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High critical temperature superconductivity discovery in cuprates superconductors in 1986 revolutionized this sector and questioned the BCS theory. In spite of great improvement in critical temperatures, no theory fully explains the properties of these materials.The critical temperature evolution according to strontium’s content in La2-xSrxCuO4 and La1,6 xNd0,4SrxCuO4 superconductor solid solutions show a superconductive dome for 0,05 < x < 0,30. However, around x = 0,125, there is a magnetic anomaly where the critical temperature decreased. With the help of more efficient technologies, new physical analysis might help to give a better interpretation of the superconductivity in the cuprates materials.Therefore single crystals of great quality are needed. The travelling-solvent floating-zone method is particularly compatible with non-congruent fusion materials as cuprates.First, a preliminary study on polycrystalline samples was conducted and gave interesting information about structure and physical properties. X-rays powder diffraction experiments were used to analyze the crystalline structures and the cell parameters of these materials. The La2-xSrxCuO4 solid solutions crystallize in the orthorhombic Bmab system if x < 0,11, and in the tetragonal I4/mmm system if x > 0,11.With the substitution of Lanthanum ions by Neodymium ions, the orthorhombic phase is stabilized and the transition occurs only for x near 0,19. The critical temperatures of these compounds are extracted from the magnetic properties analysis, obtained by a SQUID magnetometer. These properties are more complicated for the La1,6 xNd0,4SrxCuO4 solid solutions because of the paramagnetic nature of Nd3+ ions.In this work, the crystals are synthesized in an optical furnace with small growth rates (1 mm.h-1). For the selected compositions of La1,875Sr0,125CuO4 and La1,475Nd0,4Sr0,125CuO4, the solvent rod must be optimal. Crystalline quality of the crystals is checked by polarized light microscopy and by the LAUE method. Elemental analysis by electron probe micro analysis and the results of the cell parameters and the critical temperatures, compared with those obtained from the polycrystalline materials are used for the composition determination of these crystals.; La découverte de la supraconductivité à haute température dans les cuprates supraconducteurs en 1986 a révolutionné le secteur et remis en cause la théorie conventionnelle (BCS) de la supraconductivité. Malgré des avancées importantes dans la course aux plus hautes températures critiques, à ce jour, aucune théorie ne fournit une interprétation satisfaisante des propriétés de ces matériaux.L’évolution de la température critique en fonction du taux de strontium dans les solutions solides supraconductrices La2-xSrxCuO4 et La1,6 xNd0,4SrxCuO4 forme un dôme supraconducteur pour 0,05 < x < 0,30. Cependant ce dôme montre une anomalie autour de la teneur x = 0,125, où la température critique est abaissée. Grâce aux technologies de plus en plus performantes, de nouvelles analyses physiques pourraient aider à comprendre la supraconductivité dans les cuprates.Pour cela, des monocristaux d’excellente qualité sont nécessaires. La technique de croissance cristalline par la méthode de fusion de la zone solvante est très adaptée à la synthèse de ce type de matériau à fusion non congruente.Une étude préliminaire sur des céramiques a permis de collecter de nombreuses informations. Une analyse par diffraction des rayons X permet de connaitre l’évolution des structures cristallines et des paramètres de mailles en fonction de la teneur en strontium. Les solutions solides La2-xSrxCuO4 cristallisent dans le système orthorhombique Bmab lorsque x < 0,11 et dans le système quadratique I4/mmm pour x > 0,11. En substituant une partie des ions lanthanes par des ions néodymes, la phase orthorhombique est stabilisée et la transition intervient quand x est proche de 0,19. Les températures critiques de ces composés sont déterminées par l’analyse des propriétés magnétiques réalisées à l’aide d’un magnétomètre à SQUID. Ces dernières sont plus complexes pour les solutions solides La1,6 xNd0,4SrxCuO4 en raison du caractère paramagnétiques des ions Nd3+.Les solides cristallisés présentés dans ce travail ont été élaborés dans un four à concentration de rayonnement avec des vitesses de croissance très lentes (1 mm.h-1). Pour atteindre les compositions visées La1,875Sr0,125CuO4 et La1,475Nd0,4Sr0,125CuO4, il est indispensable d’identifier une composition optimale de la pastille solvante. La qualité cristalline des solides cristallisés est observée par l’intermédiaire d’un microscope à lumière polarisée et par la diffraction des rayons X par la méthode de LAUE. La détermination des paramètres de maille, des températures critiques, des analyses élémentaires par microsonde, couplés aux résultats obtenus sur les céramiques permettent d’obtenir la teneur en cation de ces cristaux.

