Substitution reactions of TCNQ (7,7,8,8-tetracyanoquinodimethane) with suitable electron donor moieties extended the range of gamma-bridged adducts R(4)Q3CNQ and R(4)P3CNQ of which the short chain Z-beta-(l-butyl-4'-quinolinium)-alpha-cyano-4-styryldicyanomethanide (C[4]H[9](4)Q3CNQ) is a typical example. These zwitterionic D-pi-A materials (where D and A are electron donors and acceptors respectively) exhibit properties such as solvatochromism, molecular rectification and second harmonic generation. Further synthetic work concentrated on modifying the donor, the acceptor and the substitution position within these zwitterions to create a series of diverse materials for non-linear optic research. Modification of the picolinium and quinolinium systems where the TCNQ substitution is in the alpha-position, has created the extensive analogous series-R(2)Q3CNQ and R(2)P3CNQ, their properties being compared and contrasted to the original gamma-bridged adducts. The behaviour of the materials on the subphase and their resultant Langmuir-Blodgett (LB) film fabrication was studied. The abrupt change in molecular orientation in LB films of the gamma-bridged adducts occurring at R= C[15]H[31] and above was not seen in the more flatly orientated alpha-bridged adducts. Further characterisation of the LB films was performed using reflectance-absorption infrared spectroscopy (RAIRS) and surface plasmon resonance (SPR).A number of N-alkylpyridinium benzimidazolate betaine derivatives and the related betaine-TCNQ adducts were prepared and their LB film forming properties were studied.

学位授与大学：京都大学 ; 取得学位: 博士(工学) ; 学位授与年月日: 2007-09-25 ; 学位の種類: 新制・課程博士 ; 学位記番号: 工博第2864号 ; 請求記号: 新制/工/1421 ; 整理番号: 25549 / Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第13393号 / 工博第2864号 / 新制||工||1421(附属図書館) / 25549 / UT51-2007-Q794 / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 引原 隆士, 教授 小林 哲生, 准教授 中村 武恒 / 学位規則第4条第1項該当

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Os supercondutores de alta temperatura crítica apresentam propriedades diferentes dos supercondutores convencionais, uma destas propriedades importantes é a simetria não isotrópica do parâmetro de ordem. Neste trabalho se apresenta um modelo que mostra a presença de simetria d-wave em uma classe de supercondutores de alta Tc, assim como a condição para a existência de ondas de densidade nos estados de equilíbrio. Se analisa primeiro um sistema de três corpos, um bipolaron e dois elétrons, com o objetivo de estudar o efeito da repulsão coulombiana local e não local sobre a simetria do parâmetro de ordem para o caso exato e também o limite de acoplamento forte. Se mostra também a construção de um Hamiltoniano efetivo que desacopla a interação el etron-bipolaron com o fim de abordar o problema coletivo de infinitos corpos e vemos como o modelo proposto prediz a formação de ondas de densidade para uma região especifica dos parâmetros físicos do Hamiltoniano estudado. / High-temperature superconductors have different properties than conventional superconductors, one of these important properties is non-isotropic symmetry of the order parameter. In this work we present a model that shows the presence of symmetry d-wave in a class of superconductors of high Tc , as well as the condition for the existence of density waves in the states of equilibrium. We first analyze a three-body system, a bipolaron and two electrons, in order to study the effect of local and non-local Coulomb repulsion on the symmetry of the order parameter for the exact case and also the strong coupling limit. It is also shown the construction of an effective Hamiltonian that decouples the electron-bipolaron interaction in order to approach the collective problem of infinite bodies and we see how the proposed model predicts the formation of density waves for a specific region of the physical parameters of the studied Hamiltonian.

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The vortex fluctuations have proved to be responsible for the onset of dissipation in thin type-II superconducting Aims. There is also growing evidence that dissipation in high- temperature superconductors exhibits the same kind of two-dimensional (2D) behavior. However, a proper analysis of these materials requires a thorough understanding of the two-dimensional fluctuations. This thesis may be considered to consist of two parts. The first is concerned with two models that have often been used as models for 2D superconductors, the 2D Coulomb gas and the 2D XY model. The second part contains analyses related to high-temperature sup er conductivity. Through analysis of some renormalization equations for the Kosterlitz-Thouless (KT) transition, it is shown that the region governed by the KT critical behavior is very small and only applies at very low values for the flux-flow resistance. It is concluded that this critical behavior not is observable in superconductors, and, furthermore, that the only available method to test for 2D fluctuations at the onset of resistance, is through comparison with the 2D resistance scaling function. The critical temperature for the 2D XY model is determined by means of a finite- size scaling relation for the helicity modulus. The linearly screened potential in the XY model is written in terms of a correlation function. The analogy to the 2D Coulomb gas is found to be exact with a temperature-dependent bare interaction and a new expression for vorticity. It is also demonstrated that the Coulomb gas scaling concept may be applied to XY-type models. An analysis of resistance data for YBCO/PBCO superlattices in terms of the 2D resistance scaling function gives evidence for 2D behavior in the cases with large separation of the superconducting layers. In the superlattices with stronger interlayer coupling, the crossover to three-dimensional behavior is seen as a deviation from the scaling function as Tc is approached from above. The anisotropic three-dimensional (3D) XY model is examined as a model for high- temperature superconductors. It is shown that the density of vortices above Tc are closely equal in the anisotropic 3D model and the 2D model. This is taken as evidence that the 3D to 2D crossover found in the superlattices also is present in the anisotropic 3D XY model. / digitalisering@umu.se

