Hiromoto Uwe

The Lower Frequency Soft-Mode in the Ferroelectric Phase of K2SeO4

In the ferroelectric phase below T 3 (93 K) of K 2 SeO 4 a new soft-mode, belonging to the B 2 representation, has been observed by means of Raman scattering experiment. This mode is different from the soft A 1 mode previously reported to soften as T 2 (129.5 K) was approached from below. The frequency of this newly observed mode is abotut 13.2 cm -1 at 9.8 K, and decreases as the temperature increases. The observed result is discussed in connection with the successive phase transitions in the K 2 SeO 4 crystal.

Raman-Scattering Study of the Soft Phonon Modes in Hexagonal Barium Titanate

The Raman spectrum of hexagonal BaTiO 3 has been investigated in the temperature range from 2 K to room temperature. Below the structural-phase-transition temperature of T a (∼222 K), we find the corresponding underdamped soft-phonon modes; the mode frequencies are proportional to ( T a - T ) β with β∼1/3 in a wide temperature range (0.5< T / T a <0.95). Although, in the ferroelectric phase below T c (∼74 K), several new features are observed, no soft mode which correlates with the ferroelectric transition is found.

Electrostriction and Stress-Induced Ferroelectricity in KTaO3

The dielectric constant of KTaO 3 has been measured as a function of the applied uniaxial stress normal to the (100), (110) and (111) faces. Under the stress normal to (100) or (110), the induced ferroelectric phase transition has been observed at liquid-helium temperatures. From the analysis of the stress dependence of the dielectric constant in the paraelectric phase, the electrostrictive constants for this crystal have been determined as Q 11 =9.69±0.7, Q 12 =-2.55±0.2 and Q 44 =3.3±2.6 in 10 -13 cgs at 4.2 K. Their magnitude was found to decrease at the rate of 8×10 -4 in 10 -13 cgs/K with increase of temperature. Characteristics of the dielectric constant in the stress-induced ferroelectric phase are also discussed.

Ferroelectric microregion in KTa1−x Nb x O3 and SrTiO3

Abstract Ferroelectric microregion (FMR) in SrTiO3 has been studied by means of Raman scattering. With lowered temperature, FMR, shape of which is ellipsoidal, increases the size concurrently with the correlation length. The ferroelectric transition of solid solution system such as Sr1−x Ca x TiO3 sets in when the FMR occupy the whole crystal volume. These characteristics are common to FMR in KTa1−x Nb x O3.

Stress-induced quantum ferroelectricity in SrTiO3

Through observation of the optical second harmonic generation, the stress-induced ferroelectric transition temperature of SrTiO 3 was determined as a function of the applied uniaxial stress along the [100] direction. The characteristics of the temperature-stress phase diagram are in agreement with the theoretical prediction of the critical behavior of the quantum ferroelectricity.

Elastic Properties and the Phase Transitions in Hexagonal Barium Titanate

Elastic properties of haxagonal BaTiO 3 have been investigated through measurements of elastic compliances by the composite-bar method. At the structural-phase-transition temperature T a ∼215 K, the effective elastic compliance is found to exhibit a stepwise anomaly. Phenomenological theory of this phase transition is given by using a model that a silent E 2u mode in the high-temperature hexagonal phase might be the soft-phonon mode of which frequency vanishes at T a . At another transition temperature T c ∼54 K, measured elastic compliances are found to exhibit a divergent increase. Together with other experimental evidences, the phase transition at T c is interpreted to be of a ferroelastic nature.

Dielectric property of epitaxial films of BaTiO3 synthesized by laser ablation

Epitaxial BaTiO3 films have been synthesized on (100)SrTiO3 at the substrate temperature of 700° C and 100 mTorr of oxygen by the method of laser ablation. The surface of the film is found to be flat with a surface roughness of about 7 nm. For the film with the thickness of 2000 A, the relative dielectric constant, e r, exhibits a broad peak at about 150° C; on the other hand, the e r exhibits a sharp peak at the tetragonal to orthorhombic transition.

Uniaxial Stress Effect on the Superconductivity of Grain-Aligned Composites of La2-xSrxCuO4

The uniaxial stress effect on the superconductivity of grain-aligned samples of La 2- x Sr x CuO 4 prepared by a “magnetic-field-induced alignment” method has been investigated. The T c decreases with increasing uniaxial stress along the “ c -axis”. The result indicates that the in-plane O-O or Cu-O bond length should significantly affect its superconductivity.

Conduction Band Structure of SrTiO3

Shubnikov de Haas(SdH) oscillations have been measured for the semiconducting single domain SrTiO3 with Carrier concentration of 3.4×1018 cm-3 and 5.9×1018 cm-3. From the temperature dependence of the SdH amplitude, the cyclotron mass values are determined for the [001] and [110] magnetic field directions. We have found, from the angular dependence of the SdH oscillation, that there exist several branches of the frequencies among which the highest and lowest branches originate from the Fermi spheres at the Brillouin zone center, and the other ones from the extremal area which is interconnected through the magnetic breakdown tunneling paths. Energy band parameters based on the kp perturbation method are determined.

Preparation and Property of La1.85Sr0.15CuO4 Single Crystal

A single crystal of the high Tc superconductor, La1.85Sr0.15CuO4, was prepared using the solid phase reaction, and the magnetization was investigated. In the superconducting state, strong anisotropic diamagnetism was observed for external magnetic field parallel and perpendicular to the c-axis. This result supports the two-dimensional character of Sr-La-Cu-O system.