Phonon softening in the CDW systems NbSe2 and TiSe2

We present new results on the soft-mode behaviour of phonons in the Charge Density Wave (CDW) systems NbSe2 and TiSe2. Our theoretical predictions from Density Functional Theory (DFT) based on ab-initio phonon calculations coincide with the CDW instability behaviour that we observed experimentally by means of high resolution Inelastic X-ray Scattering (IXS). While TiSe2 shows a rather sharp phonon anomaly at T=190 K, the anomaly in NbSe2 at T=33 K is much broader than expected for a Fermi surface nesting driven CDW instability. For NbSe2, we exclude Fermi surface nesting as main origin of the phonon softening. For TiSe2, the phonon softening seems to be well described within the framework of DFT.