Charge transfer empties the flat band in 4H$_b$-TaS$_2$ -- except at the surface

Abstract: The 4H\textsubscript{b} polytype of TaS$_2$ is a natural heterostructure of H and T-type layers. Intriguing recent evidence points towards a possibly chiral superconducting ground state, unlike the superconductivity found in other polytypes where the T layers are absent, requiring understanding of the possible contributions of electrons from the T layers. Here we use micro-focused angle resolved photoemission spectroscopy to reveal that the T termination of the 4H\textsubscript{b} structure is metallic, but a subsurface T layer -- seen below an H termination and thus more representative of the bulk case -- is gapped. The results imply a complete charge transfer of 1 electron per 13 Ta from the T to adjacent H layers in the bulk, but an incomplete charge transfer at the T termination, yielding a metallic Fermi surface with a planar-chiral character. A similar metallic state is found in an anomalous region with likely T-H-H' stacking at the surface. Our results exclude cluster Mott localisation in either the bulk or surface of 4H$_b$-TaS$_2$ and point to a scenario of superconductivity arising from Josephson-like tunneling between the H layers.