Ce travail a concerné la préparation des premières nanoparticules de supraconducteurs moléculaires dont la croissance en solution a pu être contrôlée par l'ajout de molécules amphiphiles neutres (amines, imines, acides carboxyliques). Notre étude s'est principalement focalisée sur les sels de Bechgaard (TMTSF)2X (TMTSF : tétraméthyltétrasélénafulvalène, X = ClO4 ou PF6) et sur le dérivé (BEDT-TTF)2I3 (BEDT-TTF : bis(éthylènedithio)tétrathiafulvalène). La taille, la morphologie et l'état de dispersion des nanoparticules ont été contrôlés par la nature et la concentration du structurant amphiphile. Ces nanoparticules ont été caractérisées par des méthodes spectroscopiques (IR, Raman, UV-visible, spectrométrie de photoélectrons), diffractométriques et microscopiques (microscopie électronique à transmission et AFM). Elles présentent une transition vers un état supraconducteur mise en évidence par des mesures de résistivité électrique ou de susceptibilité magnétique. Finalement, le pouvoir thermoélectrique des nano-objets de (BEDT-TTF)2I3 a été évalué. Des mesures préliminaires de facteur de mérite thermoélectrique (ZT) en font des candidats potentiels pour la réalisation de modules thermoélectriques organiques. / In this work, we described the preparation of the first nanoparticles of molecule-based superconductors for which the growth has been controlled by the addition of neutral amphiphilic molecules (such as amines, imines, carboxylic acids). Our study focused on Bechgaard salts (TMTSF)2X (TMTSF: tetramethyltetraselenafulvalene, X = ClO4 or PF6) and (BEDT-TTF)2I3 (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). The size, the morphology and the state of dispersion of the nanoparticles have been controlled by the nature and the concentration of the amphiphilic structuring agent. The particles have been characterized by spectroscopic methods (IR, Raman, UV-visible, photoelectron spectroscopy), X-Ray diffraction and microscopy (TEM and AFM). Small particles underwent a transition to a superconducting state, as evidenced by electrical resistivity or magnetic susceptibility measurements. Finally, the thermoelectric power of nano-objects of (BEDT-TTF)2I3 has been evaluated. Preliminary measurements of the thermoelectric figure of merit (ZT) make them potential candidates for future organic-based thermoelectric generators.

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This thesis describes the design, construction and application of a novel vacuum system for the preparation of thin films of organic charge-transfer compounds. The method of thermal evaporation was used for four materials: tetrathiafulvalene (TTF) and three of its derivatives, dimethyltetrathiafiilvalene (DiMe-TTF), trimethyltetrathiafiilvalene (TriMe-TTF) and bis(ethylenedithio)tetrathiafiilvalene (BEDT-TTF). The resulting thin layers were characterised using optical and electron microscopy, infrared/visible spectroscopy and dc conductivity measurements down to 77K.Thin films of tetrathiafulvalene, after doping with iodine, exhibited a maximum value of dc, in-plane room temperature conductivity σ of 8.0+2.4 S cm(^-1). Semiconducting behaviour was exhibited over the range 77-300 K with AE = 0.09+0.02 eV. The effect of the deposition rate on fihn morphology is reported. TTF iodide layers were also prepared by co- evaporating the two components. These films exhibited a maximum conductivity of 2.9+0.4 S cm(^-1) at room temperature. Again, semiconducting behaviour was noted over the range 77- 300 K with AE = 0.2+0.02 eV. A comparison of the optical, structural and electrical properties of the two types of films is made. DiMe-TTF and TriMe-TTF thin films were also successfully prepared. Doping with iodine resulted in in-plane, dc room temperature conductivities of 10(^-6) and 10(^-7) S cm(^-1), respectively. These values, together with data from optical spectroscopy, suggested that both salts were in the full charge-transfer state. (BEDT-TTF) iodide thin films were deposited by evaporating the organic compound and subsequent doping. Doped films possessed a dc, in-plane room temperature conductivity of 10(^-3) S cm(^-1).Annealing these layers at 60ºC resulted in an increase in conductivity with a final value of 1.6 S cm(^-1). Semiconducting behaviour over the range 77-300 K was exhibited by the annealed films (ΔE = 0.028 eV).Finally, thin film transistors, incorporating TTF and BEDT-TTF doped layers, were fabricated and their electrical characteristics measured.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22685号 / 理博第4626号 / 新制||理||1665(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)准教授 前里 光彦, 教授 吉村 一良, 教授 有賀 哲也 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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Les sels conducteurs organiques basés sur la molécule BEDT-TTF présentent une gamme de comportements dont la diversité s'exprime par application de contraintes de température et de pression. La connaissance des prorpriétés structurales de ces sels en tout point du diagramme de phase s'avère indispensable à la compréhension de leurs propriétés physiques. La diffraction X à très basse température et haute pression demeure encore une voie d'investigation pionnière. Nous avons étudié les comportements structuraux et électroniques de plusieurs sels de BEDT-TTF aux conditions ambiantes mais aussi en variation de température (de 340K à 10K) et en variation de pression (de 1bar à 16kbar). En particulier, les structures cristallines sont déterminées en différents points du diagramme de phase et reliées aux changements de propriétés physiques.

