Anharmonic suppression of charge density waves in 2H-NbS 2

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 phonon bands appears along the ΓM symmetry line in reciprocal space. In sharp contrast to the isoelectronic compound 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. © 2012 American Physical Society.