Thermopower Of Metals Research Articles

Theory of record thermopower near a finite temperature magnetic phase transition in IrMn

The effect of scattering of conduction electrons by dynamical spin fluctuations on the thermopower in metals near a thermal phase transition into an antiferromagnetic phase is considered. We are interested in a transition at room temperature, as has been studied in a heterostructure involving layers of IrMn. We show that the electrical resistivity exhibits a narrow but low peak at the transition, which may be difficult to detect on top of the main contributions induced by phonons and impurities. By contrast, the thermopower is found to exhibit a prominent peak both as a function of temperature $T$ for fixed layer thickness ${t}_{\text{AFM}}$ and as a function of ${t}_{\text{AFM}}$ for fixed $T.$ We conjecture that the transition temperature ${T}_{c}$ is a function of both ${t}_{\text{AFM}}$ and the Fermi energy ${\ensuremath{\epsilon}}_{F}$. Both dependencies give rise to a sharp peak of the thermopower as a function of $T$ or ${t}_{\text{AFM}}$ near the transition. The estimated magnitude and width of the peak for the case of either three- or two-dimensional longitudinal spin fluctuations is in good agreement with experiment.

Towards an ab initio calculation of elemental thermocouple output

A simple model is presented for the calculation of the temperature dependence of the thermopower of metals on an ab initio basis, which includes an accurate calculation of the electronic density of states. The aim is to predict the temperature dependence of the electromotive force generated by the elemental thermocouples Au/Pt and Pt/Pd, by calculating the thermopower of Pt, Au, and Pd. The model is compared with the measured thermopower in each case, and found to be qualitatively accurate. It is suggested that the accuracy of the model is limited principally by the ab initio calculation of the electronic density of states of the model.

Impurity band conduction in FeSi1−xAlx

Precise resistivity and thermopower measurements across the metal insulator transition inAl doped correlated semiconductor FeSi are reported. Doping of carriers results in theemergence of electronic states at the Fermi level. For sufficient concentration of dopant,these states form into an impurity band. The properties of such systems are governed bythis impurity band at low temperatures and the semiconducting bands at highertemperatures. Here we show the applicability of such a two-band structure to account forthe temperature dependence of resistivity and thermopower in metals near the metalinsulator transition.

VO: A strongly correlated metal close to a Mott-Hubbard transition

Here, we present experimental and computational evidences to support that rocksalt cubic VO is a strongly correlated metal with non-Fermi-liquid thermodynamics and an unusually strong spin-lattice coupling. An unexpected change of sign of metallic thermopower with composition is tentatively ascribed to the presence of a pseudogap in the density of states. These properties are discussed as signatures of the proximity to a magnetic quantum phase transition. The results are summarized in an electronic phase diagram for the $3d$ monoxides, which resembles that of other strongly correlated systems. The structural and electronic simplicity of $3d$ monoxides makes them ideal candidates to progress in the understanding of highly correlated electron systems.

Some problems in understanding the electronic transport properties of carbon nanotube ropes

Abstract The resistance of single-wall carbon nanotube (SWCN) ropes or mats, and some individual tubes, typically shows a crossover from non-metallic to metallic temperature dependence as temperature increases. This systematic pattern is consistent with a series heterogeneous model involving metallic resistance and tunnelling through barriers such as defects and inter-rope contacts. The metallic resistivity term increases linearly with temperature for the ropes or mats, but faster for the individual nanotubes. In contrast to the almost vanishing thermoelectric power expected from electronic band structure calculations, the measured values for mats or films (including recent measurements in a vacuum) are even larger than for typical metals. The thermopower increases with temperature as for metals, but has a characteristic non-linear shape. This temperature dependence can be modelled, for example, with parallel conduction in metallic and semiconducting tubes, but the size of the metallic thermopower required is anomalously large.

Thermoelectric effect in the hollow superconducting cylinder of Pb and Pb-In alloy

The technique for measuring of the thermoelectric effect in superconductors near the critical temperatureTC has been elaborated. The measured magnetic flux was induced by temperature gradient (ΔT∼1K) between joints. The experimental study of the thermoelectric effect has been carried out in the hollow bimetallic superconducting cylinder consisting of the Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function. The magnitude order of the thermopower estimated from the experimental data coincides with the known low temperature thermopowers of metals.

Thermopower of ultra-high purity YBa 2Cu 3O 7−δ single crystals

We have measured the thermopower of Y 2O 3 crucible grown, ultra-high quality twinned YBa 2Cu 3O 7−δ single crystal in both the a- b (in-plane) and c axis (out-of-plane) orientations. We report negative values for the in-plane thermopower and positive values for the out-of-plane thermopower of overdoped YBCO single crystals. The in-plane measurements correlate with results obtained from sintered polycrystalline YBCO [1] where the a- b plane dominates the electronic conduction process. We discuss the implications of these intrinsic results in terms of the high temperature metallic thermopower relation.

Critical Anomaly in Thermopower near the Spin-Density-Wave Transition in Quasi-One-Dimensional Conductors, (DMET)2AuI2and (DMET)2Au(CN)2

The thermopowers of (DMET) 2 AuI 2 and (DMET) 2 Au(CN) 2 were measured between liquid helium and room temperatures. In addition to the metallic thermopower above the SDW transition temperature (16 and 24 K for (DMET) 2 AuI 2 and (DMET) 2 Au(CN) 2 , respectively), a large excess contribution probably due to the SDW fluctuation was detected below about 70 K.

