Magnetic field dependence of the thermopower of dilute aluminum alloys

Measurements are presented of the thermopower S 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 equation S(H)=A(H)T+B(H)T³, allowing a separation of the electron diffusion thermopower component S_e(H)=A(H)T from the phonon-drag component S_g(H)=B(H)T³. Zero-field characteristic values of S_e were obtained for the impurities Cu, Cd, Tl, and Sn:S_e(Cu) =-0.6 T×10^-8 V/K;S_e(Cd)=-4 T×10^-8 V/K;S_e(Tl)=-3 T×10^-8 V/K;S_e(Sn) =-2 T×10^-8 V/K. The phonon-drag coefficient B 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 of A(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 of H up to 20 kG, the highest field used. This value is in good agreement with the expected value :L₀=2.45×10^-8 W·Ω/K².