Photoinduced Phase Transitions and Nonequilibrium Order Formation in Molecular Materials

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In mixed-stack charge-transfer DMTTF-QBr_nCl_<4-n> complexes, the neutral-ionic phase transition takes place at zero temperature if the composition n and pressure are tuned. From the pressure dependence of the transition temperature and the paraelectric behavior following the Barrett formula, it is regarded as a quantum phase transition. Quantum paraelectricity generally appears when quantum fluctuations destroy the ordering of local polarizations. In the above materials, local polarizations appear in the ionic phase, but the quantum paraelectricity appears in the neutral phase only. To consider its origin, we represent polarizations in the ionic phase by Sz=+1/-1, a neutral site by Sz=0, and obtain mean-field solutions in the quantum Blume-Emery-Griffiths model. The quantum tunneling term leads to charge-transfer fluctuations, which cause partial ionicity in the neutral phase. As a consequence, the dielectric permittivity follows the Barrett formula in the neutral phase near the quant … More um critical point.In order to study consequences of the differences between the ionic-to-neutral and neutral-to-ionic transitions in the one-dimensional extended Peierls-Hubbard model with alternating potentials for the TTF-CA complex, we introduce a double pulse of oscillating electric field in the time-dependent Schrodinger equation and vary the interval between the two pulses as well as their strengths. When the dimerized ionic phase is photoexcited, the interference effect is clearly observed owing to the coherence of charge density and lattice displacements. The two pulses constructively interfere with each other if the interval is a multiple of the period of the optical lattice vibration, while they destructively interfere if the interval is a half-odd integer times the period, in the processes toward the neutral phase. Meanwhile, when the neutral phase is photoexcited, the interference effect is weakly observed, where the photoinduced lattice oscillations are considerably incoherent due to random phases. Less