Phonon anharmonicity-induced decoherence slowing down in exciton–phononsystems

Abstract

Based on a generalized Fröhlich model, a time-convolutionless master equation is establishedfor studying the dynamics of an exciton coupled with anharmonic phonons. Specialattention is paid to describing the influence of the phonon anharmonicity on specificelements of the exciton reduced density matrix. These elements, called coherences,characterize the ability of the exciton to develop quantum states that are superimpositionsinvolving the vacuum and the local one-exciton states. Whether the phonons are harmonicor not, it is shown that dephasing limited-coherent motion takes place. The coherencesirreversibly decrease with time, the decay rate being the so-called dephasing rate,so that they experience a localization phenomenon and propagate over a finitelength scale. However, it is shown that the phonon anharmonicity softens theinfluence of the phonon bath and reduces the dephasing rate. A slowdown inthe decoherence process appears so that the coherences are able to explore alarger region along the lattice. Moreover, the phonon anharmonicity modifies theway the dephasing rate depends on both the adiabaticity and the temperature.In particular, the dephasing rate increases linearly with the temperature in theweak anharmonicity limit whereas it becomes almost temperature-independentin the strong anharmonicity limit. Note that the present formalism is appliedto describe amide-I excitons (vibrons) in a lattice of H-bonded peptide units.