Goniopolarity of Thermal Transport Behavior in Layered 2D Materials

NaSn2As2 has recently been synthesized and was found to be an exfoliatable van der Waals Zintl phase, opening new opportunities for electronic and spintronic design on the few-atom-thick scale. Although the band structure may suggest NaSn2As2 to be in the range of metal to semi-metal, it shows strong anisotropy especially in its “polarity”, characterized by its dominant carrier type, which strongly affects its electronic and thermal properties. We used DFT calculations to investigate band structure and Fermi surface, which agree well with ARPES measurements. In addition, we employed BoltzTraP code to calculate the transport behavior in in/cross-plane directions, which predicts strongly anisotropic carrier transport and directionally dependent polarity – “goniopolarity” – in this layered material. It is confirmed by experimental thermopower measurements which find opposite sign for two directions. It indicates thermal transport is based on both electrons and holes, making this layered material an intrinsic 2-carrier system. We show from simulations on a model band structure that this can happen for a single-band system with appropriately shaped Fermi surface with the right fraction of concave vs. convex areas, which allows exploration and design of other new goniopolar materials.