Low temperature thermoelectric properties of tungsten

Abstract

A critical analysis of reported measurements of the thermoelectric properties of tungsten is presented. Particular interest lies in the temperature region below 10°K where the thermopower, S, of this metal is distinctly non-linear even for ultra pure samples, rR ~ 10,000. An attempt is made to identify the dominant low-temperature scattering mechanism, normally assumed, on the basis of T2 terms in the electrical resistivity P and the analogous thermal quantity WT, to be electron-electron scattering. In this thesis I reject e-e scattering as the origin of such terms on the basis of magnetoconductivity measurements, the electronic structure of tungsten, and reported measurements of a new thermoelectric function. G(T). It is suggested that in an ideally pure sample e-ph Umklapp scattering on the hole octahedron may cause the appearance of the T2 term in electrical resistivity p and, hence, a positive thermopower. There appears to be some anomalous impurity-related effect at the lowest temperatures, the thermopower of very pure samples showing no systematic variation with residual resistance ratio. This may be analogous to the Kondo effect observed in the thermopowers of noble metals. Difficulties in interpreting measurements of all four thermoelectric coefficientsP,WT, S and G(T) in terms of e-e or e-ph normal Umklapp scattering on either surface, or normal impurity scattering are discussed in some detail.