K -edge absorption spectra of selected vanadium compounds

Organic semiconductors are attracting increasing interest as flexible thermoelectric materials owing to material abundance, easy processing and low thermal conductivity. Although progress in p-type polymers and composites has been reported, their n-type counterpart has fallen behind owing to difficulties in n-type doping of organic semiconductors. Here, we present an approach to synthesize n-type flexible thermoelectric materials through a facile electrochemical intercalation method, fabricating a hybrid superlattice of alternating inorganic TiS2 monolayers and organic cations. Electrons were externally injected into the inorganic layers and then stabilized by organic cations, providing n-type carriers for current and energy transport. An electrical conductivity of 790 S cm(-1) and a power factor of 0.45 mW m(-1) K(-2) were obtained for a hybrid superlattice of TiS2/[(hexylammonium)x(H2O)y(DMSO)z], with an in-plane lattice thermal conductivity of 0.12 ± 0.03 W m(-1) K(-1), which is two orders of magnitude smaller than the thermal conductivities of the single-layer and bulk TiS2. High power factor and low thermal conductivity contributed to a thermoelectric figure of merit, ZT, of 0.28 at 373 K, which might find application in wearable electronics.

Flexible n-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS2

The dependence of the electrical resistivity of Pb on hydrostatic pressure is determined over major portions of the temperature and pressure ranges 1-300K and 0-10 GPa (100 kbar), respectively, and for Sn at 300K. The results are compared with a simple Bloch-Gruneisen law including volume changes due to thermal contraction. It is demonstrated that both Pb and Sn are ideally suited for use as accurate resistive manometers, enabling a reliable continuous determination of pressure over a wide temperature range. The agreement between the pressure at low temperatures indicated by both resistive and superconducting (Tc) Pb manometers is excellent. Tc(P) calibration curves are given to 22 GPa for Pb and to 5 GPa for Sn and In. A general method is presented which shows how pressure can be used to test for the presence of electron-electron scattering; in the temperature range studied, T>or=7.2K, electron-phonon scattering constitutes the dominant scattering mechanism in Pb, as expected.

https://iopscience.iop.org/article/10.1088/0305-4608/11/3/010/pdf

Pressure and temperature dependence of electrical resistivity of Pb and Sn from 1-300K and 0-10 GPa-use as continuous resistive pressure monitor accurate over wide temperature range; superconductivity under pressure in Pb, Sn and In

Electron dynamics in the bulk and at the surface of solid materials are well known to play a key role in a variety of physical and chemical phenomena. In this article we describe the main aspects of the interaction of low-energy electrons with solids, and report extensive calculations of inelastic lifetimes of both low-energy electrons in bulk materials and image-potential states at metal surfaces. New calculations of inelastic lifetimes in a homogeneous electron gas are presented, by using various well-known representations of the electronic response of the medium. Band-structure calculations, which have been recently carried out by the authors and collaborators, are reviewed, and future work is addressed.

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Theory of inelastic lifetimes of low-energy electrons in metals

On an admissible sign of the static dielectric function of matter

We calculate ${V}_{\mathrm{ee}}$, the effective screened interaction between two electrons in a simple metal. The result is derived by a self-consistent perturbation theory based on Hartree-Fock theory. Exchange and correlation are treated in local approximation. We consider the lattice to be a smooth but elastically deformable, charged medium in which the electrons move. Our results is a generalization of the Fr\"ohlich interaction to include exchange and correlation. The effective interaction is spin dependent and may be written ${V}_{\mathrm{ee}}(q,\ensuremath{\omega})={V}_{0}(q,\ensuremath{\omega})\ensuremath{-}J(q,\ensuremath{\omega}){\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\sigma}}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\sigma}}}_{2}$. Explicit expressions for ${V}_{0}$ and $J$ are given in terms of the lattice stiffness and the same local exchange and correlation potentials of the electron gas which are needed to calculate the dielectric function and the spin susceptibility.

Electron-electron interaction in simple metals

We report measurements of the electrical resistivity, Hall coefficient, magnetoresistance, thermoelectric power, infrared reflectivity, and $c$-axis lattice parameter of single crystals of titanium disulfide ${\mathrm{Ti}}_{1+x}{\mathrm{S}}_{2}$ with varying degrees of nonstoichiometry. The strong correlations we find between different measurements made on the same sample allow us to conclude that titanium disulfide is a semiconductor rather than a semimetal. Even though this fact is established, our most stoichiometric samples continue to exhibit metallic behavior, and the source of these conduction electrons is unknown. In addition, none of the scattering mechanisms examined here is capable of explaining the unusual temperature dependence of the electrical resistivity which varies as ${T}^{3}$ at low $T$ and as ${T}^{y}$ above 100 K where $y$ ranges from 1.85 for the least stoichiometric samples to 2.2 for the most stoichiometric.

Transport and optical properties of Ti 1 + x S 2

The interaction between two-electron quasiparticles is approximated in terms of the dielectric and vertex functions of the uniform electron gas. These functions must satisfy the compressibility sum rule, and this fact makes the interaction at metallic densities much stronger than the Thomas-Fermi screened Coulomb interaction. A problem arises in applying this theory to real metals because the compressibility of the electron gas at densities appropriate to rubidium and cesium is negative. This anomaly is removed by taking into account the real metal effect of core polarization. The effective interaction is used to calculate the electron-electron scattering rate and its contribution to the thermal resistivity. The result is consistent with the single experimental measurement presently available on sodium (new results on potassium, rubidium, and cesium became available after this paper was completed; these are reported in Table III), whereas the Thomas-Fermi interaction predicts a thermal resistivity that is too small by a factor of 7. The scattering rates and thermal resistivities of all the alkali metals are calculated to enable comparison with future experimental values.

Electron-electron scattering in simple metals

Systematic structure in the K-edge photoabsorption spectra of the 4 d transition metals

A series of measurements of the Hall coefficient, infrared reflectivity, thermoelectric power and electrical resistivity of Ti1+xS2 single crystals with various degrees of stoichiometry is described, where, for the first time, each measurement was made on the same crystal (or crystals from the same batch). None of these measurements taken alone can distinguish between the semimetallic or semiconducting models of TiS2. However, by making all four measurements on each sample, it has been possible to establish correlations between the results for different samples. It was found that the product of the Hall coefficient and the square of the plasma frequency is the same for all samples, a result that is consistent with a semiconductor model, but is inconsistent with a semimetal. Nevertheless the most stoichiometric samples remain metallic with electron concentrations of 2*1020 cm-3. It was also found that the resistivity data cannot be explained by carrier-carrier or optical phonon scattering. Therefore, both the source of the residual conduction electrons and the scattering mechanism in TiS2 remain unknown.

https://iopscience.iop.org/article/10.1088/0022-3719/12/13/007/pdf

Carl A. Kukkonen

Papers

Electron-electron interaction in simple metals

Transport and optical properties of Ti 1 + x S 2

Electron-electron scattering in simple metals

Systematic structure in the K-edge photoabsorption spectra of the 4 d transition metals

Hall coefficient and reflectivity evidence that TiS2 is a semiconductor