Nonmonotonic magnetic-field dependence of transport coefficients for n-Bi-Sb semiconducting alloys

The transport coefficients of tellurium-doped n-Bi_{1 - x} Sb _x semiconducting alloys (0.07 ≤ x ≤ 0.15) are studied for single-crystal samples in the temperature range 1.5 ≤ T ≤ 40 K and in magnetic fields 0 ≤ H < 20 kOe. The theory developed in this study attributes the specific features in the behavior of the transport coefficients observed in a magnetic field to a strong anisotropy of the electron spectrum and anisotropy in the electron relaxation time. It is found that the dependences of the transport coefficients on the magnetic field for H → C ₃ can be theoretically expressed through one anisotropy parameter δ, and those for H → C ₂, by means of several anisotropy parameters, namely, γ, η, ζ, and m ₃/m ₁. It is established that the anisotropy parameter δ in the n-Bi-Sb semiconducting alloys can be estimated from measurements of the electrical resistivity ρ₂₂(∞)/ρ₂₂(0) ≅ δ and the Hall coefficient R _12.3(∞)/R _12.3(H → 0) ≅ δ in a magnetic field H → C ₃. It is shown that the observed increase in the thermoelectric efficiency by a factor of 1.5-2.0 in the transverse magnetic fields H → C ₃ and H → C ₂ originates from the nonmonotonic dependence of the diffusion component of the thermopower Δα₂₂(H)(∥T → C ₁) on the magnetic field. The nonmonotonic dependence of the diffusion thermopower in n-Bi-Sb semiconducting alloys is associated with the strong anisotropy of the electron spectrum, the anisotropy in the electron relaxation time, and the many-valley pattern of the spectrum.