OBSERVABLE ZERO SOUND IN STRONGLY CORRELATED METALS

The slow zero-sound mode expected near the Mott transition in strongly interacting two-dimensional Fermi systems that are neutral is shown t o persist as the physical sound mode in the case that the fermion carr ies electronic charge and is embedded in a positive ionic background. The latter sound velocity softens completely precisely at the Mott tra nsition, indicating that a zone-center structural transition will occu r in the system. We suggest that this phonon-softening mechanism is re lated to the structural transitions commonly observed in the oxide sup erconductors.