Yamakawa H, Miyamoto T, Morimoto T, Yada H, Kinoshita Y, Sotome M, Kida N, Yamamoto K, Iwano K, Matsumoto Y, Watanabe S, Shimoi Y, Suda M, Yamamoto H M, Mori H, Okamoto H
Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
Department of Applied Physics, Okayama University of Science, Okayama 700-0005, Japan.
Sci Rep. 2016 Feb 11;6:20571. doi: 10.1038/srep20571.
In electronic-type ferroelectrics, where dipole moments produced by the variations of electron configurations are aligned, the polarization is expected to be rapidly controlled by electric fields. Such a feature can be used for high-speed electric-switching and memory devices. Electronic-type ferroelectrics include charge degrees of freedom, so that they are sometimes conductive, complicating dielectric measurements. This makes difficult the exploration of electronic-type ferroelectrics and the understanding of their ferroelectric nature. Here, we show unambiguous evidence for electronic ferroelectricity in the charge-order (CO) phase of a prototypical ET-based molecular compound, α-(ET)2I3 (ET:bis(ethylenedithio)tetrathiafulvalene), using a terahertz pulse as an external electric field. Terahertz-pump second-harmonic-generation(SHG)-probe and optical-reflectivity-probe spectroscopy reveal that the ferroelectric polarization originates from intermolecular charge transfers and is inclined 27° from the horizontal CO stripe. These features are qualitatively reproduced by the density-functional-theory calculation. After sub-picosecond polarization modulation by terahertz fields, prominent oscillations appear in the reflectivity but not in the SHG-probe results, suggesting that the CO is coupled with molecular displacements, while the ferroelectricity is electronic in nature. The results presented here demonstrate that terahertz-pump optical-probe spectroscopy is a powerful tool not only for rapidly controlling polarizations, but also for clarifying the mechanisms of ferroelectricity.
在电子型铁电体中，由电子构型变化产生的偶极矩会排列整齐，预计其极化可通过电场快速控制。这一特性可用于高速电开关和存储设备。电子型铁电体包含电荷自由度，因此它们有时具有导电性，这使得介电测量变得复杂。这给电子型铁电体的探索及其铁电性质的理解带来了困难。在此，我们使用太赫兹脉冲作为外部电场，展示了典型的基于ET的分子化合物α-(ET)2I3（ET：双(乙二硫基)四硫富瓦烯）的电荷有序（CO）相中的电子铁电性的确凿证据。太赫兹泵浦二次谐波产生（SHG）探测和光反射率探测光谱表明，铁电极化源于分子间电荷转移，且与水平CO条纹呈27°倾斜。这些特征在密度泛函理论计算中得到了定性再现。在太赫兹场进行亚皮秒极化调制后，反射率中出现了显著的振荡，但在SHG探测结果中未出现，这表明CO与分子位移耦合，而铁电性本质上是电子性的。此处给出的结果表明，太赫兹泵浦光探测光谱不仅是快速控制极化的有力工具，也是阐明铁电机制的有力工具。
