Fermiology and Electron-Phonon Coupling in the 2H and 3R polytypes of NbS₂

We revisit the low-energy electronic structure of 2H-NbS₂ and 3R-NbS₂ combining Angle-Resolved Photoemission Spectroscopy (ARPES) and density functional theory (DFT), demonstrating the metallic ground states of both phases. We focus particularly on the Fermi surfaces, which reveal a considerably smaller size in the 3R phase. We attribute this difference to the additional Nb interstitials, without which this polytype cannot be stabilized, which yet act as electron donors, filling thus more Nb orbitals and resulting a n doped electronic structure. We estimate the bands to be shifted downwards by 250 meV into high binding energies compared to the stoichiometric system and the off-soichiometry to be 3R-Nb_(1+x)S₂ with x = 0.13. This off-stoichiometry is believed to be responsible for the absence of any instabilities in this phase. The different origin of the band splitting in each phase is carefully explained. Finally, our high-resolution data on the 2H phase reveals that the electron-phonon coupling is highly dependent on the orbital character of the bands, and this variation in electron-phonon coupling naturally links to the two-gap superconductivity.