic continuation based on stochastic sampling on a graphics processing unit (GPU).
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Peets, Darren. "Why be normal? : single crystal growth and X-ray spectroscopy reveal the startlingly unremarkable electronic structure of Tl-2201." Thesis, University of British Columbia, 2008. http://hdl.handle.net/2429/2365.
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High-quality platelet single crystals of Tl₂Ba₂CuO₆±δ (Tl-2201) have been grown using a novel time-varying encapsulation scheme, minimizing the thallium oxide loss that has plagued other attempts and reducing cation substitution. This encapsulation scheme allows the melt to be decanted from the crystals, a step previously impossible, and the remaining cation substitution is homogenized via a high-temperature anneal. Oxygen annealing schemes were developed to produce sharp superconducting transitions from 5 to 85 K without damaging the crystals. The crystals' high homogeneity and high degree of crystalline perfection are further evidenced by narrow rocking curves; the crystals are comparable to YSZ-grown YBa₂Cu₃O₆₊δ by both metrics.
Electron probe microanalysis (EPMA) ascertained the crystals' composition to be Tl₁.₉₂₀₍₂₎Ba₁.₉₆₍₂₎Cu₁.₀₈₀₍₂₎O₆₊δ; X-ray diffraction found the composition of a Tc = 75 K crystal to be Tl₁.₉₁₄₍₁₄₎Ba₂Cu₁.₀₈₆₍₁₄₎O₆.₀₇₍₅₎, in excellent agreement.
X-ray refinement of the crystal structure found the crystals orthorhombic at most dopings, and their structure to be in general agreement with previous powder data. Cation-substituted Tl-2201 can be orthorhombic, orthorhombic crystals can be prepared, and these superconduct, all new results. X-ray diffraction also found evidence of an as yet unidentified commensurate superlattice modulation.
The Tl-2201 crystals' electronic structure were studied by X-ray absorption and emission spectroscopies (XAS/XES). The Zhang-Rice singlet band gains less intensity on overdoping than expected, suggesting a breakdown of the Zhang-Rice singlet approximation, and one thallium oxide band does not disperse as expected. The spectra correspond very closely with LDA band structure calculations, and do not exhibit the upper Hubbard bands arising from strong correlations seen in other cuprates. The spectra are noteworthy for their unprecedented (in the high-Tc cuprates) simplicity.
The startling degree to which the electronic structure can be explained bodes well for future research in the cuprates. The overdoped cuprates, and Tl-2201 in particular, may offer a unique opportunity for understanding in an otherwise highly confusing family of materials.
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Peng, Chong. "Coupled-Cluster Methods for Large Molecular Systems Through Massive Parallelism and Reduced-Scaling Approaches." Diss., Virginia Tech, 2018. http://hdl.handle.net/10919/82971.
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Accurate correlated electronic structure methods involve a significant amount of computations and can be only employed to small molecular systems.
For example, the coupled-cluster singles, doubles, and perturbative triples model (CCSD(T)), which is known as the ``gold standard" of quantum chemistry for its accuracy, usually can treat molecules with 20-30 atoms.
To extend the reach of accurate correlated electronic structure methods to larger molecular systems, we work towards two directions: parallel computing and reduced-cost/scaling approaches.
Parallel computing can utilize more computational resources to handle systems that demand more substantial computational efforts.
Reduced-cost/scaling approaches, which introduce approximations to the existing electronic structure methods, can significantly reduce the amount of computation and storage requirements.
In this work, we introduce a new distributed-memory massively parallel implementation of standard and explicitly correlated (F12) coupled-cluster singles and doubles (CCSD) with canonical bigO{N^6} computational complexity ( C. Peng, J. A. Calvin, F. Pavov{s}evi'c, J. Zhang, and E. F. Valeev, textit{J. Phys. Chem. A} 2016, textbf{120}, 10231.), based on the TiledArray tensor framework.
Excellent strong scaling is demonstrated on a multi-core shared-memory computer, a commodity distributed-memory computer, and a national-scale supercomputer.
We also present a distributed-memory implementation of the density-fitting (DF) based CCSD(T) method.
(C. Peng, J. A. Calvin, and E. F. Valeev, textit{in preparation for submission} )
An improved parallel DF-CCSD is presented utilizing lazy evaluation for tensors with more than two unoccupied indices, which makes the DF-CCSD storage requirements always smaller than those of the non-iterative triples correction (T).
Excellent strong scaling is observed on both shared-memory and distributed-memory computers equipped with conventional Intel Xeon processors and the Intel Xeon Phi (Knights Landing) processors.
With the new implementation, the CCSD(T) energies can be evaluated for systems containing 200 electrons and 1000 basis functions in a few days using a small size commodity cluster, with even more massive computations possible on leadership-class computing resources.
The inclusion of F12 correction to the CCSD(T) method makes it converge to basis set limit much more rapidly.
