Fermiology and electron-phonon coupling in the 2H and 3R polytypes of NbS2

Zakariae El Youbi,Christine Richter,Matthew D Watson,Karol Hricovini,Sung Won Jung,Cephise Cacho

doi:10.1103/physrevb.103.155105

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$_2$.

Highlights

Transition metal dichalcogenides (TMDs) are well-known for hosting a variety of instabilities arising from the interplay of electron-electron and electron-phonon coupling
The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in 3R-Nb1+xS2 compared to 2H-NbS2, which we attribute to the presence of additional interstitial Nb, which act as electron donors
For the in-plane S 3px,y valence bands, we find a small splitting of the band dispersions in the 2H phase compared to the 3R, since for these orbitals the interlayer hopping terms are relatively weak compared to the in-plane hoppings

Summary

Introduction

Transition metal dichalcogenides (TMDs) are well-known for hosting a variety of instabilities arising from the interplay of electron-electron and electron-phonon coupling. A phonon mode exhibits significant softening with temperature [11], and 2H-NbS2 can be viewed as being close to a lattice instability. This presents an interesting theoretical challenge, as naïve density functional theory (DFT) calculations would predict a lattice instability [12], and the absence of any CDW phase is attributed to the anharmonic phononic effects [11,13]. The experimental electronic structure of bulk 2H-NbS2 has hardly been

Methods

Results

Discussion

Conclusion