Relativistic topological molecular crystals

Topological phases are normally difficult to realize in molecular systems, because the inter-molecular interactions are in general too weak to mediate band inversion. In this talk, however, we shall present a general strategy to hunt for relativistic topological molecular crystals (RTMCs), where the band topology emerges from the band inversion between the interstitial electron states and molecular orbitals, as well as from the strong spin-orbit coupling of heavy molecular building blocks. As an example, we identify the three-dimensional molecular crystal K₄Ba₂[SnBi₄] as a strong topological insulator in a first-principles manner. This RTMC system manifests exceptional characteristics as follows. (1) It hosts cleavable ends along all three directions, with clean pronounced topological surface states. (2) Strong response to external pressure or strain, and hence topological phase transition under relatively low pressure. (3) High-efficiency thermoelectricity, with the Seebeck coefficient S ~ 220 μV/K, and lattice thermal conductivity κ_L ~ 0.22 W/mK. (4) Ultralow work function (~2.3 eV) caused by the interstitial states. Several other RTMCs will be introduced as well. Our work opens an avenue of realizing topological phases in molecular systems, which could be a novel playground of topological and versatile other physics.