  The Seebeck coefficient and electrical conductivity are two critical
quantities to optimize simultaneously in designing thermoelectric materials,
and they are determined by the dynamics of carrier scattering. We uncover a new
regime where the co-existence at the Fermi level of multiple bands with
different effective masses leads to strongly energy-dependent carrier lifetimes
due to intrinsic electron-phonon scattering. In this anomalous regime,
electrical conductivity decreases with carrier concentration, Seebeck
coefficient reverses sign even at high doping, and power factor exhibits an
unusual second peak. We discuss the origin and magnitude of this effect using
first-principles Boltzmann transport calculations and simplified models. We
also identify general design rules for using this paradigm to engineer enhanced
performance in thermoelectric materials.
