  Charge and spin density waves, periodic modulations of the electron and
magnetization densities, respectively, are among the most abundant and
non-trivial low-temperature ordered phases in condensed matter. The ordering
direction is widely believed to result from the Fermi surface topology.
However, several recent studies indicate that this common view needs to be
supplemented. Here, we show how an enhanced electron-lattice interaction can
contribute to or even determine the selection of the ordering vector in the
model charge density wave system ErTe3. Our joint experimental and theoretical
study allows us to establish a relation between the selection rules of the
electronic light scattering spectra and the enhanced electron-phonon coupling
in the vicinity of band degeneracy points. This alternative proposal for charge
density wave formation may be of general relevance for driving phase
transitions into other broken-symmetry ground states, particularly in multiband
systems such as the iron based superconductors.
