Kunimitsu Uchinokura

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.

Hall effect and transverse magnetoresistance in a low-dimensional conductor HfTe5

Abstract Temperature dependence of Hall coefficient and transverse magnetoresistance of a low-dimensional conductor HfTe 5 with 1.5 wt. % Zr is reported for the first time up to 12 kOe in magnetic field. Hall coefficient changes sign from positive to negative at the temperature at which the peak of the resistivity has been observed. Temperature dependence of the transverse magnetoresistance has an anomaly. Two-carrier model has been proposed to explain the data qualitatively. These results suggest the change of the electronic structure associated with the charge density wave formation or some other kind of phase transition. Simple chemical bonding consideration shows that the carrier originates from p-like bands.

X-ray diffraction study of a structural phase transition in (NbSe4)3I

Abstract The second-order structural phase transition was observed by X-ray diffraction technique in (NbSe 4 ) 3 I. The transition temperature (T c ) was 274.2 K. No evidence of the formation of the charge-density wave was found out. Temperature dependence of the intensity of (0,5,12) reflection obeys the Landau theory of the second kind of phase transition near T c . Temperature dependence of the integral breadth of (0,0,12) reflection indicates the effect of the soft-phonon mode below T c . The existence of the structural phase transition, probably not associated with charge-density-wave formation, suggests the unique nature of (NbSe 4 )Z 3 I in the new series of compounds (MSe 4 ) x I (M=Nb, Ta).

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.

Shubnikov-de Haas oscillations and Fermi surfaces in transition-metal pentatellurides ZrTe5 and HfTe5

Shubnikov-de Haas (SdH) oscillations are reported for well characterised crystals of the anisotropic conductors ZrTe5, HfTe5 and Hf0.88Zr0.12Te5. In each material, only one cross sectional frequency dominates the oscillations below the temperature of the resistivity anomaly. Shapes of the Fermi surface (FS) are deduced from the dependence of the extremal FS cross sections on the angle between the crystallographic axes of the sample and the magnetic field. The FS obtained is a long cigar-shaped ellipsoid, which indicates the quasi-two-dimensional character of the electronic properties in these pentatellurides. The temperature dependence (0.32<T<4.2 K) of the amplitudes of the SdH oscillations has been studied to determine the cyclotron effective masses for dominant FS cross sections. The high-field Hall coefficient at 1.2 K shows clearly that at least two types of electrons contribute to the low-temperature conduction. The observed FSS in ZrTe5 and HfTe5 supply 78 and 68% of the total carriers estimated from the high-field Hall coefficient. The conductivity calculated by using the values of the carrier density and the Dingle temperature is the major part of the conductivity at low temperatures in ZrTe5 and HfTe5. Low-temperature electronic band structure is discussed on the basis of the electronic band calculation of Whangbo et al. (1982).

Strong Resistivity Anomaly in a New Low-Dimensional Conductor HfTe5

Abstract Temperature dependence of the resistivity, Hall coefficient and magnetic susceptibility of HfTe5 with 1.5 wt. %, Zr is reported. Anisotropic large diamagnetism is reported and has the peak in the vicinity of 76 K. Temperature dependence of Hall coefficient under 117 kOe elearly shows the destruction of Fermi surface at about 90 K. HfTe5 can be considered to be a semimetal and observed resistivity anomaly may be accompanied by the CDW formation along c-axis. We also discuss the mixed crystal effect with ZrTe5 and present the data of quantum oscillation observed at low temperature.

Raman spectra and soft modes in ferroelectric SbSBr

Abstract The polarized Raman spectra of SbSBr were measured in both ferro- and paraelectric phases. The phonons having Ag and B1g symmetries were determined to be Davydov pairs with the interaction energy of about 60 cm−1. In ferroelectric phase below Tc = 22.8 K, A1 soft mode having a critical exponent 0.23, is observed.

Structural Phase Transition In (Nbse4)3I

Abstract We have studied the second-order structural phase transition of (NbSe4)3I on the basis of dc-electrical-resistivity and x-ray-diffraction measurements. No evidence of the charge-density-wave formation has been observed. Typical dc resistivity and Hall coefficient at 295 K are 1.H ohm-cm and -6 × 10−6 m3c−1, respectively. (NbSe4) has the aspects which differ from those observed in other related compounds, (NbSe4)10/3I, (NbSe4)2I and (TaSe4)2I.

Intrinsic optical bistability near the phase transition temperature in semiconductor ferroelectric SbSI

Abstract We have observed anomalous interference pattern in thin SbSI crystals near the phase transition temperature under the He-Ne laser light illumination. This phenomenon was attributed to the intrinsic optical bistability. We also observed large shift of the transition temperature under illumination. These come from the photo-induced carriers.