The thermoelectric power of monoclinic TaS3. Comparison with orthorhombic TaS3 and NbSe3.

The thermoelectric power of monoclinic TaS₃ was measured over the temperature range of 90K to 430K. It is positive at high temperatures, negative between the two Peierls transitions and positive at low temperatures. The changes in sign occur at the transitions. The results obtained at high temperatures are compared with those for NbSe₃ and orthorhombic TaS₃.     

On the nonlinear charge density wave conductivity of TaS3

The conductivity of the charge-density-wave semiconductor TaS₃ is shown to consist of temperature dependent ohmic, and field dependent but temperature independent, contributions at temperatures below the Peierls transition T_P = 215 K. The field dependent conductivity can be described by a tunneling formalism proposed by Bardeen.     

Strandlike microstructures in the charge-density wave states of orthorhombic TaS3

Strandlike domains are observed in the commensurate charge-density wave states of orthorhombic TaS₃ using dark field electron microscopy. The images of the strandlike domains are found to be time-dependent and sensitive to the radiation damage induced by the incident high energy electrons. Our observation suggests that the appearance of the strandlike structure may be common to this class of materials regardless of the commensurability of the charge-density waves.     

Current induced deformation of charge density waves in orthorhombic TaS3

The low electric field ohmic resistance R of orthorhombic TaS₃ measured at 90 and 120 K well below the Peierls transition temperature depends on the product of a temperature difference ΔT applied along the sample and the sign of a previously applied current pulse if this pulse is larger than threshold for non-ohmic conductivity. This resistance change is about ΔR/RΔT ∽ 1×10^-3 K^-1 for a pure sample and ΔR/RΔT ∽ 6×10^-3 K^-1 for a slightly electron irradiated one at 90 K. The relative resistance change is insensitive to the sample length. We deduce that the CDW current changes inhomogeneously the Peierls gap E_g. ΔE_g < O at the contact where the CDW current enters and ΔE_g > O at the exit. The effect is attributed to a CDW current induced inhomogeneous deformation of the CDW itself.     

Pinning of charge density waves by irradiation induced defects in orthorhombic TaS3

Effects of electron irradiation induced damage on dc conductivity, threshold field and complex dielectric constant at 9 GHz are investigated in a wide temperature range. Whereas the single particle gap in the low temperature phase is not influenced by irradiation, transport properties associated with collective excitation of charge density waves (CDW) are sensitive to the presence of new pinning centres. The results are discussed in the framework of the semiclassical model of Grüner, Zawadowski and Chaikin. The pinning frequency determined from the threshold field agrees well with that of calculated from the complex dielectric constant.     

Nonlinear conductivity of orthorhombic TaS3 under uniaxial strain: Kink versus charge-density-wave transport

The nonlinear conductivity in a Pierls state of orthorhombic (o-)TaS₃ is found to be depressed by small uniaxial strain and almost disappears at some temperature dependent value of a strain while being restored at higher deformations. The phenomena observed are hard to interpret in terms of sliding charge-density-wave (CDW) transport. A model attributing the nonlinear conductivity at high fields to the kink movement, accompanied by free carrier drag, is discussed. The model does account for strain dependence of the linear and nonlinear parts of conductivity, and some other properties of unstrained o-TaS₃ as well.     

Thermoelectric power of TaS3

Thermoelectric power measurements have been made on well characterized crystals of orthorhombic TaS₃ as a function of temperature down to 80 K. A negative thermopower as for electrons is observed over the entire range, and the Peierl's transition is readily observable at 215 K. A sudden increase occurs around 160 K thereafter the magnitude of thermopower rises upon further cooling to values as high as 800 μV/K. This giant thermopower suggests that below about 160 K solitons or charge density wave dislocations dominate the low field electrical conduction instead of thermally excited electrons.     

Peierls transition in TaS3

Transition-metal trichalcogenide, TaS₃, with an orthorhombic structure was found to undergo a metal-semiconductor transition at 270 K. Electron diffraction reveals diffuse scattering above room temperature and superlattice spots at a lower temperature. These phenomena are interpreted in terms of Peierls transition.     

Non-linear conductivity of monoclinic TaS3

Strongly non-ohmic conduction is observed in the semiconducting charge-density-wave (CDW) state of monoclinic TaS₃ above a very sharp threshold field. These features could be interpreted in terms of depinning or tunneling of pinned CDW under applied electric field. We also found that large noise appears with the onset of the non-linear conductivity and fades away as the current through the samples is increased, which behaves as the decreasing function of frequency.     

The thermoelectric power of TaS3

The thermoelectric power of TaS₃ filaments was measured as a function of temperature between 90–400K. Its positive sign in the whole temperature range and its linear dependence on reciprocal temperature between 120–200K are discussed in connection with the temperature dependence of the resistance.     

