Quantum breathers in electron-phonon systems

Quantum breathers are studied numerically in several electron-phonon coupled finite chain systems, in which the coupling results in intrinsic nonlinearity but with varying degrees of nonadiabaticity. As for quantum nonlinear lattice systems, we find that quantum breathers can exist as eigenstates of the system Hamiltonians. Optical responses are calculated as signatures of these coherent nonlinear excitations. We propose a new type of quantum nonlinear excitation, "breather excitons," which are bound states of breathers and excitons, whose properties are clarified with a minimal exciton-phonon model.