Correlation-induced phonon softening in low-density coupled bilayer systems

We predict a possible phonon softening instability in strongly correlated coupled semiconductor bilayer systems. By studying the plasmon-phonon coupling in coupled bilayer structures, we find that the renormalized acoustic phonon frequency may be softened at a finite wave vector due to many-body local-field corrections, particularly in low-density systems where correlation effects are strong. We discuss experimental possibilities to search for this predicted phonon softening phenomenon.