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.     

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.     

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.     

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.     

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.     

Pressure dependence on the charge density wave transitions in monoclinic TaS3

The resistivity of one-dimensional TaS₃ in the monoclinic structure has been measured under pressure. It is shown that the upper charge density wave transition at 240 K at ambient pressure increases at the rate of +9.9 K/kbar and the lower one at 160 K decreases at the rate of -0.5 K/kbar.     

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.     

a.c. Conductivity of nearly commensurate charge density waves and application to NbSe3 and TaS3

The a.c. conductivity of an overdamped one dimensional sine-Gordon system with a low density of kinks is evaluated. This corresponds to a nearly commensurate charge or spin density wave with a kink-lattice representing the deviation from commensurability. The results explain the unusually broad crossover regime of NbSe₃ as compared with the similar but more commensurate compound TaS₃. When TaS₃ becomes commensurate (T ? 130 K) we predict that its a.c. response increases and its shape slightly sharpens.     

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.     

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.     

Thermal expansion of lT-TaS2 and 2H-NbSe2

The thermal expansion of the a and c axes of lT-TaS₂ and of the a axis of 2H-NbSe₂ have been measured between 4 K and 360 K. Discontinuities in the lattice parameters of TaS₂ were observed at the known charge density wave phase transitions near 200 K and 352 K, and a new transition was found near 283 K. These results are used to estimate the entropy changes occurring at the phase transitions. At the charge density wave onset temperature in NbSe₂ we find an upper limit to any discontinuity in the a axis of 2 × 10^-7 and to any discontinuity in the expansion coefficient of 3 × 10^-7 K^-1.     

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.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

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.     

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.     

On the nonlinear field-effect of orthorhombic TaS3

The true d.c. measurement of electrical field effect on the resistivity of orthorhombic TaS₃ in the CDW state was performed for the first time with extremely short distances between the contacting leads. Very weak nonlinear effect was observed up to 1300 V cm^?1. A distinct resistivity — increasing effect was found, which cannot be understood within the framework of any currently available theories. We propose a transverse tunneling model to account for this anomaly.     

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.     

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：     

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.     

Proton spin lattice relaxation in single crystal of NaH3(SeO3)2

Proton spin lattice relaxation time in NaH₃(SeO₃)₂ was measured at temperatures 300-77 K at a frequency of 8.01 MHz. Except near the temperatures of the ferroelectric transitions at 196 and 100 K the relaxation rate is governed by paramagnetic impurities, and the formula derived by Blumberg for diffusion-limited case is applicable. A logarithmic divergence above 196 K is related to the anomaly of the dielectric constant in the same temperature range.