Yuichi Takasu

Low Frequency Raman Profile of Type II Clathrate Hydrate of THF and Its Application for Phase Identification

Low frequency Raman profiles of type II clathrate hydrate (CH) of various guest molecules have been studied and their common feature, the sharp peak at 60 cm -1 , was identified as the most dominan...

New Analysis of Low Frequency Raman Specrta of THF Aquesou Solution and its Coexisting Phase with Clathrate Hydrate

The coexisting phases of Clathrate Hydrate (CH) and low concentration aqueous solution of tetrahydrofuran (THF) or ice I h has been studied using low frequency Raman spectroscopy. Observed low freq...

Raman scattering investigation of β-pyrochlore osmium oxides, AOs2O6 (A=K, Rb, and Cs)

Raman scattering spectra of RbOs2O6 and CsOs2O6 have been measured from 4 K to 300 K. Among six Raman active modes, we focus on T2g mode of alkaline atoms and Eg mode of oxygen. Width of the T2g mode becomes quite large in KOs2O6. This is caused by very lower-frequency fluctuation and a candidate of the fluctuation is T1u mode, which is another vibration of alkaline atoms. Large frequency changes of the Eg mode are found in KOs2O6 and RbOs2O6 in the temperature dependence. The large changes relate to low-frequency oxygen modes, which will be Eu mode. Therefore, the frequency of the T1u mode becomes very small in KOs2O6, and the frequency of the Eu mode becomes small in KOs2O6 and RbOs2O6. Since only KOs2O6 is considered as a strong-coupling superconductor, the T1u mode will play a very important role in superconductivity.

149Sm nuclear resonant inelastic scattering of SmB6

149Sm nuclear resonant inelastic scattering was carried out in SmB6 in order to investigate the lattice dynamics due to Sm atoms. A sharp phonon excitation associated with Sm atoms is observed around 10 meV, suggesting the presence of a Sm dispersionless mode. The Sm phonon energy shows significant temperature dependence, correlated with temperature dependence of Sm valence state. The nuclear resonant inelastic scattering spectra also indicate that the electronic configuration at the Sm site is 4f5 hybridized with the conduction electrons.

The Study of Dynamical Structure of Water Molecules in Aqueous Solutions of Acetone and DMSO by Low Frequency Raman Scattering

The dynamic network structures of water molecules surrounding acetone molecule and DMSO molecule are investigated by low frequency Raman spectroscopy. Acetone molecule and DMSO molecule have very similar molecular structures with only one difference in the central atom; it is a carbon for acetone and is a sulfur for DMSO. These differences manifest themselves as the differences of dipole moment of each molecule which result in a very different behavior of their aqueous solutions. Observed Raman spectra are analyzed by the linear combination of Raman spectrum of pure water and that of pure solute. We found that besides the linear combination of two spectra the extra damped harmonic oscillation mode was required for the cancellation of the systematic deviation from observed spectra. It is also found that the extra oscillation mode shows strong solute dependence. Using this solute dependence of the extra oscillation mode, we found that acetone molecules promote the ordered structure of water molecules while ...

Raman scattering on KOs2O6

KOs2O6 with a β-pyrochlore structure undergoes a superconducting transition at TC = 9.6 K, which is highest transition temperature among a series of compounds, AOs2O6(A=K, Rb, and Cs). To investigate structures and anharmonicity of vibrations of K, Raman scattering spectra were measured from 4 K to room temperature. A result at room temperature showed that the observed spectra were completely consistent to a structure with a space group Fdm among previously reported two structures. A vibration of K, T2g mode with 70 cm-1, exhibits large anharmonicity, and the similar large anharmonicity has been also found for oxygen Eg and T2g modes around 250 cm-1. Therefore, not only K vibrations but also the oxygen vibrations are important to discuss the superconductivity and/or another transition at 7.5 K.

Raman scattering of type-I clathrate compounds: A8Ga16Ge30(A = Eu, Sr, Ba) and Sr8Ga16Si30-xGex

Dynamical motions of a guest ion in type-I clathrate compounds have been studied by Raman scattering measurements. The anomalous temperature dependence of the guest phonon energy has been found not only for A8Ga16Ge30(A=Ba, Sr, Eu), but also for A8Ga16Si30-xGex(A = Ba, Sr). It has been found that the 4th order anharmonic potential is important to the guest ion motion. For Sr8Ga16Si30-xGex, the off-center rattling arises with the increase of Ge concentration, and it is confirmed that the off-center rattling plays an important role to the suppression of the lattice thermal conductivity.

Raman scattering study of the guest ion motion in caged crystals

The guest ion motion has been investigated by Raman scattering for the following caged crystals: La₃Pd₂₀X₆ (X = Ge, Si), X₈Ga₁₆Ge₃₀ (X = Ba, Sr, Eu), and KOs₂O₆. The observed Raman peaks have been well assigned by the polarization dependence and the first principles calculation. It has been found that the energy of the guest ion anomalously decreases with decreasing temperature for X₈Ga₁₆Ge₃₀, La₃Pd₂₀Ge₆, and KOs₂O₆, where the rattling motion of the guest ion has been reported. However, such anomalous decrease has not been observed in La₃Pd₂₀Si₆ (no rattling). The origin of the energy decrease is understood by the existence of the large quartic anharmonic potential. Furthermore, the existence of carriers is important for the rattling mode

Raman scattering investigation of Pr-based filled skutterudites

Temperature, pressure, and magnetic field dependence of the Raman scattering spectra of Pr-based filled skutterudites PrFe4P12 and PrOs4Sb12 have been measured. In the insulator phase of PrFe4P12 under a pressure, an additional peak is observed. On the other hand, in the A-phase below T=6.5 K at ambient pressure, no additional peak is observed. By means of a factor group analysis, we have evaluated the crystallographic symmetry of the insulator and A-phases as Immm and R3, respectively. In the spectra of PrOs4Sb12, the crystal field excitation and Einstein oscillations of Pr ion are observed in addition to the first order Raman active phonons. The Einstein oscillation is observed in the second-order Raman scattering spectrum owing to a strong anharmonicity of Pr vibration with the large displacement.

Dynamical motion of a guest ion studied by Raman scattering and the lattice thermal conductivity in A 8 Ga 16 Si 30−x Ge x (A = Ba, Sr)

Dynamical motions of a guest ion in type-I clathrate compounds have been investigated using Raman scattering. It has been found that the 4th order anharmonic potential is important for the guest ion motion not only for A₈Ga₁₆Ge₃₀ (A = Ba, Sr, Eu), but also for A₈Ga₁₆Si_30-xGe_x (A = Ba, Sr). The substitution of Ge expands isotropically the cage at 6d-site for Ba₈Ga₁₆Si_30-xGe_x, however, anisotropically for Sr₈Ga₁₆Si_30-xGe_x. Especially, for Sr₈Ga₁₆Si_30-xGe_x, the off-center rattling develops with increasing Ge concentration and plays an important role to the suppression of the lattice thermal conductivity.