Unified Theoretical Approach on High-Tc Superconducting and Ferroelectric Phase Transitions

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Ferroelectric and superconducting transitions are studied theoretically from a unified picture for various phase transitions, and the origin of high transition temperature is discussed (T_f, T_c) . First, in the mixed crystals of ferro-and antiferro-electrics, and a model of NMR-NQR spectra is proposed to define the phase transition points. Next, to explain the phase transition in one-dimensional (1D) hydrogen-bonded ferroelectrics, a cluster theory is developed by taking into account the role of host lattice on the proton configurations. It is shown that rather high T_f is related with the large anisotropy among proton-proton correlations and the renormalization of the host lattice into the proton system. Furthermore, the general equation for T_f is presented including the information of bond lengths for various hydrogen-bonded ferroelectrics. It is discussed that the mechanism to produce high T_f is common to X-dimensional (X=0,1,2,3) substances. Basing on the above-mentioned theoretical results, the equation for T_p is presented as the proton-induced ordering point among CDW chains in quasi-1D electron systems. The competition between CDW (T_p) and superconductivity(T_c) is discussed in halogen-bridged mixed-valence transition-metal complexes. The light proton motion on the interchain hydrogen bonds is introduced in a strongly coupled form with the 1D-electron system. It is predicted that protons modulate the electron-electron interaction, and that the electron-proton coupling induces a strong attraction to lead the superconductivity surpassing the CDW state. Finally, in alkali-doped fullerene, I propose a model of phonon-mediated superconductivity with rather high T_c by making use of the intrinsic large fluctuation. It is the interesting result that both high T_c and high T_f (T_p) are obtained under the similar condition.