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.     

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.     

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.     

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.     

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.     

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     

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.     

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.     

Density-of-state effects in the low temperature properties of TiBe2

The importance of exact theoretical and experimental determination of the band structure near E_F for an understanding of the properties of TiBe₂ is pointed out. Precise density-of-state data, obtained from ab-initio self-consistent band calculations, are used to determine low temperature variations in the susceptibility χ and specific heat γ. The influence on the field and temperature variations on χ is striking and qualitative agreement with experiment is possible without invoking spin-fluctuations.The field and temperature variations of γ are important enough to be considered, but are smaller than for χ and considerably smaller than observed experimentally by Stewart et al. Thus evidence of spin-fluctuations are more convincing when extracted from γ-data than from χ-data.     

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.     

The low temperature heat capacity and electrical resistivity of ReO3

Low temperature heat capacity and electrical resistivity measurements are reported for ReO₃. The heat capacity data give an acoustical mode Debye temperature θ = 327 K, and an electrronic density of states parameter γ = 2.83 mJ/mole-K². The observed temperature dependence of the resistivity is consistent with the existence of electron scattering both from acoustic mode phonons and from optical mode phonons of characteristic temperature θ_E = 1080 K. The above measurements are used to evaluate the electron-phonon interaction parameter λ = 0.24.     

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.     

The influence of substrate temperature on electrical properties of Cu/CdS/SnO2 Schottky diode

Polycrystalline CdS samples on the SnO₂ coated glass substrate were obtained by vacuum evaporation method at low substrate temperatures (T_S=200 and 300 K) instead of the commonly used vacuum evaporation at high substrate temperatures (T_S>300 K). X-ray diffraction studies showed that the textures of the films are hexagonal with a strong (0 0 2) preferred direction. Circular Cu contacts were deposited on the upper surface of the CdS thin films at 200 K by vacuum evaporation. The effects of low substrate temperature on the current-voltage (I-V) characteristics of the Cu/CdS/SnO₂ structure were investigated in the temperature range 100-300 K. The Cu/CdS (at 300 K)/SnO₂ structure shows exponential current-voltage variations. However, I-V characteristics of the Cu/CdS (at 200 K)/SnO₂ structure deviate from exponential behavior due to high series resistance. The diodes show non-ideal I-V behavior with an ideality factor greater than unity. The results indicate that the current transport mechanism in the Cu/CdS (at 300 K)/SnO₂ structure in the whole temperature range is performed by tunneling with E₀₀=143 meV. However, the current transport mechanism in the Cu/CdS (at 200 K)/SnO₂ structure is tunneling in the range 200-300 K with E₀₀=82 meV.     

Low-temperature electrical properties of V2Ga5

Electrical and magnetic measurements are reported for V₂Ga₅ crystals. A transition to the superconducting state was observed at 4.2 K. The temperature dependence of the critical fields and the Ginzburg-Landau-parameter ? were also determined. Information is also given about the temperature dependence of the ideal electrical resistivity between 10 and 300 K.     

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.     

Low temperature thermodynamical properties of ErCu2Si2

ErCu₂Si₂ crystallises in the tetragonal ThCr₂Si₂-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k=(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.     

Electronic structure, magnetic and electrical properties of multiferroic PbFe1/2Ta1/2O3

Structural analysis of the synthesized lead iron tantalate, PbFe_1/2Ta_1/2O₃ (PFT) is performed by the refinement of the X-ray diffraction data at room temperature using the GSAS code. Energy dispersive X-ray spectrometry analysis is done to find out the chemical composition. The electronic structure of PFT is calculated by the first principles full potential linearized augmented plane wave method. The spin polarized density of states shows the insulating nature. The magnetic moment of 4.3 μB per Fe ion is obtained from the electronic structure calculation using the GGA+U method and compared with the available experimental data. The electronic structure of the PFT is verified by X-ray photoemission spectroscopy. The dielectric spectroscopy is applied to investigate the electrical properties of PFT in the frequency range from 100 Hz to 1 MHz and in the temperature range from 183 to 253 K. The frequency dependent electrical data are analyzed by conductivity formalism. The relaxation mechanism is explained using the Cole-Cole approach.     

Low temperature thermal properties of PbI2

The heat capacity and thermal conductivity of a large (56.5 g) crystal of PbI₂ have been measured in the temperature region 0.5 < T < 3.9°K. Analysis of the heat capacity data yields a value of the limiting apparent Debye characteristic temperature θ₀^c = 99.4 (±0.3)°K, which corresponds to an average lattice wave velocity of 1.151 (±0.005) × 10⁵ cm sec^?1. It is consistent with a wave velocity estimated from neutron scattering experiments, but not with one determined from Brillouin spectra. The heat capacity data also show that dispersion of the low frequency waves is not unusual, as might have been expected for a layer-type crystal. The apparent thermal conduction is found to be surprisingly small in the crystal.     

The electrical resistivity of ThRu2

A simple procedure is given for fitting experimental electrical resistivity data to the parallel resistor model. The procedure is applied to the C-15 compound ThRu₂.