Lithium bis(amido)cuprates are an important class of bimetallic base, which can chemo- and regioselectively metalate aromatic compounds, via directed ortho cupration (DoCu). This thesis begins with an introduction to aspects of the chemistry of organolithium compounds, group 11 organometallic compounds and their lithium 'ate complexes. Examples of such synergic bases are presented and the introduction is concluded with a discussion of lithium bis(amido)cuprate bases, which along with their silver congeners, are the subject of this dissertation. In general, syntheses involve the addition of a lithium amide to a group 11 salt, resulting in the formation of a lithium bis(amido)cuprate or argentate. Structurally focussed work commences with the use of new amide ligands to develop heteroleptic bis(amido)cuprate systems. The reaction of mixtures of lithium amides with CuBr provides a series of novel Lipshutz-type and Gilman cuprates. Interesting structural features are uncovered, which are rationalised in terms of altered steric demands in the newly introduced amide ligands in these systems. CuSCN and CuOCN are investigated as inexpensive and safer alternatives to CuCN in cuprate formation. In the solid state, a series of Lipshutz-type cuprates (TMP)2Cu(SCN)Li2(L) (L = Et2O, THF, THP) are revealed, whose molecular conformations are infuenced by the identity of the Lewis base. However, in benzene solution, in situ conversion of Lipshutz-type to Gilman cuprate is found to occur. Moving to the synthetic setting, derivatisation of chloropyridines is attempted and gives functionalised halopyridines in 51-71 % yield. CuOCN is found to behave quite differently when reacted in the same way as CuSCN, whereby X-ray crystallography reveals structures in which Cu-Li substitution is apparent. The unique reactivity of CuOCN is interpreted with the aid of multinuclear NMR spectroscopy. A new route to Lipshutz-type cuprates is explored by the synthesis of (TMP)2Cu(OCN)Li2(THF) from Gilman cuprate and LiOCN. This avoids Cu-Li substitution. Meanwhile, reaction of lithium N,N-diisopropylamide with CuOCN also avoids metal disorder, to give a novel lithium cuprate-lithium amide adduct. Further advances in our understanding of group 11 'ate complexes are made by introducing silver as a spectroscopically active nucleus in the lithium argentates (TMP)2AgLi and (TMP)2Ag(CN)Li2(THF). In the solid state, these parallel the structures known for Gilman cuprate (TMP)2CuLi and Lipshutz cuprate (TMP)2Cu(CN)Li2(THF), respectively. In solution, NMR spectroscopy reveals features consistent with retention of these structures. Lastly, the formation of mixed Cu-Li aggregates from combining TMPLi and TMPCu in aromatic solvent are investigated. Surprising reactivity is uncovered, in which the aromatic solvent is metalated and incorporated into mixed-metal aggregates. This thesis concludes with a summary of the findings and suggestions for future work, including how the findings presented herein may be transformed into practical improvements to cuprate systems. In particular, the possibility that Gilman cuprate may be activated towards the metalation of aromatic substrates by the addition of sub-stoichiometric or catalytic amounts of a lithium salt additive is explored.

Le diabète de type II (T2D), maladie dont la prévalence augmente parallèlement au vieillissement, à l'urbanisation et au développement de l'obésité, constitue un problème mondial de santé publique. C'est dans ce contexte, et dans le cadre d'une collaboration avec les Laboratoires Servier, que nous nous sommes intéressés au FR225654, récemment isolé (en 2005). Ce composé est caractérisé par une action inhibitrice de la gluconéogenèse intense in vitro et une action hypoglycémiante significative in vivo après administration par voie orale. Son mécanisme d'action n'est pas connu. Sur le plan structural, il s'agit d'une trans-décaline substituée par un beta-céto-énol et une chaîne latérale comportant un acide carboxylique insaturé. Nous avons développé une voie de synthèse multi-étapes de ce composé, reposant sur une réaction de Diels–Alder pour former la structure bicyclique. La synthèse du précurseur ainsi que de la chaine latérale ont fait l'objet de ce travail de thèse. Notre stratégie de synthèse le décompose en trois fragments et repose sur l'utilisation de réactions originales, efficaces et stéréosélectives permettant : une mise en place du centre asymétrique quaternaire ; une transformation de triple liaison en double liaisons (E) par réduction et/ou addition stéréosélective ; l'introduction d'une double liaison (E) trisubstituée par transposition dyotropique ; une réduction régio- et chimiosélective d'un système de type diène ester. Ce travail a également permis l'élaboration d'une nouvelle méthodologie de déprotection d'éther de paramethoxyphényl par oxydation anodique.

