Magnetic field dependence of the thermopower of dilute aluminum alloys

Abstract

Measurements are presented of the thermopowerS of pure Al and dilute Al alloys from 2 to 6 K in magnetic fields up to 20 kG. For all samples the data are consistent with the equationS(H)=A(H)T+B(H)T ³, allowing a separation of the electron diffusion thermopower componentS _e (H)=A(H)T from the phonon-drag componentS _g (H)=B(H)T ³. Zero-field characteristic values ofS _e were obtained for the impurities Cu, Cd, Tl, and Sn:S _e (Cu) =−0.6T×10⁻⁸ V/K;S _e (Cd)=−4T×10⁻⁸ V/K;S _e (Tl)=−3T×10⁻⁸ V/K;S _e (Sn) =−2T×10⁻⁸ V/K. The phonon-drag coefficientB was found to vary from impurity to impurity, a variation attributed to anisotropic electron-impurity scattering. Upon application of a transverse magnetic field,A(H) was found first to become more positive, and then to saturate in value at high fields. The quantity ΔA=A(H→∞)−A(H=0) was found to be very nearly the same for all impurities, varying in value only from 2.1–2.6×10⁻⁸ V/K. Both the general behavior ofA(H) and this magnitude for ΔA are shown to be determined primarily by the form of the Fermi surface of pure Al.B(H) was also observed to vary with magnetic field. This variation is tentatively attributed to a combination of a changing importance with magnetic field of electrons in the second and third Brillouin zones of Al and anisotropic electron-impurity scattering, but no detailed explanation is yet available. Finally, the Wiedemann-Franz ratio of an impure Al-Cu sample is found to have the value 2.43⁵±0.02×10⁻⁸ W·Ω/K² at 4.5 K, independent ofH up to 20 kG, the highest field used. This value is in good agreement with the expected value :L ₀ =2.45×10⁻⁸ W·Ω/K².