The large-scale parallel explicitly correlated coupled-cluster program makes the accurate estimation of the coupled-cluster basis set limit for molecules with 20 or more atoms a routine.
Thus, it can be used rigorously to test the emerging reduced-scaling coupled-cluster approaches.
Moreover, we extend the pair natural orbital (PNO) approach to excited states through the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method.
(C. Peng, M. C. Clement, and E. F. Valeev, textit{submitted})
We simulate the PNO-EOM-CCSD method using an existing massively parallel canonical EOM-CCSD program.
We propose the use of state-averaged PNOs, which are generated from the average of the pair density of excited states, to span the PNO space of all the excited states.
The doubles amplitudes in the CIS(D) method are used to compute the state-averaged pair density of excited states.
The issue of incorrect states in the state-averaged pair density, caused by an energy reordering of excited states between the CIS(D) and EOM-CCSD, is resolved by simply computing more states than desired.
We find that with a truncation threshold of $10^{-7}$, the truncation error for the excitation energy is already below 0.02 eV for the systems tested, while the average number of PNOs is reduced to 50-70 per pair.
The accuracy of the PNO-EOM-CCSD method on local, Rydberg and charge transfer states is also investigated.Ph. D.
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Li, Zheng [Verfasser], and Oriol [Akademischer Betreuer] Vendrell. "Correlated Nuclear and Electronic Dynamics in Photoionized Systems studied by Quantum and Mixed Quantum-Classical Approaches / Zheng Li. Betreuer: Oriol Vendrell." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2014. http://d-nb.info/1059237792/34.
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Kroll, Thomas. "On the electronic structure of layered sodium cobalt oxides." Doctoral thesis, [S.l. : s.n.], 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1182443937257-74133.
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Kroll, Thomas. "On the electronic structure of layered sodium cobalt oxides." Doctoral thesis, Technische Universität Dresden, 2006. https://tud.qucosa.de/id/qucosa%3A24889.
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The discovery of an unexpectedly large thermopower accompanied by low resistivity and low thermal conductivity in NaxCoO2 raised significant research interest in these materials and let to a number of experimental and theoretical investigations. This interest has strongly been reinforced by the discovery of superconductivity in the hydrated compound Na0.35CoO2 •1.3H2O in 2003, and thus, NaxCoO2 experiences an again increasing attention. The similarity of the Na cobaltates to the high temperature superconductors (HTSC) - both are transition metal oxides and adopt a layered crystal structure with quasi-two-dimensional (Cu,Co)O2 layers - is an important aspect of the research activities. In contrast to the HTSC cuprates however, the CoO2 layers consist of edge sharing CoO6 octahedra which are distorted in such a way that the resulting local symmetry is trigonal. The trigonal coordination of the Co sites results in geometric frustration which favours unconventional electronic ground states. The geometrically frustrated CoO2 sublattice also exists in the nonhydrated parent compound NaxCoO2, which has been investigated in this work. The intercalation of water into the parent compound is expected to have little effect on the Fermi surface beside the increase in two dimensionality due to the effect of expansion. Upon lowering the symmetry from cubic to trigonal, the t2g states split into states with eg_and a1g symmetry. Measurements of polarisation and temperature dependent soft X-ray absorption have been performed on a wide doping range of NaxCoO2 single crystals. Beside the Co L_2,3-edges, the O K-edge and the Na K-edge have been measured. These measurements show strong polarisation dependencies especially for the excitations into the lower lying a1g energy level. In addition to that, also an unexpected polarisation dependence for higher energies has been observed, which should be absent in trigonal symmetry. These results point towards a deviation of the local trigonal symmetry of the CoO6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co3+ and Co4+ sites, which leads to a polaronic electron transport. Furthermore, a strong hybridisation between the Co and O ions has been found. In order to shed further light on the electronic structure of NaxCoO2, the electronic spectrum of a CoO6 cluster has been calculated including all interactions between 3d orbitals as well as hopping processes between Co and O and O and O ions. The ground state for two electronic occupations in the cluster (i.e. Co3+ and Co4+) that correspond nominally to all O in the O−2 oxidation state, and Co+3 or Co+4 has been obtained. Then, all excited states obtained by promotion of a Co 2p electron to a 3d electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large Co-O covalence and cubic crystal field effects, that result in low spin Co 3d configurations. The results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states. Beside these points, results of photoemission spectroscopy, magnetisation measurements as well as resonant and non-resonant X-ray diffraction using high energy X-rays are shown and discussed.Die Entdeckung unerwartet großer Thermokraft bei gleichzeitigem niedrigem Widerstand und niedriger thermischen Leitfähigkeit in NaxCoO2 führte zu einem großen wissenschaftlichem Interesse an diesen Materialien und zu einer großen Anzahl an experimentellen und theoretischen Arbeiten. Dieses Interesse steigerte sich noch einmal nach der Entdeckung von Supraleitung in der hydrierten Verbindung Na0.35CoO2 •1.3H2O im Jahre 2003. Die Ähnlichkeit der Na Kobaltate zu den Hochtemperatur-Supraleitern (HTSL) – beides sind Übergangsmetalloxide mit einer geschichteten Kristallstruktur in der quasi zwei dimensionale (Cu,Co)O2 Ebenen enthalten sind – ist ein wichtiger Aspekt moderner wissenschaftlicher Arbeiten. Im Gegensatz zu den HTSL Kupraten bestehen die CoO2 Ebenen aus CoO6 Oktaedern die über ihre Kanten verbunden sind und in der Art verzerrt sind, dass die resultierende Symmetrie trigonal ist. Die trigonale Anordnung der Co Plätze führt zu einer geometrischen Frustration und unkonventionellen elektronischen Grundzuständen. Diese geometrisch frustrierten CoO2 Untergitter existieren ebenfalls in den nicht hydrierten Mutterverbindungen NaxCoO2, welche in dieser Arbeit untersucht wurden. Interkalierung von Wasser in die Mutterverbindung hat nur einen kleinen Einfluss auf die Fermi Oberfläche und führt zu einem Anstieg des zwei dimensionalen Charakters durch den Effekt der Ausdehnung. Durch Verminderung der Symmetrie von kubisch zu trigonal splitten die vormals entarteten t2g Zustände auf in Zustände mit eg und a1g Symmetrie. Zur Bestimmung der elektronischen Struktur von NaxCoO2 wurden polarisations- und temperaturabhängige Messungen der Röntgenabsorption im weichen Röntgenbereich für einen großen Dotierungsbereich durchgeführt. Neben den Co L_2,3-Kanten wurden auch die O K-Kante und die Na K-Kante gemessen. Sie zeigen eine starke Polarisationsabhängigkeit speziell für Anregungen in die niederenergetischen a1g Level. Zusätzlich wurde eine unerwartete Polarisationsabhängigkeit bei höheren Energien gefunden, die für trigonalen Symmetrie so nicht auftauchen dürfte. Diese Ergebnisse weisen auf eine Abweichung von der lokalen trigonalen Symmetrie der CoO6 Oktaeder hin, welche Temperatur unabhängig ist in einem Temperaturbereich zwischen 25 und 372 Kelvin. Diese Abweichung ist unterschiedlich für Co3+ und Co4+ Ionen was wiederum auf einen polaronischen Transport hinweist. Zusätzlich wird deutlich, dass eine starke Co-O Hybridisierung existieren muss. Um weiteres Informationen über die elektronische Struktur von NaxCoO2 zu erhalten, wurde das elektronische Spektrum eines CoO6 Oktaeders berechnet in dem alle Wechselwirkungen zwischen 3d Orbitalen sowie Hüpfprozesse zwischen Co und O sowie O und O Ionen enthalten sind. Der Grundzustand für zwei elektronische Besetzungen in einem Cluster (d.h. Co3+ und Co4+) wurde bestimmt für O Ionen mit einer nominellen O-2 Oxidation sowie Co3+ und Co4+ Ionen. Im angeregten Zustand werden die entsprechenden Anregungen eines Co 2p Elektrons in einen unbesetzten 3d Zustand berücksichtigt und die entsprechenden Matrixelemente berechnet. Ein Fit an den experimentellen Daten ist gut und weist auf eine starke Co-O Kovalenz und auf einen starken Einfluss des kubischen Kristallfeldes hin, der zu einer Low-Spin Co 3d Konfiguration führt. Die Ergebnisse zeigen, dass ein effektives Hüpfen zwischen benachbarter Co Ionen eine große Rolle für die Symmetrie des Grundzustandes im Gitter spielt. Quantitative Unterschiede zwischen den experimentellen und theoretischen Daten kommen anscheinend von itineranten Effekten im Grund- und angeregten Zustand. Zusätzlich zu den oben kurz beschriebenen Ergebnissen werden in dieser Arbeit weitere Ergebnisse der Photoemissionsspektroskopie, der Magnetisierung sowie aus resonanter und nicht resonanter Röntgenbeugung mit harter Röntgenstrahlung gezeigt und diskutiert.
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Ceeh, Hubert [Verfasser], Christoph Pascal [Akademischer Betreuer] [Gutachter] Hugenschmidt, Jonathan J. [Gutachter] Finley, and Friedrich [Gutachter] Reinert. "The new 2D-ACAR spectrometer for spin-resolved measurements of the electronic structure in correlated systems / Hubert Ceeh ; Gutachter: Jonathan J. Finley, Friedrich Reinert, Christoph Pascal Hugenschmidt ; Betreuer: Christoph Pascal Hugenschmidt." München : Universitätsbibliothek der TU München, 2015. http://d-nb.info/1120584221/34.
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Andrade, Eric de Castro e. "Cálculos numéricos de sistemas eletrônicos desordenados correlacionados." [s.n.], 2010. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277676.
