  Thermoelectricity was discovered almost two centuries ago in bismuth. The
large and negative Seebeck coefficient of this semimetal remains almost flat
between 300 K and 100 K. This striking feature can be understood by considering
the ratio of electron and hole mobilities and the evolution of their equal
densities with temperature. The large and anisotropic magneto-Seebeck effect in
bismuth, on the other hand, has not been understood up to the present day.
Here, we report on a systematic study of the thermopower of bismuth from room
temperature down to 20 K upon application of a magnetic field of 13.8 T in the
binary-bisectrix plane. The amplitude of the Seebeck coefficient depends on the
orientation of the magnetic field and the anisotropy changes sign with
decreasing temperature. The magneto-Seebeck effect becomes non-monotonic at low
temperatures. When the magnetic field is oriented along the binary axis, the
Seebeck coefficient is not the same for positive and negative fields. This
so-called Umkehr effect arises because the high symmetry axes of the Fermi
surface ellipsoids are neither parallel to each other nor to the high symmetry
axes of the lattice. The complex evolution of thermopower can be accounted for
in a large part of the ($T,B,\Theta$)-space by a model based on semiclassical
transport theory and incorporating Landau quantization. The employed energy
dependence of the scattering time is compatible with electron-acoustic phonon
scattering. We find that the transverse Nernst response plays an important role
in setting the amplitude of the longitudinal magneto-Seebeck effect.
Furthermore, Landau quantization significantly affects thermoelectricity up to
temperatures as high as 120 K.
