Observable zero sound in strongly correlated metals.

The slow zero-sound mode expected near the Mott transition in strongly interacting twodimensional Fermi systems that are neutral is shown to persist as the physical sound mode in the case that the fermion carries electronic charge and is embedded in a positive ionic background. The latter sound velocity softens completely precisely at the Mott transition, indicating that a zone-center structural transition will occur in the system. We suggest that this phonon-softening mechanism is related to the structural transitions commonly observed in the oxide superconductors. PACS Indices: 71.27.+a, 71.45, 63.20.K, 74.20.Mn * Permanent address: Dept. of Physics and Astronomy, California State University, Los Angeles, CA 90032.