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.     

Angle-resolved photoemission spectra of 2H-NbSe2

Angle-resolved photoemission measurements have been made on the metallic layer compound 2H-NbSe₂. From the results, E vs k_∥ dispersion curves have been obtained along two principal symmetry directions in the Brillouin zone. The experimental energy bands are found to be in good agreement with the results predicted by one-dimensional density of initial states along k_⊥.     

Transverse energy gap in 2H-NbSe2

The transverse energy gap in 2H-NbSe₂ has been measured using a Nb flat “point” contact tunneling technique. Its value of 1.15±0.07 milivolt yields a value of 3.78 for the ratio 2Δ(0)/kT_c.     

Anomalous electrical resistivity in HfTe5

The group IVB transition-metal pentachalcogenide, HfTe₅, exhibits a peak of the electrical resistivity in the vicinity of 76 K.     

Intensity variations of angle-resolved photoemission spectra of 2H-NbSe2

Angle-resolved photoemission spectra of 2H-NbSe₂ are calculated on the one-step model with a single plane wave final state and within the constant matrix element approximation. The results are compared with the experimental spectra obtained for ?ω = 21.2 eV. It is found that the intensities of the observed peaks are predicted reasonably well by this model (especially by one-dimensional density of initial states along k_?).     

Experimental investigation of superconductivity in 2H-NbSe2 single crystal

The basic quantities characterizing the superconducting behaviour of pure 2H-NbSe₂ single crystals have been determined from specific heat measurements, performed between 0.3 and 10 K in magnetic fields up to 55 KG. When Ti impurities are added, changes are observed both in the superconducting parameters and in the onset of charge density waves, strengthening the idea that a connection exists between the two phenomena.     

Electrical resistivity of UMn2

The electrical resistivity of polycristalline UMn₂ has been measured between 4.2 and 300 K. There is a very small anomaly around the distortion (T_D = 2212 K). At low temperature the T² dependence is tentatively attributed to 3d spin fluctuations.     

Anomalous phonon thermal resistivity in superconducting Ti55Nb45 alloys

The thermal conductivity of cold-worked and heat-treated Ti₅₅Nb₄₅ alloys has been measured in the temperature range between 0.6 and 1.5 K. Anomalously strong scattering of phonons for every sample and the remarkable enhancement in the thermal resistivity caused by the annealing at 500°C on cold-worked alloys have been found. These behaviors indicate the existence of an anomalous phonon scattering mechanism besides dislocation scattering in Ti₅₅Nb₄₅ alloys. Electron diffraction patterns show the circular diffuse streaks characteristic of the precursory lattice distortion for the ω atomic configuration. Two kinds of models which correlate the anomalies in the phonon scattering with the lattice instability of the β phase of Ti-Nb matrix are described.     

Electron diffraction study of the charge-density wave superlattice in 2H-NbSe2

In addition to the well-known 3a₀ CDW superlattice, we have observed in 2H-NbSe₂ a 2a₀ superlattice and a well-defined elliptical contour of diffuse intensity between the 3a₀ CDW spots. The 2a₀ superlattice is indicative of a CDW formation through the saddle-point nesting on the Fermi-surface.     

Elastic constants of 2H-MoS2 and 2H-NbSe2 extracted from measured dispersion curves and linear compressibilities

Recent neutron data on the dispersion curves and X-ray measurements of the linear compressibilities of the 2H polytypes of MoS₂ and NbSe₂ have been used to obtain approximate values of the five independent elastic constants of these materials. In the case of NbSe₂ sufficient information is available to over-determine the elastic constants and the results are self consistent within estimated uncertainties, although the uncertainties are especially large for c₃₃ and c₁₁. Additional related considerations such as Debye temperatures and model calculations of c₃₃, and c₄₄ are also made. It is found that there is significant and unexplained disagreement between the value of the low temperature specific heat Debye temperature of NbSe₂ and the value determined on the basis of the elastic constants, but that the model predictions of c₃₃ and c₄₄ are in satisfactory agreement with the values extracted from the neutron data for both MoS₂, and NbSe₂.     