This thesis is focused on studies of substituted Hg-based superconducting copper oxides ((Hg1-xMx)Ba2Can-1CunO2n+2+δ). These compounds are promising objects of investigation, not only from a fundamental point of view but also because of their high values of superconducting transition temperature (Tc) and irreversibility field (Hirr). The first part of the thesis is devoted to optimization of the synthesis procedure for Hg-based cuprates. The influence of different parameters (T, t, p(Hg), p(O2)) on the synthesis of these compounds in sealed silica tubes was studied. Optimal conditions yielded samples containing up to 95% of HgBa2Ca2Cu3O8+δ (Hg-1223). The formation of solid solutions with the formula (Hg1-xCux)Ba2Ca2Cu3O8+δ (where x <= 0.5) was also established. Another technique was developed, using LiF as a flux, for synthesis of samples containing up to 90% of the HgBa2CaCu2O6+δ (Hg-1212) phase. The second part concerns synthesis and studies of oxyfluorides using Hg-1212 and Hg-1223 as starting materials together with XeF2 as a fluorinating agent. It was found that oxyfluorides of both phases have a parabolic dependence of Tc vs. a parameter as well as enhanced Tc values (ΔT ≈ 3-4 K) in comparison with optimally doped non-fluorinated analogues. The crystal structure of Hg-1223 oxyfluoride was studied by X-ray powder and neutron diffraction methods. It is suggested that chemical modification of the crystal structure leads to a decrease in Cu-O distance without noticeable change in Cu-O-Cu angle (in the (CuO2) layers), which may be the significant factors influencing this Tc increase. Hg-1223 oxyfluoride was also studied under high pressure for first time. It was found that this compound has a record-high Tc value (≈ 166 K) at P ≈ 23 GPa. The last part describes the investigation of substituted Hg-based superconductors in the series (Hg0.9M0.1)Ba2CuO4+δ {(Hg,M)-1201}, where M = Tl, Pb, W, Mo, Nb and V. A comprehensive study of these compounds by various methods (X-ray powder diffraction, EDX, IR-, EXAFS- and XANES -spectroscopy) indicated that the change of charge carrier doping level is a crucial factor determining the irreversibility line. (Hg0.9Mo0.1)Ba2CuO4+δ showed the most improved irreversibility line position among the (Hg,M)-1201 compounds studied in this series.

The quest for obtaining higher Tc superconductivity led to the discovery of cuprates about 20 years ago. Since then, they continue to puzzle the scientific community with their bizarre properties like non-BCS superconductivity, pseudo gap, Fermi arcs, linear T resistivity etc. Since these materials show unusually high Tc, a novel mechanism is at play and strong correlations are believed to play an important role. The theme of this thesis work is to study physics of such strongly correlated systems in two dimensions at T = 0 along with development of new theoretical tools necessary for the study. The focus of the thesis is on the ground state studies of strongly correlated models like t-J and Hubbard models using variational Monte Carlo (VMC) and renormalized mean field theory (RMFT). The general method is to propose a variational wave function, motivated by the physics ideas, to be a candidate ground state of the system. Methods to efficiently evaluate the ground state energy and minimizing it with respect to the variational parameters are developed in this work. Antiferromagnetism-superconductivity competition and electron-hole asymmetry in the extended t-J model is investigated. The main result of this work is that increasing the magnitude of the next neighbor hopping (t') on hole doped side strengthen superconductivity while it stabilizes antiferromagnetism on the electron doped side. It is also shown that it is possible to characterize the T = 0 phase diagram with just one parameter called as Fermi Surface Convexity Parameter (FSCP). Next, the possibility of phase separation in the t-J model on a square lattice is investigated using local RMFT technique. It is found that for certain doping, the system phase separates into regions with antiferromagnetic and superconducting orders. Next, the role played by crystalline anisotropy of orthorhombic YBCO cuprates on their properties is examined using anisotropic tx-ty-J model and this ground state study suggests that the anisotropies seen in their properties are plausible solely due to the crystalline anisotropy. A new general method to study strongly correlated systems with singlet ground states is developed and tested in this thesis work. The last part of the thesis explores the possibility of high Tc superconductivity in graphene which is a intermediate coupling resonating valence bond (RVB) system. It is found that undoped graphene is not a superconductor, consistent with the experiments. On doping, the ground state of graphene is found to be a superconductor with “d+id” symmetry whose strength shows a dome as a function of doping which is reminiscent of RVB physics.

