Takeo Ishidate

Raman scattering in hydrogenated amorphous silicon under high pressure

Abstract The first studies on the pressure dependences of the first-order Raman spectra in plasma-deposited a-Si:H films are reported. With increasing pressure up to 25 kbar the TO optical phonon band shows a shift in peak to higher frequencies with a sharpening of width while the TA acoustic phonon band shows a shift in peak to lower frequencies with a broadening of width. The LO optical phonon band shows a shift in peak to higher frequencies whereas the LA acoustic phonon band remains unchanged. These pressure effects are discussed with changes in force constant and structural disorder. The alloying effect of H atoms on the Raman spectra is also discussed while comparing the pressure effects.

Second Harmonic Generation in Li2B4O7

Second harmonic generation (SHG) has been studied in lithium diborate (Li 2 B 4 O 7 ) single crystal from room temperature up to the melting temperature. The magnitudes of the SHG coefficients d 33 and d 31 , at room temperature are, respectively, 3.1 and 0.4 relative to d 11 of quartz. The decrease of the SHG signal in the high temperature range suggests the possibility of the phase transition to a nonpolar phase.

Brillouin scattering of SrTiO3 under high pressure

Abstract The first study of pressure dependence of Brillouin scattering spectra in SrTiO3 up to 9 GPa is reported. The longitudinal and transverse acoustic velocities increase with increasing pressure up to 6 GPa, and show an anomaly near 6.5 GPa associated with a cubic-to-tetragonal structural phase transition. The transition pressure is in good agreement with the value expected from other data, showing the validity of the mean field approximation-in SrTiO3. The pressure dependence of the elastic constants is determined by means of the pressure-dependent density derived from the acoustic velocities in the cubic phase. The acoustic mode Gruneisen parameters of SrTiO3 are determined and discussed in comparison with those of NaCl.

Lattice Vibrational Properties of Crystalline Si-Ge Alloys

The alloying and pressure dependences of Raman spectra due to optic modes of lattice vibration in crystalline Si-Ge alloys have been measured. The effect of alloying on the mode frequencies is interpreted by introducing the heteropolar term due to the effective charge into the interatomic interaction as well as taking account of the bond lengths expected from the EXAFS measurements. The asymmetry of the observed Raman spectral-line shape is discussed on the basis of the structural disorder with deviations of the bond lengths.

Structural studies of crystalline and amorphous SiGe alloys using EXAFS and Raman scattering

Abstract Local order of crystalline and amorphous SiGe alloys has been studied by measurements of Ge K-edge EXAFS and Raman scattering. The analysis of the EXAFS data shows the structural disorder with deviations of bond lengths. The effects of alloying and pressure on the Raman spectra which relate to the TO phonons of SiSi, SiGe and GeGe pairs are dscussed in terms of the structural disorder and the charge transfer from Ge to Si.

Anomalous birefringence of cubic BaTiO3

Abstract The birefringence in the cubic phase of BaTiO 3 has been measured as a function of temperature. The results have been interpreted in terms of the anisotropy of polarization fluctuation that appears as a precursor of the phase transition.

Phase transition of SrTiO3 under high pressure

Abstract The Raman spectrum of SrTiO3 as a function of hydrostatic pressure up to 15 GPa was studied at room temperature. The structural phase transition of cubic to tetragonal phase occurred at pc = 6.4 GPa. In the high pressure phase four first-order Raman active modes, i.e. two soft Alg and Eg modes and two high frequency Eg and Blg modes were observed. The second-order Raman modes decreased in intensity with increase of pressure and disappeared at 13 GPa. The high frequency Eg and Blg modes increased in intensity with pressure. The soft Alg and Eg mode frequencies were analysed with the expression ω2 = ω0 2(p/pc - 1) with ω0 = 82.4 cm−1 for Alg and 33.4 cm−1 for Eg . The result was discussed on the basis of the phenomenological theory.

High Pressure Raman Study of KNbO3–KTaO3 and KNbO3–NaNbO3 Mixed Crystals

High pressure Raman spectra of KNbO 3 –KTaO 3 (KTN) and –NaNbO 3 (KNN) mixed crystals have been measured to study the behavior of the anomalous mode coupling as well as the phase transition and analyzed on the basis of the two-mode coupling model. The anomalous mode coupling has been observed for all samples under pressure. The mode coupling was observed without compression for K(Ta x Nb 1- x )O 3 crystals with x more than 40%. The upper B 1 (TO) mode frequency of these coupling modes decreased with increase of Ta concentration for the KTN crystals, but remained constant independent of the Na concentration for the KNN crystals. Further, Ta doping plays an equivalent role to the pressure effect with respect to the upper B 1 (TO) mode.

Second Harmonic Generation of Epitaxially-Grown GaN Crystal

The magnitudes of the SHG coefficients, d31, d33 and d15 of GaN and the relative sign of d31 and d33 have been measured, using an epitaxially-grown crystal sample. The observed values of d31/d33 and d15/d33 were -0.48 and -0.58, respectively, and |d33|=88×|d11(quartz)|. The reliability of the Maker fringe technique in SHG measurement for an epitaxially-grown sample as thin as 30 µm is discussed from the experimental point of view.

Coupled Acoustic Modes in Tetragonal BaTiO3

The dependence of the acoustic-phonon velocities in tetragonal BaTiO 3 on the propagating direction in the (010) plane has been measured by means of Brillouin scattering. The behavior of the acoustic velocities is well interpreted by the coupled mode theory due to the interaction between the acoustic mode and the soft optic mode. The values of all elastic moduli except for C 12 C have been determined and the coupling strength is discussed.