Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy

• Published in: Scientific reports Vol. 6; no. 1; p. 20571
• Main Authors: 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.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 11-02-2016; Nature Publishing Group
• Subjects: 639/301/1019/385; 639/301/1019/584; 639/301/119; Electric fields; Ferroelectrics; Humanities and Social Sciences; multidisciplinary; Oscillations; Polarization; Science; Spectroscopy; Spectrum analysis
• ISSN: 2045-2322; 2045-2322
• 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)
2
I
3
(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.