title: Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy.
abstract: 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.
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: H
authors: H
authors: Yamakawa
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: T
authors: T
authors: Miyamoto
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: T
authors: T
authors: Morimoto
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: H
authors: H
authors: Yada
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: Y
authors: Y
authors: Kinoshita
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: M
authors: M
authors: Sotome
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: N
authors: N
authors: Kida
authors: Department of Applied Physics, Okayama University of Science, Okayama 700-0005, Japan.
authors: K
authors: K
authors: Yamamoto
authors: Institute of Materials Structure Science, Graduate University for Advanced Studies, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan.
authors: K
authors: K
authors: Iwano
authors: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan.
authors: Y
authors: Y
authors: Matsumoto
authors: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan.
authors: S
authors: S
authors: Watanabe
authors: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan.
authors: Y
authors: Y
authors: Shimoi
authors: Division of Functional Molecular Systems, Research Centre of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, Okazaki 444-8585, Japan.
authors: M
authors: M
authors: Suda
authors: Division of Functional Molecular Systems, Research Centre of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, Okazaki 444-8585, Japan.|RIKEN, Wako 351-0198, Japan.
authors: H M
authors: HM
authors: Yamamoto
authors: The Institute for Solid State Physics, The University of Tokyo, Chiba 277-8581, Japan.
authors: H
authors: H
authors: Mori
authors: Department of Advanced Materials Science, The University of Tokyo, Chiba 277-8561, Japan.
authors: H
authors: H
authors: Okamoto