We present the results of analytical and numerical studies of the linear conductance of a model of a single-molecule transistor. The model incorporates the effects of the interactions between the electrons and those between the electrons and the vibrational degrees of freedom of the molecule (phonons). The electron-phonon interaction produces a dynamical modulation of the energy levels of the molecule and of the tunnelling barrier between the latter and the electrodes, effects that are absent in artificially structured devices such as quantum dots.
We show that the Kondo effect also occurs in the presence of electron-phonon interactions and leads in all cases to perfect transmission at T=0 when the average number of electrons in the molecule is an odd integer as seen in quantum dots. However, when tunnelling-barrier modulation is present it leads in general to the appearance of asymmetries in the dependence of the conductance on gate voltage that can be very deep in some regions of parameters.
The cases of weak and strong energy-level modulation are qualitatively different. Whereas in the first case the Kondo effect is driven by spin fluctuations just as in quantum dots, in the second case it is driven by charge fluctuations. In this regime the molecule becomes strongly charge-polarized at low temperature by application of a small gate voltage. This effect leads to rectification in the current-voltage characteristics of the molecular junction.
SPSCI – CEA/Saclay