Electron-phonon contribution to the thermopower of metals.

A theorem is proved for the thermopower when limited by the electron-phonon interaction. It is shown for simple metals that the thermopower is given by the Mott formula plus a correction term. The correction term is evaluated and shown to be small for simple metals. Thus we are able to show that Mott's formula is valid for phonon scattering by electrons, even when the energy dependence of the phonons is taken into account. This proof extends our earlier result, which only applied to static impurities, and to phonons in the adiabatic approximation.

Thermoelectric power and conductivity of heterogeneous conducting polymers.

We discuss the interpretation of thermoelectric power in heterogeneous media, and using a compilation of many sets of data, we analyze the thermopower of conducting polymers as a function of conductivity and of temperature. For samples of very high conductivity, the thermopower (but not the conductivity) shows typical metallic temperature dependence, which is consistent with a heterogeneous model of metallic fibrils separated by thin electrical barriers. Metallic thermopower is expected rather generally to show significant nonlinearities as a function of temperature, and we demonstrate that the observed thermopower in some highly conducting polymers is very similar to the diffusion thermopower of metals in which a knee is produced at low temperatures by the electron-phonon interaction. The thermopower of moderately doped conducting polymers, like their conductivity, is generally consistent with a significant contribution from variable-range hopping.

Variation of thermopower of alkali metals at high temperatures

We have calculated, in an empirical form, the temperature variation of the thermopower of alkali metals in the 50–300 K region and at their melting points using the finite mean free path correction to the Ziman formulation as suggested by Ferraz and March. The structure factor was evaluated using the phenomenological models of Sharma-Joshi and Krebbs. Three different types of form factors were used in order to give a comparative test of the applied theories. Our results agree to a considerable extent with available experimental results. Finally, we give a brief discussion of some of the factors that could affect the thermopower of the metals investigated.

Thermopower in Ce Kondo Systems

The giant thermopower in metals with Ce 3+ impurities is theoretically studied. The relaxation time of a conduction electron is calculated as a function of energy in the selfconsistent ladder approximation by applying Abrikosov's pseudofermion technique to the Coqblin-Schrieffer exchange model with crystal field. The thermopower as well as the electrical resistivity exhibit universal behaviors if the crystal field is neglected. The crystal field induces the characteristic features and the anisotropy in the thermopower versus temperature curve. However, the sign of the thermopower is always positive in Ce system in contrast with Yb systems.

Semiconductor-Metal Transition of [Ch(Fec14)y]x; a Magnetic Impurity Doped Polyacetylene

Abstract Results of electrical conductivity and thermopower measurement on [CH(FeCl4)y]x are presented, where y covers the full doping range. The semiconductor-metal transition is evident and leads to a qualitative change in temperature dependence of the conductivity and thermopower. The analysis of the metallic thermopower data indicates the Kondo-like effect originated from the coupling between the (CH)X chain and the dopant located near the chain. As a consistent interpretation of our data, we propose a mechanism of the semiconductor-metal transition in doped (CH)X; the one dimensional intersoliton electron tunneling model.

Metallic nature of heavily doped polyacetylene derivatives: Thermopower

Metallic thermopower data on ${\mathrm{FeCl}}_{3}$-doped polyacetylene are presented. We compare the results with the data on iodine- and ${\mathrm{AsF}}_{5}$-doped polyacetylene in the heavily doped regime. General features of the results are consistent with those of a simple metal. However, details are different. We interpret these differences as originating from the different coupling strengths between the metallic chain and the dopant in each polyacetylene derivative. The disorder induced by the dopant ion seems to be more important than the three-dimensional interchain coupling. Thus, the charge-transport mechanism may be quasi one dimensional at the high-concentration limit.

Thermodynamic Basis of Thermopower in Metals and Semiconductors

Thermodynamic Basis of Thermopower in Metals and Semiconductors

On the amplitude of the quantum oscillations in the thermopower of metals

A simple relationship is derived between the amplitudes of the oscillations in the magnetothermopower and the transverse magnetoresistivity. The relationship is found to be in reasonably good agreement with the available experimental data.

Diffusion thermopower of metals

The second order effect of the intrinsic two-phonon processes in the electron-phonon interaction gives rise to a new diffusion thermopower of metals. It is suggested that a higher order electron-phonon scattering process gives an additional term to the diffusion thermopower. In contrast to the Nielsen-Taylor effect (1970), such a process is given by the third order effect of the usual one-phonon processes. The absolute magnitude and temperature dependence of the additional term is, however, similar to the Nielsen-Taylor effect. Thus, in a quantitative study of the thermopower of metals, it is necessary to take into account the term suggested here as well as the Nielsen-Taylor term, the usual diffusion term and the phonon drag term.

Measurement of thermoelectric power at low temperatures

Abstract The superconducting galvanometer of Clarke has been applied to measurements of thermopower in metals. Experimental results are presented to show that the sensitivity is at least 100 times greater than has been previously achieved in such measurements.

Anisotropy of electron relaxation time and phonon-drag thermopower

Recent experiments on the thermopower of metals indicate an increase in the phonon-drag contribution to the thermopower on alloying. This can be explained by the anisotropy of the relaxation times for electron scattering by impurities and by phonons.