| Research Abstract |
In (DCNQI)_2Cu, conductivity and magnetism are correlated through hybridization of π and d orbitals. In the insulator phase under high pressure, three-fold lattice distortion and paramagnetism coexist. Peierls and Mott mechanisms work cooperatively. Even the level difference between these orbitals is not optimal, self-doping takes place to stabilize this lattice-distorted phase. This fact is demonstrated by the exact diagonalization studies for a two-band Peierls-Hubbard model.
In organic charge-transfer complex, TTF-CA, a transition takes place from the paraelectric neutral phase to the ferroelectric ionic phase at a low temperature or under high pressure. Photoexcitation also triggers the transition. The time-dependent Schrodinger equation is solved for an extended Peierls-Hubbard model with alternating potentials. Dynamics of charge density and lattice displacements from the ionic phase is studied to reproduce a threshold in phtoexcitation intensity. The Fourier analysis of the time-
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dependent ionicity clarifies the fast excitonic motion, the slow lattice vibration, and the even slower collective motion of neutral-ionic domain boundaries. Photoexcitation produces ionic domains with the opposite polarization to the initial one, by which the lattice order quickly decays. This is consistent with the faster evolution of the second harmonic generation than the photoreflectivity.
In halogen-bridged binuclear metal complexes, various charge- and lattice-ordered phases appear depending on the ligand, halogen ion, and counter ion. The mechanisms of the respective electronic phases are investigated in Peierls-Hubbard models. Especially with iodine, platinum, and ligand pop, halogen ion, and counter ion. The mechanisms of the iodine, platinum, and ligand pop, there appear charge-density-wave (CDW) and chargepolarization (CP) phases. Photoexcitation within the hysteresis loop triggers a transition phase does not transfer charge among binuclear units at low energies, and does not grow CDW nuclei even at high energies. Less
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