Cooling a Vibrational Mode Coupled to a Molecular Single-Electron Transistor

We consider a molecular single electron transistor coupled to a vibrational mode. For some values of the bias and gate voltage the electronic transport is possible only by absorption of one or more phonons. The system acts then as a cooler for the mechanical mode at the condition that the electron temperature is lower than the phonon temperature. The final effective temperature of the vibrational mode depends strongly on the bias conditions and can be lower or higher than the reservoir in contact with the oscillator. We discuss the efficiency of this method, in particular we find that there is an optimal value for the electron-phonon coupling that maximizes cooling.