Fermiology and electron-phonon coupling in the 2H and 3R polytypes of NbS$\boldsymbol{_2}$

TL;DR

This study compares the electronic structures of 2H and 3R NbS$_2$ polytypes, revealing that stoichiometry influences their properties more than stacking order, and highlights electron-phonon interactions relevant to superconductivity.

Contribution

It provides the first detailed comparison of Fermi surfaces and electron-phonon coupling in 2H and 3R NbS$_2$, emphasizing the role of stoichiometry over stacking arrangement.

Findings

3R-Nb$_{1+x}$S$_2$ has larger Fermi surfaces due to extra Nb donors.

Electron-phonon coupling is stronger in bands with Nb 4$d_{x^2-y^2,xy}$ character.

Stoichiometry significantly affects electronic properties more than stacking order.

Abstract

We investigate the electronic structure of the 2H and 3R polytypes of NbS$_2$. The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in 3R-Nb$_{1+x}$S$_2$ compared to 2H-NbS$_2$, which we attribute to the presence of additional interstitial Nb which act as electron donors. Thus we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the 2H phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb 4$d_{x^2-y^2,xy}$ character than for the Nb 4$d_{3z^2-r^2}$. Our results provide an experimental framework for interpreting the two-gap superconductivity and "latent" charge density wave in 2H-NbS$_2$.