Strong Polaronic Effect In A Superatomic Two-Dimensional Semiconductor

Crystalline solids assembled from superatomic building blocks are attractive functional materials due to their hierarchical structure, multifunctionality, and tunability. An interesting example is Re6Se8Cl2, in which the Re6Se8 building blocks are covalently linked into two-dimensional (2D) sheets that are stacked into a layered van der Waals solid. It is an indirect gap semiconductor that, when heavily doped, becomes a superconductor at low temperatures. Given the finite electronic bandwidths (300-400 meV), carrier properties in this material are expected to be strongly influenced by coupling to phonons. Here, we apply angle-resolved photoemission spectroscopy to probe the valence band edge (VBE) of Re6Se8Cl2. We find that dispersion of the VBE is a strong function of temperature. The bandwidth is W = 120 +/- 30 meV at 70 K and decreases by one order of magnitude to W similar to 10 +/- 20 meV as temperature is increased to 300 K. This observation reveals the dominant polaronic effects in Re6Se8Cl2, consistent with the Holstein polaron model commonly used to describe molecular solids.