Thermoelectric power generation

Although the polarisation of the light was discovered at the beginning of the nineteenth century, the Vikings could have used the polarised light around the tenth century in their navigation to America, using a ‘sunstone’ evoked in the Icelandic Sagas. Indeed, the birefringence of the Iceland spar (calcite), a common crystal in Scandinavia, permits a simple observation of the axis of polarisation of the skylight at the zenith. From this, it is possible to guess the azimuth of a hidden Sun below the horizon, for instance. The high sensitivity of the differential method provided by the ordinary and extraordinary beams of calcite at its so-called isotropy point is about two orders higher than that of the best dichroic polariser and permits to reach an accuracy of ±1° for the Sun azimuth (at sunrise and sunset). Unfortunately, due to the relative fragility of calcite, only the so-called Alderney crystal was discovered on board a 16th ancient ship. Curiously, beyond its use as a sunstone by the Vikings, during these last millennia calcite has led to the discovery of the polarisation of the light itself by Malus and is currently being used to detect the atmospheres of exoplanets. Moreover, the differential method for the light polarisation detection is widely used in the animal world.     

Thermoelectric power generation in a thermoacoustic refrigerator

Commercially available solid-state thermoelectric devices may be used for their electrical power generation capabilities when coupled to a thermoacoustic refrigerator or heat pump. General performance characteristics as well as bulk thermal conductivity for a selection of thermoelectric elements was first found by using a two-plate apparatus to maintain a constant temperature difference across the element. Further studies of an element’s performance when placed in series with the thermoacoustic refrigerator’s heat exchangers will be presented. Design considerations for using thermoelectric elements in a no-moving parts electrical power generation scheme will be discussed.     

Ground-state structure in a highly disordered spin-glass model

We propose a new Ising spin-glass model on Z ^d of Edwards-Anderson type, but with highly disordered coupling magnitudes, in which a greedy algorithm for producing ground states is exact. We find that the procedure for determining (infinite-volume) ground states for this model can be related to invasion percolation with the number of ground states identified as 2 ^N , whereN=N(d) is the number of distinct global components in the invasion forest. We prove thatN(d)= if the invasion connectivity function is square summable. We argue that the critical dimension separatingN=1 andN= isd _c=8. WhenN(d)=, we consider free or periodic boundary conditions on cubes of side lengthL and show that frustration leads to chaoticL dependence withall pairs of ground states occurring as subsequence limits. We briefly discuss applications of our results to random walk problems on rugged landscapes.     

Direct observation of superexchange in a disordered molecular solid

A novel observation of magnetic superexchange interactions in a disordered molecular solid is reported. Coordinated electrical conductivity and electron spin resonance measurements are made on controlled, variable concentrations of the molecule trip-p-tolylamine and its paramagnetic radical cation, molecularly dispersed in a polymer matrix. For a given concentration of spins in the concentration range 10¹⁹cm^-3 <N_s <5× 10²⁰ cm^-3, pronounced exchange narrowing is discovered as the density of neutral molecules is systematically increased. The temperature dependences of the ESR linewidth and the conductivity eliminate motional narrowing due to diffusive hopping of spins as the mechanism, since in these systems the paramagnetic ions are also the source of mobile charge.     

Thermoelectric power of heavy-fermion compounds

We calculate the diffusion thermopower of the Anderson lattice as a model for heavyfermion compounds in a semi-phenomenological theory. In this theory, the thermopower is expressed by the dynamical susceptibility which describes spin fluctuations and can be measured by neutron scattering. The Kondo effect is taken into account for a singlef-electron spin which is coupled to all other spins and to the conduction electrons. This approach neglects multiple intesite-scattering of the conduction electrons. We obtain a Kondo termS ₍₁₎ ^d (T) (in which the thermopower of non-interacting spins is multiplied by a factor which describes the spin dynamics) and a resonance termS ₍₂₎ ^d (T) of opposite sign which vanishes for vanishing interactions. The superposition of both terms leads to a broad maximum of the thermopower roughly at the Kondo temperatureT _K and to an additional minimum belowT _K . ForT0 the termS ₍₁₎ ^d vanishes asT ² and the termS ₍₂₎ ^d becomes proportional toT. We also show that the Sommerfeld expansion leads to an incorrect result for the low temperature resistivity of the Anderson lattice and that the Gorter-Nordheim relation does not hold at low temperatures.     

Thermoelectric power of bismuth nanocomposites

Because of the increase in the electronic density of states in low-dimensional systems, semiconductor quantum wires constitute a most promising thermoelectric material. We report here the first experimental observation of a very large enhancement of the thermoelectric power of composites containing bismuth nanowires with diameters of 9 and 15 nm, embedded in porous alumina and porous silica. The temperature dependence of the electrical resistance shows that the samples are semiconductors with energy gaps between 0.17 and 0.4 eV, consistent with the theoretical predictions.     

Thermoelectric power measurement in a-ZnSe

Thermoelectric power measurements made as a function of temperature are interpreted on the basis of an energy model obtained by the authors for a-ZnSe. The model includes an identification of the electrical transport processes in the two temperature ranges studied. The results obtained here are consistent with the earlier interpretation of experiments on a-ZnSe.     

Thermoelectric power of TmS and TmSe

The thermopower has been measured from 1.6 to 300 K for TmS and from 0.065 to 300 K for TmSe. The experimental results suggest a dominant scattering of the conduction electrons by heavy 4? electrons with a strongly temperature-dependent density of states at the Fermi level.     

Thermoelectric power of liquid noble metals

A model potential for electron-ion interaction is used to calculate the thermoelectric power of copper, silver and gold in the liquid state. The results obtained from this model potential agree well with experiment.     

Thermoelectric transport properties in disordered systems near the Anderson transition

We study the thermoelectric transport properties in the three-dimensional Anderson model of localization near the metal-insulator transition (MIT). In particular, we investigate the dependence of the thermoelectric power S, the thermal conductivity K, and the Lorenz number L₀ on temperature T. We first calculate the T dependence of the chemical potential μ from the number density n of electrons at the MIT using an averaged density of states obtained by diagonalization. Without any additional approximation, we determine from the behavior of S, K and L₀ at low T as the MIT is approached. We find that and K decrease to zero at the MIT as and show that S does not diverge. Both S and L₀ become temperature independent at the MIT and depend only on the critical behavior of the conductivity. Received 5 February 1999     

Density of states in a disordered solid,Borel-Padé analysis

Wegner's gauge-invariant model of the localization problem is treated on ad-dimensional lattice. Borel-Padé methods are used to analyze the perturbation series for the averaged one-particle Green function. Results for the density of states are obtained.     

Thermoelectric power in three La-Ba-Cu oxides

Using differential method, the thermoelectric power of three LaBaCu oxides was measured in the temperature range 4.2–300 K. The behaviour of higher T_c sample is more metallic. Introducing a f-factor into a formula for semiconductor, the temperature dependence of the thermoelectric power in samples with K2NiF4 structure can be reasonably well explained. Further knowledge about the scattering processes is needed to understand the behavior in samples with perovskite structure.