Cryogenic Thermoelectric Enhancement by Nonparabolic Band-Edge Transport in Bi₂Te₃.

Abstract

Selective scattering of electrons near the Fermi level is the kinetic origin of the thermoelectric effect. Pronounced band nonparabolicity near the band edge is expected to promote the decoupling of a high Seebeck coefficient from high electrical conductivity; however, accessing this band-edge transport regime at low temperatures remains challenging, as defect-dominated scattering often masks the intrinsic band-structure effects. Here, we experimentally show that single-crystalline Bi₂Te₃ can access a reduced-scattering band-edge transport regime in which the transport distribution becomes strongly energy dependent, enabling simultaneously a sizable thermopower and a high carrier mobility at cryogenic temperatures. This approach yields a record thermoelectric power factor of three times as high as that of conventional parabolic band-dominated Bi₂Te₃. Quantum oscillation measurements reveal multiband transport components consistent with the band-structure complexity of Bi₂Te₃, and magneto-thermal conductivity measurements indicate a reduced Lorenz factor and suppressed electronic thermal conductivity in the same regime. The resultant over 600% thermoelectric enhancement in conventional Bi₂Te₃ demonstrates a practical strategy of advancement by engineering band-edge transport in strong spin-orbit coupled materials.