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Orientador: Eduardo MirandaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Sistemas eletrônicos fortemente correlacionados desordenados possuem dois mecanismos básicos para a localização eletrônica e a subsequente destruição do estado metálico: o de Mott (causado pela interação elétron-elétron) e o de Anderson (causado pela desordem). Nesta tese, estudamos como estes mecanismos competem dentro da fase metálica e também como afetam o comportamento crítico do sistema, empregando uma generalização para o caso desordenado do cenário de Brinkman-Rice para a transição de Mott. Investigamos os efeitos de desordem fraca e moderada sobre a transição metal-isolante de Mott a T = 0 em duas dimensões. Para desordem sucientemente baixa, a transição mantém sua característica do tipo Mott, na qual temos os pesos de quasipartícula Zi indo a zero na transição e uma forte blindagem da desordem na região crítica. Em contraste com o comportamento encontrado para d = 8 , no nosso caso as flutuações espaciais dos pesos de quasipartícula são fortemente amplificadas próximo à transição de Mott de tal forma que eles adquirem uma distribuição do tipo lei de potência P (Z) ~ Z a-1 ,com a --> 0 na transição. Tal comportamento altera completamente as características desta transição com relação ao caso limpo, e é um indício robusto da emergência de uma fase de Griffiths eletrônica precedendo a transição metal-isolante de Mott, com uma fenomenologia surpreendentemente similar àquela do "ponto fixo de desordem infinita" encontrada em magnetos quânticos. Uma consequência imediata dessas novas características introduzidas pela desordem é que estados eletrônicos próximos à superfície de Fermi tornam-se mais homogêneos na região crítica, ao passo que estados com maiores energias têm o comportamento oposto: eles apresentam uma grande inomogeneidade precisamente nas vizinhanças da transição de Mott. Sugerimos que uma desordem efetiva dependente da interação é uma característica comum a todos os sistemas de Mott desordenados. Estudamos também como os efeitos bem conhecidos das oscilações de longo alcance de Friedel são afetados por fortes correlações eletrônicas. Primeiramente, mostramos que sua amplitude e alcance são consideravelmente suprimidos em líquidos de Fermi fortemente renormalizados. Posteriormente, investigamos o papel dos espalhamentos elásticos e inelásticos na presença dessas oscilações. Em geral, nossos resultados analíticos mostram que um papel proeminente das oscilações de Friedel é relegado a sistemas fracamente interagentes. Abordamos, por m, os efeitos das interações sobre o isolante de Anderson em uma dimensão. Construímos a função de escala ß (g) e mostramos que a escala de "crossover" g *, que marca a transição entre o regime ôhmico e o localizado da condutância, é renormalizada pelas interações. Como consequência, embora não haja a emergência de estados verdadeiramente estendidos, o regime ôhmico de g estende-se agora por uma região consideravelmente maior do espaço de parâmetros.
Abstract: Disordered strongly correlated electronic systems have two basic routes towards localization underlying the destruction of the metallic state: the Mott route (driven by electron-electron interaction) and the Anderson route (driven by disorder). In this thesis, we study how these two mechanisms compete in the metallic phase, and also how they change the critical behavior of the system, within a generalization to the disordered case of the Brinkman-Rice scenario for the Mott transition. We investigate the effects of weak to moderate disorder on the Mott metal-insulator transition at T = 0 in two dimensions. For sufficiently weak disorder, the transition retains the Mott character, as signaled by the vanishing of the local quasiparticle weights Zi and strong disorder screening at criticality. In contrast to the behavior in d = 8, here the local spatial fluctuations of quasiparticle parameters are strongly enhanced in the critical regime, with a distribution function P(Z) ~ Z a-1 and a --> 0 at the transition. This behavior indicates the robust emergence of an electronic Griffiths phase preceding the MIT, in a fashion surprisingly reminiscent of the " Infinite Randomness Fixed Point" scenario for disordered quantum magnets. As an immediate consequence of these new features introduced by disorder, we have that the electronic states close to the Fermi energy become more spatially homogeneous in the critical region, whereas the higher energy states show the opposite behavior: they display enhanced spatial inhomogeneity precisely in the close vicinity to the Mott transition. We suggest that such energy-resolved disorder screening is a generic property of disordered Mott systems. We also study how well-known effects of the long-ranged Friedel oscillations are affected by strong electronic correlations. We first show that their range and amplitude are signifficantly suppressed in strongly renormalized Fermi liquids. We then investigate the interplay of elastic and inelastic scattering in the presence of these oscillations. In the singular case of two-dimensional systems, we show how the anomalous ballistic scattering rate is conned to a very restricted temperature range even for moderate correlations. In general, our analytical results indicate that a prominent role of Friedel oscillations is relegated to weakly interacting systems. Finally, we discuss the effects of correlations on the Anderson insulator in one dimension. We construct the scaling function ß(g) and we show that the crossover scaling g*, which marks the transition between the ohmic and the localized regimes of the conductance, is renormalized by the interactions. As a consequence, we show that, although truly extend states do not emerge, the ohmic regime covers now a considerably larger region in the parameter space.
