Abstract-Visualization of a nonlinear conducting path in an organic molecular ferroelectric by using emission of terahertz radiation

M. Sotome, N. Kida, Y. Kinoshita, H. Yamakawa, T. Miyamoto, H. Mori, and H. Okamoto

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.241102

Nonlinear electric transport and switching to a negative resistance state are typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic molecular ferroelectric, α-type bis(ethylenedithio)tetrathiafulvalene iodide salt, and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of ∼100 μm and that region grows along the direction inclined by about 40° from the $b$ axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.

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Abstract-Visualization of a nonlinear conducting path in an organic molecular ferroelectric by using emission of terahertz radiation

M. Sotome, N. Kida, Y. Kinoshita, H. Yamakawa, T. Miyamoto, H. Mori, and H. Okamoto

https://journals.aps.org/prb/accepted/ba073K50P1a1e407c148870869f209b1cf6800e4b

A nonlinear electric transport and switching to a negative resistance state is one of typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic ferroelectric, α-(BEDT-TTF)2I3 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of ∼100 μm and that region grows along the direction inclined by about 40° from the b axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.

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

• http://www.nature.com/articles/srep20571
• H. Yamakawa
• , T. Miyamoto
• , T. Morimoto
• , H. Yada
• , Y. Kinoshita
• , M. Sotome
• , N. Kida
• , K. Yamamoto
• , K. Iwano
• , Y. Matsumoto
• , S. Watanabe
• , Y. Shimoi
• , M. Suda
• , H. M. Yamamoto
• , H. Mori
•  & H. Okamoto
• 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.