Anharmonic suppression of Charge density wave in 2H-NbS${}_2$

Abstract

The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS${}_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $\hat{\mathrm{I}}“\mathrm{M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe${}_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS${}_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.

6 pages, 5 figures, accepted for publication in Physical Review B