Doutorado
Física da Matéria Condensada
Doutor em Ciências
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SLEZAK, CYRILL BRANKO. "METHODS FOR CORRELATED ELECTRON SYSTEMS." University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1151419509.
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Rossi, Matteo. "La diffusion inélastique résonante de rayons X sur systèmes corrélés induit par l'interaction spin-orbite : applications scientifiques et développements instrumentaux." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY089/document.
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Les oxydes d’iridium (iridates) ont attiré particulière attention au cours de la dernière décennie grâce à l’identification d’un état isolant de Mott induit par l’action conjointe du champ cristallin, de la corrélation électronique et du couplage spin-orbite. Cet état a été intensément investigué et des phases et excitations nouvelles ont été prédites théoriquement et aussi individuées expérimentalement. Sans doute, la diffusion inélastique résonante de rayons X (RIXS) est l’une des techniques les plus adoptée pour mesurer les excitations à basse énergie des iridates. En effet, la section efficace relativement large de la spectroscopie RIXS au seuil L3 de l’iridium et la bonne résolution en énergie ont encouragé l’emploi de cette technique. Cette thèse se pose un double objectif : concevoir des développements instrumentaux ayant pour but d’améliorer les possibilités offertes par la spectroscopie RIXS, et appliquer le RIXS afin d’étudier la physique à basse énergie de certains iridates.Le principal projet de développement instrumental est un nouveau spectromètre RIXS avec résolution en polarisation. L’analyse de la polarisation des rayons X diffusés permet d’obtenir des informations sur la symétrie et donc la nature des excitations. Cependant, elle est peu employée à cause de problèmes techniques qui naissent quand l’on veut préserver aussi la résolution en énergie et l’efficience du spectromètre. Même si le polarimètre RIXS projeté n’est pas encore disponible, le schéma optique a été vérifié et validé. Le polarimètre aura une résolution en énergie et une efficience équivalentes à celles des spectromètres RIXS courants. Le second développement technique comprend l’équipement permettant de réaliser des mesures RIXS à basse température et haute pression. Ces équipements ont permis d’investiguer l’évolution en pression des excitations magnétique du composé Sr3Ir2O7 en dessous de la température de Néel et jusqu’au 12 GPa. Les mesures peuvent aider l’affinage des modèles magnétiques courants pour ce système. Ces mesures démontrent que les excitations magnétiques peuvent être acquises en haute pression par la spectroscopie RIXS, démontrant ainsi la possible utilisation de cette technique dans ce nouveau domaine.Cette thèse comprend aussi des ultérieurs travails expérimentaux. Le premier considère le composé CaIrO3, dont la structure cristalline est constituée par des octaèdres partageant un sommet et une arête dans deux directions orthogonales. Du coup, les interactions magnétiques sont très différentes selon la direction cristallographique. Spécifiquement, la suppression du couplage de type Heisenberg dans la direction où les octaèdres partagent une arête produit des interactions magnétiques principalement unidimensionnelles. La caractéristique de ceux-ci est la présence d’un continuum d’excitations avec une dépendance en énergie et quantité de mouvement typique, qui a été révélé par la spectroscopie RIXS. Les excitations électroniques de CaIrO3 ont aussi des caractéristiques propres. Enfin, j’ai étudié les propriétés électroniques du composé Rb2[IrF6]. Des calculs récents proposaient que ce système possède un état d’isolant de Mott similairement aux oxydes d’iridium. Les mesures RIXS ont aidé à éclairer les propriétés électroniques de ce composé. La solidité des propriétés électroniques a été vérifiée par rapport à la substitution du métal alcalin ou de l’halogène, et à l’application de pression.Cette thèse accroit l’importance de l’utilisation de la spectroscopie RIXS dans des domaines qui étaient précédemment inexplorés. L’analyse de la polarisation des rayons X diffusés sera avantageuse dans les cas où la nature des excitations ne peut être établie sans ambiguïté. L’équipement développé pour réaliser les mesures RIXS en conditions extrêmes permet d’étudier la dynamique électronique et magnétique dans des phases de la matière inaccessibles jusqu’à aujourd’huiIridium oxides (iridates) have raised notable attention in the last decade due to the identification of a Mott insulating state realized by the joint action of crystal field, electron correlation, and spin-orbit coupling. Such state has been intensively investigated and novel quantum phases and excitations have been theoretically predicted and experimentally found. Undoubtedly, one of the most employed techniques to elucidate the low-energy physics of iridates is resonant inelastic X-ray scattering (RIXS). At the iridium L3 edge, it benefits from a particularly good energy resolution, which matches the energy scales of the relevant excitations, and from a favorable inelastic cross-section. The aim of the present thesis is twofold: conceive challenging instrumental upgrades that contribute to the advancement of the technique itself, and apply RIXS to inspect the magnetic and electronic properties of selected iridates.The main instrumental development concerns the design of a new RIXS spectrometer with polarization resolution. Polarization analysis of the scattered X-rays provides useful information about the symmetry and thus the nature of an excitation. However, it is rather unexploited because of severe technical challenges when energy resolution and efficiency must be preserved. The designed RIXS spectrometer with polarization analysis capabilities is still under construction, however the optical scheme has been validated by preliminary tests. Full polarization analysis is expected without degradation of energy resolution or efficiency with respect to current state-of-the-art RIXS