We study unconventional superconductivity in two-dimensional systems. Unbiased numerical calculations within two-dimensional Hubbard models have found no evidence for long-range superconducting order. Most of the two-dimensional theories suggest that the superconducting state can be obtained by destabilizing an antiferromagnetic or spin-liquid insulating state. An antiferromagnet is a half-filled system because each site has one electron or hole. However, in anisotropic triangular lattices, numerical calculation finds pairing enhancement at quarter-filling but no long-range superconducting order. Many organic superconductors are dimerized in nature. Such a dimer lattice is effectively half-filled because each dimer has one electron or hole. Some theories suggest that magnetic fluctuation in such a system can give superconductivity. However, at zero temperature, we performed density matrix renormalization group (DMRG) calculations in such a system, and we find no superconducting long-range order. We also find that the antiferromagnetic order is not necessary to get a superconducting state. Failure in explaining superconductivity in two-dimensional systems suggests that only repulsive interactions between electrons are not sufficient, and other interactions are required. The most likely candidate is the electron-phonon interaction. However, existing theories of superconductivity emphasize either electron-electron or electron-phonon interactions, each of which tends to cancel the effect of the other. We present direct evidence from quantum Monte Carlo calculations of cooperative, as opposed to competing, effects of electron-electron and electron-phonon interactions within the frustrated Hubbard Hamiltonian, uniquely at the band-filling of one-quarter. Bond-coupled phonons and the onsite Hubbard U cooperatively reinforce d-wave superconducting pair-pair correlations at this filling while competing with one another at all other densities. Our work further gives new insight into how intertwined charge-order and superconductivity appear in real materials.

This thesis describes investigations into the properties of -phase BEDT-TTF charge transfer salts. Charge transfer salts are mainly studied as they are very useful test beds for fundamental physics due to the tuneability of their proper- ties and ground states. The effects of temperature and pressure on such systems have been studied, as these allow access to a wide range of different states and properties. Transport properties of these systems have been studied to obtain information about the Fermi surface and effective mass, and the effect of deuter- ation and also change of pressure media will be discussed. The interaction of infrared radiation with these systems has also been investigated and simultaneous pressure and temperature measurements will be presented, something not greatly studied due to the large technical challenges. The techniques and approaches for overcoming these are also discussed. Chapter 1 provides an introduction to the organic materials themselves with particular emphasis on the actual compounds studied. Chapter 2 provides the necessary theoretical background for studying organic charge transfer salts using magnetic quantum oscillations and their infrared re- ectivity. Chapter 3 covers the experimental techniques and also discusses some of the challenges encountered and their solutions to aid others working in this area. Chapter 4 describes an investigation into the transport properties of - (ET)2Cu(SCN)2 by studying Shubnikov-de Haas oscillations using both deuter- ated and normal samples and using two different pressure media, and comparing it to work done using a third. Chapter 5 presents an investigation into the pressure dependence of selected phonon modes in -(ET)2Cu(SCN)2 using infrared radiation on a deuterated sam- ple. Chapter 6 presents what is believed to be the first pressure and temperature dependent infrared study of an organic molecular material. In this case the or- ganic molecular material is d8--(ET)2Cu[N(CN)2]Br, but the techniques should be readily transferable to other materials.