  Since their theoretical prediction by Peierls in the 30s, charge density
waves (CDW) have been one of the most commonly encountered electronic phases in
low dimensional metallic systems. The instability mechanism originally proposed
combines Fermi surface nesting and electron-phonon coupling but is, strictly
speaking, only valid in one dimension. In higher dimensions, its relevance is
questionable as sharp maxima in the static electronic susceptibility \chi(q)
are smeared out, and are, in many cases, unable to account for the periodicity
of the observed charge modulations. Here, we investigate the quasi
twodimensional LaAgSb2, which exhibits two CDW transitions, by a combination of
diffuse xray scattering, inelastic x-ray scattering and ab initio calculations.
We demonstrate that the CDW formation is driven by phonons softening. The
corresponding Kohn anomalies are visualized in 3D through the momentum
distribution of the x-ray diffuse scattering intensity. We show that they can
be quantitatively accounted for by considering the electronic susceptibility
calculated from a Dirac-like band, weighted by anisotropic electron-phonon
coupling. This remarkable agreement sheds new light on the importance of Fermi
surface nesting in CDW formation.
