Phonon dilatation, dressed vibrons, and two-vibron bound-state localization in an adsorbed nanowire

Abstract

A special attention is paid to characterize the two-vibron bound-state dynamics of an anharmonic molecular nanostructure coupled with a set of optical phonons. It is shown that the vibron-phonon coupling is responsible for a new dressing mechanism. The vibrons are accompanied by virtual phonons which account for the scaling of each phonon coordinate and for the dilatation of the corresponding wave function. As a result, the dynamics of the dressed vibrons is governed by an effective Hamiltonian whose frequency, anharmonicity, and hopping constant depend on the number of optical phonons. The two-vibron bound states are defined according to a mean field procedure in which the number of phonons is fixed to their thermal average value. However, the thermal fluctuations of the number of phonons yield a vibron Hamiltonian equivalent to the Hamiltonian of a disordered lattice and they favor the localization of the bound states. For a weak vibron-phonon coupling, the localization results from quantum interferences and it follows a universal behavior. By contrast, for a strong coupling, the localization originates in the occurrence of infinite potential barriers which confine the bound states onto clusters whose number and size are controlled by the temperature.