Effects of Hf and W doping on the commensurate charge density waves in 1T-TaS2

The electrical resistivities of Hf-doped and W-doped 1T-TaS₂ have been measured to investigate the influence of varying the Fermi level on the formation of commensurate charge density waves. It was found that W-doping is far more effective in breaking the C-CDW phase than Hf-doping. Such results are explained by the energy consideration of the system with the Hubbard gap which is produced by the Mott-localization.     

Charge density waves effects on the phonon modes of 1T-TaS2

The effects of lattice distortion driven charge density waves on the 1T polytype of TaS₂ are reported. The results of a raman study of this transition metal dichalcogenide show features consistent with results previously reported using resistivity and electron diffraction techniques. The possibility of the presence of a phason mode in the raman spectra is suggested and the temperature dependence of this mode is fitted to the expected functional form.     

Phonons and charge density waves in 1T-TiSe2

Inelastic neutron scattering measurements have been carried out on 1T-TiSe₂ at various temperatures in order to obtain information about the dynamics of the phase transition associated with the charge density waves. A large damping of a transverse phonon at the L-point has been observed near the transition temperature. A simple lattice dynamical model was used to study the effect of the Ti-Se bonding on the transition.     

Anomalous magnetoresistance in 1T-TaS2

Transverse magnetoresistance of 1T-TaS₂ was measured in magnetic fields up to 100 kOe in the semiconductive region corresponding to the commensurate charge density wave (CDW) state.     

Elastic anomalies in 1T-TaS2

The temperature dependence of the resonant period in 1T-TaS₂ was measured using the resonant flexural vibration technique. From the close study on the effect of thermal cycles, it is suggested that the anomalous regions observed both on cooling down (183–193 K) and on warming up (218–282 K) were associated with the nearly commensurate-commensurate charge density waves (CDW's) phase transition.     

Raman scattering from 1T-TaS2

Raman measurements on the 1T-polytype of TaS₂ are reported. In the commensurate charge density wave state, a large number of Raman-active peaks are observed below 400 cm^-1. Most of these peaks are attributed to k = 0 optic phonons resulting from superlattice formation.     

Electronic states of domain walls in commensurate charge density wave ground state and mosaic phase in 1T-TaS2

Domain walls(DWs) in the charge-density-wave(CDW) Mott insulator 1T-TaS₂ have unique localized states, which play an important role in exploring the electronic properties of the material. However, the electronic states in DWs in 1T-TaS₂have not been clearly understood, mostly due to the complex structures, phases, and interlayer stacking orders in the DW areas. Here, we explored the electronic states of DWs in the large-area CDW phase and mosaic phase of 1T-TaS₂b...     

Lattice contraction in electron irradiated 1T-TaS2

A strong volume contraction of 0.2 ± 0.1 unit cell volumes per displaced atom has been observed in electron irradiated 1T-TaS₂. It happens along the c-axis and is explained as a consequence of an increase in the interlayer coupling caused by the metallic interstitials in the Van der Waals gap. Simultaneous resistivity measurements revealed the suppression of the ordered charge density wave phases. The volume effect causing this suppression is one order of magnitude smaller than under external pressure 13,14. This suggests that the defects perturb the charge density waves mostly locally.     

Tunneling study of the charge-density-wave state in 1T-TaS2 and 1T-TaSe2

The charge density wave induced energy gaps of 1T-TaS₂ and 1T-TaSe₂ have been investigated by tunneling measurements at temperatures between 4.2 and 320 K. For 1T-TaS₂, the energy gap 2Δ is about 0.4 eV in the nearly-commensurate phase, and the gap becomes as large as 1.0 eV in the commensurate phase. On the other hand, for 1T-TaSe₂, the energy gap is about 0.5 eV and almost unchanged in the whole temperature range studied. In addition, the two compounds show cusp-like zero bias anomaly at 4.2 K, which might be related to the Coulomb effects in a disordered system     

High-pressure X-ray diffraction study of 1T-TaS2

We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS₂ up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS₂ becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the P–T phase diagram of 1T-TaS₂.     

Size dependence of current oscillations in the charge density wave compound (TaSe4)2I

The volume dependence of the current oscillations has been examined in the current carrying charge density wave (CDW) state of (TaSe₄)₂I. The spectral width of the oscillations broadens as the specimen cross section is increased, indicating a nonuniform distribution of CDW velocity in the spicemens. The observed behavior is contrasted with that found in the more isotropic compound NbSe₃.     

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.     

Preparation and properties of 1TTaSe2

The preparation and physical properties of 1TTaSe₂ are described, including: magnetic susceptibility, electrical resistivity, heat capacity, pressure dependence and volume change of a first order intrapolytypic transformation at 473°K. Electron diffraction and X-ray measurements show that a superlattice exists in this compound containing 26 formula units per unit cell. The physical properties are consistent with the observation of a superlattice.     

Charge density wave transport in (TaSe4)2I

We report non linear transport properties below the metal-insulat transition temperature T_° = 263 K in the halogened metal transition tetrachalcogenide (TaSe₄)₂I. These non linear properties are similar to those of NbSe₃ and TaS₃ and indicate that (TaSe₄)₂I is a new compound exhibiting charge density wave transport.     

The low temperature electrical properties of 1T-TaS2

Measurements of the electrical resistivity of 1T-TaS₂ to 0.03 K show that the increase in resistivity below ～ 50 K is extrinsic.Below 2 K the resistivity is described by ? = ?₀ exp $\text{T}{}_0\text{/T)}\text{1}\text{3}$. Because of this fractional power law behavior, we conclude that the increase is due to Anderson localization by random impurity and/or defect potentials. Other difficulties in understanding the properties of 1T-TaS₂ are also pointed out.     

Nonlinear conduction in two-dimensional CDW system: 1T-TaS2

Non-linear electrical conductivity is observed in 1T-TaS₂ in the lowest-temperature phase (the commensurate charge density wave state). The collective excitations are suggested to contribute to the electronic conduction as in the case of the linear chain metals TTF-TCNQ and NbSe₃.     

Local distortion of pinned charge density waves in orthorombic TaS3

We found that the characteristic field E_MS, associated with the relaxation of the metastable ohmic conductivity, is distinctly different from the threshold field E_T for the nonlinear conduction. While E_T diverges with decreasing sample length ?, E_MS is shown to be independent of ?. The results are interpreted in terms of local rearrangement of 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.     

Thermal evidences for successive CDW phase transitions in 1T-TaS2

The heat capacity of 1T-TaS₂ has been measured over the temperature range including the successive phase transitions (140 K–370 K) by an adiabatic calorimeter. There are three transitions in the measured temperature range, two first-order transitions (at about 226 K (T₁) and about 353.5 K (T₃)) and one small anomaly at about 283 K (T₂) with a broad peak. The transition enthalpies are as follows; ΔH₁=52±5 cal·mol^-1, ΔH₂=7.5±2 cal· mol^-1 and ΔH₃=122±8cal·mol^-1.