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

TL;DR

This study reveals that in 4H_b-TaS_2, charge transfer empties the flat band in the bulk but not at the surface, leading to a metallic surface state with potential implications for its superconductivity mechanism.

Contribution

It provides the first detailed ARPES analysis showing charge transfer effects and surface-bulk differences in 4H_b-TaS_2, clarifying the electronic structure relevant to its superconductivity.

Findings

Complete charge transfer in the bulk empties the flat band.

Surface T layers remain metallic due to incomplete charge transfer.

No evidence of cluster Mott localization in bulk or 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.