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.     

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.     

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.     

Sign and amplitude of charge density waves in 1T-TaS2 and TaSe2

The effect of charge density waves has been observed in X-ray photoelectron spectra of 1T-TaS₂ and TaSe₂ as a perturbation of the core-electron binding energies. In the commensurate CDW range the Ta 4f levels show a marked splitting; in the quasi-commensurate and incommensurate states they show broadening of magnitude comparable to this splitting.     

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.     

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.     

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...     

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.     

Transport properties of 1T-TaS2-xSex

The electrical resistivity and Hall coefficient of pure and Se-doped 1T-TaS₂ have been measured over the range 1.3<T<250 K to investigate the influence of varying degrees of disorder on the electronic conduction mechanism. Results are consistent with the hypothesis that the anomalous resistivity of this material at low temperatures arises from disorder-enhanced carrier localization.     

Observation of Shapiro steps in the charge-density-wave state of NbSe3

We report the observation of ac-induced Shapiro-like steps in the dc I-V characteristics of NbSe₃. Analysis of the steps in terms of a classical model of charge-density-wave (CDW) transport, mathematically equivalent to the Stewart-McCumber model of Josephson tunnel junctions, indicates a highly coherent sample response, and that coupling phenomena well known in the Josephson literature also occur for CDW transport.     

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.     

The CDW phase transformation in 1T-TaS2 observed by the doppler broadening of positron annihilation

The layered transition metal dichalcogenide 1T-TaS₂ was studied by the Doppler broadening of positron annihilation. The Doppler broadened line- shape was measured over the temperature range between 77 and 292 K. The CDW phase transformation at about 200 K is discussed in terms of the W-parameters calculated from the energy spectra.     

Impedance spectroscopy study and ground state electronic properties of In(Mg1/2Ti1/2)O3

The complex perovskite oxide In(Mg_1/2Ti_1/2)O₃ (IMT) is synthesized by a solid state reaction technique. The X-ray diffraction of the sample at 30 °C shows a monoclinic phase. The dielectric properties of the sample are investigated in the temperature range from 143 to 373 K and in the frequency range from 580 Hz to 1 MHz using impedance spectroscopy. An analysis of the dielectric constant ε′ and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The Cole-Cole model is used to explain the relaxation mechanism in IMT. The scaling behavior of imaginary part of electric modulus (M″) shows that the relaxation describes the same mechanism at various temperatures. The electronic structure and hence the ground state properties of IMT is studied by X-ray photoemission spectroscopy (XPS). The valence band XPS spectrum is compared with the electronic structure calculation. The electronic structure calculation indicates that the In-5s orbital introduces a significant density of states at the Fermi level, which is responsible for a high value of conductivity in IMT.     

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

Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe₄)₂I above and below the charge-density-wave (CDW) transition temperature (T_c=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 (T_c?T)^{$\text{1}\text{2}$}. 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.     

Quantum melting of the charge-density-wave state in 1T-TiSe2

We report a Raman scattering study of low-temperature, pressure-induced melting of the charge-density-wave (CDW) phase of 1T-TiSe2. Our measurements reveal that the collapse of the CDW state occurs in three stages: (i) For P<5 kbar, the pressure dependence of the CDW amplitude mode energies and intensities are indicative of a "crystalline" CDW regime; (ii) for 525 kbar, the absence of amplitude modes reveals a metallic regime in which the CDW has melted.     

Charmonium-the 1P1 state

Based on a model which gives agreement with the observed hyperfine and fine splitting of the charmonium states, we predict the mass of ¹p₁ to be 3372 MeV, substantially lower than any previous predicted values. We have also estimated branching ratios and decay rates for the production of this state via channels which become accessible because of its low mass.     

Charge and lattice dynamics of ordered state in La1/2Ca1/2MnO3: infrared reflection spectroscopy study

We report an infrared reflection spectroscopy study of La_1/2Ca_1/2MnO₃ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T_C=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the Néel transition temperature T_N=163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2Δ₀=(320±15) cm⁻¹ in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures.