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.     

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.     

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.     

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.     

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     

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.     

Magnetic phase transitions in PrCo2Si2

Magnetic phases in PrCo₂Si₂ have been studied by measurements of magnetization, neutron diffraction and electrical resistivity. For <9 K, the magnetic structure with a propagation vector k = (0,0,1) [2π/c] is stable. Incommensurate structures k = (0,0,0.926) and (0,0,0.777) appear for 9 K < T <17 K and 17 K < T <30 K, respectively.     

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.     

Solid-solid phase transitions in (CnH2n+1NH3)2 CdCl2Br2 compounds

(C_nH_2n+1NH₃)₂ CdCl₂Br₂ compounds with n≥10 show several solid-solid phase transitions before descomposition takes place at 436 K. The results show a different behaviour between odd or even members of the series studied. The sequence of phase -- transitions can be interpreted as a dynamic order-disorder mechanism followed by a melting of the alkyl chains.     

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.     

Electron-lattice interaction and the CDW state of 1T-TiSe2

The electronic band structure in the CDW state (superlattice structure) of 1T-TiSe₂ is calculated on the basis of the band-type Jahn-Teller model by extending our theory of lattice instability in the normal phase. A strong coupling between the hole-band (Se p states) around the Λ point and the electron-bands (Ti d states) around the Λ points is caused by the electron-lattice interaction. Reflecting such a strong coupling remarkable changes appear in the dispersion curves near the Fermi energy and the largest CDW gap is obtained to be 0.2 eV. We have also calculated a change of the density of states near the Fermi energy due to the superlattice formation. The result is consistent with that observed by angle-integrated photoemmision by Margaritondo et al. It is also shown that the magnitude of the lattice distortion observed at low temperatures can be explained in a way consistent with the lattice dynamics in the normal phase.     

X-ray diffraction study of the CDW phase in (TaSe4)2I: Determination of the CDW modulation amplitude

An extensive X-ray diffraction study of charge density wave (CDW) phase in (TaSe₄)₂I is reported. We have observed the superstructure satellites at 2q in addition to those at q reported by Fujishita et al. The results imply a sinusoidal lattice modulation with polarization almost perpendicular to q (i.e. transverse) and the existence of CDW domains. At 15 K we have extracted an approximate value for the amplitude of the lattice modulation perpendicular to q to be μ_⊥ ∽ 0.087 Å.     

Evidence of phase transitions in K2PbCu(NO2)6

The heat capacity of K₂PbCu(NO₂)₆ shows anomalies at 273K and 280K, which are accompanied by length changes. At 280 K a change in the E.S.R. spectrum is observed.     

Successive jahn-teller phase transitions in Cs2PbCu(NO2)6

The sequential Jahn-Teller phase transitions in Cs₂PbCu(NO₂)₆ were studied by neutron and X-ray diffraction technique. In each phase we observed satellite reflections. The position of satellites in the reciprocal space is commonly given by $\text{k}{}_{\mathrm{<mtext>0</mtext>}}\text{= (}\text{1}\text{2}\text{1}\text{2}\text{ζ)}$ with $\text{0?ζ?0.5.}$ In Phase II and Phase IV, the values of ζ are definitely determined as ζ = 0 and ζ = 0.5 respectively. In Phase III, ζ is not definite within $\text{0.20?ζ?0.25}$ depending on samples, thermal cycling,etc. Phase III may be an incommensurate phase although there is the tendency that the value of ζ is locked in either at 0.20 ($\text{1}\text{5}$) or at 0.25 ($\text{1}\text{4}$).     

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...     

Light-induced ultrafast phase transitions in VO2 thin film

Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68 °C. As a kind of functional material, VO₂ thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400 nm on the obtained films, the phase transition (PT) occurred. The observed light-induced PT was as fast as the laser pulse duration of 100 fs. Using a femtosecond laser system, the relaxation processes in VO₂ were studied by optical pump-probe spectroscopy. Upon a laser excitation an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. The alteration is dependent on pump power. The change in reflectance reached a maximum value at a pump pulse energy between 7 and 14 mJ/cm². The observed PT is associated with the optical interband transition in VO₂ thin film. It suggests that with a pump laser illuminating on the film, excitation from the d_θ,? - state of valence band to the unoccupied excited mixed d_θ,?-π* - state of the conduction band in the insulator phase occurs, followed by a resonant transition to an unoccupied excited mixed d_θ,?-π* - state of the metallic phase band.     

Structural phase transitions in KCdF3

The successive phase transitions in KCdF₃ have been studied by using X-ray diffraction and differential type thermal analysis. Three transitions are observed at 485, 471 and 243 K; these are of 1st-, 2nd- and 2nd-order respectively. The space groups of the four phases are found, from the high temperature side, to be Pm3m, P4/mbm, Pbnm and Pbnm respectively. The first and second transitions can respectively be explained in terms of the condensation of soft-phonon modes at the M and R points in the cubic Brillouin zone. The third transition may be caused by instability due to titlings of CdF₆ octahedra.