1988 Fiscal Year Final Research Report Summary
Neutral-Ionic Phase Transition in Organic Crystals and Thin Films
| Project/Area Number |
62460026
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
固体物性
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| Research Institution | University of Tokyo |
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Principal Investigator |
KOSHIHARA Shin-ya Faculty of Science, University of Tokyo, 理学部, 助手 (10192056)
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| Co-Investigator(Kenkyū-buntansha) |
TOKURA Yoshinori Faculty of Science, University of Tokyo, 理学部, 助教授 (30143382)
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| Project Period (FY) |
1987 – 1988
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| Keywords | neutral-ionic(NI) phase transition / organic charge-transfer(CT) compound / negative resistance effect / charged soliton / NI domain wall / photo-induced effect / 負性抵抗特性 / 光誘起スペクトル |
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| Research Abstract |
The neutral-ionic (NI) phase transition in organic mixed-stack charge-transfer (CT) coupounds, that is the valence change from the neutral DA to ionic DA stack, has been investigated with particular emphaisis on its dynamical aspect. First, the anomalous charge-transport properties associated with the NI transition have been revealed, which are, for example, expontial increase of conductivity with hydrostatic pressure and strong nonlinear field-dependence followed by the negative resistance effect. The latter effect leads to the current switching phenomena with the temperature-dependent hysteresis loop, when an appropriate load resister is connected in series to the sample. These effects are considered to be correlated with the dynamics of low-energy excitations, such as charged solitons and NI domain walls, characteristic of the coumpound near the N-I phase boundary. Dynamical response was further pursued by measurements of frequency-dependent dielectric constant as well as photo-induced reflectance change near the NI phase transition temperature. The relaxation time of dielectric constant show the thermally activated behaviour with well-defined activation enrgy, which nearly coincides with that for charge transport. The dielectric constant, which is very large (>10^4) for the low dc field, show the strong nonlinearity at temperatures near the NI phase transition. These results indicate that the confined NI domain wall-pairs are responsible for such an anomalous dielectric respnse and the dissociation of these charged pair play an important role in nonlinear transport. The large photo-induced signal could be detected in the I phase where the DA stacks are dimerized due to the spin-lattice interaction. The trasient response and photoinduced spectral change can be interpreted in terms of the photo-injection of NI domain walls into the dimerized ionic DA stack.
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