spectrometers. Additional technical developments include sample-environment equipment to perform RIXS experiments in low-temperature and high-pressure conditions. The equipment has been successfully utilized to investigate the magnetic dynamics of the bilayer-perovskite Sr3Ir2O7 below its Néel temperature and up to 12 GPa. Our measurements provide additional observations that may sharpen the challenge to theoretical understanding of the magnetic dynamics of this material. Moreover, we demonstrate for the first time that RIXS experiments of the magnetic dynamics can be extended to unexplored thermodynamic conditions.Besides instrumental advances, additional experimental work has been carried out in order to study the magnetic and electronic excitations of the post-perovskite CaIrO3. Owing to its peculiar crystal structure, featuring both edge- and corner-sharing octahedra, the magnetic interactions of CaIrO3 are very different along orthogonal directions. In particular, the inhibition of the Heisenberg coupling along the edge-sharing direction induces one-dimensional magnetic behavior with characteristic fractional spinon-like excitations, which have been detected by RIXS. Electronic excitations are also found to have particular properties. Finally, I have focused on the electronic structure of Rb2[IrF6], which was theoretically predicted to realize a Mott insulating state similar to the one of iridium oxides. RIXS measurements helped to elucidate the electronic properties of this compound. The robustness of the electronic state has been tested against substitutions of the alkali metal and halogen, and application of physical pressure.The present work extends the potential of the RIXS technique to domains previously unexplored, i.e. polarization analysis of the scattered X-rays and high-pressure low-temperature experiments. I hope that the instrumental upgrades and applications of RIXS discussed in this thesis will further promote the technique as a powerful and reliable tool to characterize elementary excitations in correlated-electron systems
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Hart, Ian. "Magnetostriction in strongly correlated electron systems." Thesis, University of Bristol, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259584.
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Grosche, Freidrich Malte. "Pressure studies on correlated electron systems." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387721.
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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.
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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.
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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.
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Sica, G. "Electron-electron and electron-phonon interactions in strongly correlated systems." Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/12194.
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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.
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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.
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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.
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Fischer, André. "Advanced Cluster Methods for Correlated-Electron Systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-191382.
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In this thesis, quantum cluster methods are used to calculate electronic properties of correlated-electron systems. A special focus lies in the determination of the ground state properties of a 3/4 filled triangular lattice within the one-band Hubbard model. At this filling, the electronic density of states exhibits a so-called van Hove singularity and the Fermi surface becomes perfectly nested, causing an instability towards a variety of spin-density-wave (SDW) and superconducting states. While chiral d+id-wave superconductivity has been proposed as the ground state in the weak coupling limit, the situation towards strong interactions is unclear.
Additionally, quantum cluster methods are used here to investigate the interplay of Coulomb interactions and symmetry-breaking mechanisms within the nematic phase of iron-pnictide superconductors. The transition from a tetragonal to an orthorhombic phase is accompanied by a significant change in electronic properties, while long-range magnetic order is not established yet. The driving force of this transition may not only be phonons but also magnetic or orbital fluctuations. The signatures of these scenarios are studied with quantum cluster methods to identify the most important effects.
Here, cluster perturbation theory (CPT) and its variational extention, the variational cluster approach (VCA) are used to treat the respective systems on a level beyond mean-field theory. Short-range correlations are incorporated numerically exactly by exact diagonalization (ED). In the VCA, long-range interactions are included by variational optimization of a fictitious symmetry-breaking field based on a self-energy functional approach. Due to limitations of ED, cluster sizes are limited to a small number of degrees of freedom.
For the 3/4 filled triangular lattice, the VCA is performed for different cluster symmetries. A strong symmetry dependence and finite-size effects make a comparison of the results from different clusters difficult. The ground state in the weak-coupling limit is superconducting with chiral d+id-wave symmetry, in accordance to previous renormalization group approaches. In the regime of strong interactions SDW states are preferred over superconductivity and a collinaer SDW state with nonuniform spin moments on a quadrupled unit cell has the lowest grand potential. At strong coupling, inclusion of short-range quantum fluctuations turns out to favor this collinear state over the chiral phase predicted by mean-field theory. At intermediate interactions, no robust conclusion can be drawn from the results.
