  Topological semimetals provide new opportunities for exploring new
thermoelectric phenomena, because of their exotic and nontrivial electronic
structure topology around the Fermi surface. In this study, we report on the
discovery of giant transverse and longitudinal magneto-thermoelectric (MTE)
effects in Mg3Bi2, which is predicted to be a type-II nodal-line semimetal in
the absence of spin-orbit coupling (SOC). The maximum transverse power factor
is 2182 {\mu}Wm^{-1}K^{-2} at 13.5 K and 6 Tesla. The longitudinal power factor
reaches up to 3043{\mu}Wm^{-1}K^{-2} at 15 K and 13 Tesla, which is 20 times
higher than in a zero-strength magnetic field and is also comparable to
state-of-the-art MTE materials. By compensating Mg loss in the Mg-rich
conditions for turning carrier concentration, the sample obtained in this work
shows a large linear non-saturating magnetoresistance of 940% under a field of
14 Tesla. This is a two-orders-of-magnitude increase with respect to the normal
Mg-deficiency Mg3Bi2 sample. Using density functional calculations, we
attribute the underlying mechanism to the parent nodal-line electronic
structure without SOC and the anisotropic Fermi surface shape with SOC,
highlighting the essential role of high carrier mobility and open electron
orbits in moment space. Our work offers a new avenue toward highly efficient
thermoelectric materials through the design of Fermi surfaces with special
topological electronic structures in novel quantum materials.