Pressure dependence of the metal-semiconductor transition in TaS3

TaS₃ exhibits the metal-semiconductor transition at 218 K due to the Peierls instability. The electrical resistance was measured under pressure. It was found that both the transition temperature, Tp, and the activation energy at T=0 K, △(0), decrease with pressure at the rates of $\text{dTp}\text{dP}\text{?-1.3}\text{K}\text{kbar}$ and $\text{d△(0)}\text{dP}\text{?-4}\text{K}\text{kbar}$, respectively, while the ratio, $\text{△ (0)}\text{Tp}$, is independent of pressure.     

On the properties of the 1s-forms of TaSe2 and TaS2

The 1s-forms of TaSe₂ and TaS₂ with octahedral coordination of the metal are diamagnetic; 1s-TaS₂ is a semiconductor at low temperature. The diamagnetism is explained by taking account of spin-orbit coupling which leads to a ground state with zero magnetic moment (g = 0). This spin-orbit coupling stabilizes the d¹ configuration of the metal with respect to d² + d⁰. Thus, it can be understood that 1s−TaS₂ is semiconducting, while isostructural VSe₂ is metallic. Similarly, BaTaS₂ and BaTaSe₃ are semiconductors, but BaVS₃ is metallic.     

Kinetic study of NH3/ND3 exchange in TaS2 by neutron diffraction

Tracer exchange of TaS₂·NH₃ by liquid ND₃ was investigated at 215 and 223 K by neutron diffraction. The results suggest only a molecular exchange on the time-scale of quasielastic neutron scattering measurements.     

关于TaS3的电场非线性效应

本文首次用直流方法和非常靠近的测量电极研究了正交系TaS₃的电场非线性效应。所观察到的效应比前人所得到的要小得多。同时,我们还看到一种电阻增大现象。这种现象很难用已有的理论模型来解释。我们提出电荷密度波(CDW)畴的横向隧穿模型来解释这一结果。 关键词：     

Anisotropic properties of TaS2

The anisotropic properties of 1T- and 2H-TaS2 axe investigated by the density functional theory within the framework of full-potential linearized augmented plane wave method. The band structures of 1T- and 2H-TaS2 exhibit anisotropic properties and the calculated electronic specific-heat coefficient γ of 2H-TaS2 accords well with the existing experimental value. The anisotropic frequency-dependent dielectric functions including the effect of the Drude term are analysed, where the ε^xx（ω） spectra corresponding to the electric field E perpendicular to the z axis show excellent agreement with the measured results except for the ε1^xx（ω） of 1T-TaS2 below the energy level of 2.6 eV which is due to the lack of the enough CDW information for reference in our calculation. Furthermore, based on the values of optical effective mass ratio P of 1T and 2H phases it is found that the anisotropy in 2H-TaS2 is stronger than that in 1T-TaS2.     

Non-ohmic conductivity of TaS3 in the low-temperature semiconducting regime

Electrical conductivity of the quasi one-dimensional conductor TaS₃ was measured in the low-temperature semiconducting regime. Below 100K, the conductivity along c-axis (l-d axis) in the low field limit is characterized by the activated process with the activation energy of 250K, and the current-field (I-E) characteristic is nonlinear, $\text{dI}\text{dE}$ increasing with the field strength. These results can be explained by the excitation of phase solitons associated with pinned CDW condensates.     

Linear and nonlinear optical susceptibilities of orthorhombic rare earth molybdates RE2(MoO4)3

The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu.     

High frequency conductivity in the charge density wave semiconductor TaS3

We report the observation of frequency dependent conductivity σ (ω) in the charge density wave (CDW) semiconductor TaS₃. Based on σ (ω) and other evidences, we sugest that three different temperature regions occur in this compound: 1-D fluctuating CDW region above T_MI, a coherent CDW state below T_MI, and a CDW glass state at low temperatures.     

The ferroelectric mode in orthorhombic KNbO3

Both Raman scattering and infrared reflectivity measurements have been performed to determine the complete optic mode spectrum of orthorhombic KNbO₃ and characterize the ferroelectric modes. The low frequency, k ～ 0 mode is quasitransverse and possesses a spacial anisotropy similar to the TA branch at finite k. Of the original cubic ferroelectric mode, the component along the remaining soft direction is found at 56 cm^-1 and not overdamped (Γ/ω = 1), while the other two components have stiffened equally so that the mode is essentially isotropic in the orthogonal polarizations.     

Zone center frequencies of the orthorhombic NdMnO3 perovskite

A short range force constant model has been applied for the first time to investigate the phonons in NdMnO₃ perovskite in the orthorhombic phase. The calculations with nine stretching and eight bending force constants provide good agreement for the observed Raman frequencies. The infrared frequencies have been assigned for the first time.