This thesis represents a systematic study of electronic structure of the modulation-free Pb-doped Bi2212 superconducting cuprates with respect to interlayer coupling done by using the angle-resolved photoemission spectroscopy (ARPES), which is a leading technique in the experimental investigation of the single particle excitations in solids. The results presented in this work indicate a very different origin for the observed complex spectra lineshape. Specifically, the peak-dip-hump lineshape can be easily understood in terms of the superposition of spectral features due to bilayer band splitting, namely the splitting of the CuO2 plane derived electronic structure in bonding and antibonding bands due to the interlayer coupling of CuO2 bilayer blocks within the unit cell of Bi2212. By performing experiments at synchrotron beamlines where the energy of the incoming photons can be tuned over a very broad range, the detailed matrix elements energy dependence for both bonding and antibonding bands was determined. This gave the opportunity to study the electronic properties these two bands separately. For the first time, it was proved that the superconducting gap has the same value and symmetry for both bands. Furthermore, having recognized and sorted out the bilayer splitting effects, it became possible to identify more subtle effects hidden in the details of the ARPES lineshapes. On underdoped samples an "intrinsic" peak-dip-hump structure due to the interaction between electrons and a bosonic mode was observed. Studying the doping, temperature, and momentum dependence of the photoemission spectra it was established that: the mode has a characteristic energy of 38-40 meV and causes strong renormalization of the electronic structure only in the superconducting state; the electron-mode coupling is maximal around the (?à,0) point in momentum space and is strongly doping dependent (being greatly enhanced in the underdoped regime). From the above, it was concluded that the bosonic mode must correspond to the sharp magnetic resonance mode observed in inelastic neutron scattering experiments, and that this coupling is relevant to superconductivity and the pairing mechanism in the cuprates.

Over the past twenty-five years, condensed matter physics has been developing materials with novel electronic characteristics for a wide range of future applications. Two research directions have shown particular promise: topological insulators, and high temperature copper based superconductors (cuprates). Topological insulators are a newly discovered class of materials that can be manipulated for spintronic or quantum computing devices. However there is a poor spectroscopic understanding of the current topological insulators and emerging topological insulator candidates. In cuprate superconductors, the challenge lies in raising the superconducting transition temperature to temperatures accessible in non-laboratory settings. This effort has been hampered by a poor understanding of the superconducting mechanism and its relationship with a mysterious pseudogap phase. In this thesis, I will describe experiments conducted on topological insulators and cuprate superconductors using scanning tunneling microscopy and spectroscopy, which provide nanoscale spectroscopic information in these materials. / Physics

