Transport through a molecular quantum dot in the polaron crossover regime

We consider resonant transport through a molecular quantum dot coupled to a local vibration mode. Applying the nonequilibrium Green's function technique in the polaron representation, we develop a nonperturbative scheme to calculate the electron spectral function of the molecule in the regime of intermediate electron-phonon coupling. With increasing tunneling coupling to the leads, correlations between polaron clouds become more important at relatively high temperature, leading to a strong sharpening of the peak structure in the spectral function. The detection of such features in the current-voltage characteristics is briefly discussed.