Tomoyuki Sekine

Raman scattering in layered compound 2H-WS2

Raman scattering of layer-type compound 2H-WS2 has been studied at room temperature. The first-order Raman peaks are observed at 27.4, 357 and 423 cm-1. The low-frequency peak at 27.4 cm-1 is a rigid-layer mode, from which the interlayer shear force constant is estimated. The central force model is applied to the high-frequency phonons. The interlayer shear force constant is much smaller than the intralayer force constants. We also find several peaks due to the second-order processes.

Raman scattering of NiS2

Abstract Raman scattering measurements of NiS2 is done and five optical phonon peaks are found. These peaks are assigned to Ag, Eg and 3Tg phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS2 and MnS2.

Raman study of charge-density-wave phase transition in quasi-one-dimensional conductor (TaSe4)2I

Abstract Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe4)2I above and below the charge-density-wave (CDW) transition temperature (Tc=263 K). The Raman intensities of many peaks become intenser and two of the phonon peaks shift to higher frequency with decreasing temperature. Moreover a new broad peak at about 90 cm−1 and a new peak around 166 cm−1 appear in the low-temperature phase. The polarization characteristic shows that the former is assigned to totally symmetric mode. The damping constant of the phonon at 90 cm−1 increases markedly with increasing temperature. The frequency shifts to higher frequency as the temperature increases and the coupling coefficient is approximately proportional to (Tc−T) 1 2 . This peak becomes Raman active owing to the CDW phase transition. The temperature dependence of the damping constant and the frequency shift may have a relation to the dynamical properties of the CDW phase transition.

Electrical and thermal properties in CuV2S4

Abstract The successive phase transitions of the cubic spinel structure CuV 2 S 4 have been studied by the electrical resistivity and the specific heat. In the resistivity and the specific heat two anomalies have been observed at T 1 ∼91 K and T 3 ≈55 K. The specific heat indicates that the phase transition at T 1 is in the vicinity of a tricritical point, where the resistivity has a small sharp peak. On the other hand, the specific heat shows a step at T 3 , where a hysteresis of the resistivity indicates the existence of the first-order phase transition.

Physical properties of ternary TCNQ compound Me3S+(I3−)13TCNQ−23

Abstract Single crystal X-ray diffraction, ESR and electrical conductance were studied to clarify both the magnetic and transport properties on a radical-anion salt of ternary compound, Me 3 S + ( I 3 − ) 1 3 TCNQ − 2 3 with respect to the role of the I3− sublattice. In the single crystal ESR study, the anisotropic behaviour and the temperature dependence of g factor and linewidth were measured down to around 100 K. From the anisotropy of the g-factor it was revealed that the ESR signal arises from the electrons of the TCNQ molecules and the spin susceptibility exhibited the abrupt decrease below 170 K(Tm), which is the sign of the metal-insulator transition. Ac electrical conductivity up to 3 MHz showed that the conductivity becomes strongly frequency dependent below Tm. Single crystal X-ray study clarified that relatively large value of the Tm is due to a strong coupling between the I3− chains and the TCNQ superstructure.

Raman scattering in linear-chain compounds (NbSe4)10/13I and (NbSe4)3I

Abstract The phase transitions have been studied in linear-chain compounds ( NbSe 4 ) 10 3 I and (NbSe4)3I by Raman scattering. In the charge-density-wave state of ( NbSe 4 ) 10 3 I an amplitude mode and several new peaks were observed. The frequency of the amplitude mode scarcely shifted, but the coupling coefficient disappeared at the transition temperature. The damping constant extremely increases with increasing temperature. The temperature dependence of the damping constant agrees with Kuriharas theory. In (NbSe4)3I a new soft mode was found below 100 K. This fact shows the existence of another displacive-type structural phase transition around 100 K in this compound.

Ac conductivity and dielectric constant in (NbSe4)10/3I and (TaSe4)2I

Abstract Temperature dependence of the ac conductivity of (NbSe 4 ) 10/3 I and (TaSe 4 ) 2 I has been measured in the frequency range 10 Hz-10 MHz in the charge-density-wave states. Just below the transition temperature T c = 289 K, the dielectric constant suddenly increases and shows a maximum around 265 K in (NbSe 4 ) 10/3 I. On the other hand, well below T c =263 K the dielectric constant increases and has a maximum around 190 K in (TaSe 4 ) 2 I. Strong frequency dependence is observed in their dielectric constants. The imaginary part of the conductivity of (NbSe 4 ) 10/3 I as a function of the frequency shows a peak just below T c . It shifts to higher frequency with decreasing temperature. Below about 200 K a low-frequency relaxation process proposed by Cava et al. was observed in both compounds.

Observation of a soft phonon in linear chain compound (NbSe4)3I by Raman scattering

Abstract The phase transition of the linear chain compound (NbSe 4 ) 3 I was studied by Raman scattering. At 78 K three new peaks were observed at 73 cm −1 , 205 cm −1 and 261 cm −1 . The totally symmetric Raman peak at 73 cm −1 shows anomalous temperature dependence. The frequency decreases with increasing temperature, and at high temperatures an anticrossing occurs with another peak observed at about 58 cm −1 . The Raman intensity decreases and the linewidth broadens remarkably as the temperature increases. These properties allow us to assign this peak to a soft phonon. This fact indicates clearly the existence of a structural phase transition of a displacive type below room temperature.

Crystal structure and far-infrared reflectance at 30 K in linear-chain semiconductor (NbSe4)3I

Abstract The crystal structure and far-infrared phonon spectra were studied at 30 K in linear-chain semiconductor (NbSe 4 ) 3 I. A low-temperature X-ray study clarified the existence of a phase transition around 90 K. The refinement of the X-ray data showed that the space group at 30 K is P 4 .

Observation of increase in Raman intensity of surface phonon polaritons on rough surfaces of ZnTe

Abstract An increase in Raman intensity of surface phonon polaritons has been observed in free-standing thin slabs (thickness ∼ 7 μm) of single- crystal ZnTe with rough surfaces by a conventional Raman-scattering technique at room temperature. The samples with various root-mean- square roughness heights have been prepared by making use of final polishing powders with different mean-grit sizes, and the surface roughness has been confirmed by measuring the intensities of the diffuse scattering of the laser light. The increase of the Raman intensity is proportional to the mean-square height of the surface roughness, and the shift of the dispersion relation is also proportional to it. The frequency shift is explained by a perturbation theory of the surface-roughness-induced scattering.