Multistable transport regimes and conformational changes in molecular quantum dots

We analyse non-equilibrium transport properties of a single-state molecular quantum dot coupled to a local phonon and contacted by two electrodes. We derive the effective non-equilibrium (Keldysh) action for the phonon mode and study the structure of the saddle points, which turn out to be symmetric with respect to time inversion. Above a critical electron-phonon coupling lambda_c the effective potential for the phonon mode develops two minima in the equilibrium and three minima in the case of a finite bias voltage. For strongly interacting Luttinger liquid leads lambda_c=0. Some implications for transport experiments on molecular quantum dots are discussed.