- 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.
T. Terashige, H. Yada, Y. Matsui, T. Miyamoto, N. Kida, and H. Okamoto
Phys. Rev. B 91, 241201(R) – Published 5 June 2015
We measured the optical conductivity σ̃(ω) spectra of photodoped silicon by optical-pump terahertz-probe spectroscopy and analyzed them with a two-carrier Drude model. Taking into account the values of electron (hole)-phonon scattering rates previously reported in chemically doped silicon, we evaluated the electron-hole scattering rates γe-h. From 293 to 90K, the magnitudes and temperature dependence of γe-hwere successfully reproduced by a theoretical model including the effects of Rutherford scattering, Coulomb screening, and Pauli exclusion. This suggests that these three factors dominate electron-hole scattering processes in silicon. Below 90K, γe-hbecomes larger than that of the theoretical curve, which is attributable to a prolongation of the relaxation time of hot carriers.
