Quantum breathers in nonlinear phonon and coupled electron-phonon systems

Nonlinear quantum phonon excitations are studied in 1D lattice models for both effective nonlinear phonon and coupled electron-phonon systems. Multi-phonon bound states exist which are eigenstates of the system Hamiltonian, and which are natural counterparts of breather solutions of classical nonlinear systems. These quantum breather states form particle-like bands and are characterized by a finite coherence length. Due to their structure and dynamics, quantum breathers have specific experimentally observable responses in many correlation functions. In the pure phonon model, the dynamic structure factor is found to have significant intensity for the breather states, with a corresponding quenching of the neighboring bands of multi-phonon extended states. In the coupled electron-phonon model, IR and optical absorption spectra for breathers are reported.