  The charge-memory effect, bistability and switching between charged and
neutral states of a molecular junction, as observed in recent STM experiments,
is considered within a minimal polaron model. We show that in the case of
strong electron-vibron interaction the rate of spontaneous quantum switching
between charged and neutral states is exponentially suppressed at zero bias
voltage but can be tuned through a wide range of finite switching timescales
upon changing the bias. We further find that, while junctions with symmetric
voltage drop give rise to random switching at finite bias, asymmetric junctions
exhibit hysteretic behavior enabling controlled switching. Lifetimes and
charge-voltage curves are calculated by the master equation method for weak
coupling to the leads and at stronger coupling by the equation-of-motion method
for nonequilibrium Green functions.
