Anharmonic suppression of charge-density-wave instability in bulk and monolayer NbS₂

The superconducting transition-metal dichalcogenide 2H-NbS₂, in sharp contrast to the isoelectronic compound 2H-NbSe₂, at low temperature does not show any charge-density-wave (CDW) ordering co-existing with the superconductive phase. That is in strong disagreement with ab initio harmonic phonon calculations, which predict that 2H-NbS₂ is dynamically unstable.

In this talk we will show that anharmonicty is the key factor to reconcile numerical results with experiments. With the stochastic-self-consistent-harmonic technique [1], we go beyond the harmonic approximation including quantum anharmonic effects in the ab initio calculations and we obtain temperature-dependent phonon frequencies in quantitative agreement with experiment. In particular, the instability observed at harmonic level is removed. Furthermore, we analyze the effect of anharmonicity at low dimensions. Despite in the trigonal single layer form the CDW instability is even more incipient than in the bulk, the system still remains stable at low temperatures. Finally, we verify that the striking difference between 2H-NbS₂ and 2H-NbSe₂ is not simply ascribable to a mass effect, but it is dominated by the different electron screening felt by the ions in the two cases.

[1] R. Bianco et al. PRB 96, 01411, (2017)