Les chercheurs savent depuis longtemps que la supraconductivit?? est engendr??e par le couplage de paires d?????lectrons, appel??es paires de Cooper. Par contre, l???origine de l???inter-action conduisant ?? l???attraction effective entre les ??lectrons formant les paires demeure mal comprise pour certains mat??riaux d??couverts durant les trente derni??res ann??es. Dans ce m??moire, la supraconductivit?? a ??t?? induite dans des cuprates dop??s aux ??lectrons faisant partie de la famille des infinite-layers (IL), ayant la forme Sr[indice inf??rieur 1???x]Nd[indice inf??rieur x]CuO[indice inf??rieur 2]. Les IL ne peuvent ??tre d??pos??s en couches minces par la technique d???ablation laser que depuis tr??s r??cemment. L?????tude syst??matique de ces compos??s reste donc enti??rement ?? accomplir. Les IL ont ??galement une structure cristalline extr??mement simplifi??e qui facilite l???interpr??tation des ph??nom??nes physiques mesur??s en laboratoire. L?????tude des IL est un v??ritable tour de force. Plusieurs groupes de recherche dans le monde tentent de les synth??tiser en couches minces avec beaucoup de difficult??. Ces mat??riaux sont m??tastables et d???une sensibilit?? d??concertante. Obtenir des couches minces exemptes de phases parasites est un travail de longue haleine. Ce d??fi a non-seulement ??t?? relev??, mais il a ??t?? possible de r??aliser de la microfabrication sur les couches minces crues, rendant toute caract??risation des propri??t??s de transport beaucoup plus pr??cise. L???ablation laser puls??e est la technique qui a ??t?? utilis??e pour faire la croissance des couches minces ??tudi??es. La diffraction des rayons-X a permis l???identification des phases de la structure cristalline, mais surtout l?????tude des propri??t??s structurales des couches minces d??pos??es. Une analyse des param??tres en plan (a) et hors plan (c) de la cellule unit?? de ces couches minces en fonction des conditions de croissance s???est av??r??e essentielle. Il a ??t?? remarqu?? que le param??tre hors plan joue un r??le limitant dans l?????mergence de la supraconductivit??. Des mesures de r??sistivit?? ?? tr??s basses temp??ratures et sous champs magn??tiques intenses ont permis d?????tablir un lien entre l???apparition de la supraconductivit?? et la grandeur du param??tre hors plan de la structure cristalline. Une transition compl??te ne semble ??tre possible que lorsque le param??tre hors plan se trouve sous une grandeur critique, c = 3,41[A rond majuscule]. Des mesures d???effet Hall en fonction de la temp??rature sur des ??chantillons ayant diff??rentes valeurs du param??tre hors plan ont permis de d??terminer l?????volution des porteurs de charges pr??sents en fonction de c. Les ??chantillons supraconducteurs sugg??rent que la pr??sence de trous comme porteurs de charge est essentielle ?? l?????mergence de la supraconductivit??. Leur pr??sence semble diminuer rapidement ?? partir d???un param??tre hors plan plus grand que 3,41[A rond majuscule]. Il est possible d?????tablir un lien fondamental entre le comportement de la densit?? observ?? et celui d???une autre famille de supraconducteurs, les dop??s aux ??lectrons ayant une structure T???.

Zugl.: Hannover, Univ., Diss., 2009

Techn. Hochsch., Diss., 1999--Aachen.

Cette thèse se décompose en deux parties. Dans la première, nous nous intéressons au premier champ critique, Hc1, du titanate de strontium, que nous mesurons à l’aide d’un réseau de microsondes de Hall taille dans un gaz bidimensionnel. La valeur du premier champ critique nous permet alors d’évaluer la densité superfluide à six différents dopages couvrant l’ensemble du dôme supraconducteur. À bas dopage, nous trouvons que celle-ci correspond à la densité de porteurs dans l’état normal tandis qu’au-delà du dopage optimal, celle-ci chute drastiquement. En plaçant nos résultats dans le contexte de la loi de Homes, nous voyons que cette chute s’explique par l’entrée dans la limite sale. Un lissage multibande de Hc1(T), dans ce contexte semble également indiquer que la supraconductivité émerge de la bande la plus basse et est seulement induite dans les deux autres bandes. Dans la seconde partie, nous regardons l’évolution de la densité de porteurs, n, du cuprate Nd-LSCO. Nous mesurons ainsi six échantillons de dopages proches du point critique pseudogap, p*, via trois sondes de transport : effet Hall, résistivité et effet Seebeck. Nous trouvons que n chute de 1+p a p a l’entrée dans la phase pseudogap. En comparant les différentes sondes, nous montrons que cette chute est due à une reconstruction de la surface de fermi et qu’il existe sûrement des poches d’électrons et de trous juste en dessous de p*. Ceci est en accord, entre autres, avec un scénario antiferromagnétique. Finalement, nous trouvons que la mobilité est inchangée à l’entrée dans la phase pseudogap et que les mesures de transports semblent insensibles à la divergence de la masse effective vue par chaleur spécifique.