Ce mémoire traite des propriétés du La2CuO4 dopé en trous, le premier supraconducteur à haute température critique ayant été découvert. Les différentes phases électroniques du cristal y seront présentées, ainsi que le diagramme de phases en dopage de ce matériau. Les trois structures dans lesquelles on peut retrouver ce cristal seront décrites en détail, et leurs liens présumés avec les phases électroniques seront présentés. Il s’en suivra une étude utilisant la théorie de la fonctionnelle de la densité combinée au modèle de Hubbard (DFT+U) des différentes phases structurales, en plus des phases antiferromagnétiques et paramagnétiques. L’effet de la corrélation électronique sur la structure cristalline sera également étudié par l’intermédiaire du paramètre de Hubbard. Le but sera de vérifier si la DFT+U reproduit bien le diagramme de phases expérimentales, et sous quelles conditions. Une étude des effets de l’inclinaison des octaèdres d’oxygène sur la structure électronique sera également présentée., Presented here is a study on the hole doped La2CuO4, the first discovered high-­‐Tc superconductor of the cuprate family. The different electronic phases of this crystal are briefly reviewed. The three crystal structures present in this material are described, and the link between those phases and the electronic structure are discussed. The relationship of those structural phases with the magnetic phases is investigated with the help of calculations based on the density functional theory where an additional Hubbard term has been added (DFT+U). With the help of the Hubbard parameter, the effect of the electronic correlation’s strength on the structural parameters of the crystal is also studied. The idea here is to verify how well the DFT+U is able to reproduce the experimental phase diagram of this material. The effect of the tilting of the oxygen octahedras on the electronic structure is also addressed.

We suggest, and demonstrate, a systematic approach to the study of cuprate superconductors, namely, progressive change of ion size in order to systematically alter the interaction strength and other key parameters. R(Ba,Sr)$_2$Cu$_3$O$_y$ (R={La, ... Lu,Y}) is such a system where potentially obscuring structural changes are minimal. We thereby systematically alter both dielectric and magnetic properties. Dielectric fluctuation is characterized by ionic polarizability while magnetic fluctuation is characterized by exchange interactions measurable by Raman scattering. The range of transition temperatures is 70 to 107 K and we find that these correlate only with the dielectric properties, a behavior which persists with external pressure. The ultimate significance may remain to be proven but it highlights the role of dielectric screening in the cuprates and adds support to a previously proposed novel pairing mechanism involving exchange of quantized waves of electronic polarization., Comment: Manuscript: 5 pages, 4 figures. Supplemental material included (9 pages, 5 figures)

Using low-energy projection of the one-band t-t′-t′′ Hubbard model we derive an effective spin Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprates La2CuO4, Sr2CuO2Cl2, and Bi2Sr2YCu2O8. Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results. Among those we discuss the two-magnon Raman peak line shape, the K-edge resonant inelastic x-ray scattering 500-meV peak, and the high-energy kink in the angle-resolved photoemission spectroscopy quasiparticle dispersion, also known as the waterfall feature.

A contribution to superexchange coming from empty oxygen orbitals is pointed out. We also estimate the small anisotropy of the in-plane superexchange, as well as the superexchange between two adjacent layers in the specific YBa2Cu3O6 compound. In doped compounds, we improve the description of the transport properties of Zhang-Rice singlets over the simple t-J model. Particularly important for the bandwidth is the role of what we call t′unequalspins and t''unequalspins. When more than one doped hole is present in the system, the interaction between holes becomes rapidly non-negligible: two Zhang-Rice singlets placed on neighboring cells highly repel each other. Some aspects of oxygen spin dynamics are discussed, in connection with NMR experiments.

We have studied the three-band Peierls-Hubbard model describing the Cu-O layers in high-T(c) superconductors by using Lanczos diagonalization and assuming infinite mass for the ions. When the system is doped with one hole, and for lambda (the electron-lattice coupling strength) greater than a critical value, we found that the oxygens around one Cu contract and the hole self-traps forming a lattice and electronic small polaron. The self-trapped hole forms a local singlet analogous to the Zhang-Rice singlet in the undeformed lattice. We also studied the single-particle spectral function and the optical conductivity. We have found that the spectral weight, in general, is similar to that found in previous studies where the coupling with the lattice was absent. There is an anomalous transfer of spectral weight but, contrary to those studies, the spectral weight goes to these localized polaronic states. However, this polaronic shift does not seem enough by itself to explain pinning of the chemical potential observed in real materials. The peaks in the optical conductivity are also shifted, according to the polaronic shift, in the single-particle spectral functions. We compare our results to those obtained in inhomogeneous Hartree-Fock calculations and we discuss their relation with experiments.

