Soft-phonon Dynamics in the Isotopically Induced Ferroelectric Phase Transition of Strontium Titanate
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Soft-phonon Dynamics in the Isotopically Induced Ferroelectric Phase Transition of Strontium Titanate Toshirou Yagi, Masaru Kasahara, Yuhji Tsujimi, Hiroki Hasebe, Masashi Yamaguchi1, Ruiping Wang2 and Mitsuru Itoh3, Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, JAPAN 1 University of California Riverside, Riverside, CA 92521-0403, U.S.A. 2 Smart Structure Center, National Institute for Advanced Industrial Science and Technology, Tsukuba 305-8568, JAPAN 3 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, JAPAN ABTRACT The ferroelectric phase transition of SrTi18O3 (STO18), where the quantum paraelectricity is reduced by the exchange of oxygen atoms 16O for its isotope 18O, has been studied by both Brillouin scattering and Raman scattering. The Brillouin spectra of the c44 and c66 modes in the paraelectric-tetragonal phase show anomalous behavior in the temperature region several degrees above TC accompanied by a central peak. The characteristic shape of the central peak indicates a coupling mechanism between the acoustic mode and an overdamped soft polar mode. Though the ferroelectric soft A2u mode is Raman inactive in the paraelectric phase, it has been observed in the x(yz)-x geometry indicating a local symmetry breaking above TC. Existence of the ferroelectric micro region in the temperature range far above TC is confirmed by the broad spectral component. The soft mode dynamics is concluded in the phase transition of STO18. The doublet spectrum is observed without any of considerable change by the isotope exchange. INTRODUCTION Since the isotope-induced ferroelectricity of SrTiO3 was first discovered by Itoh et al., many researchers have been attracted to elucidate its phase transition mechanism[1-3], because SrTiO3 crystal has been a typical crystal of the quantum paraelectricity[4-6]. They successfully reduced the quantum fluctuation with the exchange of 16O atoms for its isotope 18O by the increase of the reduced mass (hereafter the exchanged crystal is denoted as STO18 and the natural one as STO16). The occurrence of the spontaneous polarization in STO18 has been confirmed by the observation of D-E hysteresis loop [2]. A large peak value of the dielectric constant also appears at the paraelectric-ferroelectrc transition point TC which depends strongly on the concentration x of 18O in the SrTiO3 crystal [3]. Increase of the reduced mass of the Ti18O6 octahedron is expected to suppress the quantum fluctuation of TiO6 ions without any change of the lattice parameters. Thus the isotopically induced ferroelectricty is considered to give a purely mechanical effect in the phase transition dynamics. In order to observe the softening behavior of phonons, the light scattering is quite adequate and established method. If the soft ferroelectric mode exists in the phase transition of STO18, we can conclude that the origin of the ferroelectricity induced isotopically in STO18 comes from a mass effect in the lattice vibration. On the other hand, if we f
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