Thermodynamic properties of superconducting 2H-NbSe2 exhibiting anisotropy and strong-coupling effect

A simple semi-empirical model which takes account of effects of the anisotropy and the strong coupling interaction in the expression of the energy gap together with the ellipsoidal energy spectrum is proposed to explain results of specific heat measurements on layer structure compound 2H-NbSe₂. The thermodynamic properties such as the electronic specific heat and the thermodynamic critical field deduced from this model give good agreements with experimental results.     

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.     

An electron diffraction evidence of charge density wave instability in 2H-NbSe2

A superlattice is observed by electron diffraction in 2H-NbSe₂ below the onset of charge density wave instability at 33.5 K.     

Magnetic disorder resistivity of RECu2Si2 compounds

The resistivity of RECu₂Si₂ compounds contains a term which is due to a mixture of spin and aspherical Coulomb scattering. The disorder resistivities and the ordering temperatures scale well with each other but deviate strongly from the de Gennes factor. The deviations scale with an anomaly of the c/a ratio which develops at low temperatures for the RE ions with L ≠ 0.     

Anisotropy of the electrical resistivity in PrAl2 and DyAl2

The anisotropy of the spontaneous resistivity is measured in the ferromagnetic range for PrAl₂ and DyAl₂ single crystals. For PrAl₂ we observed a change of sign in the temperature dependence of θ₆ ? θ_⊥ which cannot be obtained from existing quadrupole scattering models.     

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.     

Anomalous changes in CuO2 plane in YBa2Cu4O8 under pressure

We report the results of an atomistic simulation study of the pressure-induced structural changes in YBa₂Cu₄O₈ from 0 to 20 GPa. It is found that the crystal has similar compressibilities in the a- and b-directions over the whole pressure range, whereas in the c-direction there are quite different compressibilities at different pressures. Our results suggest that there exists a correlation between the charge transfer and the change in the Cu(2)---O(1) bond length, but we do not support the suggestion of a direct dependence of T_c on the length of this bond. However, we predict that there are anomalous changes in the CuO₂ plane with pressure, which we propose may lead to a charge carrier redistribution on in-plane copper and in-plane oxygens, which in turn may be related to the optimum carrier concentration in the CuO₂ plane.     

Anomalous bragg peak widths in LixTiS2

We have performed X ray diffraction experiments on Li_xTiS₂ and have found an experimental correlation between features observed in the (00?) peak width and -δx/δV data. (00?) peak broadening is indicative of imperfect order in the c direction of the lattice. This suggests that three dimensional models for lithium intercalation in TiS₂ should be investigated.     

Absence of metallicity and bias-dependent resistivity in low-carrier-density EuCd2As2

EuCd₂As₂ was theoretically predicted to be a minimal model of Weyl semimetals with a single pair of Weyl points in the ferromagnet state. However, the heavily p-doped Eu Cd₂As₂ crystals in previous experiments prevent direct identification of the semimetal hypothesis. Here, we present a comprehensive magneto-transport study of high-quality Eu Cd₂As₂ crystals with ultralow bulk carrier density(10¹³cm^-3). In co...     

Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn₂O₅ (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature T_C. This can be explained as an effect of vanishing ferroelectric ordering above T_C. It was found that the reduced sound velocity increases with increasing V⁽³⁾ (the third-order atomic force constants of the anharmonic phonons) in the interval T?<?T_C, whereas the reduced sound velocity remains unchanged in the interval T_C?<?T?<?T_N. It was also found that the reduced sound velocity increases with the increase of V⁽⁴⁾ (the fourth-order atomic force constants of the anharmonic phonons) in the interval T?<?T_N. In addition, the ferroelectric phase transition temperature T_C decreases when V⁽⁴⁾ increases in the interval T?<?T_N. Those theoretical results are in agreement with the experimental data.