We report the synthesis, x-ray diffraction, magnetic susceptibility and specific heat measurements on polycrystalline samples of undoped LiNi(2)P(3)O(10) and samples with non-magnetic impurity (Zn(2+), S = 0) and magnetic impurity (Cu(2+), S = 1/2) at the Ni site. The magnetic susceptibility data show a broad maximum at around 10 K and a small anomaly at about 7 K in the undoped sample. There is a lambda-like anomaly in the specific heat at 7 K, possibly due to the onset of antiferromagnetic ordering in the system. The magnetic entropy change at the ordering temperature is close to the value corresponding to R ln(2S + 1) expected for an S = 1 system. The temperature corresponding to the broad maximum and the ordering temperature both decrease on Zn and Cu substitutions and also in applied magnetic fields.

We show that, due to the weak ferromagnetism of La$_{2-x}$Sr$_x$CuO$_4$, an external magnetic field leads to a dimensional crossover 2D $\to$ 3D for the in-plane transport. The crossover results in an increase of the hole's localization length and hence in a dramatic negative magnetoresistance in the variable range hopping regime. This mechanism quantitatively explains puzzling experimental data on the negative magnetoresistance in the N\'eel phase of La$_{2-x}$Sr$_x$CuO$_4$., Comment: 6 pages, 3 figures; published version

Within the time-dependent Gutzwiller approximation for the Hubbard model we compute the magnetic fluctuations of vertical metallic stripes with parameters appropriate for ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_{4}$ (LBCO). For bond- and site-centered stripes the excitation spectra are similar, consisting of a low-energy incommensurate acoustic branch which merges into a ``resonance peak'' at the antiferromagnetic wave vector and several high-energy optical branches. The acoustic branch is similar to the result of theories assuming localized spins whereas the optical branches are significantly different. Results are in good agreement with a recent inelastic neutron study of LBCO.

From a spin-resolved photoemission study on the Bi(2)Sr(2)CaCu(2)O(8+delta) superconductor, we show experimentally that the first ionization state is of nearly pure singlet character. This is true both above and below the superconducting transition and in the presence of doping and band formation. This provides direct support for the existence and stability of Zhang-Rice singlets in high-temperature superconductors, justifying the ansatz of single-band models. Moreover, we establish this technique as an important probe for a wide range of cuprates and strongly correlated materials.

A contribution to superexchange coming from empty oxygen orbitals is pointed out. We also estimate the small anisotropy of the in-plane superexchange, as well as the superexchange between two adjacent layers in the specific YBa 2 Cu 3 O 6 compound. In doped compounds, we improve the description of the transport properties of Zhang-Rice singlets over the simple t-J model. Particularly important for the bandwidth is the role of what we call t ′ unequal spins and t '' unequal spins . When more than one doped hole is present in the system, the interaction between holes becomes rapidly non-negligible: two Zhang-Rice singlets placed on neighboring cells highly repel each other. Some aspects of oxygen spin dynamics are discussed, in connection with NMR experiments.

We have studied the three-band Peierls-Hubbard model describing the Cu-O layers in high-T(c) superconductors by using Lanczos diagonalization and assuming infinite mass for the ions. When the system is doped with one hole, and for lambda (the electron-lattice coupling strength) greater than a critical value, we found that the oxygens around one Cu contract and the hole self-traps forming a lattice and electronic small polaron. The self-trapped hole forms a local singlet analogous to the Zhang-Rice singlet in the undeformed lattice. We also studied the single-particle spectral function and the optical conductivity. We have found that the spectral weight, in general, is similar to that found in previous studies where the coupling with the lattice was absent. There is an anomalous transfer of spectral weight but, contrary to those studies, the spectral weight goes to these localized polaronic states. However, this polaronic shift does not seem enough by itself to explain pinning of the chemical potential observed in real materials. The peaks in the optical conductivity are also shifted, according to the polaronic shift, in the single-particle spectral functions. We compare our results to those obtained in inhomogeneous Hartree-Fock calculations and we discuss their relation with experiments.