Symmetry-breaking mechanisms within the nematic phase of the iron-pnictides are studied using a three-band model for the iron planes on a 4-site cluster. CPT allows a local breaking of the symmetry within the cluster without imposing long-range magnetic order. This is a crucial step beyond mean-field approaches to the magnetically ordered state, where such a nematic phase cannot easily be investigated. Three mechanisms are included to break the fourfold lattice symmetry down to a twofold symmetry. The effects of anisotropic magnetic couplings are compared to an orbital ordering field and anisotropic hoppings. All three mechanisms lead to similar features in the spectral density. Since the anisotropy of the hopping parameters has to be very large to obtain similar results as observed in ARPES, a phonon-driven transition is unlikely.
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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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Carter, Edwin Christopher. "Anisotropic phenomena in strongly correlated electron systems." Thesis, University of Birmingham, 2005. http://etheses.bham.ac.uk//id/eprint/83/.
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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.
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Hinczewski, Michael 1979. "Renormalization-group theory of correlated electron systems." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/34392.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.Vita.
Includes bibliographical references.
The thesis applies position-space renormalization-group theory to a variety of correlated electron systems, determining finite-temperature phase diagrams and thermodynamic properties for electron densities both at and away from half-filling. We begin by assessing the effectiveness of the Suzuki-Takano quantum decimation method on a d = 1 Hubbard model in an external magnetic field, where exact results for the specific heat, magnetic and charge susceptibilities are available at various electron densities. We find that our approach converges to the exact values at high temperature, and agrees well even at moderate-to-low temperatures. We then extend the decimation through the Migdal-Kadanoff procedure to a Hubbard model in d = 3. Phase diagrams are calculated for a range of Coulomb couplings, and two new "" phases are found for hole-dopings of 10 - 18% and 30 - 35%. The electron hopping strength renormalizes to infinity at the T phase sinks, possibly indicating superconductivity, an interpretation further supported by features of the specific heat. The next part turns to the tJ model in d = 3, where the phase was originally observed. In the vicinity of this phase we see a sharp peak in the superfluid weight, and a suppressed low temperature specific heat indicating gap formation. The doping dependence of the free carrier density is similar to that found experimentally in cuprate superconductors. Since strong anisotropy is a key aspect of high-T, materials, we also consider a d = 3 tJ model with distinct in-plane and out-of-plane couplings. We examine the evolution of the phase diagram as the interplane coupling is weakened, and find that the T phase persists even in the quasi-two-dimensional regime.
(cont.) The complex lamellar structure of antiferromagnetic and disordered phases that develops between the T phase and half-filling could be a sign of incommensurate spin ordering. While the pure d = 2 tJ model does not exhibit a phase, we see pre-signatures of it in the renormalization-group flows, and the phase becomes stabilized with a finite transition temperature upon the addition of even the smallest interplane coupling. The last part of the thesis looks at renormalization-group techniques for quenched random systems. As a preliminary step to dealing with disorder in the tJ model, we start with a simpler, yet currently important, classical system, testing a conjecture relating the locations of multicritical points on dual pairs of hierarchical lattice Ising spin glasses. Finally, we incorporate nonmagnetic impurities into the d = 3 tJ model. Small oncentrations of these impurities rapidly destroy the r phase and enhance antiferromagnetism, observations that have parallels in Zn-doped cuprates.
by Michael Hinczewski.
Ph.D.
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Li, Chunhua. "Gutzwiller Approximation in Strongly Correlated Electron Systems." Thesis, Boston College, 2009. http://hdl.handle.net/2345/917.
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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
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Cumberlidge, Anne-Marie. "Hydrostatic pressure studies of correlated electron systems." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613379.
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ARYANPOUR, KARAN. "APPROXIMATION TECHNIQUES IN STRONGLY CORRELATED ELECTRON SYSTEMS." University of Cincinnati / OhioLINK, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1069788785.
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Helme, Lucy. "Neutron Scattering Studies of Correlated Electron Systems." Thesis, University of Oxford, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491531.
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This thesis presents neutron scattering studies of three correlated electron systems, each of which exhibit different competing interactions. These include charge order, magnetic order and lattice degrees of freedom. The main focus is on magnetic excitations within the systems. In all cases the experimental data have been analysed through comparison with theoretical models. Chapter 3 presents an investigation into the Jahn-Teller effect in the rare earth oxide Pr02, through inelastic neutron scattering studies of the crystal field transitions above and below a static structural distortion temperature. The data are compared with a point-charge model of the crystal field levels. \Ve conclude that the observed temperature evolution of the crystal field levels originates from the structural distortion due to the Jahn-Teller effect. Chapter 4 describes studies of magnetic excitations in the layered charge-ordered transition-metal oxide La1.5Sro.5Co04 through inelastic neutron scattering studies, and subsequent comparison with spin-wave dispersion models. It was found that the spin-wave excitations were decoupled from the charge order. Inclusion of the strong crystal anisotropy was necessary in order to successfully describe the data. Chapters 5 and 6 present studies of the magnetically ordered phase of Nax Co02 with x rv 0.75, a metallic layered transition-metal oxide. Chapter 5 describes investigations into the magnetic excitations in the compound, which were successfully modelled by linear spin-wave theory, including terms for the anisotropy. The excitations were found to be highly three dimensional despite the layered nature of the crystal structure. Chapter 6 presents a diffraction study of a spin-flop transition in an applied magnetic field, which confirmed the magnetic order. The transition field was found to be in excellent agreement with the exchange and anisotropy parameters extracted in chapter 5.
