Bistable tunneling current through a molecular quantum dot

An exact solution is presented for tunneling through a negative-U degenerate molecular quantum dot weakly coupled to electrical leads. The tunnel current exhibits hysteresis if the level degeneracy of the negative-U dot is larger than 2. Switching occurs in the voltage range V-1<V<V-2 as a result of attractive electron correlations in the molecule, which open up a new conducting channel when the voltage is above the threshold bias voltage V-2. Once this current has been established, the extra channel remains open as the voltage is reduced down to the lower threshold voltage V-1. The current hysteresis vanishes above some critical temperature. Possible realizations of bistable molecular quantum dots are fullerenes, especially C-60, and mixed-valence compounds.