Electronic Transport through Magnetic Molecules with Soft Vibrating Modes

The low-temperature transport properties of a molecule are studied in the field-effect transitor geometry. The molecule has an internal mechanical mode that modulates its electronic levels and renormalizes both the interactions and the coupling to the electrodes. For a soft mechanical mode the spin fluctuations in the molecule are dominated by the bare couplings while the valence changes are determined by the dressed energies. In this case, the transport properties present an anomalous behavior and the Kondo temperature has a weak gate voltage dependence. These observations are in agreement with recent experimental data.