The objective of this thesis is to reach theoretical understanding of the unusual relationship between charge-ordering and superconductivity in correlated-electron systems. The competition between these broken symmetries and magnetism in the cuprate high temperature superconductors has been extensively discussed, but exists also in many other correlated-electron superconductors, including quasi-two-dimensional organic charge-transfer solids. It has been suggested that the same attractive interaction is responsible for both charge-order and superconductivity. We propose that the specific interaction is the tendency in correlated-electron systems to form spin-singlet bonds, which is strongly enhanced at the commensurate carrier density p of ½ a charge carrier per site, characteristic of all superconducting charge-transfer solids. To probe superconductivity driven by electron correlations, a necessary condition is that electron-electron interactions enhance superconducting pair-pair correlations, relative to the non-interacting limit. We have performed state of the art numerical calculations on the two-dimensional Hubbard model on different triangular lattices, as well as other lattices corresponding to K-BEDT-TTF based organic charge transfer solids, for the complete range of carrier densities per site p (0 ≤ p ≤ 1). We have shown that pair-pair correlation for each cluster is enhanced by electron-electron interaction only for p ≃ 0.5, far away from the density range thought to be important for superconductivity. Although initial focus is on charge-transfer solids, the results of the research will impact the field of correlated electrons as a whole. We believe our calculations will provide fundamental and fresh insight to the theory of superconductivity in strongly correlated systems.

No description available.

No description available.

La découverte en 2008 des supraconducteurs à base de fer a ouvert un nouveau champ d'investigation de la supraconductivité à haute température critique. En particulier, la phase nématique de ces matériaux pourrait jouer un rôle prépondérant dans le mécanisme de la supraconductivité. Nous avons étudié le composé FeSe par spectroscopie Raman, à pression ambiante et sous pression hydrostatique. Celui-ci ne possède pas d'ordre magnétique statique à pression ambiante, ce qui en fait un composé de choix pour l'étude de l'ordre nématique. Nous avons observé les fluctuations nématiques de charge. Leur évolution dans la phase tétragonale prouve l'existence d'une instabilité nématique d'origine électronique, qui gouverne la transition structurale. Dans la phase orthorhombique, le comportement des phonons souligne le rôle du couplage spin-phonon dans la transition nématique. Par ailleurs, la forme de la réponse Raman supraconductrice est compatible avec l'existence de deux gaps de symétrie s, dont un est anisotrope. Sous pression hydrostatique, les fluctuations nématiques s'atténuent rapidement. Le point critique quantique électronique associé se situe à très basse pression, peu avant l'apparition de l'ordre magnétique. Les fluctuations nématiques disparaissent complètement vers 2 GPa, quand la transition structurale passe de second ordre à premier ordre. C'est également proche de cette pression que se produit une anomalie dans le comportement des phonons, qui indique une modification de la structure électronique du système. Nos mesures révèlent en outre l'existence d'un pseudogap. Sa température d'apparition chute simultanément à la disparition de la phase magnétique, quand la température critique de supraconductivité atteint son maximum. Enfin, la réponse Raman de l'état supraconducteur à 7.8 GPa montre une signature claire d'un gap plein. / The discovery in 2008 of the iron-based superconductors opened a new field of investigation of high-temperature superconductivity. In particular, the nematic phase of these materials may play a major role in the mecanism of superconductivity. We studied the FeSe compound using Raman spectroscopy, at ambient pressure and under hydrostatic pressure. This material does not display any static magnetic order at ambient pressure and is therefore an excellent choice to study the nematic order. We observed the charge nematic fluctuations. Their evolution in the tetragonal phase proves the existence of an electronic nematic instability, which drives the structural transition. In the orthorhombic phase, the behaviour of the phonons underlines the role of the spin-phonon coupling in the nematic transition. Besides, the shape of the superconducting Raman response is compatible with the existence of two s-wave gaps, one of which is anisotropic. Under hydrostatic pressure, the nematic fluctuations reduce rapidly. The associated electronic quantum critical point is situated at very low pressure, just before the appearance of magnetic order. The nematic fluctuations completely disappear around 2 GPa, when the structural transition changes from second order to first order. An anomaly of the phonons also occurs close to this pressure, which indicates a modification of the electronic structure of the system. Our measurements additionally reveal the existence of a pseudogap. Its temperature of appearance reduces significantly simultaneously to the disappearance of magnetic order, when the critical temperature of superconductivity reaches its maximum. Finally, the Raman response in the superconducting state at 7.8 GPa shows a clear signature of a full gap.