Electrical transport through gated single molecules (also called molecular transistors) has become since a few years an active research field both theoretically and experimentally. We investigate the interplay between electrical and mechanical degrees of freedom in transport across a biphenyl molecule in the Coulomb blockade regime. In particular, we analyze the role played in the electrical transport by the twisting mode between the phenyl rings.
At low biases we can restrict our analysis to the neutral and anionic (one extra electron) state of the molecule only. The neutral molecule has two stable configurations at finite dihedral angles (ϑ ≅ +- π/4) while the anion state is planar. Charge transitions between the electrical states are thus modulated by Franck Condon amplitudes that account for the torsional degree of freedom yielding big phonon blockade effects.
We study the system using a generalized master equation for the reduced density matrix. We find that, due to the mechanically degenerate neutral state, the coherencies and not only the populations of the reduced density matrix determine the transport characteristics.