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Silva, Fernanda Deus da. "Contributions aux propriétés de transport d'un système à N Corps." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GRENY007/document.
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Nous étudions plusieurs problémes reliés aux propriétés de transport dans les systèmes corrélés. La thèse contient 3 parties distinctes, chacune d'entre elles décrivant un aspect particulier. Nous avons obtenu dans chacun des cas des résultats qui permettent une meilleure compréhension du transport. Nous étudions l'effet de la dissipation et d'une perturbation extérieure dépendant du temps sur le diagramme de phases d'un systèmes à N corps à température nulle et à température finie. En présence de perturbation dépendant du temps, la dissipation joue un rôle important dans l'évolution vers un état stable indépendant du temps. Nous utilisons le formalisme de Keldysh dans l'approximation adiabatique qui permet d'étudier le diagramme de phases du système en fonction de parameter et de la température. Dans la 2ième partie, nous étudions un concept important pour la physique des systèmes métalliques à plusieurs bandes, le concept d'hybridation, et la façon dont l'hybridation affecte la supraconductivité du métal. De façon générale, une hybridation dépendante ou non du vecteur d'onde k a tendance à détruire la supraconductivité. Nous montrons dans ce chapitre qu'une hybridation antisymétrique a l'effet inverse et renforce la supraconductivité. Nous montrons que si l'hybridation est antisymétrique, la supraconductivité a des propriétés non-triviales. Nous proposons que dans un tel système, il puisse exister des fermions de Majorana, même en l'absence de couplage spin-orbite. Le dernier chapitre de la thèse porte sur les effets du couplage spin-orbite sur le transport dans les nanostructures magnétiques. Dans les nanostructures, le couplage spin-orbite joue un rôle important en raison de la brisure de symmétrie à la surface ou aux interfaces. En particulier, nous étudions l'effet de l'interaction Dzyaloshinskii-Moriya (DM) sur le transport de spin dans un système tri-couche. Nous montrons qu'il existe une interaction DM entre les moments des couches et les électrons de conduction, et l'influence de cette interaction sur le transport est étudiée dans un modèle simplifié ou chaque couche est représentée par un pointWe study some important problems related to the transport properties of many body systems. It is divided in three parts, each one focusing in a specific topic. We obtain relevant results that improve our understanding of these systems. We investigate the effect of dissipation and time-dependent external sources, in the phase diagram of a many body system at zero and finite temperature. In the presence of time-dependent perturbations, dissipation is essential for the system to attain a steady, time independent state. In order to treat this time dependent problem, we use a Keldysh approach within an adiabatic approximation that allows us to study the phase diagram of this system as a function of the parameters of the system and temperature. We also discuss the nature of the quantum phase transitions of the system. Next, we study an important concept in the physics of metallic multi-band systems, that of hybridization, and how it affects the superconducting properties of a material. A constant or symmetric $k$-dependent hybridization in general act in detriment of superconductivity. We show here that when hybridization between orbitals in different sites assumes an anti-symmetric character having odd-parity it {it{enhances}} superconductivity. The antisymmetric hybridization in a problem study in this thesis (present in Chapter 3) allow us to propose a new system where it is possible to investigate Majorana fermions, even in absence of spin-orbit interactions. In the last part of this thesis we study the effect of spin-orbit coupling (SOC) on transport properties in magnetic nanostructures. In this system SOC plays an important role, because surfaces (or interfaces) introduce symmetry breaking which is a source of spin-orbit interaction. We study the role of Dzyaloshinshkii-Moriya (DM) interaction on spin-transport in a 3 layer system. We show that there is a DM interaction between magnetics ions in the layers and spin of conduction electrons. We study the influence of this DM interaction on transport within a simple model where each layer is represented by a point
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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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Bannister, Ross N. "Spectral properties of the Hubbard model for low dimensional finite systems." Thesis, University of Warwick, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.302691.
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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.
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Varadarajan, Vijayalakshmi. "SPECIFIC HEAT MEASUREMENTS ON STRONGLY CORRELATED ELECTRON SYSTEMS." UKnowledge, 2009. http://uknowledge.uky.edu/gradschool_diss/805.
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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).
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Weerasinghe, Gihan Lakshman. "Applications of stochastic methods to correlated electron systems." Thesis, University of Cambridge, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708110.
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