Prediction of large Seebeck coefficient in ferroelectric domain walls of GeTe

Domain walls in ferroelectric materials can have significantly different properties than their bulk counterparts, which creates new opportunities for the manipulation of material properties. GeTe is a ferroelectric material that is also an excellent thermoelectric, exhibiting relatively high electrical conductivity and Seebeck coefficient, as well as low lattice thermal conductivity. In this work, we present a first principles study indicating that ferroelectric domain walls in GeTe have a considerably larger Seebeck coefficient than bulk GeTe. Polarization discontinuity at domain walls gives rise to bound charge that localizes free charge carriers on the domain walls. We find that the two-dimensional electron gas confined to domain walls can exhibit large peaks in the local density of states near the Fermi level due to Van Hove singularities, resulting in an enhanced Seebeck coefficient. Van der Waals gaps along domain walls cause additional confinement of carriers within the domain walls that further increases the in-plane Seebeck coefficient perpendicular to the van der Waals gaps. The out-of-plane Seebeck coefficient is also significantly enhanced due to the charge localization on domain walls.