Galvanomagnetic and thermonagnetic effects, in quenched bismuth

Abstract

An experimental study of the quenching effects on the gailvanomagnetic and thermomagnetic properties of bismuth single crystals is presented. Measurements made include components of the magneto resistivity and magnetothermoelectric power tensors at low and intermediate magnetic fields for samples in both the annealed and quenched conditions, at temperatures between 77 K and room temperature. Marked effects of quenching on each set of tensor components have been observed. After quenching the anisotropy ratio p(_11)/ p(_33) of the zero field resistivity inverts from 0.9446 to 1.030 and the thermomagnetic data show a general reduction in magnetic field dependence. The quenching effects anneal out below room temperature. To allow a detailed quantitative investigation, a formalism has been developed by which the thermomagnetic data and the predictions of both the phenomenological theory and o f the band transport theory can be compared on the same basis as the galvanomagnetic effects. The expressions provide for the first time an analytical explanation for the Umkehr effect and lead to its prediction. The occurrence of Umkehr effect and sign reversal in the thermomagnetic power of bismuth directly follows from the nature o f the Fermi surface. Measurements a t low and intermediate magnetic fields have been analysed in terms of a two-band multi - valley Fermi surface model, using a least-means-square procedure. Reasonable agreement obtains between the model parameters computed from different sources. Magneto resistivity data at low and intermediate fields evidence consistent quenching-induced changes on the model parameters. Quenching results in an increase in carrier densities from 4.4 10(^23)m(^-3) in annealed state to 4.6 10(^23)m(^-3) and 5.3 10(^23)m(^-3) for electrons and holes respectively. Carrier mobilities in the xy-plane are reduced but those along the trigonal (z) axis are increased slightly. The energy separation between the band edges increases markedly. Quenched-in defects have predominantly accepter-like character.